Steeped

The Chemistry of Tea

Steeped

The Chemistry of Tea

By

Michelle Francl

Bryn Mawr College, USA & Vatican Observatory, Vatican City State

Email:mfrancl@brynmawr.edu

Hardback ISBN: 978-1-83916-591-7

PDF ISBN: 978-1-83767-038-3

EPUB ISBN: 978-1-83767-039-0

A catalogue record for this book is available from the British Library

© Michelle Francl 2024

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Preface

Tea is simple: loose-leaf tea, hot pure water, a cup.

I inhale the scent, tiny delicate pieces of the tea floating above the cup.

I drink the tea, the essence of the leaves becoming a part of me.

I am informed by the tea, changed.

—Thích Nhất Hạnh

I start my day by making a quick aqueous extraction of secondary plant metabolites. I stir in a few grams of a purified natural disaccharide, clutch the cup in my hands, inhale a hit of linalool, and take my first swallow of tea. Even if you’re not a chemist, you very likely start the day in the same way, making a cup of coffee or tea to enjoy its caffeine-fueled bite and heady aroma, even if you wouldn’t describe it in quite those terms. To be honest, even though I’m a chemist, I wouldn’t describe it that way either, at least not before the caffeine has kicked in.

This idea for this book grew out of a tweet. “Chemistry question for #ChemTwitter: is there an extractive advantage to the tetrahedral teabag over the traditionally shaped teabag? Or does it just look cooler?” mused @andrechemist. #ChemTwitter had thoughts, but I started to wonder if there was any published research about the effect of a teabag’s shape on the taste of the resulting tea. In fact, just what did chemists know about making a cup of tea? I was between two terms of teaching and so spent a couple of days rummaging around in the research literature to see what was known about the best way to make tea. Quite a lot, as it turns out, beginning with a mysterious woman chemist from the 19th century to chemists and physicists from all corners of the world in the 21st. And yes, there was research on the best size and shape for a teabag. Just before the start of the COVID-19 pandemic in 2020, I wrote a short essay for the journal Nature Chemistry about making the perfect cup of tea from a chemist’s perspective, from which this book ultimately emerged.

While that original article was written for chemists, this book is meant for tea-drinkers who may or may not be chemists. If you haven’t taken a chemistry course, or haven’t taken one recently, don’t worry. I’ve broken down the chemistry you need to know to safely plunge into the molecular world of tea and come out with a better cup. What is a saccharide? Why do I crave the aroma of linalool nearly as much as the buzz from caffeine? And, speaking of caffeine, what does the tea plant get from it? If you are a chemist, I hope you will delight in what follows as much as I did; there is a rich literature out there.

Each chapter begins with a tea pairing. I’ve given at least two options and tried to include ones that are more likely to be available at a local market, along with more specialty recommendations. Tea is a natural product and one of the joys is that every cup is different, depending on where and when the plant was grown and how it has been treated since. Even the local water used to brew the leaves can change the taste of the final brew, as we will see in Chapter 5.

Want to know more? Each of the chapters ends with a section entitled Going Further. If something in a chapter has piqued your interest, the resources here might be the next step in going deeper into the material. The books that I recommend are generally in print, widely available from booksellers and public libraries, and accessible to the general reader. Journal articles can be more difficult to access because publishers can charge quite a bit for the material. I have tried to select articles that are readable by a non-chemist and available for free. The Reference sections cite material from the primary literature that may be of more interest to those with a science background.

The bottom line is what can chemistry tell us about brewing a better cup of tea, whatever that might mean to each of us. Want to make sure you get every last bit of caffeine you can? Or is the calming floral aroma what you desire? What is the best way to keep a cup of tea hot on the way to work? Sections entitled Brewing a Better Cup have advice on how to get your optimal cup of tea based on the science of that chapter.

I read more than 500 papers while researching this book and drank at least 483 cups of tea (I didn’t start counting until I was partway through). I drank deuterated water, spiked my tea with l-theanine, and sampled tea with more than the usual amount of GABA. I wired my teapots and teacups with temperature sensors and measured cooling curves. I tried decaffeinating tea with vodka (not recommended). I did not scrape snow off the plum blossoms in my yard to melt for tea, despite the rave reviews in an 8th century manuscript.

A comprehensive review of the primary research literature on the chemistry of tea would require several volumes, so by no means should this book be considered the final word on all that scientists know about tea, its production, and its infusion. Instead, I have focused here on work that addresses the questions that interested me. I’ve had a great time steeping in the scientific literature about tea and hope this book will give you the eyes to see what’s hidden in your cup of tea too. I invite you to make a cup of tea, perhaps one that is paired with the chapter, and enjoy taking a dive with me into the molecular mash-up that is the world’s most popular beverage: tea.

Michelle Francl

Acknowledgments

Thanks must go to #ChemTwitter and, in particular, to @andrechemist, whose tweet got me started looking at the chemical literature around tea. I’m incredibly grateful to my mother, Lois Cullen Miller, who offered me my first cup of tea on a cold fall morning and who taught me to be curious about the unseen aspects of the world around me, and to my father, Eugene Miller, who always warmed the teapot for me when I visited. My husband, Victor Donnay, was a steadfast support and cheerleader throughout the writing of this book. I have shared many cups of tea and many conversations about chemistry and Star Trek with Lisa Chirlian over the years, which have enriched the writing of this book.

My thanks, too, to Stuart Cantrill, who encouraged me to rummage around in chemistry’s odder corners for the Thesis column in Nature Chemistry, including my take on the best cup of tea from a chemist’s perspective. I first encountered Wilhelmina Green and her paper detailing the chemical analysis of tea infusions while a Herdegen Fellow in 2012 at the Science History Institute in Philadelphia, then the Chemical Heritage Foundation.

Christopher Donnay checked that my combinatorics regarding sucrose were correct and read drafts of the introductory material. Michael Donnay sourced tea for me in the UK and read drafts. Andrew DiDonato read the material on ceramics. Denise Conte, Gene Miller, and Leah Miller read portions of the manuscript. Jenna Margolis read and commented on nearly the entirety of the manuscript and I appreciated her perspective on the biological aspects.

Kim Belcher introduced me to Rose Congou tea, which has improved both my afternoons and my writing. Sarah Binau pointed out that caffeine from tea has a different name than caffeine derived from coffee in some languages. I would never have known about the relationship between aluminum and hydrangeas if it were not for a tweet from Christopher Smith, SJ. A conversation with Michelle Mancini and Camilla MacKay at (ironically) a Bryn Mawr College coffee hour led me to explore some of the finer points of teapots.

I must also thank my colleagues, past and present, at Bryn Mawr College for all their support. In particular, I have had the support of the Frank Mallory Professorship in Chemistry, established by Sally Mallory. Trips to Japan with faculty colleagues Marc Schulz and Hank Glassman expanded my experience of tea and its connections to Buddhism. My colleagues at the Vatican Observatory have shared with me the delights of that other caffeinated beverage, at least when you are in Rome. The director of the Observatory, Guy Consolmagno, SJ, pointed me to Masters of Reality and their song T.U.S.A., as well as to some useful methodology for measuring the volumes of odd-shaped objects.

The beautiful photographs that accompany the text were styled and taken by Andrew DiDonato. The idea to include a tea pairing for each chapter was not mine, but that of an anonymous referee of the book proposal. It was a brilliant suggestion, thank you. And, finally, I would like to thank my editor, Helen Armes, for the invitation to spend this time steeping in the chemical literature on tea and for her patience and encouragement throughout the writing of this book.

Disclaimer

The Royal Society of Chemistry is not responsible for individual opinions expressed in this work and does not endorse or recommend the products mentioned herein. Other products are also available. Products should be used in accordance with the manufacturer’s instructions.

Readers should be aware that the products discussed in this book were those that were available in the US in 2023. The ingredients in current versions of these items may be different.

Contents

Chapter 1A Cup of Chemistry

1.1The Mysterious Wilhelmina Green

1.2What This Book Is About

1.3What This Book Is Not About

1.4An Incredibly Brief History of Tea

1.5Chemistry Lessons

1.6Chemistry TL;DR

1.7A Slightly Longer Course in Chemistry

1.8Chemistry’s Big Ideas

1.9How to Read a Chemical Structure—or What Do Molecules Really Look Like Anyway?

1.10Decoding Chemical Names

1.11Molecules in the Mirror

1.12Pouring the Big Ideas Into a Teacup

Further reading

Chapter 2Reading the Tea Leaves

2.1Tea Is a Work of Art

2.2Wither or Not?

2.3Bruised and Battered: Enzymatic Oxidation

2.4Killing the Green: Hitting the Off Switch

2.5Maillard’s Reaction

2.6Shaped and Dried

2.7Microscopic Chemists: the Making of Fermented Pu’erh Teas

2.8White and Yellow Teas

2.9From the Leaf to the Cup

2.10Brewing a Better Cup

Further Reading

References

Chapter 3The Drug in the Cup

3.1A Stimulating Beverage

3.2An Atom’s Eye Tour of Caffeine

3.3Molecular Locks and Keys

3.4Adenosine: A Molecular On/Off Switch

3.5Blocking the Signal to Sleep

3.6Caffeine and Humans: It’s Complicated

3.7From Cup to Receptor: Brewing a Dose of Caffeine

3.8Pharmacokinetics or Why Am I Still Awake at Midnight After Having Had Tea at 4?

3.9Side Hustles

3.10Can You Get Addicted to Caffeine?

3.11What Does the Plant Get From Caffeine?

3.12Making Tea Without Caffeine

3.13Beyond Caffeine

3.14Brewing a Better Cup

Further Reading

References

Chapter 4The Taste of Zen

4.1A Joy and a Delight

4.2A Molecular Inventory

4.3Reading the Tea Leaves

4.4A Taste of Zen

4.5A Potent Wizard

4.6Molecular Zen: l-Theanine

4.7The Power and Peril of Oxygen

4.8Catechins: Can You Catch Them All?

4.9Molecular Chameleons

4.10Bitter Dregs: Other Alkaloids

4.11Fluoride Is in More Than Your Toothpaste

4.12Metals and Minerals

4.13Tea Is Radioactive and So Are You

4.14Organic Acids and Vitamin C

4.15Insects Are a Feature Not a Bug

4.16Brewing a Better Cup

Further Reading

References

Chapter 5The Agony of the Leaves

5.1Extracting What You Want

5.2Making a Cup of Tea

5.3Tea is Mostly Oxidane

5.4“Tea You Have Not”: Time and Temperature

5.5Hotness Levels

5.6A Little Agitation Is a Good Thing

5.7Tea Grown Cold

5.8Exhausted Leaves Get a Second Wind

5.9Tea in an Instant

5.10Brewing a Better Cup

Further Reading

References

Chapter 6Sugar and Spice

6.1Spicing Up a Cup

6.2Milk

6.3Milk or Tea First

6.4A Taste of Honey

6.5Naturally Sweet: Sugars

6.6Sweet Surprises

6.7Tiny Proteins

6.8Molecular Love Triangles

6.9Chai: Tea Can Be Hot in More Ways Than One

6.10Tea. Earl Grey. Hot

6.11Drink This With a Grain of Salt

6.12Lemon

6.13Brewing a Better Cup

Further Reading

References

Chapter 7Earth, Wind, Fire, and Water

7.1The Technology of Tea

7.2What Comes Between You and Your Tea: Teabags, Balls, and Baskets

7.2.1A Brief History of Teabags

7.2.2Paper Teabags

7.2.3Plastic Teabags

7.2.4Spent Teabags

7.2.5Size is What Matters

7.2.6Infusers: Balls and Baskets

7.3 Objects of Desire: What Do I Want in a Teapot

7.3.1Rock, Paper, Scissors

7.3.2Rock

7.3.3Glass

7.3.4Scissors (Metal

7.3.5Paper

7.4Getting Into Hot Water

7.4.1Calorie Counts

7.4.2Burning Questions

7.4.3The Nuclear Option

7.4.4Climate Change

7.5Keeping Tea Cozy

7.6Brewing a Better Cup

References

Epilogue

Subject Index

Chapter 1

A Cup of Chemistry

Tea Pairing

Enjoy this chapter with a cup of loose-leaf black Assam tea, reminiscent of what Wilhemenia Green used for her analysis of tea, or a Twinings Irish Breakfast Tea.

1.1 The Mysterious Wilhelmina Green

In the spring of 2012, I got on a train four days a week to meet up with chemists from the 19th century, or at least their ghostly imprints pressed into the pages of scientific journals. One of those chemists was Wilhelmina Green, whose 1885 paper, “On the infusion of tea,” I first encountered while browsing The Chemical News in the Othmer Library in Philadelphia. In it she carefully and clearly laid out her chemical analysis of an infusion of black tea. Assam Pekoe-Souchong, brewed in a white porcelain pot, to be as precise as Green herself was. I was as taken with Green’s attention to these small details as I was by encountering a woman chemist in this era, publishing on her own and not tucked into her husband’s paper. Green, in turn, was surprised at how little chemists knew about the chemical composition of brewed tea, a gap in the literature she briskly set out to remedy. She deftly explored the chemistry of the brew with an eye to figuring out the best way to extract the desirable compounds.

After water, tea is the most popular beverage in the world. It is estimated that more than two billion cups a day are drunk and five million tons of dried leaves are harvested each year. Apparently, you cannot write about the chemistry of tea without making note of its popularity. The authors of virtually every journal article I read while researching this book—more than 500 of them—assert this fact in their introduction. I occasionally wonder if tea-drinkers have a bit of an inferiority complex relative to coffee-drinkers, which is why researchers are so insistent on reminding the readers that tea is important to more people than coffee. Or does the l-theanine in tea make us less voluble than our coffee counterparts?

Tea has piqued scientists’ curiosity not just because of its popularity, but because of the health benefits that for centuries have been attributed to the drinking of tea. Tea invigorates, tea calms. It has high levels of valuable antioxidants, it is neuroprotective. Tea has potential risks too. Is the caffeine it contains addictive? Does it contribute to the development of kidney stones, or gout, or liver failure? Does drinking hot tea increase the risk of cancer? Is green tea healthier than black tea?

The chemistry of tea is also entwined with more fundamental discoveries in science. In 1902, Emil Fischer was awarded the second-ever Nobel Prize in Chemistry, in part for his work on caffeine. In 1913, Hungarian chemist George de Hevesy had tea with another young scientist working at Manchester, Henry Moseley. de Hevesy wondered aloud if there was a way to follow the water molecules in the tea through his body. Moseley, a brilliant young physicist who that same year would uncover the key to the ordering of the elements in the periodic table, thought it unlikely. But 20 years later, with the help of Harold Urey’s gift of heavy water and a few dozen goldfish, not to mention liters of distilled urine, de Hevesy would crack the problem. The work would contribute to de Hevesy’s 1943 Nobel Prize in Chemistry for the development of isotopic tracing—a phenomenon that has spawned such innovations as PET scans and methods for mapping ocean currents. In 1925, Ronald Fisher’s exposition of the null hypothesis, a fundamental notion in statistics and data science, emerged from a conversation (over tea) about how to test the claim that someone could tell the difference between a cup of tea to which milk had been added and a cup where the milk had been put in first.

Like Wilhelmina Green, I want to know what is in my cup of tea and how to use what is known about the chemistry of tea to make the best cup. I’m curious, too, about what happens in my body, not just to the water in that teacup, but to the rich mix of chemicals it contains when I drink it. This book takes a dive into a cup of tea to see it as a chemist might—and yes, addresses the critical question of whether milk or tea goes in the cup first.

1.2 What This Book Is About

Chemistry and tea, of course. Which means, as a starting point, I have packaged up in this chapter a very short course in chemistry for those who may not have a background in this field. Make a calming cup of tea and join me to discover four of the basic concepts that underlie much of modern chemistry and learn how to decipher the structures and names of molecules. If even that feels like too much, there is a three-paragraph summary below (Chemistry TL;DR) that should get you started, although at points in the book you may want to backtrack and get a little more in-depth information.

Those who remember some of their secondary school or university chemistry (and chemists, biochemists, biologists, and the like) should jump right in at the second chapter, Reading the Tea Leaves, which tackles the chemistry that underlies the processing of tea leaves. How do the leaves of a single plant give rise to so many different styles of dried tea ready to be brewed? What do bruised apples, molds, and fungi have to do with the production of tea?

Arguably the molecular star of tea is caffeine, the world’s most widely used psychoactive substance. The Drug in the Cup explores what caffeine does in the brain, how caffeine ends up in the tea leaf, and how to get it out if you would prefer to have a little less zing in your cup. There are, as it turns out, thousands of other molecules in a cup of tea besides caffeine—along with some surprising minerals, including fluoride and aluminum. The Taste of Zen takes a tour of this rich molecular soup and examines the role some of these compounds play in creating the taste and aroma of tea infusions.

The conditions under which the chemical compounds in tea leaves are extracted affect the final infusion, as anyone who has suffered a cup of tea made with lukewarm water can attest. The Agony of the Leaves considers the ways in which the water temperature and the time permitted for the infusion can change the amount of caffeine and antioxidants in a cup. It also unravels the mystery of the white film that sometimes appears on tea that has been prepared using a microwave oven (and how that might be related to the ring left in your bathtub).

Milk first or tea first? People have strong feelings about this, but are there chemical reasons for choosing one way over the other? Sugar and Spice takes a molecular tour of the things we add to tea, from milk and sweeteners to the spices that warm a cup of chai. Why might you consider adding a pinch of salt to tea?

Does the shape of a teabag matter? What about the shape of a teapot? Earth, Water, Fire, and Air explores these questions, as well as how to make synthetic rocks—ceramics—to contain tea. How water gets hot and the best strategies for keeping tea that way, from tea cozies to thermos flasks, finishes our chemical tour of tea.

1.3 What This Book Is Not About

When someone offers us a cup of tea, we assume we are getting a beverage that is made by immersing the leaves or other parts of some plant in hot water for several minutes. The leaves are then removed and we might be offered a sweetener, some lemon, or a dairy product to stir into our cup. Chemists call this an infusion, while an herbalist might call it a tisane.

Tea in this book refers to an infusion made from the leaves of one particular evergreen shrub, native to Asia and now grown worldwide: Camellia sinensis. The name for tea’s Linnaean genus, Camellia, honors Georg Kamel,† a 17th century Jesuit brother and botanist who worked in the Philippines. Sinensis is Latin for “from China.” There are two principal varieties of tea, Camellia sinensis var. assamica and Camellia sinensis var. sinensis. While “herbal teas,” such as infusions of mint, rooibos, and chamomile, have their own unique and fascinating chemistry, I won’t discuss them here.

This book is also not about the history of tea, nor the sociopolitical ramifications of that history. The history of tea is many millennia long, complex, and—at certain times and in certain places—tightly interwoven into Western colonialism. The brief history I give later in this chapter is superficial in the extreme and not in any way comprehensive. It is not my intention to cover this complicated and often fraught story here because to do it justice would require its own book. Readers who want a historical context for tea may consult the Further Reading at the end of the chapter for suggestions of excellent histories of tea from a variety of perspectives.

1.4 An Incredibly Brief History of Tea

The chemistry behind a cup of tea is ancient chemistry, older perhaps than the chemistry of ceramics, or of dyes and paints, or of brewing. Archeological evidence for the medicinal use of plants by modern humans goes back at least 60 000 years, 40 000 years earlier than known human forays into the making of pottery. So, what made people think of soaking leaves in water (hot and otherwise) and drinking the resulting infusion in the first place? The short answer might be zoopharmacognosy: animals do it. Animals, including chimpanzees and porcupines,‡ have been observed to engage in zoopharmacognosy, deliberately chewing specific plants and either swallowing them or applying them externally to treat illnesses and injuries. In other words, extracting pharmacologically active material from plants is a behavior we have both inherited from our primate progenitors and observed in the natural world. (One of the origin stories for coffee, in fact, has a goatherd observing that when his goats ate the berries from a particular tree, they did not sleep at night.)

The origins of both the C. sinensis plant and the practice of making tea from its leaves are lost in the mists of time. Wild C. sinensis plants have never been identified. Evolutionary biologists see genetic evidence that implies tea plants were domesticated not in a single place, but perhaps in as many as three different places. C. sinensis plants found outside tea gardens are thought to be feral—descended from plants that had previously escaped the confines of cultivation—rather than wild. We are left with myths and indirect evidence as to the origin and history of the cultivation of C. sinensis. One legend attributes the origin of the tea plant to the sixth century Buddhist monk, Bodhidharma, who is credited with the founding of Zen Buddhism. He is said to have fallen asleep while meditating in a cave. Angry with himself, he tore off his eyelids and threw them out of the cave, where they took root and grew into a plant whose leaves, when infused, would make a stimulating beverage, keeping sleepy monks from nodding off during their meditations.

Archeologists suggest that the cultivation of C. sinensis and the practice of drinking of tea made from its leaves goes back at least 600 years before Bodhidharma. Evidence from burial sites in Xi’an, a city in the central Chinese province of Shaanxi and the easternmost outpost of the legendary Silk Road, suggests that tea was being drunk and traded as early as 2100 years ago. Even early on, tea was an exotic commodity, grown and produced in places most people could not imagine and would never visit. In the middle of the eighth century, tea master Lu Yu wrote The Classic of Tea, describing the cultivation, production, and brewing of tea using methods that the modern tea-drinker would recognize. The drinking of tea remained tightly linked with Buddhism. Tea was introduced to Japan along with Zen Buddhism by the Buddhist monk Eisai. By the 16th century, a culture of tea thrived in Japan, complete with elaborate ceremonies and dedicated spaces devoted to the Way of Tea.

Tea spread westward (and eventually space-ward). North African scholars and explorers of the 14th century, Ibn Battuta and his contemporary Sa’id of Mogadishu, wrote about the customs surrounding tea they encountered in China. Tea came to the attention of Europe much later; the earliest references to tea are not until the 16th century. Giovanni Botero, in his book On the Causes of the Magnificence and Greatness of Cities, describes tea as an herb from which the Chinese pressed “a delicate juice.” He emphasizes the health benefits of tea over alcohol, an argument that coffee-drinkers would also wield to good effect in this same period. The wholesale importation of tea into Europe began with the Dutch at the beginning of the 17th century. Shortly thereafter, tea became popular in England and from there immigrated to the nascent United States of America. The English, who had been indirectly trading opium for Chinese tea, brought Chinese tea plants to what is now modern day India and established tea gardens there, although tea was long known and already cultivated on the Indian subcontinent. Early in the 20th century, tea emerged as a crop in subtropical Africa. Tea has even had a part to play in trouble-shooting equipment on the International Space Station.

1.5 Chemistry Lessons

This book is about the chemistry of tea. While chemistry is my job, five days a week, it may have been a long time since you took a chemistry class, or you may never have taken one at all. Certainly chemistry is a rich field that you can spend years studying, but with a little background you can have a deeper appreciation of the world at a molecular level. You can learn to see into not only a cup of tea, but everything you encounter, with the eyes of a chemist.

In order to immerse ourselves in the chemistry of tea, we will need two things. The first is an overview of the four big ideas chemists use to explain how atoms and molecules behave. The second is the equivalent of a magic decoder ring, a brief lesson in how to decipher the weird and tongue-twisting names chemists give to molecules and the cryptic structural drawings we use.

To be fair, you may not want to wade through all the chemistry I have on offer before you plunge into the chemistry of tea specifically. If even a very short introduction to chemistry is not your cup of tea, the bare essentials are in the three paragraphs that immediately follow. You can always return to this chapter for more details if you are curious as you read further.

1.6 Chemistry TL;DR§

Matter, including everything in my cup of tea, is made up of atoms. Electrons are the glue that holds atoms together. The behavior of materials depends on the arrangement of the atoms, not their source, whether natural or synthetic, or their names.¶ Atoms can be rearranged to make new materials. Most of the molecules I discuss in this book are built on a carbon atom framework decorated with hydrogen atoms, oxygen atoms, and nitrogen atoms. This is what chemists mean by an organic molecule.

Molecules can react with each other when they get close. Many reactions can be understood as interactions between positive and negative charges; in the chemical world, opposites really do attract. Molecules can also hand electrons off from one to another, a process called oxidation.

Chemists show the framework of carbon atoms in organic molecules by drawing lines. So, a hexagon is really six carbon atoms arranged in a circuit (Figure 1.1). Atoms connected by two or three lines are more tightly attached to each other than those with one line. Hydrogen atoms are generally not shown. Long tongue-twisty names are often compact instructions for assembling the molecule. Most chemical names contain (for chemists at least) clues to the structure, and therefore clues to the function, of the molecule. With no apologies to Shakespeare, [(2R,3R,4S,5S,6R)-2-{[(2S,3S,4S,5R)-3,4-dihydroxy-2,5-bis(hydroxymethyl)oxolan-2-yl]oxy}-6-(hydroxymethyl)oxane-3,4,5-triol by its other name—sucrose—tastes just as sweet.

Figure 1.1(a) Cyclohexane (C₆H₁₂) and benzene (C₆H₆) represented as standard chemical line structures. (b) Cyclohexane and benzene with all their carbon and hydrogen atoms shown explicitly.  (c) Cyclohexane shown as its puckered chair shape. (d) The bond distances and angles of benzene.

Now feel free to skip ahead to the next chapter on the chemistry of caffeine, or push on to learn more about the chemistry lurking in your cup of tea.

1.7 A Slightly Longer Course in Chemistry

I have taught introductory chemistry for four decades, a course that consists of more than a hundred hours of lecture over a year’s time. Don’t tell my students, but you don’t need that many hours of instruction to qualitatively understand much of the chemistry that you encounter in daily life.‖ Here, as I do with my students, we’ll look at some of the major themes that drive chemistry and learn a bit about the primary language of the chemical world: molecular structure.

To start, what exactly is chemistry, at least relative to other sciences such as biology or physics? There’s an old joke that says if it is green or wriggles, it is biology, and if it explodes or smells, it is chemistry. More seriously, chemistry might be simply defined as the science of change, working to understand why and how one material transforms into another. Why does tea get darker in color after it sits for a time? Why does a white haze appear if you use a microwave to heat the water for your tea?

Chemistry is also the science of molecular behavior. There is some truth in that old joke, as many chemical processes release compounds that smell. Despite the joke’s subtext that smells are an unfortunate thing, when it comes to tea, scent is a desirable thing. Chemistry can help answer the question as to why some of these molecules smell the way they do, as well as many other properties. What gives Earl Grey tea its distinctive aroma? Why is green tea tinged a pale ochre and black tea deeply copper-toned? Why does caffeine give us a rush of energy?

There are not hard and fast lines between the different areas of science. Much interesting work happens at the intersection of fields. For example, biochemistry, which sits at the crossroads of biology and chemistry, tackles the molecules of life. Even though the main focus of this book is the chemistry of tea, we will often wander from chemistry into biochemistry, and even make brief forays into biology, physics, and engineering.

1.8 Chemistry’s Big Ideas

There are a few big ideas chemists wield to understand how the world works at the molecular level. We can use four of them to understand almost all of the chemistry I will discuss in this book.

The first notion is that atoms are the building blocks of matter. You can think of them like Lego blocks. You can use the same blocks to make a house, or a car, or a spaceship. They can be combined in many different ways, but not every block can be connected to every other block, and only certain orientations between blocks will work. The shapes of the blocks constrain what you can make from them. Chemistry is the science—and the art—of knowing how to combine atomic building blocks to make different materials, given the constraints nature imposes on the connections.

All matter is made up of atoms. (There is another bad joke that says you can’t trust atoms because they make up everything.) In this sense, chemistry is fundamental to understanding how the tangible universe works. A molecule is a discrete collection of atoms with a fixed structure. A formal definition of a molecule is that it is the smallest unit into which a chemical compound can be broken down and still retain its fundamental identity. Molecules are often what we think about when we think about a chemical. But matter can also take other forms as well—for example, many crystalline substances, such as table salt, are not made up of individual molecules, but of repeating patterns of charged particles called ions. Polymers, such as the common plastic polyethylene, are long chains of atoms. Diamond is a network of carbon atoms extending for long distances in all three dimensions. Molecules can be small, like water, which is made up of only three atoms. They can also be enormous; proteins are molecules that may have 10 000 or more atoms.

Atoms are built from electrons, tiny negatively charged particles, and nuclei, which are clusters of positively charged particles called protons and uncharged particles called neutrons. The identity of an atom is determined by the number of protons in its nucleus. All atoms that have one proton in their nucleus are hydrogen atoms and all atoms with six protons in their nucleus are carbon atoms, regardless of the number of neutrons they contain. The familiar periodic table lists the number of protons along with the standard chemical symbols used to represent atoms. Chemists and biologists can use subtle changes in the nucleus of an element, such as the addition of an extra neutron, as an atomic level tracking device. This is how George de Hevesy was eventually able to track the water from a cup of tea through the human body.**

One way that atoms connect to each other is through sharing their electrons. The two hydrogen atoms in water, H₂O, are each linked to the central oxygen atom by a shared pair of electrons, what chemists call a covalent bond. Sharing more than one pair leads to a shorter and stronger bond: two pairs make a double bond, three pairs a triple.

The second big idea is that opposites charges attract each other, while like charges repel each other. This is another way that atoms connect to each other. For example, in table salt, the positively charged sodium ions (Na⁺) are attracted to the negatively charged chloride ions (Cl⁻). Chemists call these electrostatic attachments ionic bonds and the resulting materials are often crystalline in nature. Water molecules have a positively charged side and a negatively charged side and so can insinuate themselves between charged ions, making many of these compounds soluble in water. The larger the charges, the more tightly held the ions are to one another and the less likely they are to be soluble in water. So, while table salt, comprised of +1 and −1 ions, readily dissolves in water, calcium carbonate, made up of Ca²⁺ and CO₃²⁻ ions, does not. Instead, it makes seashells and the scum that sometimes floats on your tea.

As anyone who has done any cooking knows, you have to stir things to get them to combine. Chemistry is just a specialized form of cooking (or vice versa) and the same principle applies. The third big idea that chemists wield is that molecules and other materials must get into close proximity to each other in order to react. This is often most easily done when things are in a liquid form, where the molecules and atoms are close together and easily able to move around. This lets them approach each other at the correct orientation. Molecules are very mobile in the gas phase, but compared with a liquid they are much further apart. Water molecules in the gas phase, as in the steam swirling above your cup of tea, are about 1500 times as far apart as they are in the liquid in the cup. Conversely, while molecules in the solid phase are about as close together as those in a liquid, they are trapped in a single spot like patrons in a movie theater and can only make contact with their nearest neighbors.

The last fundamental idea—and the most important one—we will find helpful in understanding the chemistry of tea is that the molecular shape controls molecular function and molecular properties. It is such an important idea that I tell the students in my introductory chemistry course that if this is the only thing they remember from my class in years to come, I would consider my work a success. Neither the name of the molecule, nor its  origins—natural or synthetic—has any influence on its behavior.

Shakespeare said a rose by any other name smells as sweet. The same is true of molecules. You sometimes hear that you shouldn’t eat anything an eight-year old can’t pronounce. For example, while 4-hydroxy-3-methoxybenzaldehyde sounds off- putting, perhaps even dangerous, it is just the formal name of the main chemical compound in vanilla extract. By either name it smells wonderful and you would not hesitate to use it in a recipe. We will see later in this chapter why chemists use such awkward names and how to decode them.

Just as the name we give a molecule has no effect on its behavior, neither does its source. Western chemists have known for almost 200 years that you can make compounds indistinguishable from those found in living organisms using inorganic materials—that is, starting from things found in rocks and minerals. The 19th century German chemist Friedrich Wöhler showed that you can make urea molecules from potassium cyanide and ammonium chloride that are identical to the urea molecules extracted from urine. Two decades later, Hermann Kolbe showed it is possible to make acetic acid, the main ingredient of vinegar, from non- biological materials, in this case carbon disulfide.

Some people mistakenly believe that compounds made by biological organisms have special qualities. For example, beta- carotene, the pigment that gives carrots their characteristic orange color and is also found in tea, is sometimes used to color food products as a substitute for what are called “coal tar dyes.” At the atomic level, a beta-carotene molecule made by a carrot or a tea plant is completely identical in structure to a beta-carotene molecule synthesized by a chemist in the laboratory. Once you get down to this level, there are no remnants of the carrot clinging to the naturally produced beta-carotene, nor traces of the laboratory bench stuck to the synthetic version. Since their structures are the same, we can expect them to have the same properties and for them to behave identically. In fact, most of the beta-carotene that is used to color food is made synthetically in chemical factories, not extracted from carrots. And you can make those “coal tar dyes” from plants! It is the structure, not the source or the name, that matters when it comes to molecules.

Still, it can be very hard to get past the notion that the source of a molecule matters. Psychologists have shown that just the thought of disgust can be a powerful motivator for human behavior. For example, would you stir your soup with a fly swatter that had been previously used to swat flies, but subsequently had been cleaned and sterilized? I wouldn’t, even though I understand there are no remnants of the fly remaining, nor any other contaminants on the swatter. In the same way, it can seem reasonable to assume that because beta-carotene can be extracted from something edible—carrots—it is somehow more natural, and perhaps safer to eat, than the dyes made from sticky and inedible coal tar. But atoms truly have no memories of where they came from. Just as the same Lego block can be used to build a cottage or a starship, the carbon and other atoms that make up carrots are no different from the atoms found in coal tar.

Most of the molecules I’ll discuss in this book are so-called organic molecules. These are molecules that are built on a carbon and hydrogen framework. In addition to carbon and hydrogen, these molecules may also contain oxygen, nitrogen, sulfur, fluorine, or chlorine. Chemists classify molecules that don’t contain carbon as inorganic. Most herbicides and pesticides are organic molecules, in the chemical sense of being based on a carbon frame. Organic has a quite different meaning in chemistry than it has when it referring to fruits and vegetables!

1.9 How to Read a Chemical Structure—or What Do Molecules Really Look Like Anyway?

In 2008 Joon Mo Kong created a molecular structure to illustrate the New York Times review of Mary Roach’s book Bonk about the science of sex. Chemists flipped out. They weren’t bothered by the frank discussion that opens the review, but the illustration that accompanied the article offended them to the core. They wrote letters to the editor pointing out its implausibility—“It’s complete nonsense, you can’t have five bonds to carbon!”—and asking for the structure to be corrected. But there was no correction needed. Kong had simply used the familiar elements of chemical line structures to spell out the word “SEX.” It was glaringly obvious to anyone who was not trained as a chemist—and utterly invisible to anyone who was (see Figure 1.2).

Figure 1.2The word “TEA” written using elements from chemical line structures in a way that would make any chemist wince.

Because one of the major ideas in chemistry is that molecular structure dictates molecular function, chemical structures are an important part of how chemists think about and communicate what is happening on a molecular level. We have developed a number of different shorthand methods that allow us to quickly outline the important parts of a molecule, as well as some more sophisticated methods for visualizing more subtle effects. What follows is a short course on how to see these different representations of molecules as chemists see them.

Chemists are indoctrinated from the earliest days of their training to see the atoms behind the spare and elegant line structures like those Joon Mo Kong riffed off to create his illustration. We rely on the underlying rules that atoms must follow in a molecule to read these figures. The International Union of Pure and Applied Chemistry, known as IUPAC, maintains standards for many things in chemistry. Their recommendations on drawing chemical structures take up 133 pages! Here’s a much shorter introduction to these structures and their meaning.

Go back to Figure 1.1(a) and compare the two hexagons. At a single glance, chemists see these to be two entirely different molecules: C₆H₆ (benzene) and C₆H₁₂ (cyclohexane).†† Because they have (to a chemist) such different structures, chemists expect them to have different properties. Benzene, for example, is a known carcinogen, with an almost perfume-like odor. Cyclohexane, by contrast, smells like cleaning fluid and is not thought to cause cancer in humans.

While at first glance there doesn’t appear to be any carbon (C) or hydrogen (H) in these molecules, chemists are trained to see each vertex as a carbon atom. Bonds between carbon and hydrogen aren’t usually shown, partly because they clutter up the picture and partly because they are often less reactive than other bonds. So, while we could draw these molecules as in Figure 1.1(b) with all their carbon and hydrogen atoms explicitly shown, chemists generally don’t. Since chemists know that each carbon atom must have four covalent bonds to it—that is, four possible sites for attachment—we can always mentally add in the hydrogen atoms. Covalent bonds, connections between atoms made by sharing electron pairs, are depicted by lines. One line is a single bond, two lines means the atoms are linked by a double bond, and three indicates a triple bond. The more bonds between atoms, the stronger the attachment and the harder they are to break apart.‡‡

There is still more that chemists can read into these structures. Chemists know that the atoms attached to a carbon with a double bond point at the corners of a triangle, while the atoms attached to a carbon with four atoms point at the vertices of a tetrahedron (Figure 1.3). We often use wedges and dashes to indicate that bonds are popping out of the page or fading behind it.

Figure 1.3 Left: a tetrahedral carbon atom with four points of attachment, all set at angles of 109.47° to each other. The solid wedge is read as popping out of the page; the dashed wedge as receding behind the page. Right: a trigonal planar carbon atom. It has three points of attachment, all 120° apart. The double line indicates that one point of attachment is a double bond. Both of these carbon atoms can make a total of four bonds.

As a result, chemists can intuit from these diagrams that benzene is flat like a plate, while cyclohexane is puckered like a recliner chair (Figure 1.1 (c)). Dame Kathleen Lonsdale, an Irish chemist and crystallographer, definitively determined in 1929 that benzene is planar—until then, many organic chemists believed it was chair-shaped. Lonsdale worked up the data from her experiments at home while the parent of a newborn daughter. I imagine many cups of tea were consumed in the process.

Chemists also have a rough sense of the distance between atoms. Single bonds between carbons are about 150 pm long, whereas those between a carbon and a hydrogen are a bit more than 100 pm (Figure 1.1 (d)). A “pm” is a picometer, which is one-billionth of a meter, or about one ten-thousandth the thickness of a page of this book. In other words, molecules are outrageously small and atoms are unbelievably tiny. There are billions more atoms in a teaspoon of water than there are stars in the Milky Way and far more molecules in your body than solar systems in the entire universe.

Small as molecules are, they have more heft than the line structures would suggest. Sometimes chemists use ball and stick models to get a better sense of the space that the atoms in the molecule occupy. The atoms are represented as spheres and the bonds between them as sticks (Figure 1.4). The atoms are usually color-coded, typically black for carbon, red for oxygen, blue for nitrogen, and white for hydrogen, although if you were able to see the atoms, they would all be the same color.

Figure 1.4Upper panel: ball-and-stick representations of cyclohexane and benzene. Middle panel: space-filling representations of cyclohexane and benzene. Lower panel: electron density surfaces from a quantum mechanical calculation.

If you were able to take a picture of an atom, you would see that the nucleus is extraordinarily tiny compared even to the size of the overall molecule. The diameter of the nucleus of a carbon atom is about 5 fm. An “fm” is a femtometer, which is one-quadrillionth of a meter—that is, one 1 000 000 000 000 000th of a meter. The diameter of a carbon nucleus is about one-millionth the diameter of a human blood cell, too small to see using a regular light microscope or with any imaging technique currently known. Even chemists find it difficult to imagine how small the nucleus of an atom is. The diameter of a carbon nucleus is about one 10 000th of the overall length of a benzene molecule.

The ball and stick models still aren’t as representative of reality as they could be. We can now make images of molecules using techniques such as X-ray diffraction (the technique used to discover that DNA has a double helix structure) and atomic force microscopy (a high-tech method to “feel” the texture of a surface at the atomic and molecular levels). Seen this way, molecules look like a bunch of fuzzy tennis balls smashed together. When we look at these images, what we are seeing are not the tiny nuclei, but the electron cloud that surrounds them. Chemists call the models that look most like these clouds of electron density “space-filling models” because they show us most clearly the space that a molecule occupies ( Figure 1.4). We generally color code the atoms to remind us which nuclei are which, but the images show us that all the atoms look identical.

Though all these representations of molecules imply that they are rigid and motionless, molecules are always on the move. Molecules in the gas and liquid phases never sit still, but continuously dash about their containers, bumping into each other and the container’s walls. At 100 °C the gaseous water molecules that are streaming out of a boiling tea kettle are moving on average at nearly 700 m s⁻¹ (2500 km h⁻¹ or 1500 mph), almost twice the speed of sound. As they move about in either the liquid or gas phase, the molecules are also simultaneously wiggling and tumbling. Bonds stretch and bend, behaving more like springs than lines or sticks. Even in the solid phase, where molecules are essentially pinned in a single position, they are still quivering and tumbling in place.

1.10 Decoding Chemical Names

The same chemical structure can be called by many different names. There are trivial (or common) names, trade names, and systematic names, the latter being the formal name for a molecule. For example, H₂O is commonly called water, even by chemists. Even though you will hear people call water “dihydrogen monoxide” in an attempt to sound ultra-scientific, the formal IUPAC-approved systematic name is actually oxidane. Vinegar is primarily acetic acid; the IUPAC systematic name for acetic acid is ethanoic acid. Teflon is a trade name for a polymer of tetrafluoroethylene, which is used as a nonstick coating on cooking pans. Its systematic name is poly(1,1,2,2-tetrafluoroethylene).

Chemists build the names of molecules like they build molecules: from smaller pieces. And while these names can seem like alien tongue-twisters, there can be a natural beauty hidden inside both trivial names and the building blocks used for systematic names. The systematic stem in ethanol, CH₃CH₂OH, the kind of alcohol that we drink, is eth- and indicates the presence of a two-carbon atom chain in the molecule. It is derived from the word ether, the material thought to fill the sky, the stuff of heaven. Which a beer after a long day of work might be. A butyl chain has four carbon atoms in it and gets its name from a down-to-earth food—butter—because butyric acid, which smells of rancid butter, was found to have four carbon atoms in it.

Scratch the surface of a molecule’s trivial name and you may find a clue to where it is found in the natural world, or even a dash of whimsy, as the structures in Figure 1.5 suggest. Thearubigin, the chemical responsible for the red–brown color of tea, gets its trivial name from the botanical family in which the tea plant is found, Theaceae, and its ruby color. Shikimic acid, found in some teas, gets its name from the flower of the star anise plant: shikimi in Japanese. If you squint, penguinone bears a passing resemblance to a penguin and snoutane looks like the snout of a crocodile.

Figure 1.5Standard chemical line structures for thearubigin, shikimic acid, penguinone, and snoutane.

Chemists use the complex names of molecules as a compact way to encode their three-dimensional structures. The full formal name of a molecule tells an advertent chemist precisely what the molecular structure is. These names are cumbersome even for chemists to use because they are often long and can contain a jumble of numbers and Greek letters. So, chemists use common names, essentially short nicknames, for their molecules. It is much like the way a GPS system uses latitude and longitude to precisely locate a place on a map. Generally, this is not how you would tell a friend how to find your house—instead, you would probably use your address or a local landmark. So, rather than say I had an aqueous solution of (2R,3R,4S,5S,6R)-2-{[(2S,3S,4S,5R)-3,4-dihydroxy-2,5-bis(hydroxymethyl)oxolan-2-yl]oxy}-6-(hydroxymethyl)oxane-3,4,5-triol (whew!), I am far more likely to say I have a solution of sucrose—ordinary table sugar—in water.

Given a molecule’s systematic name, a chemist can reconstruct its shape and, while we don’t need to know in detail how to go from the name to the structure, a few hints can help us decode the chemical names that I will use in this book. As a bonus, the chemical names that you see on many product labels will be less of a mystery.

Consider the key compound responsible for the delicious scent of vanilla extract: 4-hydroxy-3-methoxybenzaldehyde ( Figure 1.6). The “benz” in methoxybenzaldehyde tells a chemist that there is a flat hexagon of carbon atoms in the molecule, while hydroxy indicates the presence of an OH unit. Methoxy—a portmanteau of methyl and hydroxy—means there is also an OCH₃ unit. “Aldehyde” indicates the CHO cluster at the top of the molecule. Lots of aldehydes have pleasant aromas, so this is already a clue that 4-hydroxy-3-methoxybenzaldehyde might smell good. The numbers 3 and 4 tell a chemist where the methoxy and hydroxy units are attached to the hexagon.

Figure 1.6Structure of vanillin (4-hydroxy-3-methoxybenzaldehyde). The carbons in the hexagon are numbered 1–6, starting with the aldehyde (there are rules about where to start the numbering in those 133 pages of IUPAC instructions). The methoxy group is attached at position 3 and the hydroxy group (OH) at position 4.

Here are some general hints for decoding formal chemical names. Structures of the example molecules are shown in Figure 1.7.

1.Words ending in -yl, -ane, -ene, or -yne indicate carbon chains. The length of the carbon chain is indicated by the stem of the word. Methyl, for example, is a carbon chain with just one carbon atom, while pentene is a carbon chain with five carbon atoms. Cyclo- in front of a name indicates that the carbon chain is wrapped back onto itself. Table 1.1 lists some of the most common stems for different lengths of carbon chains, along with their etymology.

2.Numbers indicate the points of attachment of different molecular substructures—what chemists call functional groups.

3.“Phen” or “benz” mean there is a hexagonal planar substructure, as in phenylalanine or benzoic acid.

4.Hydroxy (–OH) groups are signaled by words ending in “-ol” (like alcohol, which is CH₃CH₂OH) or by the prefix hydroxy-. Sugar, as you might guess from the name, has lots of OH groups attached to it, eight to be exact. “-oxy” endings with a carbon chain prefix indicate an oxygen attached to a carbon framework, “ethoxy” means –OCH₂CH₃ is present.

5.“-oic” or “-ic” indicates an organic acid, meaning a COOH group is attached. Citric acid is an organic acid found in lemons and oranges.

6.“-ate” means there is an anion, a negatively charged ion, and, because opposites attract, there will be a positively charged counter ion—often sodium—present as well to balance it out. Sodium benzoate is a common preservative and monosodium glutamate (MSG) is used to enhance flavors.

7.For molecules that have mirror images, the letters R and S indicate which of the two reflections you have. R stands for rectus (Latin for right) and S for sinister (Latin for left). + and − or d (again from Latin, dexter for right) and l (laevus for left) are also used to indicate one of two mirror image molecules. For a longer explanation about mirror images, and their importance for biological molecules in particular, see the next section.

Figure 1.7Molecular structures illustrating the connection between names and structures.

Table 1.1List of commonly encountered stems for different lengths of carbon chains (and one uncommon stem)

No. of carbon atoms in chain | Stem | Etymology
1 | Meth- | Derived from methanol, which was coined from the Greek for alcohol and wood; methanol is also known as wood alcohol and has only one carbon atom
2 | Eth- | Takes its name from the anesthetic gas ether, which has two carbon atoms
3 | Prop- | From propionic acid, a (foul-smelling) carboxylic acid with three carbon atoms, its name derived from the Greek for fat because it produces an oily layer in water
4 | But- | From butter because the smell of rancid butter is caused by butyric acid, which has four carbon atoms
5 | Pent- | From the Greek for five
6 | Hex- | From the Greek for six
7 | Hept- | From the Greek for seven
8 | Oct- | From the Greek for eight
9 | Non- | From the Latin (for a change) for nine
10 | Dec- | From the Greek for 10
12 | Dodec- | From the Greek for 12
31 | Hentriacont- | Has nothing to do with chickens; from hen, the Greek for one, combined with triacont for 30, also from the Greek

1.11 Molecules in the Mirror

In 1848, the French chemist Louis Pasteur solved the mystery of why certain byproducts of wine-making spun polarized light in one direction, while molecules that appeared to have an identical formula and structure spun it in the other direction. When Pasteur examined crystals of the two compounds under a microscope, he noticed that their crystals were mirror images of each other—that is, one could not be superimposed on the other. The two molecules were therefore not identical, but were mirror images of each other. It rapidly became clear that living organisms were quite selective about which molecular mirror images they used and created.

Chemists call this property of having a non-superimposable mirror image chirality, from the Greek for hand. Hands are the archetypical chiral objects: your left hand is a mirror image of your right hand. The two hands cannot be superimposed on each other and, as a result, left gloves can only be put on left hands and right hands can only shake with right hands. Similarly, the handedness of a molecule can affect the handedness of things it can match up with.

Molecular chirality arises from the fact that carbon is tetrahedral. A tetrahedron has four vertices (Figure 1.8) and four faces. If you color each of the vertices a different color, you can create two different tetrahedrons that are mirror images of each other and cannot be superimposed. In the same way, a structure that contains a carbon atom with four different things attached to it, forming the vertices of a tetrahedron, will have a mirror image. Figure 1.9 shows the structures for caraway and spearmint oils with an asterisk indicating the carbon atoms that create the mirror image.

Figure 1.8Tetrahedral carbon atom with four different groups attached and its non-superimposable mirror image.

Figure 1.9Structures of R and S carvone. The * indicates the chiral carbon atom, the atom that has four different groups attached to it, creating the potential for the two mirror images.

Chemists tag one image with the letter R and the other with the letter S. (There are a set of rules for determining which letter goes with which reflection.) Structures with multiple carbon atoms, each having four different things attached, have the potential for multiple mirror images. Each of these carbon centers is designated by either the letter R or the letter S depending on its configuration. Ordinary table sugar, for example, has nine of these centers! These are shown in Figure 1.10: four are R and five are S.

Figure 1.10Structure of sucrose showing the nine carbon atoms that can have two mirror image configurations—chemists would call them stereogenic centers—each labeled with its absolute configuration, R or S.

Two older systems for labeling these molecular mirror images are still in use. Molecule names that include “+” in them rotate the plane of polarized light, as in Pasteur’s experiments, in a clockwise direction. Molecules that rotate it in a counterclockwise direction have “−” in their names. (+)Carvone is an oil that smells of caraway seeds, while (−)carvone is an oil that smells like spearmint. The letters d and l are also seen in molecular names, such as in l-theanine, an amino acid that is found in tea. These also indicate mirror images. The designations refer to the similarity of each mirror image to the mirror images of glyceraldehyde, a common biological molecule. Unlike the R and S designations, which can tell you precisely the arrangement of the four groups around each carbon, or the + and − designations that tell you which way the compound will rotate polarized light, all d and l can tell you is that the compound is chiral and its handedness relative to the d and l forms of glyceraldehyde.§§

Biological systems are very discriminating when it comes to these mirror images. Of the more than 500 mirror combinations possible for sucrose, nature makes and recognizes only one.  The structures of spearmint oil and caraway oil (Figure 1.9) are identical except that they are mirror images of each other. Despite their extraordinary similarity, your nose and taste buds would never confuse the two. Our DNA is right-handed, while proteins are built from the left-handed versions of amino acids. The sugars that our body metabolizes are all right-handed.

Makers of pharmaceuticals must be very careful about mirror images. The mirror images of some chiral drugs are less effective, or even ineffective, compared with their reflections. Others are toxic. The S mirror image of a common asthma drug, albuterol, does not work as a bronchodilator, but can cause hyperkalemia (an increase in the concentration of potassium ions that can disrupt heart action). Both the R and S versions of a common drug for combating acid reflux, omeprazole, are active, but the S version is more effective. It is marketed as a separate drug, esomeprazole for “es” + omeprazole.

1.12 Pouring the Big Ideas Into a Teacup

We now have a few key tools for unlocking the chemistry of tea. In the following chapters, we will see how we can use the four big ideas—atoms make up everything, structure dictates function, proximity, opposites attract—to understand what is happening in our cup of tea, starting from when the leaves are picked to when you take that first sip. Why is green tea green? How does a plant rearrange the carbon atoms it takes in from the air around us to make caffeine? Why does tea have that slightly astringent taste? Why should you skip the teabag? And just what is that white stuff on the top of the tea that you made using the microwave? With a little help from the four big ideas, all of these questions can be answered.

Further reading

J. A. Benn, Tea in China: A Religious and Cultural History, University of Hawaii Press, Honolulu, 2015. An engaging and rich cultural history of tea in China from its origins through the 19th century.

L. Collingham, The Taste of Empire: How Britain’s Quest for Food Shaped the Modern World, Basic Books, New York, NY, Illustrated edn, 2017. Chapters 6 and 11 take on the interplay between tea, Britain’s colonization of India, and the opium trade.

S. Rose, For All the Tea in China: How England Stole the World’s Favorite Drink and Changed History, Viking, New York, 1st edn, 2010. The tale of the British East India Company and Robert Fortune’s theft of tea plants.

P. W. Atkins, Chemistry: A Very Short Introduction, Oxford University Press, New York, 1st edn, 2015.

G. Patrick, Organic Chemistry: A Very Short Introduction, Oxford University Press, Oxford, United Kingdom, Illustrated edn, 2017.

M. Francl, Through the eyes of a chemist, Nat. Chem., 2016, 8, 1–2. A short essay covering six big ideas chemists employ to explain the world.

P. Ball, Designing the Molecular World: Chemistry at the Frontier, Princeton University Press, Princeton, N.J., 1st edn, 1994. A lively exploration of the relationship between molecular structure and function.

S. W. Smith, Chiral Toxicology: It’s the Same Thing…Only Different, Toxicol. Sci., 2009, 110, 4–30. A slightly more discussion of chirality, with fascinating examples drawn from biochemistry and drug design.

A. Brunning, https://www.compoundchem.com/2015/08/27/org-comp-names/. A graphical guide to the names of molecules.

†Kamel did not describe camellias, nor the tea plant, in any of his work, although he did publish on Strychnos ignatii, the strychnine-containing “beans of St Ignatius”. Coincidentally, Kamel was from the town in Moravia where Gregor Mendel would later do his groundbreaking work on heredity in plants.

‡In Tanzania, Babu Kalunde discovered the ability of an extract of the mulengelele plant to treat a gastrointestinal infection by observing sick porcupines eating it.

§TL;DR = too long; didn’t read.

¶It may seem obvious that what a molecule is called has nothing to do with how it works, or how safe it is, but people are often advised not consume anything that an eight-year-old cannot pronounce, or that your grandmother would not recognize. It’s not helpful advice and unnecessarily disparages grandmothers.

‖You do need that time in seat if you want to be able to deal with chemistry quantitatively. I’m leaving out all the equations.

**He also distilled many liters of his (or his laboratory assistant’s, the paper is a bit vague) urine.

††Fun fact for chemists: this wasn’t always true. Up until the middle of the 20th century, many chemists represented benzene as a simple hexagon, indistinguishable from cyclohexane.

‡‡What exactly is a chemical bond? The short answer is “an electron pair”; the long answer is “quantum mechanics”.

§§R, S designations are referred to as absolute configurations because you can absolutely construct the arrangement of the four groups around the carbon based on the code. d, l designations are called relative configurations because they are determined relative to glyceraldehyde.

Chapter 2

Reading the Tea Leaves

Tea is a work of art and needs a master hand to

bring out its noblest qualities.

—Kakuzō Okakura, The Book of Tea †

Tea Pairing

Try a pot of a pu’erh tea, such as Celestial Tribute, or a cup made from a bag of Tetley black tea.

2.1 Tea Is a Work of Art

Tea is a natural product, its leaves plucked from a tree on a mountainside perhaps half a world away. It can be easy to forget this when you open the cabinet, grab a nearly opaque teabag, and drop it into a cup of hot water. So put this book down for a moment and go get your tea out of the cupboard. If you use teabags, cut one of the bags open and dump the contents out onto a sheet of plain white paper. (If you use loose-leaf tea, take a few spoonfuls out and spread them on the paper instead.) Take a sniff. No matter whether you are using teabags or loose-leaf tea, the tea has a faintly floral, woodsy smell. If you are drinking a black tea, English breakfast perhaps, or PG Tips, what is inside the teabag likely looks less like leaves and more like coffee grounds. It is uniformly a deep brown color, though it may be spotted with smaller golden particles. Look closely and you may even see a narrow needle-shaped remnant of a leaf. Pick some up and rub it between your fingers. It may feel like tiny ball bearings.

Depending on the type of tea, loose-leaf tea may look more like, as the term suggests, leaves. A number of different teas are shown in Figure 2.1. The dried tea may be many shades of brown or gold or pale green, and shriveled like autumn leaves, as the ban-cha or Yin Zhen white tea. CTC (crush, tear, curl) teas may resemble larger versions of the pellets found in teabags, whereas some loose teas may have been pressed into wafers or blocks, others rolled into spheres ranging from the size of ball bearings (green snail tea) to ping-pong balls. A great deal has happened to these leaves to get them from the tree to your cup, altering not only what you can see and smell, but their molecular make-up  as well.

Figure 2.1Different styles of tea. From top left to bottom right: Yin Zhen white tea; matcha tea; PG Tips (removed from teabag); steamed green tea with roasted rice; Special Temple of Heaven Gunpowder tea; Rose Congou tea; whole-leaf Assam tea from Halmari tea garden; CTC Irish Breakfast tea; green snail tea; tied floral tea balls; pressed white tea wafer; ban-cha tea from Kamikatsu, Japan. Images courtesy of Andrew DiDonato.

Tea is a highly processed food. It is a work of art, as Okakura said, requiring a master’s attention to detail to bring out the possibilities hidden in the leaves. While we tend to think of processed foods as a modern invention, something produced in a factory, perhaps treated with unpleasant chemicals and generally less healthy to eat or drink, throughout history humans have processed food to preserve it as well as to alter its flavor or texture. Consider dried fruit or pickles, or toasted bread and jam. Whether a food is healthy or not, safe to consume or not, depends not on how much processing was done, but what processing was done.

Everything we eat has been processed to some extent, even chewing freshly picked leaves is a chemical process. Teeth physically shred the cells of the leaves, releasing what is inside them. Chemicals in the leaves are extracted by a solvent — saliva. And the digestive enzymes present in saliva almost certainly chemically alter some of the compounds. What happens to tea leaves when they are picked? What chemical reactions come into play as they are processed? It starts with the plant itself.

Tea plants are evergreens that grow in subtropical climates, where the temperature ranges between 20 and 25 °C. They require a considerable amount of water, between 150 and 250 cm of rain each year, and prefer slightly acidic, well-drained soil. Even before a single tea leaf is harvested, decisions about how much sun exposure a particular plant gets, and the point in the season when the leaves are to be plucked, affect the final chemical composition of the tea. Plants that are exposed to a great deal of sunlight have more caffeine and antioxidants in their leaves. The first leaves that appear in the spring, called the first flush, have a different chemical profile from leaves picked later in the season. Picking only the first two leaves and the bud — the tips of a tea plant — reduces the yield of a tea garden, but teas made from these leaves are richer in many of the desirable aromatic components. I prefer my teas to be “tippy.”

High-quality tea is picked by hand to avoid damaging the tea plant as well as to avoid prematurely crushing the plucked leaves, although machines and robots are being deployed that can, to some extent, alleviate the need for humans to undertake this arguably repetitive and monotonous task. When tea leaves are picked, they are detached from their source of nutrition. Water and other nutrients no longer flow up from the roots and many of the chemical processes in the leaves grind to a halt. Chemical changes are already underway. There are many more to come.

For a long time, Europeans thought green tea and black tea came from different plants, not realizing that it was the method of processing the leaves that led to the different colors of tea. Chemists desire to have control of their chemical reactions, tweaking reaction conditions to change the products that they get. Cooks do the same thing; the difference between steaming Brussels sprouts and roasting them is huge. Tea masters are equally master chemists, reading the appearance and aroma of the tea leaves, then modifying the conditions to achieve a particular tea.

Figure 2.2 shows a flowchart for the processing of tea leaves, beginning with the fresh leaf. The basic steps are withering, enzymatic oxidation, firing, sweltering, shaping and drying, and fermenting. Each processing step has its own unique chemistry. Not every tea passes through every step. Different paths through the chart lead to six broad styles of tea: black, white, oolong, green, pu’erh, and yellow tea. Let’s explore the chemistry of these steps one at a time and see how they can transform the leaves into a work of art, ready to steep in a cup.

Figure 2.2Flowchart showing the different steps used to produce the six basic styles of tea.

2.2 Wither or Not?

The split between the treatment of green teas and traditional black teas occurs at the very first step. Tea leaves destined to become black, oolong or white teas are allowed to wither or wilt for a short period of time, typically less than a day, while green teas are immediately heated. Withering can be done by simply exposing the leaves in a layer to the ambient air, or hot air can be blown across them to speed the process along. The leaves are handled carefully at this time to prevent premature crushing. The temperature must be tightly controlled, both to prevent the heat from driving off some of the desirable chemical compounds responsible for taste and aroma and to prevent the destruction of the enzymes in the leaves required in subsequent steps.

Chemical changes occur along with the drying. Withering removes some of the water in the leaves. As the amount of water in the leaves decreases, one of our big ideas of chemistry comes into play — proximity. When less liquid is present, the ability of molecules to move around and react with each other decreases. The leaf starts to collapses onto itself. The membranes of the cells in the leaf begin to degrade, and on exposure to oxygen from the surrounding air, the fats and proteins within begin to decompose and free amino acids appear.

2.3 Bruised and Battered: Enzymatic Oxidation

After the leaves that will eventually become black and oolong teas have withered for a few hours, they are deliberately bruised. The leaves can be bruised simply by handling them roughly, tossing them in a basket, or tumbling them in a drum. CTC teas are shredded by machine before being rolled into hard pellets. This can seem counterintuitive, particularly for those leaves that have been picked by hand to avoid bruising or tearing. When we pick fruit, we are usually careful not to let it bruise, wanting to avoid the soft brown spots that develop where the fruit has been damaged. But this aggressive handling of the tea physically breaks down the cells in the leaves and releases what is inside. Imagine opening a bunch of bottles and spilling their contents into a vat. Inside the cells are enzymes, along with a wide variety of smaller molecules. It is the enzymes we are after.

Enzymes are large molecules that speed up chemical reactions in biological systems. Think of them as portable molecular factories, taking in one molecule and turning out another. Polyphenol oxidases are a type of copper-based enzyme found in virtually all plants, including tea. These particular enzymes are responsible for the unappetizing browning that occurs when fruits such as apples or bananas are cut. Of course, the purpose of polyphenol oxidases is not to put small children (or even some adults) off eating overly brown sliced apples or bananas. It is thought that these enzymes are part of a plant’s response to trauma, such as physical wounding or infestation‡ by a pathogen. They divert the oxygen into inoffensive pathways before it can damage critical structures within the cell. Antioxidants are as important to plants as they are to humans.

As the name suggests, polyphenol oxidases promote reactions with oxygen. To a chemist, an oxidation reaction is technically one where electrons move from one molecule to another, reactions which often involve oxygen stealing electrons from other molecules. The molecule that loses electrons is said to be oxidized, while the molecule that gains electrons is said to be reduced. Oxidation is always paired with reduction and the processes are often called redox reactions to emphasize this. It is a zero-sum game: every electron lost must find a home in another molecule. Even such a seemingly minor change—adding or removing a few electrons from the dozens in a molecule—can result in major changes in the behavior of a molecule or atom. For example, adding one electron to a fluorine atom (F) to make a fluoride ion (F⁻) transforms the atom from something that can ravage the ozone layer to an innocuous part of table salt.

Oxidation and reduction reactions often change the color of a substance. For example, copper is a bright red–brown metal, whereas oxidized copper is a minty green. This is also true in biological systems. When you slice an avocado or banana, the cut edges turn brown as a result of the oxidation of some small molecules by polyphenol oxidases. Controlling the oxidation reaction controls the color,§ and this is also true of tea. Unlike the browning of the half-eaten apple on your desk, the changes wrought in the color and composition of tea by polyphenol oxidase are welcome. Tea masters manufacturing black and oolong teas take advantage of this oxidation reaction to control a tea’s color and flavor. Black teas are fully oxidized, whereas oolong teas have their oxidation cut short.

In order to control the amount of oxidation in tea leaves, you need a way to turn the enzymatic reaction with oxygen on and off. Turning the reaction on relies on one of the four big ideas of chemistry: proximity. The leaves are deliberately bruised and damaged to release more of the polyphenol oxidases sequestered in the cells, giving them access to oxygen from the surrounding air and freeing the molecules targeted by the enzyme for oxidation.

Polyphenol oxidases in tea leaves target a particular type of polyphenol, called catechins. We will learn more about these molecules and what they’re doing in our cup of tea in Chapter 4 — The Taste of Zen. For the moment, it is sufficient to say that these molecules are converted by the enzymes into two other types of molecules called thearubigins and theabrownins. As their names suggest, these molecules are reddish and brownish in hue and give infusions of black tea their characteristic color.

Other oxidases in the leaves contribute to the aroma of tea. You probably associate beta-carotene with carrots. It is a naturally occurring orange–red pigment that gives carrots and other orange vegetables their deep color. But tea leaves also produce beta-carotene and it, too, is a target for oxidative enzymes. The molecules produced by these oxidation reactions contribute to the aroma of tea. Beta-carotene is primarily converted to beta- ionone, which has a woodsy scent associated with violets and raspberries. A related compound, neoxanthin, is converted to beta-damascenone, a fruity–floral scent reminiscent of apples found in roses and whiskey, as well as tea. The beta-ionone itself undergoes conversions in the leaf to give rise to theaspirone, another aromatic compound smelling of fruit.

The names of these molecules give chemists a clue that they might have a noticeable aroma. The ending “-one” indicates that they are ketones. Examining the structures of these molecules in Figure 2.3, you can see the common C=O in each of them, the structural element that makes them a ketone. Many ketones have a pleasant scent. Acetone, often found in nail polish removers, is a ketone with a sweet odor. The structures of ketones make them less likely to stick¶ to each other than many similarly sized molecules. This makes them more volatile—that is, more able to get into the gas phase and waft into your nose. In order to smell a chemical compound, it must first be volatile. Once again, we see how molecular structure drives molecular function.

Figure 2.3Two of the carotenoids found in tea (beta-carotene and neoxanthin), along with three of the scented ketones produced by their oxidation: beta-ionone (scent of violets); theaspirone (floral tea scent) and beta-damescenone (scent of roses).

2.4 Killing the Green: Hitting the Off Switch

How do you turn the browning enzymes off to keep an oolong tea from oxidizing all the way to a black tea? The easiest way to prevent enzymatic oxidative browning is to prevent oxygen from reaching the enzymes in the first place. This is why cut potatoes are immediately immersed in water or plastic film is placed over an avocado half. However, neither immersing the tea leaves in a liquid nor wrapping them in a film is particularly practical. A slightly more sophisticated technological approach would place the fruit in an oxygen-deprived atmosphere, say under nitrogen gas. For this reason, dried tea leaves are sometimes packed in nitrogen gas to keep them fresh. Partially processed tea leaves can be held under nitrogen to temporarily pause the oxidation process until the manufacturer is ready for them.

Enzymes can also be stopped in their tracks by adding an inhibitor, something that locks into the enzyme and prevents molecules from reaching the processing site. Polyphenol oxidases can be inhibited by ascorbic acid, found in many citrus fruits, including lemons. As a result, a little lemon juice squeezed on an apple or an avocado will keep it from browning. Of course, creating fruits that do not contain polyphenol oxidases, such as the Arctic apple, would be another way to prevent browning.

Enzymes can be permanently deactivated by heating them to high temperatures and this is the approach taken in the processing of tea leaves. Once again, the relationship between molecular structure and function comes into play. Enzymes are long strings of amino acids, where the string is wound into a particular three-dimensional structure. Heating an enzyme disturbs the connections that hold the three-dimensional structure in place, like unlacing a shoe. Since enzymes (or any type of protein) depend on having a particular structure to work, when its structure is lost—biochemists say a protein has been denatured—it can no longer perform its normal tasks and many of its properties are changed. Considered cooked egg whites. These contain many proteins that, when heated, denature and tangle, turning a clear gelatinous liquid into a white solid.

Polyphenol oxidases are the secret that the Europeans did not know. Green teas are from the same plant as black teas and oolong teas, just treated to freeze them in their green state rather than allow the enzymes to oxidize them. Tea leaves destined to be green tea are immediately heated to destroy the polyphenol oxidase enzymes in the leaves—a process sometimes called, rather contrarily, “killing the green.” Proteins are delicate things when it comes to heat and they can begin to unravel at temperatures well below that of boiling water. The enzymes in tea can be deactivated by steaming them, heating them to 100 °C; the leaves can be heated to higher temperatures by baking them or tossing them in a very hot pan or heated drum for a few minutes. Either approach prevents oxidative browning from taking place, preserving the green color of the fresh leaves in the case of green tea and halting oxidation for oolong teas.

2.5 Maillard’s Reaction

The browning you get in the early steps of tea production is mostly the result of enzymatic oxidation, but there are other non-enzymatic chemical processes at play that can enrich the taste, aroma, and color of a cup of tea. One such process is the Maillard reaction, which occurs when teas are fired—heated to high temperatures. The reaction is named for the French chemist Louis Maillard, who first described it in 1912, and it is a powerful and ubiquitous part of a kitchen chemist’s arsenal. It is the reason why toast tastes toasty, why a well-grilled burger or steak has that delicious brown crust. It also helps give whiskey—and some teas—that wonderful malty, smoky, not-quite-burned taste.

Some chemical reactions are what an organic chemist would call “clean”.║ Mix some things together, stir and heat, and they are completely converted to the desired product. No left-over ingredients, no unwanted byproducts. The Maillard reaction is not a clean reaction in this sense. Rather, it is collection of reactions that occur at high temperatures and transform the amino acids, proteins, and sugars found in foods into many new chemical compounds. Many of these molecules enhance the flavor of the cooked food (think roasted Brussel sprouts versus the steamed version). A few are potentially carcinogenic. Some are both.

Tea leaves have all the necessary ingredients for Maillard reactions to occur, such as lots of amino acids, sugars, and proteins. The high temperatures at which some green teas and all oolong teas are fired create the perfect conditions for the reaction. The basic chemistry behind the Maillard reaction leverages another of our big ideas: opposites attract. The nitrogen atom in the amino (NH₂) group on the amino acids has a partial negative charge. The carbon of the carbonyl (C=O) group on sugar molecules is considered to be slightly positively charged. The nitrogen noses in behind the carbonyl to form a new bond (see Figure 2.4). Chemists call this reaction a nucleophilic attack. The creation of this new linkage sets off a number of different reactions,** producing a spate of new compounds, some of them sporting a new molecular ring.¹

Figure 2.4Diagram showing the nucleophilic attack of a nitrogen atom on a carbonyl group. The balloon on top of the nitrogen is an electron pair, making it slightly negatively charged and therefore seeking the slightly positive charge on the carbon in the carbon–oxygen double bond. A new bond is formed between the nitrogen and the carbon, priming the molecule to undergo further reactions.

A selection of the products known to be formed through Maillard reactions in tea leaves are shown in Figure 2.5. Since we now have new molecular structures, we also have new properties, including new tastes, smells, and colors—and new potential risks. The formation of thiophene and thiazole contribute to the aroma of tea. Molecules that contain sulfur, as these two do, often have quite potent odors, even at very low concentrations. This is why sulfur-containing compounds are used as an odorant in natural gas. The characteristic smell that alerts you to a methane gas leak arises not from the methane–methane itself is odorless—but from these added sulfurous molecules, which humans can smell at the parts per billion level. Thiophene brings a slightly sulfurous aftertaste to tea, while thiazole adds a smoky coffee-like note to the aroma.

Figure 2.5Structures of some of the molecules found in teas that are formed by Maillard reactions.

Both furan and acrylamide are known to form in fired teas. Acrylamide is thought to be a potential carcinogen, as is furan. Furan contributes a spicy cinnamon-like flavor to tea. Acrylamide is tasteless and odorless, undetectable without resorting to sophisticated analytical techniques. Acrylamide often forms when starchy foods, such as potatoes, undergo the Maillard reaction. In recent years, the amounts of both furan and acrylamide in foods have been a matter for concern by regulatory bodies in both the USA and Europe. The amounts of acrylamide can be quite high in fried foods like potato crisps and chips, but it is also found at significant levels (parts per million) in fired teas.² It is very soluble in water, so if you drink tea you are likely to be ingesting not insignificant amounts of acrylamide. I hasten to add that the current consensus is that dietary exposure to acrylamide does not increase the risk of cancer in humans.

2.6 Shaped and Dried

Before they are dried, the tea leaves are still pliable enough to shape. Some teas, including one of my favorites, green snail tea, have individual leaves rolled into tight balls. (No snails, green or otherwise, are used to make this tea; the name refers to the shape of the rolls, which resemble pea-sized snails.) Teas can be tied into tight bundles or balls that “bloom” into the shapes of flowers when steeped. Most teas are left as whole or broken leaves.

The leaves are eventually dried. Removing as much water as possible at this stage helps to preserve the leaves. Since molecules need to be able to move around to react, something easily accomplished in a liquid medium such as water, removing the water keeps molecules from finding each other and reacting, effectively preserving the molecular composition of the tea. At the end of this step, 100 kg of fresh leaves has been reduced to 15–25 kg of dried tea leaves, ready to steep.

2.7 Microscopic Chemists: the Making of Fermented Pu’erh Teas

We would rather not think about fungi or bacteria growing in food or drink of any sort. But cooks, tea-producers and brew masters have long harnessed such microbial minions to do their bidding and alter foodstuffs at the molecular level. The yeast Saccharomyces cerevisiae turns carbohydrates into carbon dioxide and alcohol, making beer fizzy and boozy and breads rise. In concert with a (literal) raft of bacteria, S. cerevisiae also can turn a sweetened infusion of black tea into kombucha, a slightly alcoholic, effervescent drink that has recently become widely popular.

Fungi are ubiquitous in the environment, found in soils and on plants, suspended in the air and on decaying organic matter. Molds are a type of structured fungal colony, in contrast with yeasts, which are single-celled organisms. The growth of mold is something we often want to discourage when trying to preserve food. But certain varieties of mold can produce delicious results. The mold Penicillium roqueforti produces the blue–green veins in blue cheeses and Penicillium camemberti is essential to the production of brie and related cheeses.

Molds can also work their magic on tea. Once dried, green teas can be taken a step further and allowed to mold and ferment to become pu’erh teas. These dark brown teas take their name from the city of their origin in Yunnan, China. To create a pu’erh tea, dried green tea can be piled up or pressed into bricks and left to age for up to 10 years. As it ages, molds that are naturally found on the tea leaves jump into action. Alternatively, a pile of the leaves can be dampened, which will speed up the molding process. More control can be exercised by deliberately contaminating the tea with particular molds. While many molds are found on the leaves that will become pu’erh teas, the primary organism is Aspergillus niger.††

A. niger is a common contaminant on many foods. If you have seen the black dust on an onion, that is A. niger. It is an aerobic organism—in other words, it requires oxygen to grow. It is tolerant of a wide pH range and it is a xerophile—that is, it is able to grow where there is very little water. It is sometimes called black mold, although it should not be confused with the toxic black mold that can be found in damp basements and walls (Stachybotrys chartarum). A. niger is generally recognized as safe (GRAS) by the US Food and Drug Administration. Enzymes produced by A. niger oxidize the polyphenols present in tea leaves, producing more theabrownins. The tea is darker as a result, and less astringent. Aspergillus sydowai is another species of fungus found on tea leaves. It feeds on caffeine, merrily pulling off the methyl groups as it goes.

2.8 White and Yellow Teas

You might have noticed I have not mentioned the production of two of the teas in the flowchart in Figure 2.2: white and yellow tea. These pale—and sometimes spectacularly expensive—teas are uncommon on supermarket shelves. They are appreciated not just for their relative rarity, but also for their subtly complex flavors.

Yellow teas are rare and costly in part because their production is more arduous than typical green teas. Like their green counterparts, yellow teas are heated to destroy the polyphenol oxidative enzymes. Then, before they are shaped and dried, they are sweltered. The leaves are gathered up and tied in a cloth or paper. They are then placed in a container, covered with a damp cloth and kept warm for several days.

Non-enzymatic oxidation occurs under these humid conditions. The yellow teas are generally sweeter and mellower than green teas as a result of the oxidation of their catechins to compounds that are less astringent. The chlorophyll in the leaves degrades, giving the tea its characteristic yellow color. Proteins also break down, yielding free amino acids that add sweetness to the tea. The tea becomes less bitter and its floral scent is accentuated. The leaves are dried, then wrapped up and left to age a second time. They are then shaped and thoroughly dried.

Of all the tea types, white tea is the least processed. It is merely picked, withered, and then shaped and dried. The name refers to the color of the dried tea leaves themselves, not to the color of the tea that they produce. The leaves appear to be covered in tiny white hairs or dust. The tiny hairs are called trichomes or pekoe and appear on the surface of the young leaves of many plants, including tea. White teas are sometimes described as lightly oxidized because, as with black and oolong teas, some oxidation does occur during the withering process. These teas can be stunningly expensive, costing upwards of 400 euros per kilogram.

2.9 From the Leaf to the Cup

The ways in which the tea leaves are handled, and the conditions to which they are subjected, control the chemical composition of the final dried leaves. The processes by which different teas are made can be simple, as in the case of white teas, or more complex, as in pu’erh or yellow teas. I have touched lightly on the molecular profile that is shaped by the manufacturing process to give an overall view of the different types of tea and how they’re made. In the next two chapters we will explore how the molecules that are developed during the creation of a tea contribute to its physiological effects and to its taste and aroma. We will start with a look at caffeine: what it is doing in the leaf and what it does in our bodies. And we will unpack what happens with the polyphenols—the  catechins—that are oxidized in black and oolong teas.

2.10 Brewing a Better Cup

If you are concerned about acrylamide, then choose a steamed green tea or a pu’erh tea.

Further reading

K. Okakura, The Book of Tea, Officina Libraria, Rome, Italy, Annotated edn, 2022, A beautiful long-form essay written by a Japanese scholar visiting Boston in the early 20th century that situates tea within Zen Buddhism and Japan’s cultural context.

M. Hartings, Chemistry in Your Kitchen, Royal Society of Chemistry, Cambridge, UK, 1st edn, 2016, A lucid explanation of the Maillard reaction and its importance in many aspects of kitchen chemistry, written for the non-chemist. There’s also a section on that other beverage, coffee.

J. W. Uhl, The Art and Craft of Tea: An Enthusiast’s Guide to Selecting, Brewing, and Serving Exquisite Tea, Quarry Books, Beverly, Massachusetts, Illustrated edn, 2015, A clear exposition of the process of making tea, from plucking the leaves to roasting them, with many beautiful color photographs.

P. Coucquyt, B. Lahousse and J. Langenbick, The Art and Science of Foodpairing: 10,000 flavour matches that will transform the way you eat, Firefly Books, Buffalo, N.Y, 1st edn, 2020, This exhaustive book on combining flavors includes a section on the basic structural motifs found in flavor molecules.

References

1. P . Yang, H. Wang, Q. Cao, H. Song, Y. Xu and Y. Lin, J. Food Compos. Anal., 2023, 115, 104954.

2. Y. Mizukami, Y. Sawai and Y. Yamaguchi, J. Agric. Food Chem., 2008, 56, 2154.

3. https://teaepicure.com/oriental-beauty-bug-bitten-teas/

†K. Okakura, The Book of Tea (茶の本, Cha no Hon) A Japanese Harmony of Art, Culture, and the Simple Life, Fox, Duffield & Company, New York, 1906.

‡The taste of a tea variety called Oriental Beauty is often attributed to infestations of small insects called leafhoppers. The damage inflicted by the leafhoppers supposedly jump-starts the oxidation process by prompting the plant to release polyphenol oxidases in response. But the story is more complex, involving pheromones that attract spidersto eat the leafhoppers.³

§And the texture and flavor as well, although you may have been told as a child that the browning doesn’t affect the flavor of the apple, so eat it!

¶The lack of stickiness has to do with the inability of ketones to form hydrogen bonds with each other.

║Here, both “organic” and “clean” are used in their chemical context. Organic chemists work on molecules that are built on a carbon–hydrogen framework. Clean chemistry does not produce many unwanted side-products.

**Chemists will recognize that I have left out many steps, including the Amadori rearrangement, an elegant imine–enamine tautomerization reaction.

††This was named by the 18th century Florentine priest and mycologist Pier Antonio Micheli for its resemblance to an aspergillum, a branched instrument used in religious ceremonies to sprinkle holy water.

Chapter 3

The Drug in the Cup

Tea tempers the spirit and harmonizes the mind;

dispels lassitude and relieves fatigue,

awakens thought and prevents drowsiness.

—Lu Yu, The Classic of Tea †

Tea Pairing

Enjoy this chapter with a black tea from the Halmari tea garden in Assam or brew a cup of Lipton Green Tea. Both are excellent sources of caffeine.

3.1 A Stimulating Beverage

Before I sat down to write this chapter, I made a pot of tea. (A tippy black tea from the Halmari tea garden in Assam, if you must know. No milk, plenty of sugar.) It was late in the afternoon and after a busy day of teaching i was just a bit desperate for the virtues of a cup of tea extolled by the 8th century tea master Lu Yu: a touch of calm and a jolt of energy.

Today, we think of tea primarily as a beverage, something to warm us up on a cold day or coolly quench our thirst on a summer afternoon—or offer us a bracing start to the morning. It can be part of a social ritual, like the green tea offered to pilgrims at temples in Japan or an afternoon tea served with cakes and dainty sandwiches. But in the eyes of a biochemist, a cup of tea is essentially a drug delivery system and the primary drug it delivers is caffeine. Caffeine acts on the central nervous system, moderating mood, improving physical and cognitive performance, and counteracting drowsiness. As Kakuzō Okakura opened his essay on tea, “Tea began as a medicine and grew into a beverage.” This chapter explores the drug in the cup, how it gets into a tea leaf and how to get it out.

Caffeine is thought to be the world’s most widely used psychoactive substance; 90% of adults in the world consume caffeine, while only 50% drink alcohol. It’s not hard to imagine why. As Lu Yu observed 12 centuries ago, tea can alleviate fatigue and sharpen our wits. The use of caffeinated beverages as stimulants is linked in legend to the need for monks and mystics to stay alert through long periods of prayer and meditation. Early Buddhist monks drank tea to sustain them in long vigils and their Sufi counterparts brewed coffee; indigenous peoples in pre-colonial North America turned to extracts of caffeine-containing yaupon holly to support them through long rituals. Today, the restorative effects of these beverages are as appreciated by the exhausted parents of newborns and students cramming for exams as they were by the Buddhist monks in 7th century China.

By the middle of the 19th century, caffeine was recognized as the active ingredient in a number of these stimulating beverages, including coffee and tea, as well as yerba-maté (Ilex paraguariensis). A medical text from 1848 describes it as a “nervous stimulant and tonic of considerable power.” Caffeine has been used since the 1880s as a treatment for migraine and as a diuretic. Although today most caffeine is taken PRN—pro re nata or “as needed”—without a physician’s prescription, caffeine remains part of the standard Western pharmacopeia. Premature infants routinely receive intravenous infusions of caffeine to stimulate their breathing and prevent episodes of apnea.

Caffeine is a natural substance, found in more than 60 plants around the world, including tea’s source, Camellia sinensis, as well as in other plants commonly used to make beverages. Coffee (Coffea), kola (Cola acuminata) and yerba-maté all contain substantial amounts of caffeine, which can be extracted using widely available methods and materials. Curiously, none of these plants are native to Europe, which was caffeine-free until coffee crept its way into Hungary and Austria in the early 16th century. In North America, only two plants are known to produce caffeine: yaupon holly‡ and one of its close relatives.

Even the caffeine in sodas may come from plants. While much of the caffeine added to sodas and energy drinks is produced in chemical plants, not natural plants, caffeine extracted from coffee during the process of making a decaffeinated version is purified and then sold to drink manufacturers.

European chemists first engaged with caffeine in the early 19th century, about 200 years after tea was first introduced to the continent. In 1820, physician and analytical chemist Friedlieb Ferdinand Runge extracted caffeine from coffee—the beans apparently a gift from the poet and novelist Johann Wolfgang von Goethe. Runge called it Kaffebase, literally the base (in the sense of the opposite of an acid) from coffee. Less than two years later, the pure compound was independently isolated by two teams of French chemists: one headed by Pierre Jean Robiquet and Pierre-Joseph Pelletier and the other by Joseph Bienaimé Caventou. Pelletier gave the compound the name that has stuck in English: caffeine. In 1827, another French chemist, M. Oudry, isolated a compound he called théine§ from tea. A decade later it was shown that Oudry’s théine was actually the same as caffeine. The famous German chemist Emil Fischer first synthesized caffeine in the laboratory in 1895 and two years later puzzled out its molecular structure. In 1902, Fischer won the second Nobel Prize ever awarded in chemistry, in part for his work on caffeine.

Before we tackle the effects of caffeine in the human body, it is worth stopping to note that the plants from which we get caffeine are not altruistic. They don’t kindly produce molecules to act as a central nervous system stimulant in humans. Plants make caffeine for their own reasons. So, what is the primary biological function of caffeine in plants? As it happens, our favorite drug is actually a pesticide, albeit an all-natural one. Caffeine is toxic to insects and other pests, including slugs and snails, that might try to eat plants.

We aren’t slugs or snails, so what happens in our bodies when we drink a cup of tea containing a dose of 50 mg or so of caffeine? How does caffeine keep us awake? What else can caffeine do for us and what are its risks? How can we remove it from tea? Just as for any other molecule, the particular arrangement of the atoms in caffeine dictates what it can do. So, get yourself a dose of caffeine and we’ll start by taking a guided tour of its molecular structure and then see how that controls its activity in humans—and plants. Just in case the caffeine keeps you up at night, we’ll take a look at how to get the caffeine out of a cup of tea.

3.2 An Atom’s Eye Tour of Caffeine

Given the relationship between the structure of a molecule and its function, it is unsurprising that chemists organize molecules into families by their structure. Caffeine is a member of the alkaloid family. Alkaloids are a broad class of compounds that contain nitrogen atoms, often embedded into rings of carbon atoms. As you can see in the structure in Figure 3.1, caffeine has four nitrogen atoms (denoted by N) tucked tidily into two  carbon rings.

Figure 3.1Molecular structure of caffeine, also known as 1,3,7-trimethylxanthine.

Like caffeine, many other alkaloids are phytochemicals— molecules produced by plants—although fungi and some animal species also produce them. Alkaloids can have a powerful effect on human physiology, even at very low doses, and many have been used medicinally for centuries. Quinine, the tonic in tonic water, is an alkaloid. It is extracted from chichona bark, native to the tropical Andes mountains, and was used by the Quechua as a treatment for malaria as early as the 16th century. The anti- cancer drug taxol was discovered in the bark of the Pacific yew tree and, famously, the antibiotic penicillin was found in the mold growing on Alexander Fleming’s petri dishes. Both are alkaloids. Chemists have made many synthetic alkaloids, often by riffing on naturally occurring structures. For example, a group of industrial chemists created amoxicillin by replacing the six-carbon “tail” of a naturally occurring penicillin with a ring of six carbon atoms (Figure 3.2).

Figure 3.2Amoxicillin can be created from one of the naturally occurring penicillins (penicillin F) by exchanging the hydrocarbon “tail” in the box for a different structural group.

Alkaloids are also among nature’s most infamous poisons. It is alkaloids that make deadly nightshade (Atropa belladonna) so deadly and what make kissing poison dart frogs or tickling a blue-ringed octopus such exceptionally bad ideas. In the case of belladonna, it’s a cocktail of alkaloids—atropine, scopolamine, and hyoscyamine—that interferes with important pathways in the nervous system, such as those that control heart rhythm. Both the frogs and the octopuses produce tetrodotoxin, an alkaloid neurotoxin that can paralyze those who come into contact with it. This is a reminder to never confuse “all natural” with “really safe!”

Many alkaloids are psychoactive. Extracted from the leaves of Erythroxylum coca, cocaine is, like caffeine, an alkaloid and an addictive stimulant. Psilocin, an alkaloid found in psychedelic mushrooms, can trigger synesthesia.¶ The hallucinogen LSD—lysergic acid diethylamide—is derived from ergotamine, an alkaloid found in ergot. Ergot is a fungus that grows on grains other than wheat, such as rye. History is replete with episodes of ergot poisonings, which can trigger hallucinations, presumably due to the ergotamine produced by the fungus.

Structurally, alkaloids are a vast class. You can see in Figure 3.3 how diverse their structures can be. Chemists find it more helpful to break down the alkaloid family into smaller groups with more closely related structures. Caffeine falls into a subset of alkaloids called purines. If you have ever suffered from gout, you might have been advised to avoid eating food rich in purines, such as shrimp or bacon. The name was coined by the 19th century organic chemist Emil Fischer, who first synthesized the parent compound of this subclass in 1898 from uric acid. He fused the words purum and uricum to emphasize that purine derived purely from urine.‖ The reverse reaction, the breakdown of purine to form uric acid, is what leads to a painful attack of gout when crystals of uric acid form in the joints. Familiar purines besides caffeine are the bases adenine and guanine that make up our DNA and RNA. In fact, it is the breakdown of these DNA bases that contributes to the uric acid in our urine—and to the synthesis of caffeine in tea and coffee plants.

Figure 3.3Molecular structures of a variety of naturally occurring alkaloids.

All purines have a five-membered ring fused to a six-membered  ring and each ring has two nitrogen atoms embedded in it. Figure 3.4 shows examples of different purines. The two fused rings in a purine are flat. The purine rings in adenine and guanine make the “steps” you see in a DNA helix (Figure 3.5).

Figure 3.4Molecular structures of a variety of purines.

Figure 3.5The purine bases adenine (A) and guanine (G) are shown in a snippet of DNA. The bases are linked together by a backbone of alternating phosphate and sugar groups that twist into the iconic double helix.

Caffeine is a particular type of purine called a xanthine (pronounced ZAN-theen), where two oxygen atoms (denoted by O)  are attached to the bigger ring. The behavior of the various xanthines is dictated by their structure, as we recall from one of our big ideas of chemistry. Replacing various hydrogen atoms on the rings with other structures subtly alters the biochemistry of these compounds. What is the difference between a stimulant and a diuretic (a drug that will increase the rate at which water is removed from the body)? In the case of two xanthines, it is a single atom.  For example, compare the two molecules in Figure 3.6. The molecule on the left (chlorotheophylline) is a stimulant much like caffeine. Swap out the chlorine atom (Cl) on the “nose” of the purine for a bromine atom (Br) and the compound on the right (bromotheophylline) becomes a very effective diuretic. Sold under the generic name pamabrom, bromotheophylline is often used to treat menstrual bloating and is found in over-the-counter preparations such as Midol. It is a more powerful diuretic than either the chlorinated version or caffeine itself.

Figure 3.6The molecular structures of chlorotheophylline and bromotheophylline.

In the late 19th century, cycling coach “Choppy” Warburton was reputed to be doping his riders with a mysterious substance he kept in a small glass vial in his pocket. Some reports suggest that the solution contained strychnine.** Others wondered if it could be a stimulant known colloquially as “trimethyl” that gave Warburton’s riders their edge. The death of cyclist Arthur Linton two months after winning a brutal single-day Bordeaux–Paris  race in 1896 coached—and likely drugged—by Warburton has been attributed to the contents of that little bottle. While it is unlikely that Linton died because of any drugs administered by Warburton (he appears to have contracted typhoid fever), the incident has been widely held up as one of the early examples of doping in modern sports.

Warburton never revealed his secret formula, but it is possible that the “trimethyl” that cyclists might have added to their water bottles to give them a boost was caffeine. Caffeine is known to chemists as trimethylxanthine, named for the three methyl (–CH₃) groups that decorate the outside of the xanthine rings (Figure 3.1).

3.3 Molecular Locks and Keys

The key to understanding caffeine’s ability to wake us up and sharpen our wits lies in our cellular machinery. Proteins are a key part of the apparatus that makes up the chemical factories and signaling systems found in living cells. DNA and RNA hold the codes for making these molecular machines. A protein is simply a string of hundreds—or even thousands—of amino acids, which are folded up into a specific three-dimensional shape (Figure 3.7).

Figure 3.7(a) A generic amino acid. The name comes from the –NH₂ (amino) group and the –COOH (carboxylic acid) group on the right. Different groups can be attached at the circle. For example, if a hydrogen is there, it is glycine; a C₆H₅CH₂ group at that position makes phenylalanine. There are 22 naturally occurring amino acids. (b) The first few amino acids of the 369 in the primary sequence for a protein found in tea that converts caffeine to theacrine (6LYI).¹⁴ Biochemists use either a single-letter code for each amino acid or a three-letter code. Both are shown. (c) The three-dimensional structure of the protein, which is a dimer. The image was created using Mol*,¹ a visualization tool for large molecules.

Each protein has a shape that is uniquely determined by its sequence of amino acids. Even changing one amino acid in a sequence can change the shape of the protein and, since shape dictates function, these tiny alterations can change the way a protein works. Sickle cell anemia is caused by just such a change. A single glutamic acid is changed into a valine in the hemoglobin sequence, creating a form called hemoglobin S. The new sequence folds differently from normal hemoglobin and, as a result, warps the red blood cells that contain it.

Biochemical systems are incredibly efficient. Enzymes, which are a type of protein that accelerate chemical processes, can speed up a reaction by a factor of a billion or more over what even the best chemists can achieve in the laboratory. Enzymes achieve these incredible reaction rates by pulling in a molecule and positioning it so that everything that is needed for the reaction is at hand. Biochemists discovered enzymes long before they understood what their structure was or how they worked, at about the same time as caffeine was first isolated. It wasn’t until the early 20th century that chemists understood that enzymes were actually proteins, long chains of amino acids. (The 1946 Nobel Prize in Chemistry went to James Sumner, John Northrup, and Wendell Stanley for this discovery.)

Proteins can also shuttle molecules in and out of cells, ignoring the undesirable molecules while escorting necessary compounds through the membranes of the cell wall. Another critical function that proteins serve is to act as switches, turning functions in the body on and off, or sending signals to other parts.

When it comes to their targets, most proteins are incredibly discriminating and will only bind to particular molecules. Emil Fischer, who discovered the purine family of molecules to which caffeine belongs, proposed in 1894 that this specificity arose because the enzyme and its substrate—the molecule it  targets—have complementary shapes. Proteins and their substrates match like a lock and key. Although Fischer did not know what the chemical structure of the enzymes looked like, his “lock and key” hypothesis turned out to be essentially correct. Proteins have binding pockets that match their substrates, both in their shape and in the pattern of electrical charges that act like atomic Velcro to hold molecules in their proper place in the pocket.

Once again, we see how structure and function are related. The structures of enzymes and other biochemical molecules have evolved to match the particular molecules they bind. These keys can be very specific; even a subtle change in the structure of the substrate, for example taking away a methyl group, can prevent a molecule from binding and the reaction from happening. Chemists can now use techniques such as X-ray crystallography and NMR†† to see the precise structure of an enzyme or other biochemical receptor and can, on occasion, even catch a substrate in action, trapped in the binding site of the enzyme.

What does all this molecular machinery have to do with caffeine? Many drugs work by binding to specific receptors in the body and preventing the enzyme’s usual target from getting into the binding pocket. For example, the blood pressure drug captopril works by blocking an enzyme that usually produces a molecule that raises blood pressure. With the site blocked, the body can’t produce as much of the compound, keeping the blood pressure low. Caffeine, we shall see, works in much the same way, blocking the molecule adenosine from accessing its receptors.

3.4 Adenosine: A Molecular On/Off Switch

Adenosine acts as sort of a molecular on/off switch for humans (and other mammals). There are adenosine receptors distributed throughout the body, including in the brain. The binding of adenosine to its receptors is a key step in regulating many human physiological processes, including the perception of pain, blood flow in the brain, respiration, and, of interest to us here, sleep.

The adenosine receptors are proteins. They are part of a large family of proteins called GPCRs, (G protein-coupled receptors). These proteins are widely used in mammals as signaling systems. Many processes in the human body rely on GPCR signaling,‡‡ including critical processes such as respiration and heart rate, as well as sensory signals such as taste and sight. Rhodopsin, a GPCR found in the eye, makes it possible to read this page.

Adenosine is produced in the body when adenosine triphosphate (ATP) is chewed up. ATP is the body’s energy currency. When we are awake and active, our bodies are burning a lot of energy to move and to think. In other words, we are using up a lot of ATP and subsequently producing a lot of adenosine. This adenosine makes its way to a receptor. There are four different subtypes of these receptors in the human brain. Biochemists call them A₁, A2A, A2B, and A₃. The A₁ type receptor is found primarily in the brain as well as in the spinal cord and heart. The binding of caffeine to the A2A receptors is thought to be responsible for increased alertness, while binding to A₁ receptors handle signals to the cardiovascular system.

Like all GPCRs, the adenosine receptors are embedded in cell membranes. These are large molecules—for example, human A2A contains more than 300 amino acids strung together like pearls on a necklace. It weighs over 32 kDa,§§ roughly the same as 1800 water molecules. A small loop on the top of the receptors containing the adenosine binding site—sometimes called the mouth of the receptor—sits outside the cell (Figure 3.8). When a molecule, such as adenosine or caffeine, binds to this loop, it causes a change in the structure on the part of the protein protruding into the cell. This tail of the protein flips like a physical switch and the resultant change in structure kicks off a cascade of processes inside the cell. The linking of binding on one side of the cell to structural change on the other is why these proteins are called coupled receptors.

Figure 3.8(a) The human A2A receptor with caffeine bound to it created using Mol*,¹ a visualization tool for large molecules. From the Protein Structure Bank, 3RFM.² (b) Schematic diagram of the receptor embedded in a cell membrane.

When lots of adenosine builds up on these receptor sites, they start sending a signal that we need rest. We feel tired and groggy. One way to clear out the adenosine is to take a nap or get a good night’s sleep, giving the molecular machinery that sweeps adenosine out of the receptors a chance to catch up. But, for many of us, a mid-afternoon nap or an uninterrupted night’s sleep are not possible. We are working during the day, perhaps caring for small children or a spouse at night. We have a project due and need to pull an all-nighter. Whatever the reason, we can’t take the time to let our adenosine receptors clear out.

3.5 Blocking the Signal to Sleep

The solution is as simple as a dose of caffeine, in a hot cup of tea or coffee, or a soda. Caffeine is a bit of a molecular bully, squatting in the same receptor sites in the brain that adenosine would like to use, unwilling to budge. Caffeine can block up to 50% of the adenosine receptors in the brain and, as we’ll see, can sit there for many hours. As a result, the adenosine that is building up as we go about our day can’t find a way to bind and send a signal to rest. We don’t feel tired, although, in fact, our body is tired. While we can use caffeine to override our body’s message to stop and take a rest, it’s a temporary fix. Like the dishes from lunch in the sink, which eventually must be washed up, the molecular debris is still building up and ultimately will need to be cleared out for our cells to properly function.

Why does caffeine have such an affinity for the adenosine receptors? The answer lies, of course, in the structures of caffeine and adenosine. As you can see in Figure 3.9, the structure of caffeine can be superimposed on top of the structure of adenosine. If adenosine fits in the binding pocket of a receptor’s “mouth”, it is likely that caffeine will also fit.

Figure 3.9The caffeine and adenosine molecules shown superimposed over each other. Note the near perfect overlap of the purine motifs.

In 2011, a team led by Fiona Marshall determined the structure of the A2A receptor with caffeine inside the binding pocket.²  The basic structure of the pocket is essentially two helices set askew like pincers, wider at the top and narrow at the bottom. Caffeine molecules slide in with the six-membered ring at the bottom of the pocket (see Figure 3.8). The caffeine is held in place by non-covalent bonding interactions. There is a hydrogen bond from one of the amino acids¶¶ in the pocket to one of the carbonyl oxygens, which anchors the caffeine firmly in place. The structure of the A₁ receptor with caffeine bound has not been determined, but, given the strong similarity between the sequences of the binding site, the binding should be similar.

While caffeine binds strongly to these adenosine receptors, it’s possible to craft molecules that will bind even more tightly—super-caffeines, if you will. One of these molecules is PSB36, which has the same xanthine core as caffeine, but also includes a long tail coming of the nitrogen that can extend deeply into the bottom of the binding pocket (Figure 3.10).³ This helps bind it more tightly in the pocket. PSB36 binds 10 times‖‖ more tightly than caffeine.

Figure 3.10PSB36 is a “super-caffeine” that uses the carbon chain tail in the box to anchor itself tightly in the adenosine binding pocket.

Caffeine can cross the so-called blood–brain barrier, a feat few drugs manage. While the blood–brain barrier sounds a bit like a plastic shield for the brain, it is in fact a complex web of structures and biochemical machinery that keeps most pathogens and toxic chemicals from penetrating into the brain. Caffeine’s ability to slip past the barrier is due to its structural similarity to adenosine. The search for drugs for neurodegenerative diseases, such as Parkinson’s disease, has capitalized on this, looking at drugs with a structure similar to caffeine.

Surprisingly for a drug so widely used, scientists have not entirely unraveled the various biochemical pathways responsible for all of caffeine’s effects on the human body. Another pathway that caffeine is thought to inhibit is the phosphodiesterase pathway, although toxicological studies suggest that this pathway is unimportant until toxic doses have been reached. Other molecular targets are not significant until very high concentrations of caffeine are reached either.

3.6 Caffeine and Humans: It’s Complicated

Caffeine is primarily a psychoactive substance, something that when ingested can result in temporary changes in cognition, behavior, perception, mood, and consciousness. Perhaps the most desirable and well-known effect of caffeine is alertness. But it has a range of other effects on our health, both positive and negative, all tied to its ability to bind to various receptors in the human body. Once again, its structure dictates its function.

As a stimulant, caffeine can both temporarily counteract the effects of a lost night of sleep and improve reaction times. It is important to note that caffeine will not counteract declines in performance due to the effects of alcohol or long term sleep deprivation. Despite all the movie scenes where a well-meaning friend tries to sober up their drunken companion with coffee and a cold shower, drinking a strong cup of coffee does not make it safe to drive when intoxicated. Nor will tea-swilling parents of newborns be at their sharpest, something I can personally attest to.

Other appreciated psychoactive effects of caffeine include the improved cognitive performance noted by Lu Yu in the 8th century, a benefit my exam-taking students can certainly appreciate. Caffeine can also improve mood. Not only can it lessen depression, but caffeine has a calming effect, part of what makes a cup of tea so soothing in moments of stress. Perhaps most intriguingly, it can also be used to treat pain.

A few years ago, I took a fall skiing, my ski going in one direction, my knee in the other. Soon I had a swollen and painful knee. I retreated to a fireplace with a bag of ice, two tablets of ibuprofen and a large mug of tea. While the warmth and aroma of the tea were soothing, I was really drinking it for the dose of caffeine it was delivering.

Like caffeine, ibuprofen works by blocking specific receptors, in this case ones that signal pain. Caffeine appears to increase the availability of the ibuprofen at the signaling receptor, helping more of the pain medication bind and stay bound. As a result, taking 100 mg of caffeine (roughly what was in that mug of black tea) in conjunction with 400 mg of ibuprofen makes it two to three times more effective in relieving acute pain. There is an additional bonus to this regimen. The onset of pain relief is faster and its duration is markedly increased. Compounds that boost the effectiveness of a drug or vaccine are called adjuvants. Caffeine is a potent adjuvant not only for ibuprofen, but also for other drugs in the same family, such as aspirin and naproxen. It’s worth adding a teabag or two to your first aid kit.

There are other apparent health benefits of consuming caffeine. There is evidence to suggest that caffeine is “neuroprotective.” For example, the regular consumption of caffeine reduces the risk of developing Parkinson’s disease. There is some research suggesting that caffeine can protect against other neurodegenerative diseases, including dementia and Alzheimer’s disease.

3.7 From Cup to Receptor: Brewing a Dose of Caffeine

Dry tea leaves contain up to 2.8% by weight caffeine. Soaking the tea leaves in water, particularly hot water,*** extracts the caffeine. The polar water molecules grab onto the polar “handles” (the carbon–oxygen bonds), efficiently washing the caffeine out of the leaves and into the tea. In just three minutes, fully 25% of the caffeine can be extracted from the dried leaves and, after five minutes, more than 90% of the caffeine is in solution, an easily ingested form.

Because tea is a natural product, the dose of caffeine will vary from cup to cup. (Both brewing time and water temperature will also affect the total caffeine content of a cup of tea; see Chapter 5.) A 2016 study of 37 different commercially available teas by researchers at the University of Szeged revealed that the caffeine content is quite variable.⁴ Boros, Jedlinszki and Csupor showed that a 200 mL cup of tea brewed from 2 g of loose leaves might contain anywhere from 16 to 80 mg of caffeine. For comparison, the same amount of caffeinated soda might contain 20–30 mg of caffeine, while a cup of coffee will have a bit less than 100 mg.

I had long thought that green teas contained less caffeine than black teas—perhaps because I associate them with the serene atmospheres of the Japanese Zen monasteries I have visited—but the same study found that the caffeine content was unrelated to the type of tea. The authors found a range from 25 to nearly 50 mg of caffeine per 200 mL cup in various samples of black tea and a very similar range in green teas, 20–45 mg in each cup. The average cup of tea contained about 35 mg of caffeine. I was surprised to discover that decaffeinated teas are not caffeine-free, although they generally have less than 10 mg per cup. Only herbal teas are truly caffeine-free.

One way pharmacologists look at drug dosages is to calculate the ratio of milligrams of drug administered to kilogram of body weight, or mg kg⁻¹ (pronounced “migs per kig”). The average daily dose in the US is about 3 mg kg⁻¹ or, put another way, an average 70 kg woman ingests 210 mg of caffeine per day, the equivalent of five to six cups of tea. On the high end, the top 10% of caffeine consumers ingest about 500 mg a day. More than 85% of adults in the US use caffeine on a daily basis. Even low doses of caffeine have a noticeable effect on human physiology. Sixty milligrams of caffeine, about what is in my morning mug of Assam tea, can measurably improve stamina.

Once consumed, caffeine is rapidly and completely absorbed from the small intestine. Caffeine appears to reach peak plasma concentrations within 30 minutes to two hours of consumption.⁵ While sleepy adults have occasionally joked that they would prefer their first cup of coffee to come intravenously, assuming that would be more effective, research has shown that this wouldn’t help. Caffeine is just as available from an oral solution, as in coffee or tea, as it is intravenously. Almost 20% more caffeine reaches the bloodstream from an oral solution or IV dose than if taken in pill or capsule form.⁵ Neither alcohol nor nicotine, nor age nor gender seem to affect the absorption rate.

Once caffeine is absorbed, it is well distributed throughout our body and ends up in the water in our intracellular tissues. Caffeine crosses the blood–brain barrier, which few drugs do. Once in our tissues, caffeine doesn’t typically accumulate in the body—our livers clear it effectively, although perhaps not as rapidly as we might wish.

3.8 Pharmacokinetics or Why Am I Still Awake at Midnight After Having Had Tea at 4?

I am probably not the only tea-drinker who at 11 pm is regretting the tea I drank to stay awake at the 5 pm seminar. Caffeine’s effects can be long lasting and the length of time it remains in the body is affected by a variety of factors, including how much broccoli you had for lunch and whether you eschewed the cookies for carrot sticks at the seminar tea.

One way to measure how long a drug will be effective is its half-life. The half-life is the time that it takes for one-half of the drug to leave the body. We often hear of half-life in the context of the decay of radioactive materials, but the principle also applies to many other natural processes. It turns out that for most drugs the size of the original dose does not affect how long it takes for half of it to be used up or eliminated, making the half-life a useful way to estimate a drug’s duration of action. A good rule of thumb is that a drug is essentially gone after about 10 half-lives; only 0.1% of the drug remains in the body at that point.

The average half-life of caffeine in the body is about four hours, but the range is wide and dependent on many individual factors. For example, the half-life of caffeine in pregnant people can be as long as 15 hours, while smokers metabolize caffeine twice as fast as non-smokers. It can take fully 24 hours for some people to clear enough caffeine from their system to no longer feel any effects, which explains why you can still be wide awake at midnight after a late afternoon cup of tea. Is there anything to be done?

Is there a way to either encourage the caffeine to vacate the receptor sites it is lounging in, or hustle it out of the body before it has a chance to settle in? It’s hard to nudge the caffeine out of the receptors—the binding is tight and whatever displaces the caffeine is likely to have a similarly stimulating effect. There are ways to speed the removal of caffeine from the bloodstream, which at least means that once an adenosine receptor is empty it is less likely to get refilled by caffeine, reducing the length of time caffeine will keep you awake.

Humans break down caffeine in their livers using an enzyme from the family biochemists call cytochrome P450 (specifically one with the code name CYP1A2). In the liver, one or more of the methyl groups is removed by one of these enzymes, converting it first into other xanthine molecules. In the end, caffeine is transformed into uric acid, which is eventually excreted in urine. Ultimately, less than 5% of the caffeine is excreted untouched. Drugs and foods that interfere with this pathway can increase the time caffeine remains in the bloodstream, while compounds that boost CYP1A2 activity can speed up the process.

Those who consume significant amounts of vegetables from the Brassica family—including cabbage, cauliflower, kale and broccoli—also boost their liver’s ability to break down caffeine using CYP1A2 by about 20%.⁶ Meanwhile, fans of grapefruit and vegetables from the Apiaceae family (carrots and celery) beware.⁷ The effectiveness of cytochrome P450 enzymes are reduced by these foods, which means that caffeine will persist longer in the bloodstream. Find a good book or something to binge on Netflix, you’ll be awake a while longer.

3.9 Side Hustles

As with all drugs, caffeine also has undesirable side-effects, beyond just keeping you up at night when you’d rather be sleeping. Chemists often say the dose makes the poison. Because caffeine is a central nervous system stimulant, at higher dosages, over 15 mg kg⁻¹, it can lead to an uncomfortably rapid heart rate as well as feelings of restlessness and anxiety. If you’ve ever tried to sleep after drinking an after-dinner shot of espresso, imagine if you tossed back 15 of those tiny cups. Those who aren’t habituated to caffeine may experience a rise in blood pressure. Ingesting high doses of caffeine—over 1000 mg—can result in more than nervousness and irritability. High doses of caffeine can result in delirium, vomiting, convulsions, and even death. As a result, energy drinks and caffeinated alcoholic beverages with a high caffeine content have recently come in for scrutiny.

As plant alkaloids go, caffeine is not terribly toxic to humans. Unlike ricin, found in castor beans (Ricinus communis) and the cicutoxin in water hemlock (Cicuta maculata), both of which are toxic at very low doses, caffeine at the levels found in beverages is classified as GRAS (generally regarded as safe) by the US Food and Drug Administration. Likewise, the European Food Safety Authority notes that caffeine can be safely used in moderation by most people. Still, as with any substance, even pure water, it is possible to consume a lethal dose of caffeine. Consuming more than 10 g of pure caffeine, roughly a tablespoon, is deadly in about 50% of cases. That’s the equivalent of rapidly drinking more than 200 cups (40 L) of a very strong tea. Even if you could drink so much so quickly, under these conditions the water (yes, the water) is likely to kill you first. A lethal dose of water is about 6 L, and of alcohol about 500 mL, while only 60 mL—four tablespoons—of pure nicotine can be fatal.

There is some evidence that drinking caffeinated beverages, including tea, can lead to gastrointestinal distress, including acid reflux. Since adenosine receptors are implicated in bone formation, it’s also possible that caffeine could have an adverse effect on the skeletal system. It appears that caffeine can both promote bone reabsorption and decrease bone formation. A large observational study of Swedish women showed that drinking four or more cups of coffee a day was associated with a small reduction in bone density, although this didn’t translate to an increased risk of fractures or osteoporosis.⁸ Tea-drinking appeared to boost bone density in post-menopausal women, suggesting that either caffeine does not play a part, despite the proposed mechanism, or that other compounds in the tea affect bone growth and override the effects of caffeine.

Caffeine is often thought to be a diuretic—in other words, that it will cause you to excrete large quantities of water in your urine. While this is true at high doses, the regular use of caffeine in moderation has no effect on hydration levels in the body. Some loss of water does occur at higher doses.⁹ For example, one study showed that people who were given 600 mg of caffeine per day—the equivalent of more than 10 cups of tea—experienced a loss of 0.7 kg of water over the day, with the concomitant loss of useful electrolytes such as sodium and potassium.

It is generally agreed that a total daily intake of 400 mg of caffeine is unlikely to pose a risk to most adults. Children and adolescents are recommended to keep their daily dose to 2.5 mg kg⁻¹. Those who are pregnant may wish to draw the line somewhere between 300 and 200 mg per day. Caffeine crosses into breastmilk, but studies suggest that babies are unaffected at daily doses below 400 mg. That said, individual sensitivity to caffeine varies in infants as in adults, and some babies will experience fussiness and a disruption in their sleep patterns when exposed to lower amounts caffeine while nursing. I gave up caffeine for the duration of nursing my youngest. It kept me awake, but alas what crossed into my milk kept him awake as well, an infinite loop of sleeplessness.

3.10 Can You Get Addicted to Caffeine?

If I skip my morning cup of tea, I am sometimes reminded by a headache that my excess adenosine receptors are missing something. Habitual caffeine users find that missing their daily dose of caffeine can leave them feeling exhausted and depressed as well as with a headache. Symptoms can be alleviated by filling up those receptors with caffeine or by waiting a few days for the body to readjust the number of adenosine receptors.

If withdrawal is unpleasant, does this mean caffeine is addictive? Many mental health professionals would say that addiction results when users both build up a tolerance to a substance and experience withdrawal symptoms when they quit. These criteria are accompanied by repeated failed attempts to quit and continuing to use the substance in the face of adverse effects on well-being.  Often addictive substances, like amphetamines and opiates, are tied into the overproduction of dopamine. Even though caffeine doesn’t provide a rush of dopamine, the patterns of use and abuse can be similar to other addictive drugs. The World Health Organization currently classifies caffeine withdrawal as a psychological disorder, although not all professionals would agree.

As we learned earlier, caffeine principally works by blocking adenosine receptors in the brain. If caffeine molecules are regularly camped out in these receptors—if you drink tea every day—then the body responds by making additional receptor sites. As a result, the body once again has empty spots for adenosine to bind to and you would need to take more caffeine to fill the spots and feel alert again. This overproduction of receptors is the mechanism that allows humans to build up tolerance to caffeine. The process is rapid and tolerance appears in as few as four days.

Studies in rats have shown that 20% more adenosine receptors were formed in the brain after two weeks of being given 10 mg kg⁻¹ caffeine per day (the equivalent of a human ingesting 700 mg of caffeine). The development of new receptors is rapid. It takes three to five days for new receptors to appear in mice treated with caffeine.¹⁰ The extra receptors vanish in about a week after the caffeine is withdrawn. Humans can reset their adenosine receptor array nearly as quickly: in a week to 10 days.

Tolerance to caffeine varies with the effect. It develops more rapidly for alertness, for example, than it does for increases in stamina. Studies in humans have suggested that a dose of caffeine before exercise, specifically cycling, increases performance for the first 15 days, then a gradual tolerance appears to build up. Interestingly, there is some evidence that the effects of caffeine on cognitive performance are not blunted by habitual use, an effect that my students can be particularly grateful for. (This also implies that the biochemical mechanism responsible for improved cognitive performance is distinct from the pathway that leads to increased alertness.)

3.11 What Does the Plant Get From Caffeine?

As with any chemical we extract from plants—from the bright orange of the beta-carotene sometimes used to color macaroni and cheese to the compound responsible for the marvelous scent of vanilla in my favorite cookies—plants make molecules for their own purposes. Plants neither know nor care what humans might do with the compounds their cellular factories churn out.

What do plants want with caffeine? Brace yourself: it’s a pesticide. Caffeine seems to serve as a chemical shield, a deterrent to pests who would like to dine on the tea plants’ leaves and roots. I was surprised to discover that the role of caffeine in plants is still not fully understood. In addition to conferring insect resistance, caffeine also appears to inhibit the infection of plants with a fungus spread by adult female shot-hole borer beetles (Xyleborou fornicatus ††† ). Caffeine may also help tamp down local competition for resources. Decomposing and fallen plant matter deposit caffeine in the soil around the base of the tea plant, which seems to inhibit the germination of seeds, a sort of automatic weeding system.¹¹

Although caffeine was first isolated from coffee plants by Friedlieb Runge in 1820, it took scientists nearly two centuries to trace the biosynthetic pathways plants use to produce caffeine and related compounds. The genes encoding the enzymes for the production of caffeine were first identified in tea leaves in 2000 by Misako Kato and co-workers at Ochanomizu University in Japan.¹² The same genes were subsequently identified in coffee and cacao plants; the pathway appears to be similar in other plants that produce caffeine.

The biological feedstock plants use to make caffeine is xanthosine, which has the same planar purine structure as caffeine (Figure 3.11), but is missing its methyl groups and has a five-membered ring tail attached to one of the nitrogen atoms. The plants begin by adding one of the methyl groups, followed by clipping off the tail. The final two methyl groups are then tacked on to produce caffeine.

Figure 3.11Scheme for the synthesis of caffeine in tea plants.

The xanthosine comes from the degradation of the adenine and guanine nucleotides that make up DNA and RNA (Figure 3.11). (It can also be produced de novo, in other words from scratch, and by a couple of other pathways.) Tea plants begin producing caffeine when they are quite young and, since it doesn’t degrade easily, it quickly builds up in the plant.

The genetic codes for the production of caffeine have been transferred to other plants, such as tobacco and chrysanthemums, to provide them with built-in resistance to insects. The resulting plants are less likely to be ravaged by leaf-eating caterpillars and aphids. The latter are one of the most devastating agricultural pests.

Caffeine is not evenly distributed throughout tea plants. Radioactive tagging experiments have shown that caffeine synthesis in tea only occurs in new growth, typically in the spring. External conditions also influence the caffeine concentrations in a tea plant. Triggered by a complex web of biochemical reactions, tea plants that are grown in the shade produce higher levels of caffeine. There are even a few species of tea and related plants that do not produce significant amounts of caffeine, although they do synthesize some closely related compounds.

3.12 Making Tea Without Caffeine

The floral fragrance of tea and its warmth can be enticing at the end of a long day, but the stimulating effects of the caffeine dose it delivers are not always welcome. Whether it is a good night’s sleep or health issues you are concerned about, sometimes a cup of decaffeinated tea is what you need or want. How do you get the caffeine out of a cup of tea?

Chemistry is as much the art of separating out molecules from a mixture as it is the art of making new molecules. Since we can’t use atomic tweezers‡‡‡ to pick out the molecules we want to get rid of, we sift molecules out depending on their properties. For example, you might have a filter on your tap to purify your water. The filter contains material that some molecules will stick to and others will not. Water molecules come through, lead ions don’t. Or you might add a solvent that dissolves the caffeine, letting you wash it out while leaving behind the rest of the compounds.

A common characteristic chemists use to separate out molecules relies on the fact that different amounts of different compounds dissolve in different solvents. You don’t need to be a chemist to know this, just doing the dishes will have taught you that grease will not come off your pans with plain water, but will if you add a dollop of dish soap.

Oily things dissolve better in oils, while non-oily things tend to dissolve well in water. Chemists use the term hydrophilic—water-loving—for things that do dissolve in water and hydrophobic—water-fearing—for things that don’t. For example, in tea we find that the caffeine in the leaves dissolves very rapidly and completely in water. Caffeine is hydrophilic. Some of the bitter compounds—for example, tannins—do not dissolve as well or as quickly. You may have noticed that if you leave tea leaves in water too long, then the tea tastes bitter: we often say it’s “over brewed.”

Like any other property, whether a molecule is hydrophilic or hydrophobic depends on the structure of the molecule. Water tends to dissolve compounds that are what chemists call polar. Like the two poles of a magnet, the electrons in a polar molecule are distributed such that one side of the molecule is slightly negatively charged relative to the other side of the molecule. When molecules have this disparity of charge, they tend to line up head to tail, with the positively charged ends aligning near the negatively charged side of another molecule, much like the opposite poles of magnets attract each other. Water can also make a special type of bond, called a hydrogen bond (Figure 3.12). While these bonds are not as strong as the ones that hold a molecule together, they can, and do, effectively hold one molecule to another in solution.

Figure 3.12A hydrogen bond between caffeine and water. A hydrogen bond uses a hydrogen atom to bridge between two electronegative atoms—atoms such as oxygen (O) or nitrogen (N) that attract electrons more strongly than most other atoms. These bonds are always linear, with the electronegative atoms and the hydrogen (H) in a row. Chemists use dotted lines to show these bonds, indicating that these intermolecular bonds are weaker than the bonds between atoms inside a molecule. Although they are not strong, they are important, helping to keep DNA zipped up and the cellular machinery in proteins in the correct shape to operate.

Chemists often say “like dissolves like.” So, we expect water will dissolve things that are polar because it is polar. Water will also dissolve molecules that it can make hydrogen bonds with. Caffeine is polar because the nitrogen and oxygen atoms it contains are more attractive to electrons than the carbon and hydrogen atoms. The nitrogen atoms and the two oxygen atoms sticking out from the ring can also make hydrogen bonds with water. Both these factors make caffeine attractive to water (hydrophilic).

What does all this have to do with removing the caffeine from tea? It means that you don’t need a chemistry degree or any special equipment beyond what you need to make tea in the first place: you can decaffeinate tea right in your own kitchen in less than five minutes. Take a teaspoon of tea and add about a scant quarter cup of boiling water (50 mL). After the tea leaves have steeped in the water for three minutes, most of the water-loving caffeine is now in the water and no longer in the leaves. (If you are using a teabag, steep the tea for just 30 seconds before removing it and re-steeping.) Now discard that solution and re-brew the tea leaves for five minutes in a fresh 175 mL of boiling water. Since there isn’t much caffeine left in the leaves to extract, the resulting cup of tea will contain little caffeine. This method can remove about 80% of the caffeine from loose green tea leaves and nearly all the caffeine from a teabag,¹³ while leaving behind most of the other molecules in green tea that contribute to the taste.  The taste will not be the same as the caffeinated version because caffeine, like most alkaloids, has a noticeably bitter taste. For this reason, decaffeinated teas often tastes bland and watery compared with their caffeinated versions.

This simple home technique for decaffeinating tea does not work as well for black teas as it does for green. Black teas have a different molecular profile from green teas and the balance of compounds left behind doesn’t taste as good. Commercial tea manufacturers have turned to other solvents to remove the caffeine from a broader range of teas. The choice of solvent hinges on finding one that will selectively remove caffeine, but not disturb the balance of the other compounds. The solvent must also be easy to remove and any residue left behind must be non-toxic and tasteless. Water, of course, fills all of these requirements. Alcohols,§§§ specifically ethanol, might also seem a good choice. For example, we use ethanol to extract the wonderfully aromatic compounds from vanilla beans. And, indeed, a 50 : 50 mixture of ethanol and water can be used at room temperature to decaffeinate tea.

Commercial decaffeination leans toward solvents that dissolve caffeine many times more effectively than water or water– ethanol mixtures. Originally, methylene chloride (CH₂Cl₂), which like water is polar, was used, but we now know that these chlorocarbon compounds are involved in the destruction of the ozone layer, are toxic at high levels, and are probably potential carcinogens. Even though most of the methylene chloride evaporates, there are residues left behind in the tea leaves that most tea-drinkers (including me) would find unacceptable. Ethyl acetate (C₄H₈O₂) is an alternative to methylene chloride. It is found naturally in many plants, including tea. While ethyl acetate leaves behind many of the desirable compounds, its residue can give an off-flavor to the tea.

Modern decaffeination processes use carbon dioxide and water. We are familiar with carbon dioxide gas; we breathe it out and it is used to carbonate drinks. Seltzer is a solution of carbon dioxide and water. But in this application it is not seltzer that is being used, but a supercritical fluid. This is an interesting state of matter that is typically found at high pressures and high temperatures,¶¶¶ where the material has the properties of both a liquid and a gas. In this decaffeination process, the tea is soaked in water or a water–alcohol mixture and then placed in a chamber where the supercritical fluid is produced. The supercritical fluid extraction of caffeine uses pressures that can be 300 times higher than normal air pressure, roughly the pressures found on the seafloor at depths of 3000 m. The advantages of this method are that no toxic residue is left behind, the carbon dioxide returns to the gas phase at room temperature and floats away, and the water evaporates, leaving behind the leaves. The resulting flavor profile is better than that for many other solvents.

Decaffeinated teas produced by extraction should really be renamed low-caffeine teas. Chemical analysis shows that some of these teas have as much as 10 mg of caffeine per cup, about one-quarter that of a standard cup of tea. If you want or need to drink a tea that contains no caffeine at all, tea brewed from C. sinensis will simply not do. Instead, choose a tisane, an herbal tea brewed from a plant that does not produce caffeine, such as peppermint or rooibos.

What if we could grow tea that naturally has no caffeine to begin with? It turns out that there are naturally decaffeinated teas. While most C. sinensis plants produce caffeine as the major purine alkaloid, there are a few related species that instead accumulate theobromine, the main alkaloid in cocoa. Theobromine (Figure 3.11) is just one stop earlier in the pathway that leads to the synthesis of caffeine. Camellia ptilopylla, aptly known as cocoa tea, produces theobromine in roughly the same amounts that normal tea leaves contain caffeine; the dry leaves are about 7% theobromine by weight.

A variety of Chinese tea called kucha makes theacrine instead of caffeine.¹⁴ And honyacha, a tea found in the province of Fujian, China and first described in 2018 by Liang Chen from the Tea Research Institute of the Chinese Academy of Agricultural Sciences, is naturally free of caffeine. Chen and co-workers showed that it lacked the genes for caffeine synthesis.

There is also a naturally caffeine-free coffee plant, Coffea humblotiana, found on the islands in the Comoro archipelago off the east coast of Africa near Mozambique.¹⁶ Its genome has been mapped and, like honyacha, it was shown to lack the genes that control the production of caffeine. Unfortunately, the plant also produces some unpleasant tasting and highly soluble compounds (diterpene glycosides) that render the resulting coffee undrinkable.

Since the genes for caffeine synthesis in plants have been identified, one can imagine using genetic engineering to create tea plants that do not have these genes. The leaves could then be harvested and processed in the usual ways to produce naturally caffeine-free tea. These teas presumably would not lack the other compounds that are removed in a chemical decaffeination process and so should have a less lack-luster taste. Another genetic tactic would be to find a way to increase the efficiency of the pathways in the plant that degrade caffeine, preventing it from collecting in the leaves.

At the time of writing this book, scientists have been unable to silence the caffeine synthesis genes in tea plants, although researchers have succeeded in creating coffee plants in which the genetic code to create caffeine has been removed. The leaves from these plants have roughly half the caffeine of the so-called wild type. The beans from these plants, dubbed Decaffito, are expected to have a reduced caffeine content once they mature. Other than a lack of caffeine, these genetically edited coffee plants appear to have the usual distribution of other compounds.

Should researchers manage to delete the requisite genes from C. sinensis, it is unclear how receptive tea-drinkers would be to drinking teas that have had their genes tinkered with.¹⁵ Although there is no known risk to such manipulations—I would drink a genetically modified tea—it may be that the market is too small to justify the investment.

Of course, nature herself tinkers with genes, with and without human assistance. Conventional cross-breeding could produce low- and no-caffeine strains. Zeno Apostolides, a biochemist at the University of Pretoria in South Africa, is heading a team screening tens of thousands of plants from tea plantations to identify low-caffeine “sports,” individual plants with genetic mutations that result in little or no caffeine being produced. These could be used to breed reduced-caffeine teas or even caffeine-free teas. Researchers estimate that such a breeding program might take up to 25 years, so for now drinkers of decaffeinated tea are limited to chemically decaffeinated teas.

3.13 Beyond Caffeine

First thing in the morning, caffeine may seem the most important molecule in my cup of tea, but there are thousands of other chemical compounds that contribute to both my psychological and physical well-being in that cup as well. In the next chapter, we will explore some of the structures and functions of those molecules. Why is a cup of tea so calming, even given the caffeine it contains? What makes tea an antioxidant? Are there health risks as well as health benefits to drinking tea?

3.14 Brewing a Better Cup

To decaffeinate tea at home, take a teabag (or a teaspoon of loose tea) and add about a quarter cup of boiling water. Let the teabag steep in the water for 30 seconds (three minutes for the loose tea). Discard the water and re-brew the tea leaves for five minutes in a fresh cup of boiling water. This will remove more than 80% of the caffeine.

Letting tea steep longer than recommended will not give you an extra-caffeinated cup of tea, just an increasingly  bitter-tasting brew.

The hotter the water, the more caffeine will be extracted; for maximum caffeine, use boiling water.

Never reuse a teabag if you want any caffeine in your second cup; virtually all of the caffeine has been extracted after steeping for less than a minute.

Further reading

M. Pollan, This Is Your Mind on Plants, Penguin Press, New York, 2021. Devotes an entire chapter to the story of caffeine, in particular its introduction to Europe and the impact it had on European intellectual life.

C. Willson, Toxicol. Rep., 2018, 5, 1140–1152. A good overview of the risks and benefits of caffeine from a pharmacological perspective.

References

1. D. Sehnal, S. Bittrich, M. Deshpande, R. Svobodová, K. Berka, V. Bazgier, S. Velankar, S. K. Burley, J. Koča and A. S. Rose, Nucleic Acids Res., 2021, 49, W431–W437.

2. A. S. Dore, N. Robertson, J. C. Errey, I. Ng, K. Hollenstein, B. Tehan, E. Hurrell, K. Bennett, M. Congreve, F. Magnani, C. G. Tate, M. Weir and F. H. Marshall, Structure, 2011, 19, 1283–1293.

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5. Institute of Medicine Committee on Military Nutrition, Pharmacology of Caffeine, National Academies Press, US, 2001.

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7. J. W. Lampe, I. B. King, S. Li, M. T. Grate, K. V. Barale, C. Chen, Z. Feng and J. D. Potter, Carcinogenesis, 2000, 21, 1157–1162.

8. N. K. Berman, S. Honig, B. N. Cronstein and M. H. Pillinger, Osteoporosis Int., 2022, 33, 1235–1241.

9. R . M. van Dam, F. B. Hu and W. C. Willett, N. Engl. J. Med., 2020, 383, 369–378.

10. D. Shi, O. Nikodijević, K. A. Jacobson and J. W. Daly, Arch. Int. Pharmacodyn. Ther., 1994, 328, 261–287.

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12. M. Kato, K. Mizuno, A. Crozier, T. Fujimura and H. Ashihara, Nature, 2000, 406, 956–957.

13. S. Suteerapataranon, J. Butsoongnern, P. Punturat, W. Jorpalit and C. Thanomsilp, Food Chem., 2009, 114, 1335–1338.

14. Y.-H. Zhang, Y.-F. Li, Y. Wang, L. Tan, Z.-Q. Cao, C. Xie, G. Xie, H.-B. Gong, W.-Y. Sun, S.-H. Ouyang, W.-J. Duan, X. Lu, K. Ding, H. Kurihara, D. Hu, Z.-M. Zhang, I. Abe and R.-R. He, Nat. Commun., 2020, 11, 1473.

15. S. K. Yadav and P. S. Ahuja, Plant Foods Hum. Nutr., 2007, 62, 185.

16. N. Raharimalala, S. Rombauts, A. McCarthy, A. Garavito, S. Orozco-Arias, L. Bellanger, A. Y. Morales-Correa, S. Froger, S. Michaux, V. Berry, S. Metairon, C. Fournier, M. Lepelley, L. Mueller, E. Couturon, P. Hamon, J.-J. Rakotomalala, P. Descombes, R. Guyot and D. Crouzillat, Sci. Rep., 2021, 11, 8119.

†Y. Lu, The Classic of Tea (茶经), 760–762 CE.

‡Yaupon holly’s Latin name is the unappealing Ilex vomitoria; the tea is available commercially. It was used in the ceremonies of some North American indigenous communities, where its consumption after fasting apparently on occasion led to vomiting.

§In Spanish, teína is used for the caffeine in tea, while cafeína describes the caffeine in coffee. They are, of course, the same molecule, regardless of source.

¶Synesthesia is a confusion of the senses. For example, numbers might have colors—three is always seen as orange or perhaps six is violet—or shapes might be “heard” as music.

‖The development of modern organic chemistry is tied to urine in a number of ways. Friedrich Wöhler showed that molecules from biological sources could be made from non-biological materials. He synthesized urea from a mineral source and showed it to be identical to urea extracted from urine.

**Although strychnine is used to poison rodents, including rats, in smaller doses it is a muscle stimulant. It was previously used in the treatment of heart disease and gastrointestinal disorders, and to enhance athletic performance.

††NMR stands for nuclear magnetic resonance, essentially MRI (magnetic resonance imaging) for molecules. NMR is a standard technique for determining molecular structure. MRI is actually an offshoot of NMR. Both work by putting molecules in a strong magnetic field and then tickling them with low-energy radio waves. The nuclei respond to the radio frequencies and the strength and patterns of their responses gives chemists clues to how the nuclei are connected in a molecule.

‡‡About one-third of drugs on the market target GPCRs.

§§A kDa is a kilodalton, a unit of mass for large molecules. The dalton is named after the British chemist John Dalton, who first proposed the notion of atomic mass. For comparison, a water molecule weighs 18 dalton. One dalton is about 0.000000000000000000000000002 kg (2 × 10⁻²⁷ kg)

¶¶The hydrogen bond is to Asn 253, the 253rd amino acid in the protein, which is an asparagine.

‖‖For the biochemists reading this, K i is 9560 nM for caffeine and 908 nM for PSB36.

***Caffeine’s solubility is highly temperature dependent. It is 30 times more soluble in boiling water than in water at room temperature.

†††Xyleborus derives from the Greek for wood (xylem) mashed up with borer; fornicatus is not what you think it is, but refers to the beetle’s characteristic arched shape—they look like tiny beans, about 2.5 mm in length.

‡‡‡Atomic tweezers do exist, but their primary use is in nudging individual atoms around a surface, not fishing out many molecules from a solution. Louis Pasteur famously used physical tweezers to pick out tiny crystals of one compound from a mixture, but the phenomenon that let him do that sort of separation is rare.

§§§Chemists use the word alcohol for a large class of compounds, anything with an –OH group attached. Unlike ethanol, most of these alcohols are not safe to drink.

¶¶¶For the chemists and physicists reading this, the critical temperature for CO₂ is 31.0 °C and the critical pressure is 73.8 bar.

Chapter 4

The Taste of Zen

The first cup moistens my lips and throat.

The second shatters my loneliness.

The third causes the wrongs of life to

fade gently from my recollection.

The fourth purifies my soul.

The fifth lifts me to the realms of the unwinking gods.

—Lu Tong, Song of Tea or Seven Bowls of Tea †

Tea Pairing

Read this chapter with a cup of Yunnan green snail tea, which is not made from green snails, but has had its leaves rolled into shapes that resemble snail shells. It is very fragrant and a bit spicy. For a cup that is likely to be high in soothing l-theanine, brew up a cup of Tetley Black & Green tea.

4.1 A Joy and a Delight

I, at least, drink tea for more than the pleasant zing of caffeine. I relish the full experience: the scent, the color, the taste, the warmth of the cup in my hand, and even the way the steam twirls in the late afternoon sun streaming through my office window. Tea can wash away the dregs of the night, soothe my soul, and a superb cup might lift me to those dazzling celestial realms.

Marcel Proust famously opened his seven-volume novel, The Remembrance of Things Past, with the narrator’s description of being catapulted back in time to his aunt’s room by the taste and smell of a small French cake, the madeleine,‡ and the tea his mother serves him. Traditionally, the madeleine gets all the credit for evoking these tender memories, but the tea plays more than a supporting role. Proust’s narrator recounts how the whole of his childhood village, from the water lilies on the Vivonne to the parish church, “sprang into being, all from [his] cup of tea.” The smell and taste of even the least drop of tea opened a vast reservoir of memory.

Many people begin drinking tea for health reasons. They switch from coffee to take advantage of tea’s lower caffeine content, or they’ve heard that something in tea moderates the caffeine-induced jitters. Perhaps they have read about the value of the antioxidants found in both green and black tea. But I will admit that the perceived health benefits were not my reason for taking up the habit. Growing up in the Midwest region of the US, I should have been a coffee-drinker. Instead, my first taste of tea was a Proustian moment. I was eight years old and camping with my family in the late fall in the Wisconsin Dells. It was frosty in the morning and when I asked for something to drink, my mother offered me a hot cup of milky, sweet black tea. More than five decades later the scent of the tea dust from a Lipton teabag unleashes a veritable tsunami of memory: the rushing noise of the gas fueling the propane stove my mother boiled the water on, the musty smell of the army green canvas tent, the faded blue color of the hoodie sweatshirt I wore, the creamy color of the milk tea, and the hint of bitterness in the brew that made me feel quite grown up.

My present-day interest in the composition of tea on the molecular level has as much to do with understanding its aroma and taste, what food chemists call the organoleptic experience, as it does with assessing the health benefits—and risks—associated with drinking a cup. That said, there is a lengthy list of health benefits attributed to the drinking of tea that would never have occurred to my eight-year-old self: a reduced risk of heart disease, stroke, type 2 diabetes, cancer, and anxiety. Evidence for these benefits varies and much is epidemiological, looking at trends in populations, rather than the gold standard of double-blind trials.

Tea has a long history as a medicinal beverage. Lu Yu in his Classic of Tea notes that tea can ease joint pain, cure headaches, and quell indigestion. In the 16th century, a compendium of medical treatments by Li Shizhan cataloged a long list of tea’s benefits, including the treatment of everything from malaria to dysentery. Interestingly, this collection observes that regular tea-drinking strengthens the teeth, an effect that we now know is due to the fluoride found in tea. By the 12th century, tea-drinkers were also admitting to enjoying their cup: Lu Yu called it a joy and a delight.

Nothing we eat or drink is free from risk. Even water is hazardous when enough is drunk. Tea-drinkers may find themselves at higher risk of developing anemia. Compounds in green tea have been linked to liver damage, while those who are affected by kidney stones are often advised to forgo a cup of tea. Drinking scaldingly hot tea may increase the risk of developing esophageal cancer.

In her 1885 paper titled “The Infusion of Tea,” chemist Wilhelmina Green thought it “remarkable how little was known regarding the composition of tea infusion as drunk, and of the conditions which determine the extraction of the various soluble constituents.” Fast forward more than a century and chemists have identified more than 2000 different molecules in tea infusions, an enormous library of chemical function. It is also important to remember that tea is not a uniform manufactured product. Rather, it is a natural product where every type of tea, and even every leaf, is different. As a result, every cup of tea that is brewed will be slightly different in its molecular make-up. In this chapter, we will take a look at the molecules found particularly in black, oolong, and green teas.

Since simply listing all the different molecules known to be in an infusion of tea would take most of the chapter, we will instead get acquainted with the families of common molecules found in tea. We will explore the structures of the most important compounds in each class and consider what we know about the sorts of molecules that are tumbling about in a tea cup. What are their structures and functions? Why is a cup of green tea the color of new spring grass and one of black tea a deep ruby red? What makes tea taste bitter, but smell of orchids? Can a cup of tea really reduce my anxiety and improve my cardiovascular health? Let’s dive into a cup and find out how tea works its molecular magic.

4.2 A Molecular Inventory

If you asked a random chemist what, aside from caffeine, is in a cup of tea, they may start their list much as Wilhelmina Green did, with tannins. Tannins are an enormous class of molecules found in plants, including in seeds, leaves, and fruit, as well the bark of trees.§ Their chemical structures feature flat six-membered rings of carbon with polar hydroxyl groups attached (Figure 4.1). Like caffeine, this class of molecules serves to protect plants from being eaten by insects and other animals. Tannins often have a puckery, unpleasantly dry taste, like a piece of unripe fruit, part of the reason they are so effective as a defense. Tannins can also act as antioxidants, able to surround and sequester chemical species with unpaired electrons.

Figure 4.1Molecular structure of tannic acid. The 10 hexagons with circles in them are phenols, hence the classification of this molecule as a polyphenol. Although I have drawn this to look like it is planar, it is not.

What other kinds of molecules have chemists found lurking in a cup of tea? Water, of course, is the most common molecule in brewed tea, though, like goldfish who don’t notice the water they swim in, chemists tend to ignore water when they make molecular inventories. Tannins and related compounds make up the bulk of the non-water molecules found in brewed tea by weight. Alkaloids, primarily caffeine, but other alkaloids as well, including a dash of quinine, are bustling about in a cup. There are sugars and amino acids. Carotenoids (yes, like in carrots) and other pigments are the palette tea paints with. Vitamins and minerals—including aluminum and fluoride—are also extracted from the tea leaves into the brew. All in all, it is a rich molecular mix that ultimately gives rise to a fragrant, pleasurable cup of tea. And one that offers both health benefits and risks.

4.3 Reading the Tea Leaves

Exactly how do chemists know what is in a complex mixture such as a cup of tea? We can’t just put a drop under a microscope and check out the molecules floating around. Molecules are so small that we can’t see them, even with the most powerful of standard light microscopes. To know what is in something we first have to take it apart. Chemists start an analysis of a complex mixture like tea by first breaking it down into individual components and then working out what each compound is.

You might be surprised to learn that chemistry is as much the art of taking things apart as it is the art of putting things together. The very word “analysis” comes from the Greek root “lysis”—to take apart. Analytical chemists take mixtures apart and identify their ingredients. In Wilhelmina Green’s day, this was an arduous process. Consider how Green painstakingly separated the tannins from her tea infusions. She began by adding copper acetate to the solutions. This formed a solid copper tannate, which she filtered from the solution. Green then combined the copper tannate with sulfur¶ and set it aflame in a stream of hydrogen gas. It must have been quite a sight—and the smell literally hellish. (Sulfur is otherwise known as brimstone.) The fiery reaction transformed the copper encapsulated by the tannins to copper sulfide, which Green weighed. She could use the amount of copper to back-calculate the original amount of tannins present.‖ The isolation and identification of each chemical compound in tea required a unique, multi-step protocol to be developed. Imagine doing that for hundreds or even thousands of different compounds.

Modern analytical chemists have less malodorous and more efficient techniques for separating out the different components of a mixture so they can be identified. (Also generally less exciting than burning brimstone.) While it can still be a daunting task, modern chemistry has several tools that Green did not have. One powerful approach to chemical analysis is chromatography, which literally means color writing. You can observe the phenomenon that this technique is based on by writing on a coffee filter or paper towel with a non-permanent marker. Wet one end of the paper with water and watch the moisture spread across the paper. You will notice that the various colored components of the ink move at different rates, separating them. Chemists make use of this property of different materials traveling at different rates through a medium to separate them out. In combination with techniques that allow the identification of each of these different components, chromatography enables chemists to figure out what is in even a very complex mixture, such as tea.

To identify the individual compounds once they’ve been separated, chemists must resort to looking for the ghosts of molecules. These spectroscopic** techniques measure the light absorbed by different molecules to identify them. Different structural features have different spectral signatures.  For example, molecules like caffeine that have a C=O (carbonyl) group in them strongly absorb infrared light. The infrared spectrum for caffeine is shown in Figure 4.2, where you can see the large peak that to a chemist’s trained eye says, “there must be carbonyls in this structure.” Nuclear magnetic resonance, which uses radio-frequencies to probe the connections between nuclei, is another spectroscopic technique chemists use to uncover the structure of a molecule. Every molecule has its unique spectral signature, like a fingerprint, that can ultimately be traced back to its structure.

Figure 4.2Infrared spectrum of caffeine. The large peaks on the left indicate the presence of the two carbonyl (C=O) groups. The larger of the two peaks results from the carbon–oxygen double bonds stretching in concert with each other (a symmetric stretch), while the slightly smaller peak is associated with the two bonds stretching 180° out of phase with each other (an asymmetric stretch).

Chemists can also weigh molecules using a technique called mass spectrometry (or mass spec). Mass spectrometry sorts molecules by their weights using magnetic and electric fields. Combining mass spectrometry and chromatographic techniques gives chemists a window into complex mixtures. It is what is used to analyze the urine of athletes to check for banned substances, and what chemists use to peer into a cup of tea.

4.4 A Taste of Zen

When I was in elementary school in the 1960s, I remember being taught that you could map different categories of taste onto specific areas of your tongue. Sweet was tasted at the front, while bitter and sour tastes were experienced further back. It turns out this map that I had in my head all these years is entirely wrong. By the mid-1970s, the work of psychologist Virginia Collings and others showed that all the taste sensations were experienced across the entirety of the tongue. Today’s sensory scientists—psychologists, biologists, biochemists, and chemists—have a much better understanding of how taste and smell arise. At their root, taste and smell are chemical processes. Humans have evolved to express molecular receptors for five basic tastes: sweet, sour, salty, bitter, and umami. These receptors are clustered in the mouth and when a triggering molecule binds to them, they send a signal to the brain that we register as a taste. Even though chemists cannot yet draw a direct line from a molecular structure to its unique taste or smell, there is much we do understand about how particular molecules give rise to particular flavors.

Of the five fundamental tastes—bitterness—is the one most associated with tea infusions. In his 8th century treatise Classic of Tea, tea master Lu Yu repeatedly characterized the drink as bitter. Evolutionarily, bitterness is a warning. Alkaloids often taste bitter and, given the number of naturally occurring toxic alkaloids, it is hardly surprising that humans and other mammals have evolved a marked sensitivity to bitterness. Caffeine and other alkaloids help give tea its bitter bite.

Biochemists have identified as many as 30 different individual taste receptors for bitter in humans. Perceptions of bitterness vary widely, depending on genetic factors. For example, not everyone can taste the bitter compound phenyl thiocarbamide (PTC). This was discovered in 1931 when Dupont chemist Arthur Fox accidentally dispersed some PTC powder into the air. While his nearby colleague complained that the dust tasted bitter, Fox couldn’t taste it at all. The two chemists then proceeded to deliberately taste the compound. It sounds shocking to modern ears, but chemists used to routinely taste and smell their  compounds—150 years ago, you could find hundreds of references to the taste and smell of molecules and reaction mixtures in major chemistry journals. In 1919, the Journal of the American Chemical Society featured a catalog of the tastes of nearly 100 molecules in an attempt to understand what made a molecule sweet.

Some people—including me—express a preference for bitter tastes. Decaffeinated tea tastes thin and bland to me without the bitter alkaloid in it. Given the dangers many bitter compounds pose, it seems somewhat surprising that people would have such a preference. It is possible that we associate the desirable effects of caffeine, such as alertness, with the bitter flavor and so override the safety warning. Or perhaps we put up with the bitterness in foods, whether in broccoli or tea, because of the perceived health benefits.

Personality may have something to do with these taste preferences because people open to novel experiences or to seeking out new sensations often have a preference for bitter tastes. I admit to finding the research of Christina Sagioglou and Tobias Greitemeyer at the University of Innsbruck in Austria in this regard more than a little disturbing.¹ They found a link between a preference for bitterness and psychopathic or narcissistic personality traits. This seems at odds with the traditional image we have of tea-drinking as a calm and meditative practice. As the 15th century Japanese Zen priest Murata Juko put it, “Zen and tea have the same taste.”

Other taste sensations result from molecular interactions beyond those with the standard suite of receptors. Astringency, a dry or puckery feeling in the mouth, is, next to bitterness, the most commonly experienced sensation when drinking tea. There is no receptor for astringency; rather, the sensation is thought to arise from the reactions of molecules in the tea with the proteins commonly found in saliva.†† These proteins help thicken saliva and improve its ability to lubricate the mouth. The resulting protein complexes are not very soluble in water, so this process effectively removes the proteins from the saliva. As a result, saliva becomes less viscous and less able to coat the mouth.

A sweet aftertaste, unrelated to the activation of sweet receptors, often follows an initial bitter or astringent sensation. It is a bit like the shadow of the sensation, or the rush of relief experienced when a painful sensation ceases. This phenomenon was observed in infusions of tea at least 2000 years ago. In an herbal dating from the 1st or 2nd century and attributed to Shennong (a legendary Yan emperor), tea is described as a bitter drink with a sweet aftertaste.

Whether perfume or wine or tea, trained tasters and “noses” can deconstruct a taste or smell, characterizing it using descriptors drawn from standard vocabularies developed for different substances. While we might be most familiar with such descriptions of wines, which might be oaky or fruity, these lexicons have been developed for many foods, including tea. A tea might be described as fresh or rustic, sultry or sweetish, or it might be said to have the scent of almonds or spinach, or cocoa. Less pleasant tastes might lurk in a cup of tea as well. In 2013, University of Kansas researchers Jeehyun Lee, Delores Chambers, and Edgar Chambers reported that a panel of trained tasters evaluating hundreds of green teas characterized some as having a burnt taste and others as animalic—having an earthy scent some find reminiscent of a horse stable.² Tooth-etch—the sensation that your teeth are grooved or etched after swallowing a substance‡‡ —was one of the most frequently invoked flavor descriptors of tea in this study.

One test of our understanding of the molecular basis for the taste of tea would be to take all the compounds that had been identified as responsible for the taste of a cup of tea and put them together in the correct concentrations and then see if this duplicated the taste. In 2005, German chemists Susanne Scharbert and Thomas Hofmann did just that. Specifically, they blended 51 compounds identified as contributing to the taste of black Darjeeling tea.³ These included 15 different amino acids, along with more than a dozen flavonol glycosides, various theaflavins, organic acids, sugars, and, of course, caffeine in amounts reflective of a natural cup of tea. A panel of trained testers assessed the mixture, concluding that it accurately mimicked the taste of Darjeeling tea brewed from leaves. They further reduced the list to only 12 compounds and testers still rated it as very similar to brewed tea.

4.5 A Potent Wizard

“Smell,” wrote Helen Keller, “is a potent wizard, that transports you across thousands of miles and all the years you have lived.” The perception of odor in humans is a complex phenomenon and, while indisputably chemical in nature, is not easily reduced to simple relationships between molecular structure and smell, in part because of the strong associations with the memories of past events it can evoke. In 1970, when asked about the molecular basis for smell, organic chemist Robert Luft responded that if you could answer that question, the Nobel prize would be yours. Twenty years later molecular biologists Richard Axel and Linda Buck would do just that: elucidate the basic molecular underpinnings for the perception of smell in humans. And yes, they did win the Nobel Prize for Medicine in 2004 for  their work.

To understand the connection between smell and molecular structure, we need to back up a bit. To smell something, it must first get into your nose and, to do that, it needs to be in the gas phase. Chemists call molecules that can easily go from a liquid or solid phase to a gaseous state “volatile.” The volatility of a molecule depends on how tightly the individual molecules are held together by what chemists call intermolecular forces. These forces depend on the structure of the molecule. Large molecules tend to be stickier than smaller ones. Molecules that are polar also tend to be held together more tightly because opposite charges attract each other, one of the big ideas in chemistry. Molecules that can make hydrogen bonds with each other are also sticky (see Figure 3.12). So, molecules that we can smell are likely to be smaller, have a low molecular weight, and are often not polar nor able to form hydrogen bonds. That said, the receptors need something to match with and recognize, which often means polar bonds or hydrogen bonds. As a result of trying to balance these competing structural requirements, the determination of the odor of molecules is a bit of a sticky business for chemists.

We smell with both our nose and our brain. Once a molecule enters our nose, Axel and Buck showed there are a plethora of odor receptors there that connect to signaling pathways for the perception of smell in the brain. High up in our nasal cavity, roughly between our eyes, are some 50 million neurons tasked with detecting molecules in the air drawn up into our nose. These neurons are crowded into roughly one square inch in each nostril. With them, we have the ability to detect and identify thousands of different odors, some of them at incredibly low levels. For example, the rotten fish smell of trimethyl amine can be detected at a one part per billion level, while the scent of a sulfur compound found in grapefruits can be perceived at less than one part per trillion. This is the equivalent of being able to taste a pinch of sugar stirred into an Olympic-sized swimming pool. Smell is indeed a potent wizard.

At the ends of the olfactory receptors are little fingers that contain proteins. These proteins, when they bind to a molecule, trigger a signal that is sent to our brain. Humans have a large and diverse complement of these receptors. Most of these receptors are based on the family of proteins that biochemists call GPCRs (G protein-coupled receptors), the same class of proteins that comprise the adenosine receptors that bind caffeine. Recently, researchers Josefina Del Mármol, Mackenzie Yedlin, and Vanessa Ruta at The Rockefeller University in New York showed that individual receptors are flexible.⁴ Rather than binding one specific molecule, a given receptor can be triggered by any of a suite of related compounds.§§

Despite humans’ poor reputation for odor detection, at least compared with mice and elephants¶¶ (and, yes, dogs), we devote more than 1% of our genome to coding for olfactory receptors. It is the biggest family of genes we possess, coding for roughly 400 different types of receptor. Compare this to the number of receptor types we use for vision—only three—or for taste, where there are only five general types.

Although huge strides have been made in understanding the biological mechanism for the detection of smell in humans, much of the way in which we process the scent of complex mixtures remains a mystery. Unlike a musical chord, which is the sum of the frequency of the individual notes and where louder notes can drown out quieter ones, odors are not simply additive. One odor can change the way that the cells in the nose respond to other odors.

Language—or our lack thereof—plays into our perception of aromas. While Proust details the memories that the taste and smell of the spoonful of tea and the madeleine surfaced, he fails to describe the taste and smell themselves. It is notoriously difficult to capture taste and, even more so, an aroma in words. Thomas Morton, in an essay entitled “Archiving Odors,” suggests that one reason for our inability lies in how few words many languages have to describe smells. The classic Roget’s Thesaurus lists more English synonyms for the color red than it has total words to suggest for the description of odors. As a result, we resort to allusion, referring to objects and experiences, speaking of the scent of a wet dog or of the air before a snowstorm, or a sultry cup of tea.

It is helpful to remember that taste and smell are more than straightforward chemical signaling processes. When dealing with complex natural sources, various signals can interfere with each other, which often changes our perception of the intensity of the individual signals. Given that almost 500 different compounds have been identified that contribute to the aroma of tea, the potential interactions are legion. And, as Proust so evocatively reminded us, our own recollections of previous experiences are layered onto the objective taste and aroma of a food. After a memorable incident more than a decade ago involving a stomach bug, an evacuation off a mountain by the ski patrol, and maple syrup, I find even the hint of maple in food repellent.

The smell of a cup of tea is a complex affair, starting with the molecules in the cup, moving into the molecular milieu of the nose, and finally waving a wand to bring forth those potent memories Helen Keller—and Proust—describe. What is in that evocatively fragrant brew that dances between bitter and sweet on my tongue?

4.6 Molecular Zen: l-Theanine

A few years ago, a friend sent me a link to something called mindful mints, candies containing spearmint, caffeine, and a compound extracted from tea, l-theanine. These sweets were meant to give you the boost of caffeine without the usual jitters. Theanine was touted as a way to moderate caffeine’s undesirable impacts and enhance its positive effects, including increasing that cognitive boost. It sounded like the ultimate bio-hack, pulling out just two of the molecules in tea and trying to optimize their effects. (Why the mint? I suspect it is an attempt to cover the bitter taste of the caffeine.) What does l-theanine do in tea and in the human body? Is it why tea can “cause the wrongs of life to fade gently from my recollection,” as mystic Lu Tong described?

Yajiro Sakato first isolated the amino acid l-theanine (Figure 4.3) from tea leaves, hence its name, in 1949. Intriguingly, it is not one of the amino acids used to make proteins and is found in only a few plants, most notably tea, although it is also found in a mushroom, Xerocomius badius ‖‖ and in the yaupon holly plant (Ilex vomitoria) used to make a caffeinated drink in North America. The theanine content of the tea plant is high, contributing as much as 3% to the dry weight of tea leaves, and found in higher concentrations in the roots. l-Theanine accounts for half of the amino acids in tea.

Figure 4.3Molecular structures of three sweet amino acids: lugduname, L-theanine, and aspartame. The –COOH groups make them acids and, combined with the amino (–NH and –NH₂) groups, they become amino acids.

Theanine concentrations in brewed tea are highly variable, ranging from about 25 mg of theanine a cup (200 mL) to less than 10 mg per cup. The amount of theanine is not correlated with the type of tea: a given black tea might have two or three times the concentration of a cup of green tea, or the reverse. A British study of supermarket tea observed that cheap teas often contained far more theanine than their more expensive counterparts.⁵ Only pu’erh teas have virtually no theanine—the thorough oxidation of the leaves during fermentation is thought to destroy it. A longer brewing time leads to higher theanine concentrations in a cup. A 90-minute infusion extracts the maximum amount of theanine, although I can personally testify to the undrinkability of such a brew.

Amino acids are the building blocks of proteins, which are the molecular machines that do everything from digesting our food, to transporting oxygen from our lungs to the rest of the cells in our body. The codes stored in our DNA allow us to essentially 3D-print these huge molecules that regulate everything from our blood pressure to our circadian rhythms. Between 10 and 15% of the mass of a human body consists of amino acids, mostly assembled into proteins. As we saw in Chapter 3, amino acids have both an amine (–NH₂) group and a carboxylic acid (–COOH) group in their neutral state (see Figure 3.7). Of all the millions of possible amino acids, only 22 are used to construct proteins in all terrestrial life. Amino acids can take two forms, mirror images of each other like left and right hands. The structures are then tagged l for laevus (Latin for “left”) and d for dexter (Latin for “right”). Virtually every protein is built exclusively from left-handed amino acids,*** consequently the molecular machinery that makes theanine makes it exclusively in the L form.

Even though tea plants don’t use theanine to build proteins directly, it is produced as part of a pathway to convert ammonia, which is toxic to the plant, into a form that makes the nitrogen more accessible for the synthesis of other biomolecules,††† including proteins and catechins. Theanine is primarily synthesized in the plant roots and then migrates to the leaves. Plants make theanine from ethylamine and glutamic acid using an enzyme aptly named theanine synthetase. Radioactive labeling experiments have shown ethylamine is the key to theanine synthesis: plants that don’t make ethylamine will not make theanine either. (Intriguingly, most plants do have theanine synthetase, so if fed ethylamine they will also produce theanine.) Growing conditions affect the amount of theanine produced in a given tea plant. Teas grown in sunnier conditions produce less theanine. First flushes will have more theanine than later flushes because the theanine that has spent the winter stored in the roots is transported to the early shoots, leaving less for sprouts that appear later in the season.

Unsurprisingly, given its structure and high concentrations, theanine is potentially a major contributor to the taste and aroma of tea, said to add a touch of sweetness and an earthy flavor to green teas in particular. In reconstructing the flavor of Darjeeling tea, however, Scharbert and Hofmann found that theanine does not significantly contribute to the taste. I spiked a cup of black tea (an Assam from the Halmari estate) with 125 mg of l-theanine to see if it affected the taste. I am not a trained taster by any means, but this tea is in my regular rota and, as I can taste the difference between tea made from the water at home and the water in my office, I thought I had a passing chance of seeing if high concentrations of l-theanine made a difference to the taste or scent. The short answer is that in a blind tasting I can tell the difference between a spiked cup and an untouched cup. The theanine adds a distinct undertone of sweetness to the flavor.

Many amino acids are sweet because their structures have features on them that match up with taste receptors on the tongue that signal sweetness (see Chapter 6) Theanine is no exception—and, yes, I’ve tasted it as the pure talc-like powder as well as in my tea. The sweetest substance known—lugduname—is also an amino acid and the artificial sweetener aspartame is built from two linked amino acids, aspartic acid and phenylalanine (Figure 4.3).

Theanine triggers the earthy umami taste of green teas because of its structural similarity to another amino acid, glutamic acid. Amino acids are often encountered in their ionic forms—for example, glutamic acid can transfer a positively charged hydrogen atom to water or another molecule to form a negatively charged glutamate ion (Figure 4.4). Glutamate triggers the so-called fifth taste sensation, umami. Food high in glutamate and structurally similar compounds have a rich, earthy taste. A number of protein receptors‡‡‡ for glutamates have been identified and binding to these receptors triggers the umami sensation. Monosodium glutamate (MSG) is a solid in which each glutamate ion is matched by a positively charged sodium ion. When added to a liquid or used in a recipe with liquid ingredients, the sodium ions separate from the glutamates and go their own way. Thus naturally occurring glutamates, whether in tomatoes, Parmesan cheese or anchovies, are indistinguishable from glutamates added in the form of MSG.

Figure 4.4Molecular structures of glutamic acid and monosodium glutamate. The sodium ion is what chemists call a spectator. The active species is glutamate and the sodium atom floats freely away.

Beyond its potential contribution to the taste and smell of a cup of tea, there is evidence that theanine has some health benefits in humans.§§§   ⁷ The structurally similar glutamate, in addition to signaling umami taste, is a ubiquitous neurotransmitter, found in the nervous system of every animal that has one, as well as in creatures that don’t, like sponges. Glutamate also serves as a signal in plants and is involved in processes ranging from seed germination to wound healing. It should come as no surprise that theanine affects the human central nervous system. Interestingly, while glutamate does not cross the blood–brain barrier, theanine, like caffeine, apparently can.

While studies have shown that theanine can reduce anxiety, improve cognition, reduce blood pressure, improve the quality of sleep, and, like caffeine, has neuroprotective properties, the status of theanine in various national pharmacopeias varies. In the US, it is considered GRAS (generally recognized as safe) and, in Japan, it is approved as a food additive. Germany forbids adding exogenous theanine to beverages and, more generally, the European Union does not consider theanine to have any health benefits.

Is theanine the source of the Zen in a cup of tea? In 2020, a group of researchers in Canberra, Australia looked at the available data on the effects on mood and cognition of l-theanine in humans.⁸ They concluded there is some evidence that theanine can reduce the stress response in humans, at least in the short term. It was not shown to be effective in reducing clinical anxiety, although it may have uses in addressing mild subclinical anxiety. It appears to have few side-effects at the dosages used in many of these studies. Theanine seems to work in synergy with caffeine, smoothing out the caffeine jitters and moderating the spikes in blood pressure caffeine can produce. It appears to increase the cognitive boost caffeine gives and enhances its neuroprotective capabilities. Reported effective doses range between 200 and 400 mg per day, the equivalent of drinking 10 or more cups of tea a day. Or you can do what I did while I wrote this section and stir 250 mg¶¶¶ of theanine into my tea. I felt pretty chill.

4.7 The Power and Peril of Oxygen

What, aside from caffeine, is in tea that humans find desirable? If you stopped a random person in the tea section of the grocery store, they might say “antioxidants.” Tea has been considered to be a medicinal beverage for as long as humans have brewed it. In the modern era, the health benefits of tea are often attributed to the high concentrations of antioxidants it contains. Before we embark on a tour of tea’s rich collection of antioxidants, let’s take a moment to consider what an antioxidant is, and why we should want them in our diet.

Life on Earth depends on oxygen. Oxygen is a highly reactive molecule, which is both its power and its peril. While some reactions of oxygen in the cell are what produce the energy to sustain an organism—why we can’t live without oxygen—others can damage critical structures such as DNA. The goal is a balance, suppressing damaging pathways without turning off the metabolic processes that keep us alive. Molecular oxygen, the O₂ gas we breathe, can be transformed by various pathways into what are called reactive oxygen species, such as hydrogen peroxide and free radicals.

A free radical is a molecule that has an unpaired electron. These unpaired electrons are so desperate to find another electron to pair up with that they will rip one away from other molecules. Ripping away an electron, part of the glue that holds a molecule together, disrupts the bonding in a molecule, often fundamentally altering its structure in the process. This change in structure leads to a change in function, and not always for the better. Free radicals can fragment the strands in a DNA molecule, destroying some of the genetic information stored within, and chew up proteins and other essential molecules.

Imagine a chain-reaction crash on a highway and you have some sense of the damage even a single free radical can do. Every time a radical tears an electron away from another molecule to satisfy its own needs, it creates a new radical in turn, which now seeks out a new victim, leaving a long trail of molecular wreckage in its wake. Some antioxidants work by sopping up the precursors to reactive oxygen species. Others surround and isolate these molecular marauders where they can’t do any damage. Antioxidants come in many forms, as we shall see, and an infusion of tea has more than its share of these defenders.

4.8 Catechins: Can You Catch Them All?

The tannins that Green so painstakingly isolated from her pot of tea are a major source of polyphenols in the human diet. A single cup of tea contains around 200 mg‖‖‖ of polyphenols, in the form of a family of molecules called catechins (pronounced  CAT-ah-kins). Catechins contribute to both tea’s puckery astringent flavor and its color. They are also powerful antioxidants, a significant source of tea’s medicinal benefits.

Polyphenols contain a number of six-membered aromatic rings (phenyls) with –OH groups (alcohols) attached, like the tannic acid shown in Figure 4.1. Catechins have two of these phenol rings; the base structure is shown in Figure 4.5. The name of this chemical family derives from catechu,**** an astringent extract from the bark of acacia trees, from which the first catechins were obtained.

Figure 4.5Molecular structure of the parent catechin, along with two catechins found in high concentrations in tea: epigallocatechin (EGC) and epigallocatechin gallate (EGCG). The phenyl rings are the six-membered rings with three double bonds.

Catechins are found in many foods derived from plants. Both chocolate and prune juice contain significant amounts of catechins, as does beer. The two most abundant catechins found in tea are epigallocatechin gallate and epigallocatechin (Figure 4.5). Although these chemical names do not roll off the tongue (even biochemists tend to call them by their initials, EGCG and EGC respectively), to a chemist they underscore that these are plant- derived materials. The names are a mash-up of terms referring to the galls (balls of fungus) often found on oak trees and the catechins derived from acacia trees. These two molecules account for about three-quarters of the catechins in tea, although at least 10 other catechins have been identified.

Catechins contribute to the astringent†††† and bitter taste of teas. The astringent quality of the catechins is thought to arise from their reaction with proline-rich proteins in saliva. Recall that these proteins help to thicken the saliva and improve its ability to lubricate the mouth. The –OH groups sticking out from the sides of the catechins act like molecular Velcro, grabbing onto the proteins. The protein–catechin complexes are not very soluble in water, so this process effectively removes the proteins from saliva (which is mostly water), making it thinner and less effective at coating the inside of the mouth. The result is a drier mouth and a puckery feeling.

What work do catechins perform for the plant? Like caffeine, catechins can act as local herbicides, discouraging the growth of plants around the base. They also have antibiotic properties, a defense against infections. Catechins appear to discourage herbivores from browsing on the plants by disrupting oxidation–reduction reactions in their gut and upsetting their digestion.

Gallocatechins are extraordinarily powerful antioxidants. They are essentially molecular vacuum cleaners, able to hoover up damaging reactive oxygen species, such as superoxide and singlet oxygen. These catechins are strong antioxidants due to the –OH groups sprouting from the phenyl rings (the hexagons with three double bonds). These so-called phenolic hydroxyl groups react with the radical oxygen compounds and produce innocuous molecules, stopping the free radical chain reactions before they can do more damage to cellular structures, such as DNA. This is a likely mechanism for why black tea in particular is associated with a reduced risk of cardiovascular disease in meta-studies.⁹

The amount of catechins in a particular cup depends on the type of leaf as well as the growing conditions. Tea leaves grown in the shade have a lower quantity of catechins and produce a less astringent beverage. Green tea contains more catechins than oolong, black or pu’erh teas. During fermentation, oxidation turns the catechins in green tea into new compounds— theaflavins and thearubigins—making those teas darker. But you can have too much of a good thing and over-long fermentation of tea leaves results in the formation of theabrownins, which can change the flavor profile of the resultant brew.

Natural does not necessarily equate with having benefits for human health, or even “safe.” Drinking tea is known to deplete iron in the blood. Iron is the most abundant of the transition metals in the human body and is used to transport oxygen and to break down molecular invaders. A 70 kg human contains about 4 g  of iron (along with 2 g of zinc and traces of cobalt, copper, and molybdenum). Most of the iron in the body is used to make the active core of hemoglobin, which binds oxygen in red blood cells. Unfortunately, the catechins compete with hemoglobin for iron ions. The same phenolic hydroxyls groups that make catechins such powerful antioxidants can also wrap around iron ions, keeping them out of circulation and making less iron available to populate the hemoglobin oxygen-binding sites. If iron in the diet is not increased to compensate, tea-drinking can lead to, or exacerbate, anemia. Given that the World Health Organization estimates almost a third of women worldwide are anemic, tea is not necessarily a healthy beverage choice for many populations.

Catechins may also damage the liver.¹⁰ Experiments with rats suggested that EGCG, which is the primary catechin in green tea, was the most toxic of the catechins. Another study showed that liver enzymes were elevated in a small portion of a group taking doses of green tea extract in excess of 850 mg per day over a long time period. Interestingly, these elevations were not observed in a group who were ingesting a similar dose by drinking green tea, rather than taking a pill. As a result, the European Food Safety Authority examined the potential toxicity of catechins in green tea infusions. While they could not rule out that rare individuals might have an idiosyncratic reaction to the catechins in tea, they concluded in 2018 that drinking a cup—or even many cups—of green tea was unlikely to result in liver damage in humans.

4.9 Molecular Chameleons

The most vivid memory I have of taking first-year chemistry at university is one of pure panic. I was taking the section of the course for chemistry majors, taught by Francis Sherwood Rowland (who would win the 1995 Nobel Prize in Chemistry for his work on the ozone layer and chlorofluorocarbons). I turned over the exam paper to find just four questions, none of them of the numerical sort I had been used to solving in previous courses. And none that I had the faintest idea of how to answer, at least at first glance. It made me wonder if I really was going to be a chemist.

More than 40 years later, I can still recall three of the four questions. The first question stated that the color of tea was observed to lighten perceptibly when lemon was added and asked why that should be so and for the pertinent chemical reactions. In addition to the panic, I can also recall my delight when I realized I could answer the question. We had learned about acid–base indicators and I realized that, even though I didn’t think of tea as being in the same chemical group as the phenolphthalein‡‡‡‡ I had used to titrate acids in the laboratory, it might work the same way. Maybe I could be a chemist after all.

The key to this chameleon-like behavior of tea is a class of compounds called the thearubigins. Thearubigins are red or red–brown in color, while theaflavins impart a bright red-orange tint, accounting in part for the color of black teas when brewed. The color comes from the benzotropolone ring, the fused six- and seven-sided rings in Figure 4.6. When a proton from the citric acid in the lemon attaches to a thearubigin, a new compound is formed that is lighter in color. These sorts of chemical chameleons that change color in solutions of different acidities are called indicators. This is the chemistry behind the pH strips used to test aquarium water and the litmus paper that detects acids and bases. Along with the catechins, theaflavins add to the astringent taste of black tea and bring a desirable bright golden undertone to an infusion.

Figure 4.6Molecular structures of a theaflavin and a thearubigin. Note the seven-membered ring found in each of these, glued in the theaflavins to a six-membered ring. These are the sub-structures responsible for the color of these molecules, what chemists call a chromophore.

Thearubigins are formed from catechins, particularly EGC and EGCG, during the oxidation of tea leaves, as are the related class of molecules, theaflavins (Figure 4.6). As a result, infusions of black tea contain large amounts of both types of compounds. Researchers at the Tea Research Foundation of Kenya showed in 2009 that as the catechin concentration in a tea leaf fell, the amount of theaflavins and thearubigin in the tea leaf increased proportionately.¹¹ Both theaflavins and thearubigins are formed when catechins combine in processes chemists call condensation and polymerization reactions. These are polyphenols and therefore antioxidants.

Nikolai Kuhnert at Bremen University explored the structures of thearubigins found in tea. He estimated that there might be as many as 1500 different thearubigins in a tea infusion.¹² As an organic chemist, he was amazed by the structural diversity and complexity of the compounds formed from the catechin building blocks, a result of incredibly unselective chemical reactions. Even though these are polymers, unlike the polymers of nucleic acids and proteins, which string together simple small molecules like beads, the catechin building blocks themselves undergo significant changes in their structures during assembly. Kuhnert suggested that this lets the plant develop its own chemical library similar to what pharmaceutical chemists assemble to find compounds that might lead to new drugs. Essentially, the tea plant has on hand a stock of potential herbicides and pesticides.

4.10 Bitter Dregs: Other Alkaloids

The main alkaloid in tea is, of course, caffeine, discussed at length in Chapter 3. Other purine alkaloids similar to caffeine in their structure and properties and found in tea infusions include theobromine, theophylline and theacrine (Figure 4.7). All are found at much lower concentrations than caffeine. Although all three are pharmacologically active, principally due to their ability to bind to various adenosine receptors, none is found at levels high enough in a tea infusion to produce a significant effect in humans.

Figure 4.7Molecular structures of common alkaloids found in tea: theacrine, theophylline, theobromine, and caffeine.

Tea contains only about 1% as much theophylline as it does caffeine. Unlike theobromine, which is a molecular way station on the caffeine synthesis pathway, theophylline is produced by the breakdown of caffeine in tea leaves. Theobromine and theophylline are also minor human metabolites of caffeine and about 15% of the caffeine you consume is converted into theobromine and theophylline in the liver. Theophylline was first extracted from tea in the late 19th century and gets its name from a portmanteau of the Latin root for tea (theo) and the Greek root for leaf (phyllon). Its structure is nearly identical to caffeine.

Theophylline is a bronchodilator, acting to relax muscles, including those that control the airways, as well as being an anti-inflammatory and diuretic. Theophylline has been used to treat asthma since the 1930s, although it has a very narrow window of therapeutic utility. Overdose may result in arrhythmias and a rapid heartbeat. While better drugs are now available with fewer undesirable side-effects, theophylline remains in use as a treatment for chronic obstructive pulmonary disease and asthma in both humans and domestic animals. The levels in tea are far below any that would affect the airways, less than 0.1 mg compared with a typical therapeutic dose in adults of 300–400 mg. As comforting as I find a cup of tea when my asthma flares up, my tea isn’t doing anything to treat it.

Cocoa beans are the primary dietary source of theobromine, not tea. The limited amounts in tea are left over from the biosynthesis of caffeine (see Chapter 3). Depending on the type of tea, concentrations of theobromine can range from 2% to slightly less than 10% of the caffeine levels. Despite the name, theobromine contains no bromine. The name comes from the genus of the cacao tree, Theobroma, which derives from the Greek roots theo—god—and broma—food. (As far as I’m concerned, both tea and chocolate are divine.)§§§§

Like caffeine, theobromine interacts with the adenosine receptors in the brain. Dog-owners might recognize theobromine as what makes chocolate toxic to dogs. Doses of 100 mg per kg of a dog’s body mass are extremely hazardous. Caffeine is similarly toxic to dogs, although they clear it far more quickly than they do theobromine. The half-life in canines is about 17.5 hours for theobromine and 4.5 hours for caffeine. Theobromine and caffeine are also toxic to cats, but cats are generally picky eaters and appear less likely to ingest chocolate or lap up their owners’ coffee or tea than dogs.

Theobromine has no current pharmacological use, although it has been implicated in chocolate’s ability to trigger gastroesophageal reflux disease. Like theophylline, it acts to relax muscles, allowing acid to escape from the stomach into the esophagus. It has also been suggested that theobromine is responsible for chocolate’s ability to increase the levels of beneficial high- density lipoprotein (HDL) cholesterol.

Theacrine has an additional methyl group added to it, as well as an extra oxygen atom popped onto the “nose” of the purine. Given their structural similarities to caffeine, it is unsurprising that theobromine and theacrine also bind to adenosine receptors, although not as strongly as caffeine. They both therefore behave as central nervous system stimulants. It is reasonable to assume that both are toxic to the pests that afflict tea plants.

4.11 Fluoride Is in More Than Your Toothpaste

Fluoride is the anionic (negatively charged) form of fluorine, F⁻.  We tend to associate it with toothpaste and municipal water systems, not tea. So, you may be surprised to learn that tea is a natural source of fluoride. Tea is one of the few plants that accumulates fluorine and one of the very few where the fluorine is in a non-toxic form.¶¶¶¶ While other foods, such as grapes and potatoes, also contain small amounts of fluoride, tea is the primary dietary source of fluoride. The Toxicarium family of plants also extracts fluorine from the soil, but converts it into toxic fluoroacetates, a poisonous variant of acetic acid, the main ingredient in vinegar. Cattle and other herbivores find the plants tasty, but browsing on them is fatal. While plants from the Camellia family also hyper-accumulate fluorine, it is in the form of the fluoride ion, which is non-toxic to humans at the levels it is found in these plants.

There’s a lot of fluorine to be found on Earth, so it is perhaps not surprising that it shows up in plants and animals. By weight, it is the 13th most abundant element in the Earth’s crust, more abundant than carbon. The fluoride in a tea plant originates in the ground, as do all the metals and minerals that are found in plants. It is not added when the leaves are processed or necessarily derived from contaminated soil. The more fluoride in the soil, the more fluoride is found in the tea plants grown in it. Volcanic soils are often naturally richer in fluoride, but fluoride can also be present in the soil as a contaminant. Common polluting sources of fluoride include some fertilizers as well as the byproducts of metal smelting operations, power plants, and glass factories.

It is not entirely clear what the molecular mechanism is that transports fluoride from the soil into a plant, or why the tea plant would want to imbibe fluoride. Fluoride is not good for the plants. It can combine with positively charged calcium and magnesium ions in plant cells and disrupt their proper functioning. High fluoride concentrations can result in “tip burn,” where the leaves gradually turn yellow and are then shed. (The plants that produce the toxic fluoroacetates are thought to do so to discourage animals from eating them, although it seems a bit of a scorched-earth approach to kill off the herbivores.) Still, genes that code for proteins to create fluoride-specific channels into cells have been identified in Camellia sinensis plants. Most of the fluoride is transported either as the ion itself, or as aluminum–fluoride complexes, via non-specific channels, essentially picking up the fluoride by accident along with ions that the plant requires. (Tea also hyper-accumulates aluminum.) These complexes are stored in little holes, called vacuoles, in the leaves. Virtually all of the fluoride is stored in these vacuoles. Sequestering fluoride into a vacuole is one strategy that the plant uses to minimize fluoride damage, combining it with aluminum is another. The aluminum–fluoride complexes are less toxic to the plants than either the free fluoride or free aluminum ions.

A tea plant can have several thousand milligrams of fluoride in a single kilogram of the dry plant. The older the leaf, the more fluoride it contains. Tea leaves harvested in the spring generally have less fluoride than those harvested in the summer or autumn. Somewhere between 20 and 90% of the fluoride in tea leaves can be extracted into the brew, depending on the infusion conditions. Higher water temperatures extract more fluoride. Brewing tea from broken tea leaves or CTC (crush, tear, curl) teas, such as those found in many commercial teabags, also yields a higher concentration of fluoride in the final cup. We see another of the big ideas in play here: the higher concentrations are a result of more exposed surface area for the leaves and the resulting proximity of the solvent (water) and the substrate (the leaves). Longer brewing times likewise result in more concentrated fluoride infusions.

Studies have shown that black commercial teas in pure distilled, deionized water have on average about 2.5 mg of fluoride per liter.¹³ By comparison, the US Centers for Disease Control recommends that water is fluoridated at a level of 0.7 mg per liter and that drinking water should not exceed a fluoride concentration of 4 mg per liter. The maximum daily intake of fluoride for adults should not exceed a dose of 10 mg per day, the equivalent of drinking three liters of black tea. Even the most prodigious national tea-drinkers, the Irish and the Turkish, don’t drink that much tea in a day!

The recommendation to fluoridate water stems from the ability of micro-doses of fluoride, roughly 50 µg per kg body weight, to reduce dental decay. Because fluorine is underneath chlorine in the periodic table, the pathways that incorporate chloride into tooth enamel are also available to fluoride. As a result, fluorapatite is formed in the teeth, which protects them from decay. It is worth noting that too much fluoride can be problematic. An overdose of fluoride, fluorosis, can permanently discolor teeth. It can also affect bones and there is some evidence that it can cause damage to tissues ranging from the liver to the thyroid gland. If your drinking water contains a great deal of fluoride to begin with, using it to make tea can lead to undesirable health outcomes. In particular, children who live in volcanic regions and drink tea can consume more fluoride than they should.

4.12 Metals and Minerals

We don’t think of plants as sources of metal in our food, except perhaps spinach as a source of iron, but about 10% of tea consists of inorganic substances (materials that aren’t based on a carbon–hydrogen framework). Although we tend to view heavy metals, such as lead or cadmium, and radioisotopes, such as uranium and thorium, as unnatural contaminants, these elements also occur naturally in the environment and so find their way into plants, including C. sinensis. We have seen with fluoride and aluminum that what is in the soil can be sucked into the tea leaves and, of course, what is in the tea leaves can be extracted and find its way into the cup that we drink and, eventually, into our bodies.

Metals, including iron and uranium, have been found in tea leaves and tea infusions. It is important to remember that chemists can now detect metals at very low concentrations, so simply the ability to detect a metal does not necessarily indicate it is a hazard. Chemists say “the dose makes the poison.” Iron is a good example of this. Low doses are necessary to form blood cells, about 10 to 20 mg per day (the equivalent of a 1.5 mm diameter bead of pure iron), while at high doses it is toxic—it is one of the leading causes of poisoning in US children.

Tea is not the only plant that takes up metals from the environment. For example, Brazil nuts can contain barium at concentrations that are toxic to humans and they can be highly radioactive due to radium. Barium and radium are both found underneath calcium in the periodic table and, as a result, are expected to have similar properties. The same pathways that bring in calcium ions are also capable of pulling in barium ions and radioactive radium ions. What metals have been found in tea leaves and in the tea brewed from them, and do they present a hazard to humans? The list includes metals considered essential for human health, such as chromium, manganese, and iron, as well as less savory elements, such as cadmium, lead, thorium, and uranium.

As virtually every article on aluminum in the diet reminds you, aluminum is the third most abundant element in the Earth’s crust. There were once hints that aluminum could be implicated in Alzheimer’s disease and, although the current consensus seems to be that exposure to aluminum is not associated with the disease’s development, people today are more aware of the sources of aluminum in their diet. While additives in processed foods, particularly in baked goods, are the main source of dietary aluminum, tea contains somewhere between 0.25 and 1 mg of aluminum per cup. This is similar to what is found in wine, but higher than the levels observed in coffee and beer. Bananas and squash are also hyper-accumulators of aluminum, with many times the concentration in tea.‖‖‖‖ Studies suggest that less than 10% of the aluminum in a cup of tea (or that glass of wine) is absorbed and it is therefore not a significant source of dietary aluminum.¹⁴

While drinking tea depletes iron in the bloodstream, there is a lot of iron in tea leaves, about 100 mg per kg. This is about one-quarter of the amount found in spinach. Not much of this iron makes it out of the leaves into the final infusion. A cup might contain 1–4 mg of iron. It is worth noting that plant sources of iron are generally not as bioavailable to humans as animal or heme sources. Spinach might be famously iron-rich, but only about 2% of that iron can be absorbed by the human body. If the same applies to tea, then a cup would provide a scant dose  of iron.

4.13 Tea Is Radioactive and So Are You

Pretty much everything is radioactive. All living things are radioactive, including you and me. The granite countertop you make your tea on? Radioactive.***** And the tea? Yep, radioactive, too. So, what is in all of these things that release radiation? Every element has several isotopes, atoms that differ in the number of neutrons in the nucleus. For example, carbon-14 has six protons in its nucleus because every carbon atom has six protons, but eight neutrons. Most carbon atoms are carbon-12, with six protons and only six neutrons in their nucleus. Isotopes like  carbon-14 that are radioactive are called radioisotopes.

Living things are radioactive because they contain carbon-14 and potassium-40, both of which primarily decay by emitting beta particles.††††† Most of the radiation emitted by living plants and animals stems from the decay of potassium-40, which has a half-life of more than a billion years. The so-called banana-equivalent dose was created to help people get a sense of the magnitude of natural radiation in food and other objects and events. One banana emits about 15 becquerels of radiation (a becquerel is one decay event per second). A human being emits at a rate of about 3000–4000 becquerels.

Because tea contains potassium, it will be radioactive. However, tea is not a significant source of potassium—there are only between 30 and 40 mg in a cup of tea, resulting in about 1% of a banana-equivalent dose of radiation per cup. Tea has also been shown to contain radioisotopes of uranium, thorium, and radium. Researchers in Bangladesh have estimated the total radiation dose from drinking tea to be the equivalent of about 50 banana-equivalent doses per year.¹⁵ At this level, the researchers concluded that the radiation risks from drinking tea are minimal or non-existent. (The US Environmental Protection Agency similarly concluded that, under most circumstances, radiation from a granite countertop does not rise above the typical background level.) To put it in perspective, the annual exposure to radiation of a human living on Earth is 24 000 banana-equivalents a year.

With all of the metals found in tea leaves, the amount that actually makes it into the cup depends on both how hot the water used to make the infusion is and how long the tea leaves are left to infuse, topics we’ll take up in the next chapter, The Agony of the Leaves.

4.14 Organic Acids and Vitamin C

Infusions of tea contain several organic acids in addition to the amino acids. A broad definition of an acid for chemists is something that will relinquish one of its hydrogen atoms in the form of a proton, a hydrogen atom with no electrons and consequently the charge of +1. Organic acids are a class of molecules that have a carbon framework attached to a –COOH group (Figure 4.8). The hydrogen atom attached to the oxygen of the –COOH group can transfer to another molecule, often water,‡‡‡‡‡ leaving behind a negatively charged ion called a carboxylate. (If you see something in an ingredient list that ends in “-ate,” chances are that it is an ion left after a proton has departed.) Commonly encountered organic acids include vinegar (acetic acid) and citric acid (found in many citrus fruits). Cream of tartar, one of two active ingredients in baking powder, is the potassium salt of tartaric acid (potassium hydrogen tartrate).

Figure 4.8Molecular structures of several organic acids found in tea. Gallic acid is the most common in tea, while oxalic acid was once implicated in the formation of kidney stones.

Tea is a significant source of gallic acid, oxalic acid, and malic acid, as well as containing chlorogenic acid, shikimic acid, and a dash of ascorbic acid, better known as vitamin C (Figure 4.8). Like theobromine, which has no bromine, chlorogenic acid doesn’t contain any chlorine. Its name stems from the fact that it turns green when oxidized. Shikimic acid is named for the star anise plant from which it was first isolated in 1885. (Star anise also makes a good infusion.)

Vitamin C is critical to human health. Without it, humans develop scurvy because we cannot synthesize it ourselves. A 2017 study by a group from the US Department of Agriculture found that, while there was a significant amount of vitamin C in the tea leaves themselves, there was a scant milligram of vitamin C in a cup of brewed tea. Since the recommended daily allowance for vitamin C is of the order of 100 mg, it is clear that even the most insatiable of tea-drinkers will not consume enough vitamin C to meet their daily requirement.

Oxalic acid is also found in rhubarb, both the leaves and the stalks. When I was growing up, we had a large rhubarb patch in the backyard. My mother warned us sternly not to eat the rhubarb leaves because they were poisonous due to the oxalates they contained. (Yes, my mother did tell us it was the oxalates, she was a chemist by training.) She was right, oxalic acid and oxalates are toxic, causing kidney damage due to the precipitation of solid calcium oxalate.§§§§§ The lethal dose is quite high, somewhere between 15 and 30 g. It turns out there is less than 0.5 g of oxalate in 100 g of rhubarb, while the spinach that my mother encouraged us to eat contained twice as much oxalate as the rhubarb.¶¶¶¶¶ I would have had to eat many kilograms of rhubarb leaves to ingest a fatal dose. As much as I enjoyed foraging in the garden for berries and peas when I was a kid, my mother need not have worried. I was never tempted to browse the rhubarb leaves like a cow.

Oxalate poisoning aside, those who suffer from kidney stones are often warned off drinking tea because of the oxalic acid it contains, which was thought to lead to the formation of small calcium oxalate deposits in the kidneys. However, a 2019 study from the Sorbonne showed no evidence for an increased risk of the development of kidney stones in drinkers of green tea.¹⁶ In fact, there was a lower risk of the development of particular types of kidney stones associated with the regular drinking of green tea. A prospective study in China came to a similar conclusion, finding that green tea was actually associated with an overall lower incidence of kidney stones. A recent meta-study also showed no correlation between tea consumption and the development of kidney stones. Still, Dr Google will tell you that tea is a big no–no if you are prone to kidney stones and, of course, one should always consult one’s own physician in this regard.

4.15 Insects Are a Feature Not a Bug

It can seem a long way, both in distance and in time, from the green leaves of the tea plant to the dry, fragrant, brown leaves that I scoop from the canister on the counter into my cup. I can easily forget that these leaves were once chewed on by caterpillars and the flowers burrowed into by bees. The insects are a necessary feature of the cultivation of tea plants, from the bees that pollinate the flowers to the leaf-hoppers whose wounds can prompt the tea leaves to produce volatile aromatic compounds and, eventually, particularly fragrant teas. Insects are a critical part of the Earth’s ecosystem, as recent die-offs in various pollinator species, including bees, have made abundantly clear.

But obviously bugs are not always benign. And there are a lot of them. By some estimates, as many five million different species of insects and beetles currently populate the Earth. Total estimates of the population are in the quintillions of individual insects, somewhere on the order of 10 to the 19th power, altogether weighing more than a gigaton. Given how many insects there are, and how pervasive they are in the environment, it should come as no surprise that not all of the insects are removed when tea is processed.

Without a doubt there are the dried remains of bugs in the teabag or loose tea that you brewed. It can be difficult to know exactly what insects are on a plant, or remain on a plant after it has been harvested. Some of them can be quite small—the leaf-hoppers are only 3 mm long and the tiny wasps called fairy flies can be as small as 0.1 mm in length. Researchers at Trier University in Germany resorted to DNA profiling to identify the insects found in commercially available tea and other herbs.¹⁷ They identified DNA signatures from hundreds of different species of insects in samples of green tea purchased from grocery stores.

Am I worried about all these insects in my tea? Not at all. It is not as if there are 400 whole insects in a teabag or teaspoon of tea, just tiny bits and pieces. I consider it the price I pay for a healthy ecosystem and access to the natural products derived from it. And perhaps they add a bit of flavor of their own to my chemical brew.

Now that we have a sense of the molecular menagerie in a cup of tea, we can turn our attention to the conditions that bring out the best of these different compounds in an infusion. Must the water truly be at a boil? How long should you brew a cup? Why is it so important to agitate your teabag rather than let it sit idly in the bottom of the cup? In the next chapter, we explore the “agony of the leaves,” the moment of truth when the dry tea leaves hit the water.

4.16 Brewing a Better Cup

Choose a supermarket tea and let your tea steep a minute longer to increase the theanine content and produce a slightly more calming brew.

If you’re concerned about the amount of fluoride in your tea, use whole leaves rather than a teabag. If you live in a volcanic region, or have water that is particularly mineral-rich, you may prefer to use bottled water.

Further reading

W. M. Green, The Chemical News, 1885, vol. 52, pp. 229–231. Wilhelmina Green’s paper is available in Google books and is worth a look, even if you’re not a chemist.

M. Francl, Nat. Chem., 2015, 7, 265–266. Chemists have a mixed relationship with smell, as I explore in this essay.

H. McGee, Nose Dive: A Field Guide to the World’s Smells, Penguin Press, New York, 1st edn, 2020. Harold McGee has long written about food and chemistry in a way that not only satisfies my itch to understand what is happening at the molecular level, but has made me a better cook as well. This is a readable and intriguing book about food and smell.

http://www.compoundchem.com/2018/07/17/spinach/. Andy Brunning creates wonderful explainer graphics about chemistry. This is an excellent piece about iron and spinach.

https://xkcd.com/radiation/. Randall Munroe draws the XKCD cartoons. This panel sets radiation exposure in context using the banana-equivalent.

References

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5. K. Boros, N. Jedlinszki and D. Csupor, Pharmacogn. Mag., 2016, 12, 75–79.

6. M. Saeed, M. S. Khan, A. A. Kamboh, M. Alagawany, A. F. Khafaga, A. E. Noreldin, M. Qumar, M. Safdar, M. Hussain, M. E. Abd El-Hack and S. Chao, Poult. Sci., 2020, 99, 5625–5636.

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8. J. L. Williams, J. M. Everett, N. M. D’Cunha, D. Sergi, E. N. Georgousopoulou, R. J. Keegan, A. J. McKune, D. D. Mellor, N. Anstice and N. Naumovski, Plant Foods Hum. Nutr., 2020, 75, 12–23.

9. S. V. Jovanovic, M. G. Simic, S. Steenken and Y. Hara, J. Chem. Soc., Perkin Trans. 2, 1998, 2365–2370.

10. Scientific opinion on the safety of green tea catechins, 2018, https://efsa.onlinelibrary.wiley.com/doi/full/10.2903/j.efsa.2018.5239, accessed 26 August 2022.

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15. N. Absar, J. Abedin, M. M. Rahman, M. H. Miah, N. Siddique, M. Kamal, M. I. Chowdhury, A. A. M. Sulieman, M. R. I. Faruque, M. U. Khandaker, D. A. Bradley and A. Alsubaie, Life, 2021, 11, 282.

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†T. Lu, Tang dynasty, Song of Tea or Seven Bowls of Tea.

‡Though it probably wasn’t a madeleine at all, as Edmund Levine pointed out in an essay in Slate magazine in 2005.

§The name comes from the Latin for the bark of an oak tree, tannare. It can be used to tan leather.

¶Green’s paper says she used “flowers of sulfur,” which is a very pure, very fine, bright yellow powder of elemental sulfur. Alchemists (and later chemists) referred to any fine, flour-like powder made by first subliming the material, then recondensing the gas as “flowers.” Not because of any resemblance to blossoms, but because they had the texture of a fine-milled flour.

‖The method was developed by Josef Eder, an Austrian chemist who helped to develop the chemistry behind photographic prints that is still in use. One gram of Cu₂S is equivalent to 1.3061 g of tannin.

**Spectroscopic comes from the Latin spectre, or ghost.

††Astringency is evaluated using the half-tongue test. One milliliter of a solution with the astringent compound is placed on one side of the tongue and pure water on the other half. The taster then sticks their tongue in and out rapidly for 30 seconds and decides which side of the mouth has the astringent compound on it. I confess I have not tried this.

‡‡Concord grape juice reliably produces this sensation, which is so intense that every time I read this paragraph my brain replays the effect.

§§Biochemists call this kind of behavior promiscuous.

¶¶Those noses are not just for show. Elephants have twice as many genes for olfactory receptors as dogs and five times as many as humans. Their sense of smell is so sensitive they can identify their family members by smelling their urine.

‖‖Badius derives from brown, for the mushroom’s characteristic coloration; it is not an indication of toxicity.

***A few naturally occurring proteins incorporate d-amino acids in their sequences. Examples include platypus and spider venoms. Amino acids convert extremely slowly from the l- to d-forms at room temperature, but the conversion proceeds more rapidly at high temperatures. There is some evidence from mass spectrometry to support the formation of measurable amounts of d-theanine in an infusion of hot tea.

†††Biologists refer to the process of getting nitrogen from the air, where it exists as the inert gas N₂, into a usable form “nitrogen fixation.” By some accounts, theanine is the primary amino acid responsible for nitrogen fixation in tea plants.

‡‡‡Most of these are what biochemists call GPCRs; examples include TAS1R1 and TAS1R3.

§§§Also, it seems, in chickens, where it has been shown to promote growth and bolster immune responses.⁶

¶¶¶My kitchen scale won’t weigh such a small amount, so I dissolved 3 g of l-theanine into 120 mL of water and then put 10 mL, about two teaspoons, of that into my tea.

‖‖‖It is odd to think that as tea steeps it gets heavier, but it does. A cup of tea weighs about a gram or two more than the water used to brew it.

****Humphry Davy, a 19th century British chemist famous for his experiments with nitrous oxide, showed that catechu was a better compound for tanning than traditional oak extracts.

††††Astringent derives from the Latin astringere, meaning to pull tightly together or pucker.

‡‡‡‡Phenolphthalein is, like the catechins, a polyphenol. It is bright pink in alkaline (basic) solutions and colorless in acidic solutions. The quintessential titration of hydrochloric acid with sodium hydroxide often uses phenolphthalein as the indicator.

§§§§Despite the similarity of their names, theophylline and theobromine have different etymologies (at least according to the Oxford English Dictionary).

¶¶¶¶Hydrofluoric acid (HF) is extraordinarily toxic. Historian of science Mary Elvira Weeks called chemists Paulin Louyet and Jérôme Nicklès “martyrs to science.” Both died from exposure to HF in the race to isolate elemental fluorine.

‖‖‖‖Hydrangeas also accumulate aluminum from the soil, turning their flowers a bright blue. When grown in aluminum poor soils, the blooms are a deep pink.

*****A granite countertop contains roughly the equivalent of a pea-sized ball of uranium distributed more or less evenly throughout the material. This is enough uranium to produce measurable amounts of radon gas. Among the many good reasons to ventilate a kitchen well.

†††††About 10% of the potassium decays by electron capture and emits gamma rays. The decay of potassium-40 produces the inert gas argon and is the reason why the Earth’s atmosphere is roughly 1% argon.

‡‡‡‡‡Chemists know it is more complicated than I am suggesting here. The proton that leaves the acid may be associated with more than one water molecule.

§§§§§Part of the toxicity of ethylene glycol antifreeze derives from the metabolic conversion of the ethylene glycol to oxalic acid.

¶¶¶¶¶Parsley and chives are also high in oxalates.

Chapter 5

The Agony of the Leaves

Tea Pairing

Try a whole-leaf Darjeeling tea with this chapter. If you can, brew it in a clear glass to observe the writhing of the leaves as they unfurl. Or try making your regular tea with distilled, deionized water.

5.1 Extracting What You Want

The way tea leaves seem to writhe as they steep in hot water has been called the agony of the leaves, but it could equally describe the experience of ordering tea at a restaurant in the US. This uniquely American torture of the leaves begins when someone in the kitchen puts hot, though in all likelihood not boiling, water into a cold porcelain pot. It sits on the counter until it is brought to the table, at which point there is some small ceremony with a fancy wooden box from which you select a teabag filled with tea fannings, essentially tea dust. High-quality tea dust, perhaps, but certainly not the whole leaves prized by tea-drinkers. By this point, the temperature in the pot has dropped far below the optimal value for steeping whatever tea you have selected and the resulting brew tastes of “lukewarm brown water that looks like gnat pee,” to quote the 90s rock band Masters of Reality. I have had a far better cup at petrol stations on the highway in Ireland than I have ever had at a dining establishment in the US.†

Making a cup of tea is easy, arguably easier than making a cup of coffee (at least in the days before Nespresso or Keurig machines). Place a teabag in a cup, pour boiling water over it, and let it steep. Two or three minutes later you have a perfectly acceptable cup of tea, aromatic and caffeinated. Making a cup of tea can also be an elegant and elaborate ritual, as anyone who has experienced chanoyu, the Japanese tea ceremony, can attest. Any failure to properly steep tea cannot be chalked up to a lack of directions. Instructions can be found in sources as disparate in time and tradition as Lu Yu’s 15th century treatise Classic of Tea, an opinionated 1946 essay by writer George Orwell, and the 2019 International Organization for Standardization (ISO) standard for preparing tea.

Most tea that is consumed today is steeped black tea, by some estimates more than three-quarters of the three billion cups drunk each day. (Twenty percent of those cups are green tea and about 2% are oolong teas.) From a chemist’s point of view, making a cup of tea is a chemical extraction, wherein a solvent is used to remove some of the chemical compounds from a biological matrix, the tea leaves. Then we drink the solvent, along with the extracted chemical compounds. The particular process of steeping tea leaves is technically an infusion. It is the last step in the complex journey that turns newly harvested tea leaves into a pleasing hot beverage. Specifically, tea is an aqueous infusion, made by soaking plant material in water. (French press coffee is also an infusion.) Other common methods of extraction used to make beverages include decoction, where the material is boiled in the solvent and then filtered off, and percolation, where the solvent is allowed to trickle through the matrix, carrying with it the desired compounds. Masala chai is traditionally prepared as a decoction. Tea made in a Nespresso or Keurig is percolated, not steeped.

Chemical extractions are all about getting out what you want and leaving behind what you don’t. The control of an extraction, whether of tea in your kitchen or some chemical in an industrial plant, is primarily achieved by tweaking four parameters: the choice of solvent; the temperature of the solvent; the time permitted for the solvent to do its work; and the physical state of the matrix—how much surface area is exposed to the solvent. Such an approach to the steeping of a pot of tea sounds cold and calculating and, frankly, not the cozy ritual we might wish to start off the day. But understanding how these variables work enables me to craft a process that reproducibly results in my best cup of tea. Let’s take the parameters in turn and see how we can make the chemistry of extraction work for us.

5.2 Making a Cup of Tea

Before we delve into solvents and temperatures, let me outline the fundamental framework for making a single cup of tea by infusion. The basic method for making an infusion of tea has not significantly changed in the last century. The instructions given in my copy of Florence White’s 1932 Good Things in England: A Practical Cookery Book for Everyday Use are essentially the same that I follow to make my morning cup in 2023 and are well summed up by the Masters of Reality quote at the start of the chapter: “Pour boiling water over the tea.” These (slightly more detailed) directions are for steeping loose-leaf black or pu’erh teas.

1.Warm the teapot or the cup by swirling hot water in it, or filling it up with hot water and letting it sit on the counter. Confession: I don’t always do this, but I should, as we will discover in the section on temperature.

2.For each 200 mL of water, place one teaspoon of dry tea leaves (about 2 g) into the empty pot or put them into a tea basket and place this inside the cup or pot.

3.Pour boiling water over the tea leaves and let steep for three to four minutes.

4.Remove the basket or immediately strain the tea in the pot into a second pre-warmed pot or into pre-warmed cups.

If using green or white tea leaves, lower the temperature of the water to 80 °C. (No thermometer? If you pour boiling water into a cold pot and let it sit for eight minutes, the temperature will be close to 80 °C.)

If using a teabag, use one standard bag for every 200 mL of boiling water. Reduce the steeping time. Remove the teabag at the end. If desired, wrap the string around the bag or gently press the teabag against the side of the cup to squeeze a few more milliliters of water out. (There is a gadget for this as well, a cross between tongs and a tea strainer, although I’ve never seen the need for one. I’m also fairly sure that etiquette books would have something to say on the matter, but that is well beyond the scope of this book.)

The ISO standard for making tea differs in a few particulars.¹ However, that protocol is not designed for preparing tea to drink with your breakfast, but is meant to standardize the tasting of tea infusions to assess the quality of the leaves. The ISO standard recommends 2 g ± 2% of tea per 100 mL of water at the boiling point for either green or black tea. Black teas should be steeped for six minutes, while green tea leaves should be steeped for five minutes, slightly longer in both cases than I or Florence White would permit our tea to steep. The resulting infusion is stronger than many tea-drinkers would prefer.

5.3 Tea is Mostly Oxidane

Most people know the molecular formula for water, H₂O, and many could draw it with its characteristic hydrogen Mickey Mouse ears (Figure 5.1). While the formal International Union of Pure and Applied Chemistry (IUPAC) name for water is oxidane,‡ chemists most commonly refer to H₂O the same way as everyone else does. We call it water. Water is the major chemical component of tea, serving as the solvent to extract the compounds that give a cup of tea its flavor, fragrance, and appearance. Ultimately, water is what carries the flavors of tea to our taste buds and provides the energy to volatilize tea’s scent molecules.

Figure 5.1 Upper panel: molecular structure of water shown as both a line structure and a space-filling model. Middle and lower panels: two versions of the equilibrium between a water molecule and its ions. One shows water dissociating into a proton and a hydroxide ion (OH⁻), whereas the other shows the transfer of a proton from one water molecule to the other. Both of these equilibria are simplifications of the complex handoff of protons between water molecules in liquid water. The handoff between molecules is rapid, on the millisecond timescale.

When you put whole tea leaves into hot water, what happens? The dried tea absorbs water, causing the leaves to writhe and unfurl in metaphoric agony. (The much finer particles found in many commercial teabags simply swell.) On the microscopic level, water can now move in and out of the leaves, taking the various water-soluble constituents with it. A 350 mL mug of tea uses between 3 and 5 g of dried loose-leaf tea and will absorb 15 or so milliliters of water in the steeping process. A single commercial teabag has about 2 g of tea and should be steeped with between 175 and 200 mL of water.

It can be tempting to squeeze a teabag or compress loose leaves to reclaim some of the water that has been absorbed by the leaves. There is mixed evidence on what effect this might have on the composition of the infusion. Some data suggest that it increases the concentrations of the more astringent antioxidants, particularly if short steep times are used, which will certainly affect the taste.² There are potential health benefits to the increased amount of antioxidants and the matter of taste is one of personal preference.

Water is an ideal choice of solvent for extracting the desirable molecules from tea leaves, particularly caffeine and the flavor- packed thearubigins and theaflavins. Pure water is tasteless. It is cheap and practically non-toxic. (That said, everything has a hazardous dose, including water.) Water is sometimes called the universal solvent. It is polar and can make hydrogen bonds (see Chapter 3). Since like dissolves like, water can dissolve other things that are polar and can form hydrogen bonds, such as caffeine and the many flavor-packed polyphenols in tea. Water won’t dissolve oils, another advantage it has as a solvent for extracting tea because it leaves behind many of the overly bitter oils.

Water is readily available, although, as planets in the solar system go, the Earth is pretty dry. Uranus and Neptune have many times the amount of water of Earth. That said, water is not exactly in short supply here: there are about 10¹⁵ liters of water on our planet’s surface and perhaps six times as much as that trapped in the mantle. A few years ago, prompted by a summer workshop at the Vatican Observatory, I wondered where all the water on Earth comes from, in a galactic sense. Was it melted comets that had crashed into the Earth? Or perhaps recycled dinosaur pee,§ as some wits have suggested?

The water on Earth is caught up in a complex cycle, as the Buddhist monk Thích Nhất Hạnh noted: “You know that you are drinking a cloud; you are drinking the rain. The tea contains the whole universe.” While a cup of tea might not literally contain the whole universe, the water in it is tied into the birth of galaxies and, indeed, the universe itself. In this sense, the water in my cup of tea is unimaginably old. The protons in it, the nuclei of the hydrogen atoms, likely formed one second after the Big Bang, some 13.82 billion years ago. The recycled stellar matter that formed the solar system eventually coalesced into tiny particles, which included water made from those ancient protons. These particles crashed into each other, eventually forming the planets, including the Earth and its water.¶ In another sense, water is ephemeral. Any particular water molecule has a lifetime of a few milliseconds before it hands one or another of its hydrogen atoms off to another oxygen in the liquid (Figure 5.1).

We ought to pay close attention to the source and preparation of the water we use to brew tea. Long before the English physician John Snow linked a 19th century London cholera outbreak with sewage-contaminated water, the 8th century tea master Lu Yu recommended avoiding water taken from rivers too close to human habitation. His advice is similar to that of the early 11th century German herbalist Hildegard of Bingen, who wrote of the Rhine, “Its water, taken uncooked, aggravates a healthy person … if the same water is consumed in foods or drinks, or if it is poured over a person’s flesh in a bath or in face-washing, it puffs up the flesh, making it swollen, making it dark-looking.” Clean water was not a given through much of history. Boiling water for tea likely made it a safer drink than water pulled from a stream or river, or even the local well.

Snow melt was particularly prized by early Chinese tea- drinkers. One determined gardener scraped the snow off plum blossoms and used the resulting water to make what was by all reports an incredible tea. (Despite having a plum tree in my yard, I’ve not tried this as I suspect there are more undesirable contaminants on the leaves in this century.) In a commentary on Lu Yu’s treatise, Wu Juenong suggests that the improved flavor results from the lower concentration of heavy water in snow melt than in groundwater.³ Heavy water is water where the two hydrogens have been replaced by deuterium atoms, D₂O rather than H₂O. In almost every respect deuterium is chemically identical to hydrogen, except it weighs twice as much. This increase in mass has some effect on chemical reactions, generally slowing them down. Drinking too much heavy water slows down many biochemical processes and most organisms will not flourish if the water they drink (or swim in) contains too much D₂O.

Given that the D₂O concentration of groundwater is very low to begin with, roughly 150 ppm, it seems to be an improbable explanation for the extraordinary taste of the gardener’s tea. Pure heavy water does taste sweet.‖ I’ve done the double-blind taste test, but at the concentrations found in nature, it is undetectable by taste. But if you want to try to duplicate the legendary plum blossom tea, you can purchase, at an exorbitant cost (£40), bottles of water which are depleted in D₂O. Or you can just go scrape snow off some blossoms come spring.

Most water is not pure H₂O, but contains positively charged ions such as calcium and magnesium matched by negatively charged ions, often bicarbonate and sulfate. Water with a high ion content is often called hard water, while soft water has fewer ions. Calcium and magnesium ions have a +2 charge. Recall from our big ideas of chemistry that opposite charges attract and the larger the charge, the stronger the attraction. These ions therefore tend to clamp tightly onto negatively charged ions to make solids. As a result, hard water tends to lead to the formation of what plumbers called limescale, or solid calcium carbonate, the same material that eggshells are made of. Spring water is often hard as a result of the minerals it has moved through. Rain water and snow melt are considered soft waters, having few ions other than the minuscule amounts of hydrogen and hydroxide ions always present in water.

The ions in water can affect both the taste and color of tea. For example, if I make tea in the kitchen at the college where I work and where the water is quite hard, it is noticeably darker than the tea I make with the water at home and has a distinctly different flavor. The calcium concentration enhances the astringent taste and suppress the bitter taste of epigallocatechin gallate (EGCG) and the umami taste from l-theanine. Tea made with hard water tends to be a darker brown than that brewed in soft water. The darker shade might not be noticeable in a black tea, but can be objectionable in green teas. Research by chemist Monika Sommerhalter and colleagues at California State University, East Bay suggested that these changes in color and flavor arise because the catechins and other polyphenols in the tea can be oxidized and polymerized.⁵ The change in structure when a molecule is oxidized or polymerized changes the function of the molecule, altering both its taste and color. (This is the same reason that teas darken as they sit.) The good news? The caffeine concentration is not significantly affected by the hardness of the water. Go ahead and make that first morning cup with water from the tap or filtered water, as you wish. Either way, it’ll be the stimulant you need to start your day.

5.4 “Tea You Have Not”: Time and Temperature

The famously eccentric mathematician Paul Erdös** is credited with the saying that mathematicians are nature’s way of turning coffee into theorems. I might equally say that I was the mechanism for turning some 600 cups of tea into this book. When I was a young chemist, I would often make a cup of tea as I worked (not in the lab!). The only trouble was, I would often get so involved with what I was doing that I would forget about my steeping cup of tea until it was lukewarm and too bitter to drink. I stumbled upon a program that would bring up a photo of Big Ben and make a loud chime when your tea was ready to drink. These were the days before micro-payments, so the author asked only that you send him a sample of your favorite tea. I have no idea how many people actually complied, but I enjoyed the notion of envelopes of tea flying into his inbox from every corner of the world. These days, I time my tea with a quick command to a digital assistant. Four minutes, not a second longer, for my bracing cup of morning Assam tea, three minutes for that soothing cup of Rose Congou I prefer in the late afternoon. And the water temperature? Always at the boiling point for these black teas!

For centuries, people relied on their own time sense to decide when the tea was properly steeped.†† This was not always a success. In an 1852 encyclopedia of domestic economy, the authors note that Europeans “suffer the water to remain too long upon the tea, which causes it to extract a bitter portion.” They themselves suggest no time, beyond “sufficient.” The field of chemical kinetics considers questions about the speed and length of chemical processes and, over the last four decades, has had much to offer on the basic question of how long to steep your tea.

The simplest answer to how long to steep your tea is however long you like to steep your tea. Generally, black teas are allowed to infuse for two to five minutes, green teas for one to two minutes. Much longer infusions are used to make what is often called “sun tea,” or cold-brewed teas. These can steep for 12–24 hours. What chemical kinetics can tell you is that different components of the tea infuse at different rates. Stopping the infusion by straining the tea or taking your teabag out at different times gives you a different balance between the astringent compounds, the bitter compounds, and different amounts of caffeine.

In the 1980s, Michael Spiro and Shama Siddique at Imperial College London were among the first to explore the rates at which different compounds were extracted in an infusion of tea.⁶ They showed that tea infusions follow what chemists call “first-order” or “exponential” kinetics. These kinds of processes start off rapidly, then level off, as the graph of caffeine infusion in Figure 5.2 shows.

Figure 5.2 Graph showing the percentage of caffeine extracted into an infusion over 10 minutes. This process is first-order—that is, as more caffeine is extracted and the concentration of caffeine increases in the solution, the extraction slows down and less and less caffeine is extracted. The two points shown on the graph indicate when 50% and 90% of the caffeine have been extracted. Data taken from ref. 9 .

If you are drinking tea primarily for the caffeine, you don’t have long to wait. When steeping whole-leaf black tea, about 80% of the caffeine has been extracted after three minutes and most of the caffeine after five minutes. Doubling the infusion time does not yield a significantly higher dose. Once you reach the flat part of the curve in Figure 5.2, increasing the time doesn’t appreciably alter the amount of caffeine in solution.

While the overall caffeine concentrations are roughly the same in dry green tea leaves and dry black tea leaves, the caffeine is extracted faster from the more highly oxidized leaves. Green teas require closer to 10–15 minutes to extract the majority of the caffeine, but by that point are often too astringent and brown for most tea-drinkers. As a result, green tea infusions can be lower in caffeine.

The steeping time most significantly affects the taste and aroma of a cup of tea because the larger flavor compounds are slower to infuse than caffeine. Theaflavins infuse at almost half the rate of the smaller alkaloid molecules caffeine or theobromine. The antioxidant levels that are correlated to the health benefits of tea are also increased at longer steeping times. Recent work has shown that the longer tea is steeped, up to 10 minutes, the higher the concentration of polyphenols and the more effective the solution is at preventing oxidation.‡‡ But these brews can be too astringent for many tea-drinkers.

Once the constituents are in solution, moving around freely, another of the big ideas of chemistry comes into play. Molecules can now get into proximity with each other and with the oxygen dissolved in the water. Once they are close enough, they can react. So, in addition to time being a factor in what compounds are extracted, it also has a role in the chemical reactions that can now occur. The catechins can polymerize to thearubigins and theaflavins, changing the color of the tea. Eventually, theabrownins will form, which can lend an unacceptable astringency to a cup. Solids may begin to collect and make the tea murky, a process called creaming, a phenomenon we will take up shortly.

5.5 Hotness Levels

There is a proverb, “A true warrior, like tea, shows his strength in hot water.” While tea infusions can be made at any temperature, even in ice water, hot water will more effectively extract the desirable molecules, from caffeine to the antioxidant polyphenols. Temperature matters.

For chemists (and physicists, biologists, and any other kind of scientist), temperature is an indirect measurement of how much energy the molecules have on average. The more energy molecules have, the more varied the chemical reactions and processes they can participate in and the faster these chemical processes will proceed. A thermometer measures what mathematician James Serrin delightfully called “hotness levels.§§ ” More molecules have high energies at higher hotness levels (i.e., higher temperatures).

The “hotness” scale that scientists use to correlate energy to temperature is the Kelvin scale. This scale starts at absolute zero (0 K or −273 °C), where molecules have the lowest possible energy, and climbs from there. At the boiling point of water (373 K or 100 °C), the average speed of a molecule of water in steam is almost 2400 km per hour, while in the liquid phase the water molecules are moving far more slowly, at roughly 800 km per hour. Molecules are also twitching and tumbling, even at very low temperatures. At high temperatures, they twitch and tumble faster. More frequent vibrations and rotations can help set the molecules up for a reaction to occur.¶¶

Under some circumstances, chemists can tell the temperature of water just by looking, it’s one of our superpowers. The secret is that we are familiar with phase diagrams, which tell us that when ice and water are both simultaneously present, then the temperature of the water must be 0 °C. Boiling water is certain to be at a temperature of 100 °C.‖‖ That said, as novice cooks soon discover, it can be difficult to be certain when water is actually boiling. Hasok Chang’s wonderful book Inventing Temperature, on the history of the measurement of temperature, describes the long road to knowing the precise boiling point of water.

In his Classic of Tea, Lu Yu described three different stages as water approached the boiling point. The first was when small bubbles that resembled fish eyes appeared. Cooks and chemists alike know that at this stage the water may be hot, but has not reached a boil. The bubbles are the dissolved gases coming out of solution as the temperature rises. Keep heating, and bubbles will begin to appear around the edges. Finally, Lu Yu says, the water will rumble violently and splash about. Modern cookbooks call this a rolling boil; the Old Testament prophet Isaiah described the water as seething. At this point, the water is truly boiling and you are assured that the temperature is 100 °C. If you are making a cup of black tea, this is the temperature you want the water to be at when you pour it over the leaves to steep.

Even when the water poured over the leaves is fully boiling at 100 °C, the temperature at which the infusion is occurring is significantly lower. The water cools when it is poured into the pot because some of the energy in the hot water is transferred to the molecules of the teapot. The amount of energy transferred, and the temperature that is reached, depends on the initial temperature of the pot as well as on the material it is made of, a topic we will take up in a Chapter 7 on teaware. I measured the temperature of the water in my usual porcelain teapot, beginning after I poured boiling water over the tea leaves and then checking it every 30 seconds for the next four minutes. You can see from the graph in Figure 5.3 that, within 90 seconds, the temperature of the water dropped by almost a full degree. Over the four minutes that I usually steep my black tea, the temperature continues to fall, reaching 86.4 °C by the end.

Figure 5.3 Cooling curve for water in a 600 mL porcelain teapot weighing 440 g. The water was at 100 °C when it was poured into the pot.

From chemical kinetics we know that the rate of processes can change dramatically with temperature. As little as a 10 °C increase in temperature can double the rate of a reaction. The rate of infusion of caffeine at 70 °C is less than half that at 90 °C, the rough temperature you get when you pour boiling water over tea in a pot. Lower temperatures also lead to less caffeine being extracted in total. For green tea, the caffeine extracted at 60 °C after 10 minutes is roughly half that extracted at 90 °C. Similar results are seen for oolong tea. Increasing the infusion time at a lower temperature will not make up for the decrease in caffeine. Be assured that if you are making a cup of tea for someone else and you let the water cool too much, they will be able to taste the difference. As the Masters of Reality put it, “tea you have not”! Worse yet, there will be significantly less caffeine in it, which is no way to start a partner’s morning.

Beware the urns of hot water set out at a meeting or buffet. As I’m sure tea-drinkers know, the cup of tea they will get from these urns will be most unsatisfying. Generally, the water in these urns is insufficiently hot to brew a good cup of tea. I’ve taken to carrying around a thermometer and measuring the temperature of the hot water in my cup at such events at my college (my colleagues are remarkably tolerant). It generally ranges between 70 and 75 °C. If the water is dispensed into a stoneware mug at the frigid temperatures of many meeting rooms, then the temperature drops by 10 °C. Paper cups fare better and the temperature drop is only 2 °C. At these temperatures, the total amount of caffeine in the infusion is reduced by a half or more. If meeting organizers understood that they were depriving the tea-drinkers in the group of their usual dose of caffeine, then they might provide an electric kettle.

Sun tea, a cold infusion of tea leaves, was all the rage when I was a graduate student in California in the 1980s. The notion was that you used the sun’s rays instead of the heat of the stove or kettle to make the tea. Sun tea, as it turns out, does not require the sun. It’s simply a lower temperature infusion. It can be done in the dark at refrigerator temperatures (around 4 °C), or at room temperature. The total amount of caffeine extracted from tea at low temperatures is significantly reduced. (By contrast, cold-brewing of coffee seems to have little effect on the caffeine content.) Researchers at National Chung-Hsing University in Taiwan showed that tea steeped at 4 °C for a full 24 hours has only one-third of the caffeine of the same tea steeped for 20 minutes at 90 °C.⁸ Theanine is slightly elevated in these cold-brewed teas, as are the concentrations of other sweet-tasting amino acids. The total catechin concentration is reduced by 30–50%. Overall, sun tea, whether made in the sun or a refrigerator, is a less caffeinated, less bitter, less astringent brew.

Because changes in temperature can dramatically change the speed of reactions, chemists often want to run their reactions at a constant temperature to avoid fluctuations in the rate. Chemists in the laboratory, and even kitchen chemists these days, can avail themselves of a constant temperature bath for this purpose. Chefs call this technique sous vide and the equipment I use in my kitchen is made by the same company that makes the apparatus I’ve used in the laboratory. If I wanted a perfectly reproducible cup of tea, I could set up a constant temperature bath in my kitchen at 94 °C, let the water come to temperature along with its container, and then immerse the tea in the container. You can see what this looks like in the photo in Figure 5.4. While I assure you this is not how I make my regular pot of tea, it does produce an excellent cup.

Figure 5.4Tea steeping in a constant temperature water bath. Image courtesy of Andrew DiDonato.

Short of setting up a constant temperature bath in the kitchen and at the office, what can we do to keep the temperature up and the extraction proceeding apace? In his book, Do Nothing to Change Your Life, then Archbishop of York Stephen Cottrell suggests respecting the small rituals of life, such as pre-warming the pot for tea, as a way to slow down in a hurried world. It is certainly a way to take a little time for self-care, but, as a chemist, I would also heartily recommend it. If I fill up my pot with hot water from the tap, or with a preliminary round of hot water in my kettle, some of the energy from the water will transfer to the walls of the pot or cup. If you dump the water out, put in the tea and add boiling water, then the water being used for steeping will lose less to the pre-warmed container. The composition of the pots and cups matters as well, a topic we’ll take up in Chapter 7.

The temperature of the water also affects its acidity. Any solution exposed to the atmosphere contains dissolved gases and this includes the water used for tea. Heating the water drives some of the gases out of solution. It is often said you shouldn’t re-boil water for tea because it will remove some of the dissolved oxygen and will make the water taste flat. Heating water also drives off dissolved carbon dioxide, leaving less than 20% of the carbon dioxide behind if the water is boiled for a long time. And because the carbon dioxide that is present in water can turn into carbonate and carbonic acid, changing the amount of carbon dioxide also changes the acidity with its characteristic sharp flavor.

Heating water for tea increases the pH by between half a unit and one unit. A higher pH means a solution that is less acidic (each unit of pH is a factor of 10 on the acidity scale). It is more likely that this is at least part of what is affecting the taste of the brewed tea, in addition to the lack of oxygen in the water. Increasing the pH also lowers the concentration of catechins in solution. Since the catechins add taste and color, it is not surprising that driving out the extra carbon dioxide by boiling the water for an extended period of time makes the tea less flavorful. The catechins are less stable at high pH, oxidizing to other compounds. The amount of caffeine and theanine extracted do not appear to depend on the pH, so even if the tea tastes a bit flat, rest assured that you are still getting your caffeine kick and the molecular Zen of l-theanine.

5.6 A Little Agitation Is a Good Thing

I was sitting across from a colleague at a meeting, preoccupied by the way she absentmindedly dunked a teabag in her mug like a yo–yo while she made a point. I scribbled a note to myself to explore the effects of agitation on the infusion of tea. I, too, often find myself periodically shaking the basket containing the tea leaves before I take it out of the pot. Does it matter, or is it just a nervous tic? As it turns out, both the shape and size of the leaves influence a tea’s taste, as does whether the leaves are allowed to gently drift about in the water or are vigorously agitated. Both the leaf size and the degree of agitation affect the surface area of the leaves exposed to the solvent, and thus the efficiency of the infusion.

The increase in the rate of a chemical reaction that results from increasing the surface area can be quite substantial. For example, if you leave iron, such as a cast-iron tea pot or frying pan, sitting around in a damp environment, then it will eventually rust, although the process will take weeks or years to occur. Take that same iron and grind it up finely. When exposed to oxygen, the resulting reaction will be so fast that the iron may burst into flame. (This is the chemistry behind those disposable hand-warmers that you can tuck into your mittens on a cold hike.) In the same way increasing the surface area of the tea leaves exposed to the water speeds up the infusion.

The most straightforward way to increase the surface area that can be accessed by water is to use the same approach as the iron hand-warmers: break the leaves into tinier pieces.**** Dry teas ready to infuse can range from very small leaves, 300 µm or less in diameter, to whole loose leaves, and to tightly rolled leaves, such as snail teas. Large balls of leaves a few millimeters in diameter that are meant to decoratively unfurl in a glass pot or cup can also be infused.

Research shows that if you are using a finely ground tea, then nearly all of the caffeine is removed in the first minute, while it can take up to five minutes to extract the same amount of caffeine from a whole leaf. In the end, you get no more caffeine from the whole leaf than you do from the ground-up version of the same tea—you just get your caffeine faster. Percolated tea, such as that made in a Keurig or Nespresso machine, relies on this rapid extraction. These methods use very finely ground tea leaves, so that, as the water percolates through, it very quickly picks up the caffeine as well as the flavorful compounds.

Loose-leaf tea is often marked with a series of letters indicating the size of the leaves, which are a clue to how long they will need to brew. There is no agreed upon international standard for the codes, but some terms are in wide use in Europe, the UK and the US. Tea dust and fannings are the smallest sized teas and are found in some commercial teabags. (If you blow on dried tea leaves, or fan them, you can separate out the smaller particles from the whole leaves and larger broken pieces, hence the term “fannings.”) Teas labeled CTC, for cut–tear–curl or crush–tear–curl, are produced by mechanically shredding oxidized black tea leaves and rolling the shreds into tight pellets. These compact pellets are well suited for brewing in a teabag and can be found in many commercial teas.

Teas can also be sorted as whole leaves or broken leaves. Commercial teabags typically use smaller broken pieces, while whole leaves are sold in bulk or tins. Whole leaves are generally considered the highest quality teas. For black teas, the term Orange Pekoe (OP) is often used to refer to large whole leaves. Pekoe teas have smaller leaves. Teas where the leaves are mostly broken are tagged BOP, for Broken Orange Pekoe. By now, it should be clear that Orange Pekoe has nothing to do with oranges and pekoe is not a reference to a particular variety of tea, but rather both refer to the size and state of the tea leaves. Other letters also get attached, such as TG for “tippy golden” or GF for “golden flowery.” Tippy teas have many young buds, whereas flowery teas are crumpled and resemble a dried flower. Brew your TGOP tea longer than your TGBOP tea.

Agitation is another simple way to effectively increase the exposure of tea leaves to water during infusion. Left to their own devices, the leaves will stick together, blocking the solvent from fully accessing their surfaces. There is a remarkable amount of diversity among individuals when it comes to agitating their tea as it brews. Some people pour the water over the tea, either loose leaves or a teabag, which gently stirs the leaves. Others stir the leaves briskly in the pot, or repeatedly dunk the teabag into their mugs. Still others take a hands-off approach, leaving a teabag to float in the mug, or avoid disturbing the loose leaves.

Conrad Astill and coworkers explored the effects of different brewing techniques on the concentration of polyphenols and caffeine, including what happens when the teabag or tea leaves are agitated.⁹ They showed that caffeine was extracted efficiently regardless of whether or not the tea was contained in a teabag or whether that teabag was “dunked.” Mechanically agitating the leaves, by dunking the teabag or stirring loose leaves, extracted the aromatic polyphenols more efficiently. The resulting infusions had a higher concentration of key antioxidants relative to infusions that were made without disturbing the leaves. While brewing loose leaves provided the most antioxidants per cup, a well-dunked teabag provided nearly as much. (All this argues for investing in a laboratory “shaker” to gently, but continuously, agitate my tea in its constant temperature bath.)

Tea balls, small perforated metal or silicon replacements for teabags, can significantly reduce the exposure of the leaves to the solvent. As the leaves swell, they pack more tightly into the ball and the molecules in the leaves at the center of the ball have a tough time fighting their way into the solution in the cup. If you use one, you can improve your cup by avoiding filling it more than half-way to allow for swelling of the leaves, and then vigorously dunking it.

5.7 Tea Grown Cold

Iraqi poet Dunya Mikhail laments a lukewarm cup of tea in her poem The Iraqi Nights, “tea grown cold is worse than death.”¹⁰ I confess that I agree with her. So, I resort to reheating my cold tea in the microwave, sometimes twice. True tea connoisseurs are wincing. I apologize. Just make a fresh cup, I hear them say. But a fresh cup takes eight minutes to make, which is often six minutes more than I have. Two minutes of exposure to microwave radiation and what I have is once again hot, if not quite the same as what I made an hour ago. I suffer the scum that floats on the top, and the odd cloudiness that develops, in exchange for a  hot cup.

Tea scum consists of those threads of white particles that float on the surface of a cup of tea as it cools, particularly tea made with hard water or from water that was boiled in the microwave. When stirred, it can resembled latte art (Figure 5.5). Michael Spiro and Deogratius Jaganyi from Imperial College London explored the formation and composition of tea scum or, as it is sometimes called, tea scale.¹¹ As much as 10 mg of tea scale can appear on top of a cup of black tea. Using sophisticated instrumentation, such as electron microscopy, Spiro and Jaganyi showed that the scum that forms is a complex and heterogeneous mixture of large molecules and minerals, such as calcium carbonate and other metal carbonates and hydroxides. Tea scum is not dissimilar from the scum that forms a ring around a bathtub in areas where the water is particularly hard, something I try hard not to think about when I pull my reheated tea from the microwave. The carbonate is present in the water because of dissolved carbon dioxide. The calcium and magnesium ions primarily originate in the hard water, while the manganese and aluminum ions are derived from the tea leaves. The large molecules are probably theaflavins and thearubigins, along with some proteins, all extracted from the tea leaves.

Figure 5.5 Formation of scale on the top of a cup of tea.

Should you object to the scum on your tea, you need not despair. Spiro and Jaganyi showed that the amount of scum on a cup of tea can be significantly reduced if you use soft water, such as distilled water or, even better, distilled, deionized water. If there are no calcium or magnesium ions present, then they cannot react with the carbonates in the water to form insoluble mineral particles. Failing that, thoroughly boiling the water to remove as much of the oxygen and carbon dioxide as possible will reduce the amount of scum that forms. The number of carbonate ions in the solution are reduced when the amount of carbon dioxide in the solution is reduced. Less oxygen in the solution means that fewer of the thearubigins and theaflavins are formed by oxidation, which also reduces the amount of scum formed. Water boiled in the microwave reaches its boiling point so quickly that not much of the dissolved oxygen or carbon dioxide has a chance to escape, resulting in a higher likelihood of a scum forming.

Adding lemon juice can also effectively prevent the formation of scum on top of your tea.††† The citric acid found in lemons lowers the pH, which decreases the formation of scum. But perhaps more importantly, the citrate ions in the juice can surround the calcium and magnesium ions and prevent them from combining with the carbonate into the solids that you see floating on the top of your tea. Alternatively, adding milk to your tea increases the amount of scum formed, although it is vastly less noticeable in the murkier infusion.

In her paper on tea infusions, Wilhemia Green noted the gradual clouding and increased viscosity of tea infusions over time. The infusions begin clear, then gradually grow darker and cloudy. The clouding process is called creaming because the tea begins to look as if a small amount of milk or cream has been added.

To a chemist’s eyes, the cloudiness that develops over time reveals something about what has happened chemically to the tea: it suggests the formation of a colloid. A colloid is a suspension of tiny insoluble particles distributed through a medium. Colloids are different from solutions. A solution—for example, of sugar in water or tea—consists of individual molecules, rather than collections of molecules, distributed among the molecules of the solvent. Fog is a familiar colloid, where tiny particles of liquid water are suspended in a gas medium—the air—and scatter the light from street lamps or car beams. Technically, colloids formed from solids suspended in a liquid medium, like tea cream, are called sols.

Just what are the solids that are forming in a cooling cup of tea? In essence, they are clumps of molecules, like the scum that forms on the top of the tea, a complex mixture of different species. The largest of these molecular clusters is around 100 nm in diameter, about one-thousandth of the thickness of a human hair. What causes molecules to collect like this? Much of the attraction depends on the size of the forces between molecules compared with the force between the solvent molecules and the molecules extracted from the tea. We have seen that molecules can become attached to each other through hydrogen bonds, like one water molecule is attracted to another. Electrostatic forces, the attraction between positive and negative charges, also lead to molecular clustering. But molecules can also be attracted to each other through what chemists call dispersion, or London, forces.‡‡‡ The larger the molecule, the stronger these London forces become. The large catechin molecules and the even larger thearubigins and theaflavins are far more attracted to each other than, for example, two caffeine molecules might be.

The formation of tea cream is not completely understood, and there are likely many pathways to forming the clusters that cloud and darken a tea infusion. The main route seems to be through the association of the large theaflavins and thearubigins molecules, along with proteins. Both calcium and caffeine play some role in the formation of tea creams, though decaffeinated teas still produce the suspended particles as do many herbal infusions.

Tea cream is aesthetically unpleasing; the resulting dark, murky beverage is at odds with the bright, clear appearance of fresh infusions. But creamed tea is also less flavorful and less aromatic than a fresh infusion. Many of the compounds that contribute to the taste and aroma of tea are now caught in these molecular clumps and, as a result, hidden from our taste and smell receptors. Manufacturers of ready-to-drink teas, such as bottled iced tea, are on the hunt for methods to reduce creaming to improve both the appearance and the taste of their products.

5.8 Exhausted Leaves Get a Second Wind

Every day roughly 6000 tonnes of exhausted tea leaves, wrung of all their caffeine and much of their antioxidants, are discarded. In one year, this is enough to nearly fill Boston Harbor with dry leaves.§§§ These tea leaves, now damp, are often tossed in the trash and find their way to landfills, where they contribute to global warming, releasing methane as they decompose in these oxygen-deprived environments. Alternatively, they can be composted. My mother used to toss them into the garden, with the theory that they would make the soil more alkaline and therefore more hospitable to her azaleas and gardenias.

It’s tempting to look at a used teabag, or the leaves from the morning’s pot, and think they can be reused. But, as we have seen, these leaves are depleted of their caffeine and a large proportion of other desirable compounds. Research shows that reusing a teabag a second time results in an infusion that has virtually none of the caffeine and only about half of the polyphenols of the original infusion.¹² Reusing it for a third time yields tea that has less than a tenth of the polyphenols relative to the first infusion. Truly do not bother reusing tea leaves, unless all you want is a faintly colored, bland-tasting, caffeine-free hot drink.

The sheer amount of exhausted tea leaves has driven the exploration of alternative uses for the discarded leaves. Exhausted tea leaves have been shown to be effective at removing heavy metals, including lead and uranium, from water.¹³ Essentially, the leaves act as tiny filters. Full disclosure, there is nothing special about tea leaves. Dried coconut husks and coffee also work. But perhaps the most creative use of tea leaves was on the International Space Station. In 2019, astronauts opened a bag of tea leaves and set the contents adrift to help locate a persistent leak that more sophisticated technology had failed to find.¹⁴ Following the tea leaves led them to a small crack that they then repaired with tape.

5.9 Tea in an Instant

Steeping is the time-honored way of producing a cup of tea. While we have not yet reached the moment when we can say, as Captain Jean-Luc Picard said so many times in his ready room on the USS Enterprise, “Tea. Earl Grey. Hot.”, and have a replicator produce the perfect cup from raw matter in a second or two, the modern world has brought us other rapid methods of making a cup of tea.

For centuries, the only way to get a cup of tea was to boil water over a flame and pour it over the leaves, then separate the leaves from the water (or, in the case of matcha, to simply consume the finely powdered leaves suspended in the drink). In 1841, a recipe for making dry instant tea was published in a collection of industrial processes, The Cyclopædia of Practical Receipts. Tea leaves were to be soaked in a small amount of hot water for about half an hour, then the resultant solution evaporated under low pressure. The anonymous author¶¶¶ says the dried remains will make a “decent cup” of tea when added to a small amount of hot water. The author notes that the lower the temperature used to extract the tea, the better the quality of the reconstituted beverage. Later in the 19th century, concentrated tea, sugar, and evaporated milk were made into a paste that could be reconstituted in hot water. It was convenient, but was clearly inferior to a freshly steeped cup. By the 1940s, instant teas were being produced in the UK and subsequently in the US, helped along by a world war and the need for portable rations, as well as the popularity of instant coffee.

Today, instant tea is still produced by first concentrating the tea. Tiny droplets of the concentrate are sprayed into hot air, where they dry virtually instantaneously. This produces tiny pellets that can be reconstituted by adding cold water to make iced tea or, less frequently, hot water. What is produced is generally considered to be of lower quality than steeped tea and requires sophisticated handling to retain as much of the desirable tea aroma as possible and to maintain the clarity of the final brew. Tea creaming remains a significant issue with instant teas. Various approaches have been taken to reduce the creaming effect, including physically removing the undesirable solids before drying. Additives that break down the materials that can contribute to creaming, such as enzymes to destroy the proteins, are often used.

The heat energy used in the infusion of tea from leaves is a two-edged sword. The extra energy increases the rate of extraction, but also increases the rate at which some of the compounds degrade. Cold extraction gets around this issue, but, as we’ve seen, it takes a much longer time, by a factor of a thousand or more. Cold-brewing also reduces the caffeine content, which may not always be desirable.

Barring Star Trek’s replicator, which presumably is not infusing the tea but constructing it from its molecular components, is there a way to extract the desirable material from tea leaves quickly and gently? Tao Xia, Siquan Shi, and Xiachun Wan at Anhui Agricultural University have explored this using ultrasonic extraction.¹⁵ They repurposed an ultrasonic cleaning bath to extract the compounds from green tea leaves by cavitation, making tiny holes in the cell walls, rather than by diffusion. The best parameters were determined to be a temperature of 60 °C and an extraction time of 40 minutes. The ultrasonic extraction yielded slightly more caffeine, about 6% more, than the conventional approach.

Trained tea-tasters were used to evaluate the final brew. The panelists were impressed with the quality of the tea infusions obtained using the ultrasonic method; the resulting teas were slightly less astringent and bitter than the conventionally brewed tea. The ultrasonic methods appeared to release more of the compounds responsible for tea’s floral aroma, which may have contributed to the panelists’ preference. I’ll admit I haven’t tried this method, being reluctant to either spend the money to buy the equipment or to use the equipment in my colleague’s lab, but I remain intrigued. Will an ultrasonic kettle ever make it to market? Given the speed of the percolating machines, such as the Keurig and Nespresso, which rely on high temperatures and finely ground teas to rapidly extract the caffeine and catechins, perhaps not, but a good cup of tea might be worth the wait.

One way we control what is in a cup of tea is to control the conditions under which we brew it. The time, the temperature, and the size of the leaves all play a part in the chemical composition of an infusion of tea. But we can also enhance what is in the cup of tea by bringing other molecules into the mix. Sugar, spices, even a dash of salt can be added to tea to change its flavor and aroma. In the next chapter, we explore many of the ways to enhance a cup of tea with sweeteners, spices, and flowers. And we will at last tackle the question of whether milk or tea should go in the cup first.

5.10 Brewing a Better Cup

Heat the water to its boiling point and use immediately.

Can’t wait for the caffeine? Use a teabag with finely crushed tea or a percolating device and you have the maximum dose in 30 seconds. The brew might not be as aromatic or nuanced as your usual brew, and be a bit more bitter, but it will be as effective.

Feel free to dunk that teabag and squeeze out the tea remaining in the bag; you’ll get more antioxidants.

Do not reuse your tea leaves! The caffeine is gone and the antioxidants are seriously depleted.

Avoid using a microwave to boil the water unless there is no another option. The additional creaming that results will reduce the antioxidant concentrations and the tea will taste flatter.

To salvage your tea when the only hot water available is from an urn kept below the boiling point of water, choose a paper cup rather than a mug. It is not as sustainable an option, but it will keep the temperature of the water higher and so give you more caffeine than a reusable cup or mug. Or pre-warm a reusable mug by swirling some of the hot water in it and then discarding it. Leave the teabag in the mug slightly longer than you do when using boiling water to compensate for the slower extraction rate of caffeine at these lower temperatures. It may not be a great cup of tea, but it will be a better cup.

If you are reheating your tea in the microwave, add a bit of lemon to dissolve the tea scum and reduce creaming.

Compost your tea leaves if you can—it reduces global warming by a tiny fraction. It doesn’t improve your cup, but it makes for a better world.

Further reading

M. Francl, Nat. Chem., 2016, 8, 897–898. A short essay on the origin of water in the universe, beginning with the Big Bang and leading up to the formation of the Earth.

P. Ball, H₂O: A Biography of Water, Weidenfeld & Nicolson Ltd, London, 1999. A lively reflection on water and its many facets, from cloud formation to rivers to the pseudoscience that water attracts.

H. Chang, Inventing Temperature: Measurement and Scientific Progress, Oxford University Press, Oxford, New York, Illustrated edn, 2007. A fascinating tour of the history of temperature and its measurement. We take thermometers for granted, but their development was not straightforward.

G. Orwell, A Nice Cup of Tea, https://www.orwellfoundation.com/the-orwell-foundation/orwell/essays-and-other-works/a-nicecup-of-tea/. From the author of 1984, advice on making the best cup of tea, much of which is supported by the science in this chapter.

Lu Yu, Classic of Tea. The edition in ref. 3 is beautifully illustrated and includes a substantial amount of historical context.

References

1. Tea – Preparation of Liquor for Use in Sensory Tests, ISO 3103: 2019, International Organization for Standardization, 2019.

2. Y. Hilal and U. Engelhardt, J. Verbraucherschutz Lebensmittelsicherh., 2007, 2, 414–421.

3. J. Wu, Illustrated Modern Reader of ‘The Classic of Tea’, Shanghai Press, 2017.

4. N. Ben Abu, P. E. Mason, H. Klein, N. Dubovski, Y. Ben Shoshan-Galeczki, E. Malach, V. Pražienkova, L. Maletinska, C. Tempra, V. C. Chamorro, J. Cvačka, M. Behrens, M. Y. Niv and P. Jungwirth, Commun. Biol., 2021, 4, 1–10.

5. M. Cabrera, F. Taher, A. Llantada, Q. Do, T. Sapp and M. Sommerhalter, Molecules, 2021, 26, 3485.

6. M. Spiro and S. Siddique, J. Sci. Food Agric., 1981, 32, 1027–1032.

7. J. Serrin, in Contemporary Developments in Continuum Mechanics and Partial Differential Equations, ed. G. M. de La Penha, and L. A. J. Medeiros, North Holland Publishing Co, 1978, p. 416.

8. S.-D. Lin, J.-H. Yang, Y.-J. Hsieh, E.-H. Liu and J.-L. Mau, J. Food Process. Preserv., 2014, 38, 1234–1243.

9. C. Astill, M. R. Birch, C. Dacombe, P. G. Humphrey and P. T. Martin, J. Agric. Food Chem., 2001, 49, 5340–5347.

10. D. Mikhail, The Iraqi Nights (trans. K. J. Abu-Zeid), New Directions, 2014.

11. M. Spiro and D. Jaganyi, Nature, 1993, 364, 581.

12. P . T. Gan and A. S. Y. Ting, J. Culin. Sci. Technol., 2019, 17, 170–183.

13. S. Hashmi, A. Sengupta and R. S. Chauhan, J. Radioanal. Nucl. Chem., 2021, 329, 1361–1373.

14. J. O’Callaghan, Astronauts Plug Leak on The International Space Station With The Help of Floating Tea Leaves, https://www.forbes.com/sites/jonathanocallaghan/2020/10/20/astronauts-plug-leak-on-the-international-space-station-with-thehelp-of-floating-tea-leaves/, accessed 7 February 2023.

15. T. Xia, S. Shi and X. Wan, J. Food Eng., 2006, 74, 557–560.

†Chez Panisse outside San Francisco has had a tea sommelier on their staff, so I presume I could get an amazing cup of tea there to go with a surely astounding meal, but I’ve never had the pleasure of dining there.

‡Not dihydrogen monoxide, although IUPAC says that is also an acceptable name.

§On the other hand, if you drink tea on the International Space Station, some of the water is recycled astronaut pee.

¶Recent research suggests that at least some of Earth’s water is not allochthonous, but indeed came crashing in with asteroids that collided with the planet.

‖It is rather surprising that heavy water is sweet. Chemists think they have a good understanding of what makes things sweet and heavy water meets none of these criteria. Nevertheless, biochemists at the Hebrew University of Jerusalem have confirmed the sweetness of heavy water and proposed a molecular mechanism for the taste.⁴

**Erdös couch-surfed in the days before that was a thing, collaborating with mathematicians all over the world. Researchers are said to have an Erdös number of 1 if they co-authored a paper with Erdös, 2 if they wrote a paper with someone who wrote a paper with Erdös, and so on. My Erdös number is 6, not bad for a chemist, but my son has me beat with a 3.

††Historical directions for making some herbal tinctures sometimes suggested saying a number of Aves—Hail Mary prayers—over the mixture. While this might seem to be superstitious, the use of recitations of familiar prayers or psalms was a known strategy for timing and historians suggest that the prayers were not intended (or solely intended) to invoke divine help in making the brew more efficacious, but simply to make sure it was steeped for the right amount of time. Which would, of course, make it more likely to be efficacious.

‡‡The experiments measured the effectiveness of preventing the oxidation of iron ions, which were then extrapolated to behavior in a living system.

§§Serrin formally defined a thermometer in the following way: “There exists a topological line M which serves as a coordinate manifold of material behavior. The points L of the manifold M are called ‘hotness levels’, and M is called the ‘universal hotness manifold’.”⁷

¶¶I have two words for the chemists and physicists reading this: statistical mechanics.

‖‖This is true at sea-level; the temperatures will be different at different altitudes. The measurement of the boiling point of water was used by the explorer Nain Singh to find the altitude when the British were surreptitiously mapping Tibet in the 19th century.

****This can be taken too far. Tea dust, like the dust of many agricultural products, is highly combustible and explosive. Although not a risk in the amounts found in teabags, on an industrial scale tea dust does pose a hazard.

†††And, if you are wondering, citric acid can also remove the scum from your tub. You’re welcome!

‡‡‡These forces are named not for the city of London, but for the German physicist Fritz London.

§§§The infamous Boston Tea Party, which helped kick off the Revolutionary War in the US, dumped just over 40 tonnes of tea into the harbor.

¶¶¶Given as “A practical chemist, member of several scientific societies etc. etc.”.

Chapter 6

Sugar and Spice

Love and scandal are the best sweeteners of tea.

—Henry Fielding, Love in Several Masques †

Tea Pairing

Enjoy this chapter with a cup of Rose Congou or an Earl Grey with a dash of honey. Make a cup of chai masala, too.

6.1 Spicing Up a Cup

The 18th century novelist Henry Fielding may have sweetened his tea with love and scandal, but tea-drinkers have been enhancing their tea with flavors sweet and savory for centuries. Lu Yu added salt to his tea. Tea was prepared with onions and butter, while wealthy Dutch ladies served theirs with saffron. Today, black teas are often served with milk or lemon and some kind of sweetener. Spices, such as cinnamon or cloves or cardamom, citrus, and flowers also have long found their way into tea. How do all these additives affect the chemistry of tea?

Perhaps the biggest question that arises around what gets added to tea is whether it matters if milk is put in the cup before the tea, or the tea before the milk, and can you tell the difference? The short answers to these questions are yes and yes. It is possible to taste the difference between tea to which milk has been added and tea which has been added to milk already in the cup. And yes, it does matter in which order you add them, but perhaps not for the reasons generally assumed. Let’s tackle this critical question first.

6.2 Milk

Europeans began adding milk to tea almost as soon as tea was  introduced to them. As Richard Coulto of the History of Tea Project at Queen Mary University of London writes, historians find mention of tea drunk with milk in advertisements in England and in letters from French marchionesses as early as the 1670s. There is also a long history of butters, such as yak butter, and other fats, including milks, being added to tea outside of Europe. And, of course, there is masala chai, a milk-based decoction of tea first brewed in India and now widespread.

When we talk of adding milk to tea today, what precisely do we mean? In everyday use the term milk refers to a wide range of liquids, although the European Union defines milk quite strictly as what we obtain by milking a dairy animal, a cow unless otherwise specified. Animal milks are the liquids produced by mammals to feed their own young, but humans have been milking that supply for their own purposes for millennia. Depending on where you live and your cultural background, the animal sources of milk may be cows, goats, sheep, buffalo, or even reindeer. Plant-based milks—such as coconut, oat, or soy milks—are also ubiquitous in what we still call the dairy section of the supermarket.

When we talk of adding milk to tea, we generally think of cows’ milk. To a chemist’s eyes, milks, whether produced by farm animals or extracted from plants, are extraordinarily complex mixtures. Regardless of the source, the general composition of milk is the same. Milks are mostly water, almost 90% water in the case of cows’ milk, closer to 50% for coconut milk. Shining a light through a glass of milk reveals that it is a colloid. The same Tyndall effect that we saw caused tea to look cloudy scatters the light as it hits the milk particles suspended in the water.

While proportions vary, milks contain fats, proteins, and sugars, along with assorted minerals and some, shall we say, biological debris.‡ A dozen different chemical elements are found in milk, the primary ones being potassium, calcium, and phosphorus. While we think of animal milks as excellent dietary sources of calcium, milk is also a good source of another essential element, selenium.§ It is primarily the fats and the proteins gathered together into clumps that are suspended in the water to form the colloid and these are the major targets with which the molecules in tea interact. The majority of the proteins in milk are called caseins. These proteins repel water (are hydrophobic) and so prefer to bind to other proteins to create larger structures that become suspended in the milk.

Adding that splash of milk to your tea seems trivial, but at the molecular level it is the intermingling of two quite different and complicated mixtures. How do we sort out the millions of possible interactions? Since we know that one of the big ideas of chemistry is that similar structures have similar reactivity, one way to tackle this is to look at groups of related compounds. We’ll examine here how the catechins, the alkaloids, and the amino acids interact with the various components of milks.

Does adding milk affect the antioxidant levels in tea? In 2009, researchers Zerrin Yuksel, Elif Avci, and Yasar Kemal Erdem explored the binding between milk and the major catechins found in tea in model systems.¹ Their results suggested that, as a chemist might suspect from their structures, the catechins are able to bind to the proteins in milk. This implies that adding milk might slightly reduce the amount of antioxidants available in the tea. The same year, Lisa Ryan and Sébastien Petit at Oxford Brookes University considered the effects of the addition of milk—whether whole, semi-skimmed, or skimmed—on the concentrations of antioxidant catechins in black tea.² The fat content of the milk has some effect on the antioxidant concentration in the tea, apparently suppressing the binding of the catechins. Tea to which skimmed milk has been added has the lowest concentration of catechins.

The general consensus is that adding milk slightly reduces the concentration of antioxidants in the infusion, but it is not clear whether this reduction is significant in terms of the final concentrations in our blood. Scientists at Unilever Research in The Netherlands measured the levels of catechins in the bloodstreams of a group of human volunteers after they had drunk a cup of black tea, either with or without milk.³ The levels of catechins in their blood increased quickly after drinking a cup of tea, peaking after about two hours. The catechins were also eliminated quickly, with a half-life of roughly five hours.¶ The addition of milk to the tea had no measurable effect on the catechin levels in their blood. The slight variations in antioxidant concentrations caused by adding milk to your tea are unlikely to have significant effects on the health benefits of the tea. So, if you like milk in your tea, go ahead knowing that you are not diluting its healthy qualities.

Researchers Emma Keenan, Mike Finnie, Paul Jones, Peter Rogers, and Caroline M. Priestley at the University of Bristol examined the effect of milk on levels of l-theanine in black tea.⁴ They found that, regardless of the type of milk added, whether skimmed or full-fat, the theanine levels were unaffected by the addition of 12 mL of milk to 200 mL of tea. Quadrupling the amount of semi-skimmed milk reduced the amount of theanine in the tea by a factor of 10. They have evidence to suggest that the theanine is being bound to something in the milk, although they cannot identify what. Nor have they followed what happens when the bound theanine is ingested; it is entirely possible that the theanine is released during digestion. But if you are drinking tea for the relaxing effect of l-theanine, you might want to be sparing with the milk.

It is perhaps unsurprising that adding milk to tea reduces our perception of the bitterness of the infusion. The alkaloids, particularly caffeine, are the primary source of the bitter taste in infusions of tea. One theory holds that the fat particles emulsified in the milk sequester the caffeine away from the taste receptors. Results from Russell Keast in Australia counterintuitively suggest that tea with full-fat milk tastes slightly more bitter than tea made with skimmed milk.⁵ There is no evidence that milk reduces the availability of caffeine, so if it is caffeine’s zing you are after, add milk with abandon—or at least to taste.

6.3 Milk or Tea First?

The question of which comes first, the milk or the tea, divides tea-drinkers into two camps, perhaps even into two social classes, but from a scientist’s perspective it is the part this question played in the development of an important statistical tool, the null hypothesis, that is perhaps even more intriguing.

In the early 1920s, British statistician Ronald Fisher was taking a tea break with two colleagues, a biologist and biochemist, at the agricultural research institute where they worked. The biologist, Blanche Muriel Bristol, objected to the cup she was offered as Fisher had put the milk in first and then added tea. She preferred it the other way round. Fisher and the biochemist (William Roach, whom Bristol would later marry) suggested that she could not possibly taste the difference. They proceeded to test her and, as the story goes, Dr Bristol could indeed distinguish between the two preparations.⁶ In 1935, Fisher published his now classic text, The Design of Experiments, which introduced the concept of the null hypothesis‖ using the example of an unnamed lady who claimed she could distinguish between cups of tea to which milk had been added first and those to which it had been added after the tea was poured.

There are many theories about why the milk should go in first or vice versa and, at various times, it has been used as a marker of social class. Some suggest that milk in first helps temper the thermal shock of pouring hot tea into a cold cup. People in the lower classes could not afford the porcelain cups that were presumably less susceptible to cracking upon temperature change. Milk added last allows the drinker to adjust the amount of milk based on the appearance of the brew, a luxury afforded only to those who had the more expensive porcelain cups. Another theory has it that the upper classes had better access to fresh milk, which was less likely to curdle when added to hot tea. Putting milk in first and pouring the tea allowed a moment for the milk to temper.

The British Standard for making a cup of tea, and the current ISO standard, come down on the side of saying that the milk goes in first, but just barely. They acknowledge the alternative, suggesting that if one wishes to add the milk after adding the tea, then to wait until the tea has cooled slightly to between 65 and  80 °C to reduce the risk that the milk might curdle. The standards call for a small amount of milk, 5 mL in a 250 mL cup of tea, just one teaspoon. I expect most people measure the same way I do, as a quick splash from the milk container in the fridge or, if I’m being quite fancy, a glug from a small pitcher.

The question of which goes first is perhaps moot if you’re using a teabag or tea basket or ball to make tea directly in a mug (as most tea-drinkers in the US and the UK do). In this case, milk definitely goes in last because otherwise you’ll reduce the temperature of the water the tea is steeping in. If you make your cup of tea with the British Standard in mind, we can make a quick calculation of how much the water will cool if the milk is already in the cup. We’ll assume that the milk is at the temperature of a refrigerator, 4 °C, and that the water for the tea is fully at the boiling point, 100 °C. Using 250 mL of water and 5 mL of milk, we find that the final temperature would be roughly 2 °C cooler than it would be if you did not put the milk in the cup ahead of time. But beware if you like more than a scant teaspoon of milk in your tea: adding two tablespoons of milk cools the tea by about 10 °C, which will definitely affect the taste and amount of caffeine in your brew.

6.4 A Taste of Honey

Sweetness is a primal preference. Our closest primate cousins will brave honey bee stings for the sweet syrup they can extract from the hives. Eight thousand year old cave paintings in Coves de l’Aranya in eastern Spain show a woman scaling a ladder leaning against a cliff to raid a beehive in pursuit of the honey it surely contains. Our first food, breastmilk, is sweet and, perhaps for this reason, many of us continue to enjoy sweet drinks the rest of our lives.

The arrival of coffee and tea in Europe drove the desire for sugar to sweeten the bitter brew. And in many traditions sweet treats accompanied the tea. The bowls of green tea served in temples in Japan to pilgrims often comes with a sweet cookie, a tradition of serving something sweet alongside a bowl of tea that stretches back to at least the 1500s. Russians have long served tea with jam and, along the Silk Road, tea was served well-sweetened  and strong.

Sweet and bitter tastes are known to interact with each other. Mixing a sweet-tasting substance with a bitter substance results in a reduction of the perception of both bitterness and sweetness. Sweetened tea is not as bitter as unsweetened tea nor as sweet as the comparable solution of sugar might be. Natural sweeteners, at least sucrose, more effectively moderate the bitterness of caffeine than artificial sweeteners such as aspartame or saccharin. Fructose is better than sucrose at moderating bitterness, an effect leveraged by beverage manufacturers, who, for many reasons, prefer to use high-fructose sweeteners.

Tea can be sweetened with natural sweeteners such as table sugar—sucrose—or honey. Both honey and table sugar are nutritive sweeteners, adding to the taste, but also to the energy content of the tea. When sweeteners are metabolized, chemical bonds are made and broken, and the energy released in these processes is stored in a molecule called ATP,** which essentially acts as a battery for our cellular machinery. Some non-nutritive sweeteners, including aspartame, also transfer energy to ATP when they break down, but because they are so much sweeter than sucrose or honey, you don’t need to add very much to sweeten a cup of tea and so the amount of energy added is negligible. Other non-nutritive sweeteners are not metabolized by humans and pass through the body unchanged. Sucralose, for example, is much sweeter than sucrose and very little of it is broken down in the human body.

Sweeteners are also sometimes classified as natural—including sucrose and fructose or syrups such as honey or agave nectar—or artificial, such as aspartame or saccharin. As always, the function depends on the structure of a molecule, not on its source. Synthetic sweeteners are not inherently any more or less safe for human consumption than naturally produced sweeteners. For example, the natural sweetener erythritol, found in molasses, has recently been shown to be associated with an increased risk of heart attack or stroke, while a huge body of research has failed to show that consuming aspartame is unsafe. Recently, the World Health Organization proposed classifying aspartame as potentially carcinogenic, which will surely prompt more research. There are many sweet molecules, but I will focus here on the ones that may be used to sweeten a cup of tea.

6.5 Naturally Sweet: Sugars

Sugar to a chemist is a class of molecules that includes table sugar, but is not limited to it. Sugars are water-soluble, often sweet-tasting,  carbohydrates. As their name suggests, these molecules have chemical formulas that look like hydrates of carbon, where the ratio of hydrogen to oxygen is two to one, just as it is in water: the standard formula is Cn (H₂O)n and for glucose n = 6. Organisms—including humans and plants—use sugars as a source of energy. Sucrose is formed from two single sugar units, the monosaccharides glucose and fructose. The structures of some naturally occurring sugars are shown in Figure 6.1. The solubility of many of the sugars is driven by the –OH (hydroxide) groups they contain, which can easily hydrogen bond to water. Sucrose, with its eight hydroxide groups, is so soluble that 2 g of it will dissolve in a single milliliter of water at room temperature and more than twice that will dissolve in a milliliter of water at 90 °C. To put that in perspective, less than 0.5 g of table salt, sodium chloride, will dissolve in that same 1 mL of water at room temperature.

Figure 6.1 Molecular structures of three common sugars: fructose, glucose, and sucrose.

Honey is a natural product and, like tea and milk, it is a complex mixture of molecules and materials. Its composition varies depending on its source and the flowers the bees have harvested the nectar from to produce the honey. My local farmers market features honey produced in hives in different postal codes, which have subtly different colors and tastes. The main sugars in honey are glucose and fructose, but hundreds of other compounds are also found in honey, many of them aromatic. Honey has a low water content, so low that it is essentially a supersaturated solution of glucose. The crystals that sometimes appears in honey are a result of the glucose coming out of solution.

Stevia is a sweetener derived from the leaves of the Paraguayan plant Stevia rebaudiana. The leaves have been used in South America for centuries to sweeten beverages. Stevia leaves contain a number of sweet-tasting molecules. The sweet compounds found in the stevia plant are glycosides. A glycoside is a molecule in which a sugar is attached to another molecular substructure. Organisms use glycosides to store molecules in an inactive form. Break off the sugar and the molecule is released to do its work. Stevioside is the primary glycoside in a stevia sweetener, chemically extracted from the leaves, purified and then crystallized before formulating it into something that can be stirred into your tea. Figure 6.2 shows the chemical structures of a number of different sweeteners. Stevioside†† has three glucose units attached to a terpene unit.

Figure 6.2 Molecular structures of several non-nutritive sweeteners.

Agave nectar is extracted from one of several species of succulent plants, including the blue agave.‡‡ Agave is a caloric sweetener, like sugar, composed primarily of the monosaccharide fructose. Spoonful for spoonful it is a little bit sweeter than sugar, so you use less, theoretically reducing your caloric intake. Agave is also available as a liquid, which makes it easy to add to cold beverages, such as iced tea.

6.6 Sweet Surprises

Saccharin, or benzoic sulfimide (Figure 6.2), was the first of the commercial artificial sweeteners to be discovered. In 1879, chemist Constantin Fahlberg at Johns Hopkins University was working with benzoic sulfimide. He failed to wash his hands on leaving the laboratory one night and, when he sat down to dinner, he discovered that his bread tasted “unspeakably sweet,” as did everything else he had touched, from his napkin to his glass. He reported in an 1886 interview with Scientific American that, after this, he returned to his laboratory and tasted the contents of every beaker and dish he had been working with until he found the culprit, which he patented under the name saccharin. It wasn’t until the sugar shortages of the First World War that saccharin became widely used. About 100 years after it was discovered, research linking it to bladder cancer in rats led to the US Food and Drug Administration requiring a warning label on anything containing saccharin. The warning was eventually dropped when it became clear that, while rats were at risk, humans were not. It remains one of the world’s most popular artificial sweeteners.

In 1937, a graduate student at the University of Illinois, Michael Sveda, picked up the cigarette he’d left on the laboratory bench and took a puff. (I know, this is a bad idea for so many reasons.) He was surprised when the cigarette tasted sweet. It had become contaminated with the cyclamate he had been working with. Cyclamate (Figure 6.2) is about 30 times sweeter than sugar and, while most people do not metabolize it, some people convert it to cyclohexylamine, which has been linked to bladder cancer. Although it is unlikely to cause cancer at the levels it is ingested, the US has banned its use as a sweetener.

Sucralose, serendipitously discovered in 1976 by researcher Shashikant Phadnis at what was then Queen Elizabeth College in London, is a derivative of sucrose in which two of the hydroxyl groups have been replaced with chlorine atoms (Figure 6.2). Given that structure and function are related, it is perhaps not surprising that this molecule tastes sweet. It is about 500 times sweeter than sugar. Humans are unable to metabolize it so, while it tastes sweet, it provides no calories.

6.7 Tiny Proteins

Aspartame is incredibly sweet, roughly 200 times sweeter per spoonful than sugar. Unlike stevia or agave, which are derived from natural products and are both based on sugars, aspartame is a synthetic product and is based on proteins. Aspartame (Figure 6.2) is essentially a dipeptide, a molecule made by connecting two amino acids, in this case phenylalanine§§ and aspartic acid. Proteins, including enzymes, are polypeptides made up of hundreds or even thousands of amino acids. Aspartame was discovered serendipitously in the mid-1960s by a researcher who licked his fingers while working in the laboratory and found they tasted sweet. (Yes, there is a theme of researchers finding these compounds by doing things in the laboratory that makes modern day chemists shudder.)

While aspartame is regularly demonized and many ill effects are ascribed to it, research to date generally agrees that it is safe at the levels found in the human diet. This isn’t a surprise to chemists, given the big idea that the behavior of a molecule depends on its structure. Aspartame is a dipeptide, a two-unit protein, so our bodies break it down in the same way we break down any protein we consume, turning it into the component amino acids which can be then reused to build proteins and other cellular structures. You may have heard that aspartame “contains” methanol (wood alcohol), which can cause blindness if ingested in large quantities. The breakdown of aspartame in the body can produce methanol in tiny quantities, but the breakdown of any protein you eat produces methanol. The amount of methanol formed in the body from ingesting even very large doses of aspartame is within the natural variation of methanol found in humans. Methanol is naturally found in fruits and vegetables. There is no way to avoid methanol in the diet at these very low levels and our bodies are equipped to handle it. As chemists are wont to point out, the dose makes the poison.

There are other sweeteners besides aspartame that are derived from small peptides—for example, advantame, which is 20 000 times sweeter than table sugar, and neotame (Figure 6.2). Aspartic acid and a derivative of alanine are linked to make alitame, which, while not approved for use in food in the US, is approved by the European Union.

6.8 Molecular Love Triangles

What structurally makes a molecule taste sweet? The simple answer is the ability to bind to sweet receptors. These receptors, like those for other tastes are GPCR¶¶ (G protein-coupled receptors). Each of these receptors has binding sites for small molecules, like sucrose or saccharin, and larger binding sites for things like the sweet proteins.

Even before the sweet receptor proteins were identified and characterized, chemists had a sense of what the molecular requirements for sweetness were. In 1967, Shellenberger and Acree proposed that sweet-tasting compounds have two contact points—a hydrogen bond donor and a hydrogen bond  acceptor—spaced between 250 and 400 pm apart. In 1972, Lemont Kier modified this model to include a third contact point—a hydrophobic substructure—to make what is now called the sweetness triangle (Figure 6.3). The triangle on the molecule matches up with a complementary triangle on the receptor in the binding site. In the molecule saccharin, the imine group  (–NH) acts as the donor and the sulfoxide group (–SO₂) behaves as an acceptor. The non-polar (hydrophobic) six-membered carbon ring is the third point of attachment.

Figure 6.3 Schematic diagram of the sweetness triangle showing some of the key molecular motifs and the spatial arrangement that can lead to a sweet taste.

6.9 Chai: Tea Can Be Hot in More Ways Than One

At the beginning of the 20th century, chai masala, a spicy–sweet milk and tea drink, emerged as a popular drink in the Indian subcontinent. Today, chai masala and its cousins can be found nearly anywhere in the world. Drinking tea flavored with herbs and spices has a long history. Lu Yu’s Classic of Tea mentions adding chives to tea. Travelers to the Khiva Khanate (in what is now Uzbekistan) in the early 19th century reported seeing tea served mixed with milk, butter, and other fats and mentioned the custom of adding spices such as cinnamon and cloves.

There are many ways to make a cup of chai masala. The simplest is to bring to the boil a half-and-half mixture of milk and water, add black tea leaves and steep (or continue boiling to make a decoction), and then strain and sweeten the final mixture. Spices are often boiled along with the milk and water. The spices used in a chai masala are a matter of personal taste and can include ginger, cinnamon, cloves, black pepper, and lemongrass.

Whether chai masala is technically a decoction or an infusion depends on the preparation. It can be made as a decoction of the spices, which is then used to infuse the tea leaves. Or both the tea and spices can be decocted (boiled) in the milk and water. Either way, we expect that the chemical composition of the beverage will be slightly different from the usual infusion of leaves. Extraction at the higher temperatures obtained in the decoction will lead to additional caffeine and catechins in the prepared chai relative to tea infusions in hot water. (As we noted in the last chapter,  10 °C can make a large difference in the speed of an extraction and the amounts of different compounds extracted.) The spices add their own unique chemical signatures to the brew. Let’s take a look at some of the molecules in play in a warm, spicy cup of chai masala.

Fresh ginger root is often grated into chai masala. Like tea and most natural products, ginger contains hundreds of different compounds. At least 400 have been identified to date. Gingerols are the primary components responsible for the scent and spiciness of ginger root (Figure 6.4). The ginger plant uses a series of enzymes to convert sucrose into several related gingerols, which it uses to defend itself from various environmental challenges. When gently warmed, as in the making of chai, gingerols lose a water molecule and convert to shogaols. These piquant compounds are about as spicy as the piperines found in black pepper— shogaols rate 160 000 units on the Scoville scale used to assess the heat of spices. For comparison, habanero chilis rank similarly on the Scoville scale. Like most hot spices, gingerols and shogaols activate pain receptors in addition to taste receptors.

Figure 6.4 Molecular structures of the key flavor compounds found in ginger, cinnamon, and cloves.

Some people advise boiling the ginger before adding it to the milk because ginger contains the enzyme zingibain, which can damage the proteins in milk, causing it to curdle.‖‖ Zingibain is similar to bromelain, an enzyme in pineapple that also denatures proteins. It is the reason that gelatins will not set if they are made with fresh pineapple—they must be made with canned fruit that has been heated to a high temperature to inactivate the enzyme. Since zingibain becomes inactive when heated beyond 70 °C, boiling the ginger effectively keeps it from curdling  the milk.

Cardamom is another member of the ginger family frequently added to chai. It has a more resinous flavor than ginger, which would not surprise chemists as the main component of the essential oil that is extracted from cardamom pods is a terpene. Terpenes are compounds that are often found in evergreen trees and can have a piney, resinous fragrance.

Cinnamon is a spice with a long history—references can be found going back nearly 4000 years.**** Cinnamon is obtained from the bark of several different, albeit related, tropical trees. Most of the cinnamon used in foods is from the bark of either Cinnamomum verum trees or Cinnamomum cassia trees. While, like ginger, cinnamon contains many different compounds, cinnamaldehyde is the molecule primarily responsible for its characteristic taste and fragrance (Figure 6.4). Cinnamaldehyde is sparingly soluble in water, as might be expected from its structure. The “tail,” made up of carbon and hydrogen atoms and ending in the hexagonal phenyl ring, is hydrophobic. In other words, it would prefer to avoid interacting with any water molecules, but will nestle with the hydrophobic proteins in the milk used to make masala chai. It is the lone oxygen atom that can make a hydrogen bond with water and that lends the molecule a slight polarity that enables any of it to dissolve it all. Cinnamaldehyde has little or no known toxicity in humans.

While cinnamaldehyde is the primary ingredient in cinnamon bark, another compound found in the spice in significant amounts presents potential risks. Coumarin††† (Figure 6.4) is found at high levels in cinnamon from C. cassia. One gram of cinnamon might contain as much as 10 mg of coumarin (the bark of C. verum contains little or no coumarin). Coumarin is found in other fruits, such as apricots and strawberries, as well as in cherry blossoms and licorice root. It smells of vanilla, but has a bitter taste.

Coumarin has been shown to be toxic to the liver in rats, so, although the risk to humans seems to be quite low, it is regulated in many countries. The European Food Safety Authority set guidelines for a safe amount of coumarin in foods of roughly 6 mg per day, with a note that ingesting higher levels for a short period appears to be safe. So those holiday spice cookies my aunt and my husband adore, which might have as much as 2 mg of coumarin each, are not technically a hazard because I only make them for the winter holidays. A cinnamon stick from C. cassia might contain 6 mg of coumarin, although, of course, if it is crushed up coarsely and then strained from the tea, there is far less coumarin in the resulting cup.

Cloves are another common spice used in chai, as are black peppercorns. The oil in cloves is called eugenol (Figure 6.4) and has a distinctive taste and fragrance. It is also found at high concentrations in cinnamon. Eugenol has a long history as a topical anesthetic, particularly for dental applications. In the amounts found in spices, or used in clinical applications, eugenol is not toxic. That said, very high doses can cause liver damage. The active spicy ingredient in black pepper is piperine (Figure 6.4). Like gingerol, piperine interacts with pain receptors, part of what gives it its characteristic heat.

Examining the molecular structures of these spicy compounds (Figure 6.4), you might notice that, compared with the polyphenols, they have more carbon atoms relative to the number of oxygen atoms. These compounds are thus hydrophobic, dissolving well in the milk that forms the base of masala chai decoctions. (And also why, after eating a peppery dish, milk will wash away the spice, cooling the heat.)

6.10 Tea. Earl Grey. Hot

Jean-Luc Picard, captain of the (alas fictional) United Federation of Planets’ starship Enterprise, says to the air, “Tea. Earl Grey. Hot” and, with a swirling of CGI glitter and a brief chirp, a white porcelain cup of tea shimmers into sight in a slot in the wall. I’m jealous of the science fiction technology, but not of the particular cup of tea Picard prefers.‡‡‡ I find the scent of the oil of bergamot used to flavor Earl Grey teas to be just a bit too cloying and perfumy sweet for my taste, although it is my husband’s cup of choice. He is not alone is his preference for scented teas: floral and citrus flavors have long been added to tea to enhance their scent and provide splashes of color.

Jasmine flowers have been used to enhance the scent and taste of tea for more than a thousand years. The tea, typically a green tea, is scented by placing it in contact with jasmine blossoms for 12–24 hours. The blossoms are then removed and the tea gently dried again. The process transfers volatile compounds from the jasmine flowers to the tea. The hot water from a tea infusion subsequently releases these compounds into the air as the tea steeps. More than 70 different aroma molecules have been identified in jasmine tea. Linalool, which is found in tea leaves themselves, is also an aromatic component of jasmine flowers. Like the oil extracted from cardamom pods, linalool is a terpene, with a spicy–floral scent. Other scented compounds in jasmine include benzyl acetate, which smells of pears, and farnesene, which has an  evergreen-like odor (and is found in chamomile and cannabis).

Jasmine tea is more than a pleasant sensory experience. Researchers at Kyoto University and their collaborators have shown that breathing jasmine tea has a significant effect on autonomic activity, reducing the subjects’ heart rates as well as having a calming influence on mood. They attribute this effect to linalool.⁷

Roses, which are grown in many places around the world, are another common floral addition to tea. My favorite late afternoon tea is a Rose Congou,§§§ a blend of black tea and rose petals. Researchers at the Kunming University of Science and Technology in China have explored the volatile aromatic and flavor compounds extracted when roses are infused under the same conditions as the tea is infused.⁸

Benzyl alcohol, phenethyl alcohol, and eugenol are the three largest volatile components extracted during the infusion of roses (Figure 6.5). Benzyl alcohol, which smells of both roses and moss, is a respiratory irritant and toxic at high concentrations, but it is innocuous at the amounts found in these infusions. As you can see in Figure 6.5, phenethyl alcohol is almost identical in structure to benzyl alcohol, so we might expect it to exhibit similar behavior. It smells of roses and has a sweet taste that also evokes peaches. And, like benzyl alcohol, it is also an irritant and a neurotoxin at high concentrations, although safe at the levels it is found in rose teas. Eugenol, found in many plants, is also extracted, imparting a mildly spicy, clove-tinged scent to the tea.

Figure 6.5 Molecular structures of some floral-scented molecules.

Both jasmine tea and Earl Grey tea, while also adding their own unique notes to the experience, contain compounds also found in tea, enhancing their effect. In some senses, jasmine tea and Earl Grey tea smell and taste more tea-like than tea itself. Unlike jasmine tea or, as we shall see, Earl Grey tea, the compounds extracted when roses are infused alongside tea leaves diversify the sensory profile.

Oil of bergamot, extracted from the rind of a small citrus fruit native to southern Italy, has been used to scent black tea for at least 200 years. Although now firmly established in the pantheon of tea choices, the earliest use of the citrus flavoring may have been to cover the taste of poor quality black teas. Legends aside, Earl Grey does not appear to have been originally blended for the second Earl Grey (or any other earl for that matter). The Oxford English Dictionary traced the origin of the name Earl Grey for this blend to late in the 19th century, well after the blend’s introduction to England.

Compared with most other citrus rind oils, the oil from bergamots contain less limonene and more linalool and bergamol (see Figure 6.6). Linalool is a key component of black tea’s aroma, which may be why bergamot oil in particular was originally used to “improve” poor quality tea. The limonene in bergamots, like linalool, is a terpene. It is found not only in citrus plants, but also in various pine trees and species of mint. It has a strong citrusy odor and likely acts as a pesticide, discouraging insects from dining on the plants.

Figure 6.6 Structures of some molecules found in oil of bergamot.

Oil of bergamot also contains a photosensitizer, bergapten. (Chemists looking at its structure in Figure 6.6 will see the fused rings and think “chromophore!”—a structural feature that absorbs light.) Other citrus fruits contain similarly sensitizing compounds, which, when absorbed by the skin, can increase its susceptibility to sunburn. Limes in particular can ruin a tropical holiday if too much of the oil gets onto your skin and you then bask in the sun. When oil of bergamot is used in perfumes, the bergapten is first extracted, eliminating the risk of phototoxicity. While the amount in a cup of tea is unlikely to lead to burns in the mouth, the presence of this compound in Earl Grey tea makes it a poor choice for such home remedies as cool teabag compresses for your eyes or as a soothing sponge for sunburn—despite the potential for other compounds in Earl Grey to relieve pain.

Like the aromatic jasmine compounds, oil of bergamot has measurable physiological effects. Researchers in Japan, led by Shinobu Sakurada, showed that when oil of bergamot is injected into a mouse’s paw that has been first treated with capsaicin to provide a painful stimuli, a significant reduction in the mouse’s pain-associated behavior is seen.⁹ The relief is even more pronounced when linalool or linalyl acetate are injected. Their experiments showed that linalool activates the same pain relief pathways as opioids and that administering an opioid antagonist such as Narcan (nalaxone) blocks the effect.

A more limited relief of pain can be achieved by inhaling oil of bergamot or linalool. There is evidence from a team of researchers in Japan and Italy that the inhalation of oil of bergamot leads to a reduction in pain perception in mice.¹⁰ While the inhalation of oil of bergamot did not seem to relieve the immediate acute pain in mice, it did reduce the response to ongoing pain. The higher the dose of oil, the better the response. It is not just mice who benefit. The use of bergamot in a mental health setting also reduced the level of anxiety reported by patients.¹¹ Perhaps this is why some people find a cup of Earl Grey tea particularly soothing.

The incorporation of flowers into tea can offer more than additional notes to the flavor and aroma. It can also offer a visual benefit. Tea balls, not the metal sieves used to contain tea leaves while they steep, but rather tea leaves tightly wrapped and tied into a ball about 3 cm in diameter, often incorporate flowers such as chrysanthemums or roses. When placed in hot water to steep, these tea balls transform the agony of the leaves as they unfurl into a striking display of a blooming flower (Figure 6.7). Of course, these blooming tea balls are best appreciated in a glass teapot, which, as we will see in the next chapter, is perhaps not otherwise the best choice for steeping tea.

Figure 6.7 A floral tea ball unfurling while steeping in hot water. Image courtesy of Andrew DiDonato.

6.11 Drink This With a Grain of Salt

Lu Yu in his Classic of Tea recommends adding a dash of salt to the water before infusing the tea leaves. Likewise, some  coffee-drinkers add a pinch of salt to their brew, anecdotally claiming that it evens out the taste and, in particular, reduces the bitterness. While some think that you can rescue a terrible cup of coffee with a dash of salt, Kenji Lopez-Alt’s brief exploration of this phenomenon at Serious Eats suggests that you should just dump the coffee down the drain (or hold your nose and drink it if you must have that caffeine).¹²

Metal ions, including the sodium present in table salt (sodium chloride, NaCl) and zinc, have been shown to reduce the perception of bitterness.¹³ The bitterness was suppressed even when the concentration of the salt was too low to be tasted itself. This suggests that the mechanism of the suppression is not that you are distracted by the salty taste, somehow convincing your brain to ignore the bitterness in favor of the saltiness, but that there is some interaction with the chemical mechanism that produces the perception of a bitter taste. A computational modeling study by Chris Hendon at the University of Oregon on the interaction of sodium ions with critical flavor compounds in coffee supports this theory.¹⁴ He found that caffeine, for example, is not bound by sodium.

While sweet tastes are also known to cut the perception of bitterness, as will the addition of milk, the addition of zinc ions from a zinc lactate salt were most effective in suppressing the bitter taste of caffeine in particular.¹⁵ Alas, zinc inhibits the perception of sweet tastes, so if you like your tea sweet, zinc is no cure for the bitterness.

6.12 Lemon

Along with milk and sweeteners, lemon is perhaps the most common thing added to a cup of tea in the US and Europe. Lemon is never added along with milk because it can cause the milk to curdle. As we noted in Chapter 5, one effect the addition of lemon has at the molecular level is to hide away the magnesium ions that encourage the development of tea scum and tea cream. The citrate ions from citric acid, which is the principal chemical compound in lemon juice, have pairs of oxygen atoms (like crab claws) that can simultaneously bind to a single magnesium ion, effectively sequestering it (Figure 6.8).

Figure 6.8 Molecular structure of citrate.

Lemon juice is highly acidic. It has a pH of about 2, strong enough to strip off silver and copper oxides and bring back the original shine of these metals. A squeeze of lemon into tea amounts to the addition of about 5 mL or less of lemon juice. In addition to the citric acid, lemon juice also contains ascorbic acid (vitamin C). Vitamin C is essential to human health. It is required for the synthesis of various neurotransmitters as well as for key components of connective tissues. While the recommended daily dose of vitamin C is 90 mg, deficiencies do not become critical until consumption is less than 10 mg per day for a sustained period of time. Scurvy can result if insufficient vitamin C is consumed, but squeezing a lemon into your tea is not likely to help much. There are only about 2 mg of vitamin C in a cup of tea with a squeeze of lemon. That said, most adults and children in the US meet their dietary requirements for vitamin C. So, if you like lemon in your tea as I do, enjoy it, but know that any nutritional benefit is tiny.

Squeezing lemon above a cup of tea, or floating a lemon slice in the infusion, releases a small amount of lemon oil as well as lemon juice. The main component of this oil is limonene (Figure 6.6), but there also are small amounts of linalool and bergamol. Like rose infusion, lemon adds its own particular flavor notes to tea, but, like jasmine, it also adds to the linalool already present and so enhances the desirable floral aroma of tea.

Adding milk or sugar or lemon can elevate the taste of a cup of tea. The addition of roses or jasmine or oil of bergamot can enhance the floral components of tea, making it a yet more aromatic and calming beverage. But the choice of what we serve tea in, the type of teapot or teacup, can improve the experience as much as what we add to the tea itself. In the last chapter of this book, we explore the equipment we use to make tea and the materials that make for the best cup and pot.

6.13 Brewing a Better Cup

If you like milk in your tea, add it first for the very best cup. If you must add the milk after the tea, you would do well to gently warm the milk first.

If tea is too bitter for your taste, a sweetener can moderate the bitterness or you could add a pinch of salt.

Having a bad day? Consider making a cup of jasmine or Earl Grey tea and inhaling the aromatic steam.

Further reading

Why Do Things Taste Sweet? https://www.acs.org/pressroom/reactions/library/why-do-things-taste-sweet.html. Accessed 21 July 2023. A short explainer by the American Chemical Society on the molecular basis of sweetness.

S. Mintz, The Oxford Companion to Sugar and Sweets, Oxford University Press, Oxford; New York, Illustrated edn, 2015. Everything you could possibly want to know about sugar and sweets by a collection of more than 200 experts, including articles on the biochemistry of sugar.

D. S. Farrimond, The Science of Spice: Understand Flavor Connections and Revolutionize Your Cooking, DK, New York, Illustrated edn, 2018. An overview of culinary spices, including some material on their chemical composition.

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6. S. Senn, Significance, 2012, 9, 30–33.

7. K. Kuroda, N. Inoue, Y. Ito, K. Kubota, A. Sugimoto, T. Kakuda and T. Fushiki, Eur. J. Appl. Physiol., 2005, 95, 107–114.

8. H. Qin, X. Deng, B. Li, W. Dai, S. Jiao, Y. Qin and M. Zhang, J. Food Process. Preserv., 2018, 42, e13323.

9. T. Sakurada, H. Mizoguchi, H. Kuwahata, S. Katsuyama, T. Komatsu, L. A. Morrone, M. T. Corasaniti, G. Bagetta and S. Sakurada, Pharmacol. Biochem. Behav., 2011, 97, 436–443.

10. D. Scuteri, M. Crudo, L. Rombola, C. Watanabe, H. Mizoguchi, S. Sakurada, T. Sakurada, R. Greco, M. T. Corasaniti, L. A. Morrone and G. Bagetta, Fitoterapia, 2018, 129, 20–24.

11. X. Han, J. Gibson, D. L. Eggett and T. L. Parker, Phytother. Res., 2017, 31, 812–816.

12. Should You Put Salt in Your Coffee?, https://www.seriouseats.com/should-you-put-salt-in-your-coffee-reduce-bitterness. Accessed 6 April 2023.

13. A. Drewnowski, Nutr. Rev., 2001, 59, 163–169.

14. C. H. Hendon, L. Colonna-Dashwood and M. Colonna-Dashwood, J. Agric. Food Chem., 2014, 62, 4947–4950.

15. P. A. S. Breslin and G. K. Beauchamp, Chem. Senses, 1995, 20, 609–623.

†H. Fielding, 1728, Love in Several Masques, Act 4 Scene 11.

‡This debris can include blood cells and other cells, bacteria, hormones, and enzymes. There is a reason we pasteurize milk.

§Selenium proteins are secreted in the thyroid. Humans require about 50 µg a day. In addition to barium and radium, Brazil nuts contain selenium. One Brazil nut meets our daily requirement. Tea also is a source of selenium.

¶The bioavailability of other related antioxidant compounds, such as the flavonol quercetin, can be much longer. For example, after eating a single serving of onions, it requires almost 17 hours to eliminate half of the quercetin from your bloodstream. It is popular these days to talk about “super foods”—that is, foods that are supposedly extra healthy. One can imagine brewing tea along with onions to create a super brew with long-lasting antioxidants. Although this may sound odd to us, in the past tea has been brewed with onions and chives.

‖Briefly, the null hypothesis says that any observed differences in outcomes or populations are due merely to chance and not to a particular cause. In the case of the lady and her tea the null hypothesis would be that any success she had in distinguishing between the cups was due to good guesses and not to any particular ability.

**ATP is adenosine triphosphate. The amount of energy tied up in ATP is vividly described by the poet and eminent immunologist Miroslav Holub in his poem The Rampage as “like the explosion of a star/in a chicken coop.”

††So, what do these molecules do for the plant? A number of theories have been proposed, including acting as a pesticide, reducing oxidative stress, and regulating water uptake. The sweet taste is a biological accident, unrelated to the uses the plant makes of the molecule.

‡‡Tequila is also made from the blue agave, but I don’t recommend it in tea.

§§Because the metabolism of aspartame produces phenylalanine, people with phenylketonuria (PKU) must assiduously avoid it, hence the warning label required by many countries There is no reason to avoid phenylalanine otherwise; the proteins you eat every day contain phenylalanine. Estimates are that the average American ingests 7500 mg of phenylalanine a day, compared with the roughly 100 mg of phenylalanine contained in a serving of aspartame.

¶¶Many sensory receptors are GPCRs, including those for sight and bitter tastes. GPCRs are also the receptors for adenosine and caffeine.

‖‖Ginger is used to deliberately curdle milk in the creation of the Cantonese dish ginger milk curd.

****Cinnamon was used in mummification in Egypt and is mentioned in the Old Testament—for example, in the book of Exodus, as being used to scent sacred oils (Exod. 30:23).

†††Coumarin dyes are familiar to chemists due to their use in lasers. The name is similar to that of a drug commonly used as a blood thinner, Coumadin or warfarin. Coumadin can be synthesized from coumarin, although the latter does not have any anticoagulant effect.

‡‡‡Sir Patrick Stewart, who plays Picard in the Star Trek series, actually prefers Yorkshire Gold tea.

§§§Congou teas are the original base of the popular English Breakfast blend. The name derives from the same etymological root in Chinese as does kung fu, from a word meaning skilled.

Chapter 7

Earth, Wind, Fire, and Water

Water is the mother of tea, a teapot its father, and  fire the teacher.

—Chinese Proverb

Tea Pairing

Read this chapter with a cup made with a tetrahedral PG Tips teabag, or steep a cup of a Formosa oolong using a tea basket. Whatever you choose, be sure to warm the cup or pot first!

7.1 The Technology of Tea

Perhaps it is all the science fiction I read as a child, but I am an inveterate early adopter of technology, including when it comes to my tea. While I might still be waiting for my flying car or for the starship Enterprise’s replicators, I can now say to the air, “Siri, set a timer for four minutes!” Four minutes later, with a gentle tap on my wrist, I am alerted to stop unloading the dishwasher and take the basket of tea leaves from my teapot and pour myself a cup of fresh tea.

While we think of technology as whatever is new and improved, from the time the first person thought to repurpose an empty ostrich’s egg to store water to give a thirsty child in the night humans have been looking for ways to make what we do more efficient and more convenient. Tea is no stranger to technology. From the 7th century wind ovens that conveniently brought water to the boil for tea on a tabletop, to the pod-based brewers found, it seems, in every office kitchen, new and (sometimes) better ways of making and enjoying tea are always emerging. My kitchen cabinets reflect this. I have dozens of teapots, more mugs and travel cups and thermoses than I can count, along with tea infusers ranging from the whimsical (a yellow submarine that looks as if it were drawn from the Beatles’ eponymous 1969 album) to the high-tech (stainless steel with laser-cut perforations).

Now that we know something about what is in a cup of tea, in this chapter we will explore how technology might shape what is in that cup, and why. Does it matter what color your cup is? Do those fancy pyramid-shaped teabags steep differently— better—than the iconic rectangle? Why are teapots squat and round while coffee pots stand tall and cylindrical? We will take a look the advantages and disadvantages of various materials and shapes for teacups and teapots, along with ways to heat water up and keep the resulting infusion hot. And, of course, the effect of those teabags, balls, and baskets on what’s in a cup of tea.

7.2 What Comes Between You and Your Tea: Teabags, Balls, and Baskets

The writing of this book began with a question posed on Twitter by @andrechemist in early 2020 wondering whether a tetrahedral- shaped teabag like that shown in Figure 7.1 would have an advantage over a flat teabag when it came to extracting the tea inside. Or was it just a cool shape? (Chemists think tetrahedrons are inherently interesting because that is one of the three base shapes that the skeletons of carbon-based compounds take.) Exactly what effect does the materials and shape of a tea infuser—whether a teabag or a reusable infuser—have on the composition of brewed tea?

Figure 7.1 From left to right: a round teabag; a tetrahedral teabag; and a standard “flow-through” teabag. Image courtesy of Andrew DiDonato.

One of the big ideas of chemistry comes into play when we think about teabags and other infusers: that of proximity. How close can the tea leaves get to the water? A teabag is a barrier between the solvent—the hot water—and the tea leaves we are trying to extract, so it makes sense that it would have some effect on the resulting infusion. As we have seen in Chapter 5, the size of the leaves, whether whole, broken, or crushed nearly to the point of a powder, as in tea fannings, is one determinant of the speed and composition of the infusion. Another factor is  the exposure of individual leaves or particles to the solvent—their proximity. If the swelling leaves become tightly packed inside a teabag or infuser, then the solvent has less access to the leaves at the center. This suggests, of course, that the best teabag is no teabag at all. But convenience often trumps quality. For the same reason I often reheat cold tea in the microwave rather than make a fresh cup, I make tea with a reusable infuser. It is easier. But what does it do to my tea?

7.2.1 A Brief History of Teabags

If you drank tea today, chances are you used a disposable teabag to make an individual cup. Teabags are ubiquitous in the US and the UK. It is estimated that 75% of US tea-drinkers use teabags as their principal method of infusion. Less than 5% do as I do and turn to loose tea leaves as their first choice. In the context of the long history of tea, teabags and reusable infusers are relatively recent innovations, developed over the last century. For most of the millennia tea has been drunk, the leaves have been strained out using a lid on the tea pot or bowl, or a sieve, or not removed at all. Disposable teabags simplify every step of tea-making. They make it convenient to store, portion out, steep, and, finally, to tidily dispose of the used tea leaves.

At the risk of stating the obvious to any reader of this book, a teabag is a small porous bag either pre-filled with tea or empty and ready to fill with a tea of the infuser’s choice. A string is sometimes attached to make it easier to agitate the bag in the teacup and to remove it when the infusion is finished. By porous, I mean something that allows water and other molecules to pass through it, but prevents the passage of larger particles. I note that no material is entirely porous to every single molecule. This means that some materials used to make teabags may trap some of the compounds that we wish to find in the infused tea, altering its composition relative to something brewed with whole leaves unfiltered by the teabag material. Simultaneously, the bag must not become fragile when immersed in hot water because we want to remove the tea leaves at the end of the infusion time. And, of course, whatever the bag is made of ought not to add any flavor to the tea and should not leave behind any residues that are unpalatable† or unsafe for human consumption. The design of teabags therefore presents a surprising technological challenge.

Over the century and a half since the teabag first appeared, many materials have been used to make porous bags for infusing tea. Today, filter paper—bleached or unbleached—is perhaps the most common material used to make teabags, followed by various plastics and natural fabrics, mostly cotton. The story often told of the origin of the teabag is that in 1908 Thomas Sullivan, a tea importer in New York, sent samples of his teas to customers tied up in small silk bags. The customers found the bags useful as a makeshift infuser and so, the legend goes, the teabag was born. But teabags were clearly in use many years before Sullivan’s samples were sent out. In the mid-1880s, tea and coffee shops were using large muslin bags as infusers for pots of tea. A US patent for cotton cloth teabags was granted in 1903 to two women from Wisconsin, Roberta Lawson and Mary McLaren.¹ Their square bags made of coarsely woven cotton thread, with wire woven in as a stiffener, were intended to be used to make a single cup of tea, just as modern teabags are.

The 1930s and 1940s saw two developments in paper technology that contributed to the mass production of teabags and their widespread adoption, first in the US, then in the UK and worldwide. In 1936, the chemist Fay Osborne, inspired by the hand-made Japanese yoshino papers he had seen used as cigar wrappers, developed an inexpensive porous paper that would not shed lint. Soon after this, papers that could be heat-sealed appeared. In a process patented by William Hermanson in 1945, paper was impregnated with threads of cellulose acetate and vinyl that, when heated and pressed, created a seal. (Cellulose acetate was the first commercially produced plastic, made by taking wood pulp or cotton lint and treating it with glacial acetic acid.‡ ) An advantage of cellulose acetate over more modern plastics is that it degrades when composted. These sealed bags securely hold the tea leaves or particles inside the sachet during both shipping and steeping.

While the addition of plastic fibers to paper represented a step forward in the technology of teabags, the sachets can now be made entirely from plastic. The plastics can be made from petrochemicals (oil) or from plant-based sources, such as corn silk, and, as we shall see, can be adapted at the molecular level to better suit the steeping process.

7.2.2 Paper Teabags

If you have ever written with a marker on a piece of paper that subsequently got wet and noticed a rainbow of colors appear around the edges of the ink, you have inadvertently performed a chemical separation of the various components of the ink. Chemists call this process of separation chromatography because it was first used to separate different colored pigments.§ Different chemical compounds are absorbed differently by the paper. Compounds that are absorbed more strongly by the paper are carried along the water moving through the paper more slowly than those that have less affinity for the paper. When you use a paper or cloth teabag to infuse a cup of tea, you are perforce also doing a chromatographic separation, whether you want to or not. Some of the compounds in the tea will be carried through the material of the teabag more quickly than others, some so slowly that little will be present in the infusion when the teabag is removed. This will change the ultimate chemical composition of the infusion and, of course, its flavor and aroma.

We can use what we have already learned about the structures of the molecules in a tea infusion, along with an understanding of the underlying molecular structure of paper, to see how a paper teabag might affect a cup of tea. Paper is made from cellulose, which is a biopolymer (Figure 7.2) constructed from hundreds or thousands of sugar units, specifically glucose. It can be extracted from a wide variety of sources and is the most abundant biopolymer on Earth. Cellulose is found in wood, linen, hemp, and cotton— all of these sources have been used to make paper. We can see from the structure of cellulose, with its many hydroxyl (–OH) groups that it will more strongly absorb polar substances than non-polar substances.

Figure 7.2 A single unit of cellulose. The structure in the square brackets is repeated over and over again to make the polymer. The small “n” at the lower right of the brackets indicates that this is a repeating motif.

For many of us, the most critical question about teabags might be whether they impede the transfer of caffeine from the tea leaves to the tea infusion. As might be expected, steeping tea using a teabag slows the movement of the polar caffeine molecules through the paper. Michael Spiro and Deogratias Jaganyi have shown that the rate of caffeine infusion decreases by about 30% when a teabag is used compared with loose-leaf tea.2,3 They found that the total amount of caffeine in the infusion after about half an hour of steeping is the same, although by that point the tea is likely undrinkable and bitter. Agitation of the bag while brewing will speed the transfer, another argument for dipping and squeezing that teabag. Compared with loose-leaf tea, tea made using a teabag will have lower amounts of polyphenols and other antioxidants.⁴ Again, the structure of these molecules, which include many polar groups, suggests that they will bind to the cellulose in the paper, which will hamper their transfer into the infusion.

The microscopic structure of the material a teabag is made from also matters because molecules can move directly through the pores in the material and avoid being bound to the fibers of the material making up the teabag. Researchers at the Institute of Chemical Technology in Mumbai explored the effects of different materials used for teabags on the infusion of tea.⁵ Scanning electron microscopy images of teabags made from cellulose papers and polylactide (a synthetic plant-based material) showed a tangled web of fibers, whereas those made from woven nylon displayed a more regular grid. The woven nylon sachets were the most porous overall. On average, the pores in cellulose paper were about 40 µm in diameter (about half the width of a human hair), whereas the pores in woven nylon were four times that size. While tea infused slightly more quickly through the nylon bags, the difference between nylon and cellulose was minimal.

In 2010, Lisa Ryan and Sébastien Petit of Oxford Brookes University showed that using a teabag reduces the amount of antioxidants available in the infusion relative to using the same leaves without a bag.⁶ It also takes longer for antioxidant levels to reach their maximum—by a factor of at least three.

7.2.3 Plastic Teabags

The modern world relies on plastics to such an extent that the appearance of plastics in rock strata clearly mark them as laid down in the last century. Plastics are polymer-based materials that are, well, plastic—literally easily molded. Typically, they have low melting points and so can be easily softened in order to shape them. Once a plastic sets it can be hard, like the keys on my computer keyboard, made from a polymer called acrylonitrile butadiene styrene, or pliable, like my nylon tea strainer. Plastics are generally resistant to water because they are built on a hydrophobic skeleton, a carbon–hydrogen framework that provides no purchase for the polar water molecules to grab onto. While we think of plastics as a modern human invention, there are many naturally occurring plastics—for example, latex is a plastic produced by plants.

Polymers are built from units called monomers, which repeat over and over again. One of the simplest and most common synthetic polymers is polyethylene. It is built from the monomer ethylene, CH₂CH₂. Other common polymers encountered in teaware include polystyrene, which is used to make cups, and polyethylene terephthalate (PET). One appeal of these hydrocarbon polymers for chemists is their tunability. Chemists can change the behavior of the polymer by hanging different groups from the carbon framework. This is another way in which one of our big ideas, the relationship between molecular structure and function, plays out. Chemists can also alter the behavior of a polymer by linking strands of the polymer to each other, a process called cross-linking. A number of synthetic polymer structures are shown in Figure 7.3.

Figure 7.3Molecular structures of the monomers of polyethylene, nylon 6 (one variation of nylon), polystyrene, and polyethylene terephthalate (PET).

Plastics crumble into smaller and smaller pieces as a result of mechanical wear or exposure to ultraviolet light, producing what are called microplastics, defined as particles of plastics and polymers that are less than 5 mm in diameter. Environmental microplastic contamination is an area of growing concern and the ability of plastic teabags to shed microplastics is well documented. The study of the effects of microplastics on humans is in its early days in terms of the amounts that we ingest, the routes through which we might ingest them, and, ultimately, what health risks they might present.

While the plastic used to make a teabag might seem to be at first glance inert—that is, unreactive with the things it is coming into contact with—it can shed small pieces of the polymer into a tea infusion. Analytical chemists have also shown that plasticizers, chemicals used in the production of polymers and plastics, can be present in tea infusions.⁷ Since the health risks of microplastics may be associated with the size and shape of the particles, whether the teabag is made from a fossil fuel source or from a plant-based source may be immaterial to the risk these particles present. While some critics of plastic teabags have suggested that they somehow dissolve in hot tea, this is not the case. The production of microplastics has more to do with mechanical forces because tiny bits of plastic break off as objects rub against each other during packing and transport.

Microplastics end up in tea not just from the utensils that we use to brew it, such as teabags and infusers, or from the plastic cups that we might serve it in, but also because they can become incorporated into the tea from the very start. Microplastics find their way into tea leaves as a result of the many plastics used in their cultivation, as well as during storage and processing. Plastic sheeting is used as a ground cover, plastic tools are used to cultivate the plants, and agricultural workers leave behind traces of plastic packaging from personal items. Microplastics are also ubiquitous in the broader environment, in water as well as in the atmosphere and soil. Since some tea is initially processed by being spread out in the open air to expose it to sunlight, it is particularly susceptible to contamination of all sorts from atmospheric sources. The origins of micro-sized plastics and polymers in the air include fragments of automobile tires, which sounds unappetizing. (To be fair, there are also gem-like micrometeorites sifting down onto the tea from outer space,¶ although at a lower concentration, they likely pose similar risks as the microplastics.)

7.2.4 Spent Teabags

Teabags are a convenient way of disposing of spent tea leaves. Pull up the bag and toss it in the trash. But, given the amount of tea drunk and the percentage of tea that is prepared using a teabag, the environmental burden of disposing of the bags must not be ignored. Each of the different materials exacts its own environmental penalty. Some are more biodegradable than others. Tea sachets made from cellulose paper can be composted right along with the tea leaves, although these bags generally contain at least some plastic. Nylon teabags will not break down, while bags made from silk-like polylactic acid will, but only if industrially composted.

Another consideration is where the used teabags go after disposal. If they end up in landfills, then they decompose into methane, which is a more potent greenhouse gas by a factor of nine than the carbon dioxide produced during composting. Composting teabags and their contents is therefore a more environmentally sound method of disposal. The bags and their contents can also be separated so that a non-compostable bag can go into the trash, while the leaves are composted, although this obviously reduces their convenience.

7.2.5 Size is What Matters

The shape of the teabag has also changed over time, driven in part by the availability of materials that allowed more flexibility in shape, particularly the heat-sealable papers. Purse-shaped muslin bags gave way to the iconic rectangular-shaped bags. In the 1950s, the Lipton tea company introduced the Flo-Thru teabag, essentially two teabags connected by a paper bridge, thereby increasing the surface area exposed to the hot water. The round teabag was first mass marketed by Tetley, a more aesthetic choice, but perhaps not one that was any more functional than the rectangular bags that preceded it. Tea socks are large rectangular versions of teabags that can be filled with the tea of your choice.

The effect of teabag shape on the resulting infusion is not merely an academic question, but one that has led to a dispute between tea manufacturers over questionable advertising. In 2014, the UK’s Advertising Standards Authority ruled that a company was not misleading customers when it showed an advertisement in which they claimed their pyramidal teabag had better “brewing power” than a flat, round teabag. Data they submitted to the Advertising Standards Authority suggested that a more rapid infusion was obtained with pyramidal teabags. So, what do we know about the effect of the shape—and the size—of a teabag on the resulting infusion?

Jaganyi and Ndlovu quantitatively explored the effect of the size and shape of a teabag on the transfer of caffeine to the infusion.⁹ They found that increasing the size of a rectangular teabag from 16 to 36 cm² increased the rate of caffeine transfer by 25%. Making the teabag even larger made the transfer of caffeine slightly more efficient, although at some point increasing the size ceased to have any effect. Changing the shape of the teabag, from square to rectangular to round, all while holding the surface area constant, did not affect the rate at which caffeine was extracted. As long as the tea has enough room to expand and move freely within the bag, caffeine extraction will be as efficient from one shape to another. Pyramidal bags often offer the leaves significantly more empty space relative to a flat bag and so will be more effective. Similarly, the folded “flow-through” design effectively doubles the size of the bag, which offers more rapid caffeine transfer.

Because tea expands by factor of more than four in volume when exposed to hot water, it is vital to avoid packing too much tea into a bag regardless of its shape. In 1886, more than a century before Jaganyi and Ndlovu’s work, users of muslin teabags were being cautioned about overfilling the sachet. Like teabags, infusers need to be larger than you think, as we shall see in the next section.

7.2.6 Infusers: Balls and Baskets

A reusable tea infuser—essentially a perforated container—rather than a disposable teabag is a standard part of my tea-making  routine. I have infusers that are sized for an individual cup as well as those more suitable for a teapot (Figure 7.4). Tea-drinkers use infusers rather than teabags for a variety of reasons.  They may wish to avoid using disposable items to lessen their impact on the environment. They can be more economical to use. They may want to use their own tea, rather than be confined to the more limited selections available in pre-filled bags, or they may wish to make a pot of tea to share.

Figure 7.4 From top left to bottom: silicone refillable teabag-shaped infuser; silicone owl infuser; spring-loaded tea ball; silicone and stainless- steel flower infuser; silicone submarine infuser; tea stick; large stainless-steel laser-perforated tea basket; silver woven tea basket; nylon tea basket; glass tea basket; and silver tea strainer. Image courtesy of Andrew DiDonato.

Early 20th century reusable infusers looked like lidded scoops or spoons, closely resembling the spring-loaded tea balls in common use today. Many styles of infusers are available, from the sleek and modern to the whimsical to the very practical. There are two main types: baskets, which have an open top and are suspended at the top of a cup or pot, and tea balls, which, like teabags, are closed and fully submersible. Some tea balls have a small chain attached to facilitate agitation during steeping and easy removal of the ball at the end. Some tea baskets are small enough to easily stir the infusion, again making it easy to agitate the tea as it brews.

In choosing a reusable infuser, both the material of which it is made as well as its size will affect the final brew. As with teabags, the material should not impart any flavor to the infusion, nor should it leave behind potentially undesirable compounds. Infusers made of nylon or polyethylene or other plastics will suffer from the same issues that disposable teabags made of these materials do. While these materials are essentially inert, they will also shed microscopic particles of plastic. Glass or stainless steel are extremely durable and will not react with the materials found in tea. Perhaps more importantly, unlike plastic infusers, they will not leave unwanted compounds or microscopic particles in the infusion. By contrast, the perforations in glass infusers are often quite large, allowing smaller particles of the tea to be left in the infusion. Stainless steel is lightweight and can be finely perforated, making it possible to keep all but the smallest particles of tea out of the infusion.

As an alternative to using carbon-based plastics, silicone materials offer a number of advantages. Silicones are polymers based on a silica and oxygen framework rather than the carbon framework that polyethylenes and related plastics are based on. (Silicone should not be confused with silicon. Silicon is a crystalline chemical element found directly under carbon on the periodic table. It is what is used for making chips for electronics.) Like the carbon-based plastics, silicones repel water and are chemically unreactive. Silicone is stable over a wider range of temperatures and is generally more durable than many carbon-based polymers. Unlike stainless steel and glass infusers, the silicone versions remain cool to the touch because silicone doesn’t transfer heat nearly as well as metals or glass. Perhaps silicone’s biggest advantage is that it will not shed microplastic particles.

Size matters. To maximize the exposure of the tea leaves to the solvent in a tea infusion, it is important to avoid confining the leaves so that they pack tightly, limiting the amount of water that can get into the center. Tea-drinkers have long been chary of overpacking the leaves into a teabag or infuser. In her 1915 book The Science of Homemaking, Emma Elizabeth Piere makes the point that a tea ball should never be filled more than two-thirds full, but even that is likely to be too full to allow the water to freely circulate around the leaves in the center of the ball. Since the dried leaves expand by more than a factor of four when immersed in hot water (Figure 7.5), ideally a tea infuser should be filled no more than one-quarter full.

Figure 7.5 The swelling of the tea leaves is apparent when the dampened teabags are compared with those in . Image courtesy of Andrew DiDonato.

How much tea can you put in some of these smaller infusers? I measured the volumes of a number of different infusers, including the popular small spring-loaded tea balls, and estimated the amount of tea that can be made with them. To do so, I modified a procedure used by Guy Consolmagno SJ of the Vatican Observatory to measure the volume of meteorites.‖ Consolmagno used small glass beads to measure the volume occupied by meteorites, inspired by the coarse-grained sugar used to sweeten coffee (and tea) in Italy.¹⁰ The usual method for determining the volume of an object is to immerse it in water, or another liquid such as oil, and measure the total volume displaced. For obvious reasons, we do not want to get the one-of-a-kind meteorite samples wet or oily (many of them contain iron that will rust). The beads will not damage the meteorite samples, but they are also well-suited to my purposes: unlike water, they won’t leak out of the perforations. The results are shown in Table 7.1.

Table 7.1 Interior volumes of tea infusers determined using method of Consolmagno & Britt.¹⁰ The volume of dry tea is the interior volume divided by 4.5. Each 1 mL of dry tea makes 35.5 mL of infused tea

Infuser | Interior volume (mL) | Volume of dry tea (mL) | Volume of prepared tea (mL)
Oxo | 200 | 44 | 1577
Nylon basket | 145 | 32 | 1143
Flower | 47 | 10 | 371
Spring-loaded tea ball | 27 | 6 | 213
Owl | 26 | 6 | 208
Teabag | 24 | 5 | 189
Yellow submarine | 22 | 5 | 173
Tea stick | 13 | 3 | 105

The standard recommendation is to use one teaspoon, or 5 mL, of whole dried tea leaves for every 175 mL of water. The sleek modern tea stick holds less than a teaspoon of dry whole leaves when leaving enough room for expansion, only enough to make 100 mL of tea. (A mug of tea contains about 350 mL.) The spring-loaded tea ball, while well-perforated as it is made of mesh, holds a scant 6 mL of dry tea leaves and is really only suited for use in a teacup, not a mug. Most of the whimsical tea balls have a limited number of perforations, which makes it difficult for the water to circulate through the tea leaves, as well as small volumes. They might be lovely conversation starters on my shelf, but they will not give me the best cup of tea. While the caffeine content might not be overly reduced, the extraction of the aromatic compounds that give tea its prized floral scent will be slowed and therefore be limited. By contrast, the large, well-perforated workaday tea baskets are sized to use between 30 and 50 mL of dry tea leaves, plenty for even a pot of tea.

7.3 Objects of Desire: What Do I Want in a Teapot?

I have drunk more than 400 cups of tea while sitting at my keyboard to write this book (yes, I kept track) and many more while researching it. Unsurprisingly, I have many teacups I can use. I have a modern and a traditional teacup and saucer from the Imperial Porcelain Factory in St Petersburg, Russia, both decorated with the classic cobalt blue and gold. I have a hand-made Japanese-style scholar’s mug, fired in a wood kiln at a Benedictine abbey. I have cups made of ceramics, both porcelain and stoneware, glass teacups, cast iron tea bowls, and stainless-steel mugs. The mug I pulled from the shelf most often while writing this book is a white porcelain mug with a classic-looking (but transgressive) blue Delftware pattern on it. I own tiny cups from Japan and a capacious mug acquired in Copenhagen. More than a dozen teapots likewise sit on shelves at home and my office; a selection are shown in Figure 7.6. What makes a good teacup or teapot from a chemist’s—and perhaps an engineer’s—perspective?

Figure 7.6 A selection of teapots; note the near-spherical shape of most. From top left to bottom right: cast iron tetsukyūsu; Yixing-style teapot—note the unglazed interior visible on the lid; ceramic teapot— note the cover is lined with felt, which serves as a built-in tea cozy; double-walled glass pot designed for coffee—note the tall, slim shape; beehive porcelain teapot; and almost perfectly spherical cast iron tetsukyūsu. Image courtesy of Andrew DiDonato.

Since one of our big ideas is that the underlying atomic and molecular structure of a material dictate its properties, understanding what properties are desirable in a teapot or teacup will help us choose the appropriate materials. A good teacup or teapot should be impermeable—it shouldn’t stain or absorb flavors.** Likewise, it should be chemically inert—that is, nothing in the teacup should react with any of the compounds found in tea. Given the thousands of different molecules found in tea, this sounds like a tall order. But because similar structures will have similar reactivities, we really only have to consider the families of molecules rather than each type of molecule individually. For everyday use we would prefer that the material be not too fragile and it should be resistant to thermal shock. In other words, it ought not to crack when you pour a hot liquid into it. Travel cups with lids to prevent spills or leaks are practical, but can rob you of much the sensory experience of the aroma arising in the steam from the cup.

Heat retention is critical. If a teapot allows the water within to cool too quickly, then this will affect the composition of the infusion. How well the teacup insulates the tea, keeping it hot enough to drink, must also be considered. The shape of a teapot matters. Geometers know that for a given volume of material, the shape that exposes the least amount of surface area is a sphere, while tall thin cylinders of the same volume expose substantially more surface area. Tall cylindrical teacups expose more surface area to the tea and will cool more rapidly. At the risk of giving US readers an ear-worm, the 1939 children’s Tea Pot Song†† is spot on: short, stout cups and pots are preferable. Ceramic coffee pots are often distinguished from teapots by being taller and more cylindrical. This is because these coffee pots are used solely as serving containers, in contrast with teapots, which are used to prepare the tea as well as serve it. Temperature is critical for the extraction of both coffee and tea and the surface area of a container exposed to a liquid affects the cooling of the contents. Keeping the tea hot as it steeps is more easily accomplished in a short, stout pot that most closely approximates a sphere, a shape which has the smallest surface area for a given volume.‡‡

Of course, personal preferences as well as aesthetic and cultural considerations, rather than material science will drive some choices. When it comes to volume, a cup should not hold so much tea that it cannot be drunk before it cools to an unacceptable temperature (whatever you consider that to be), nor so little that it must be refilled multiple times from a teapot. In her 1896 book on social etiquette, Maud Cooke described the ideal teacup: “cups should hold more than an actual thimbleful, though they need not hold a pint, and should bear some relation to the laws of gravitation in their poise upon the saucer.” The Classic of Tea suggests that tea bowls should hold around 300 mL of tea (one half-sheng), roughly the size of modern mugs.

While we tend to think of the sensory experience of drinking tea as being primarily one of taste and smell, sight, sound, and touch all have parts to play as well. The translucency of the material, how much light passes through it, is a purely aesthetic choice, as is color. The tea in my Imperial Porcelain teacup is delightfully brightened by the light that passes through the thin porcelain, making my favorite Assam tea glow like rubies and bringing out the grass-green of green snail tea. It can be difficult to determine the strength of a brewed cup of tea in a dark-colored cup, although I would admit I appreciate that tea stains don’t show on my deep black mug. Blue–green cups have long been prized because they deepen the color of green teas. The 8th century tea master Lu Yu thought both pure white and yellow teacups were poor choices for black teas.

Sound may be the most subtle of the senses activated in the context of drinking tea. My mother, who lost much of her sight in midlife, used the changing pitch of a liquid being poured into a container to know how full it was. As a container fills, the pitch of the sound rises.§§ This is an example of resonance, the ways in which materials respond to particular frequencies of sound. Different materials and different sizes and shapes of containers will have different resonant frequencies. These resonances can be evoked when tea is poured into a cup or when a spoon tinks against the side of a cup. I even appreciate the particular resounding thunk when I set a favorite mug on my desk, which tells me without looking that the laws of gravitation are working in my favor and my cup is not poised to tip over.

We also need to consider touch, particularly the texture and the ability to transfer heat to one’s hand. In a cold winter kitchen, a warm mug in hand is a delight, while a glass mug might be too hot to handle. The silicone sleeve on a favorite cup provides not only a bit of insulation, but also some texture to improve my grip. The lip of the mug or cup should be smooth. Maud Cooke despaired of teacups with fluted edges and I find the texture of paper or Styrofoam cups to be particularly off-putting.

7.3.1 Rock, Paper, Scissors

What do all these material considerations tell us about what solid materials we should use to make teacups and teapots? Porcelain or stoneware? Glass or stainless steel? Like the kid’s rock, paper, scissors,¶¶ game, each has advantages and disadvantages. Let’s start with a look into what a solid looks like to a chemist and, in particular, at the types of solids used to create teaware.

Solids are one of the three classic phases of matter that many of us learn about early in our schooling. (The other two are gases and liquids; plasmas‖‖ are also a phase.) But when material scientists think about solids, they recognize that solids come in many forms, each of which constitutes its own phase. We say that ice is solid water, but, in fact, there are more than 20 different solid phases of water, including water IX made (in)famous**** in Kurt Vonnegut’s book Cat’s Cradle,¹² and only later discovered by physicists.¹³ The different phases are distinguished by different arrangements of these molecules within the solid. Imagine these as essentially different seating arrangements for a lecture. The audience could be seated in rows of chairs, or grouped around round tables. As you can see in Figure 7.7, water (a) has its molecules randomly oriented, while the molecules in water (b) are arranged in a regular hexagonal pattern.

Figure 7.7 (a) Representation of amorphous solid water. There is no overall pattern to the arrangement. (b) Representation of one plane of hexagonally packed solid water (such as ice XI). The waters are linked to each other by hydrogen bonds. The O–H moieties are not hydroxides, but indicate water in which one of the hydrogens is pointing above or below the plane of the paper, linking the planes together.

Material scientists call phases of solids where the molecules are oriented randomly amorphous. Crystalline phases of matter have molecules or ions arranged neatly in a repeating pattern. Crystalline solids can be single crystals, where the pattern repeats over the entire sample, or polycrystalline solids where different zones have different patterns. Glasses are a type of amorphous solid. Many plastics, including polyethylene, are also amorphous solids. Ceramics, such as porcelain or stoneware, are typically polycrystalline solids, as are many minerals. Perhaps surprisingly—since they don’t look like a typical mineral crystal— pure metals, such as copper, are most often crystalline at the atomic level, as are many alloys, including stainless steel.

7.3.2 Rock

Most teacups and teapots, whether a 21st century mug or a 10th century tea bowl, are made of ceramics. What is a ceramic? One way to look at ceramics is that they are essentially synthetic rocks. You take a soft, inorganic (by which chemists means something that is not principally made of carbon and hydrogen) material, shape as desired, and then expose it to very high temperatures in a fire or kiln. The resulting object is hard and chemically inert—it won’t react with many substances. Like a rock. Importantly for tea-drinkers, ceramics can be made impermeable to water. Ceramic technology is incredibly old and humans have been making pottery for more than 25 000 years. (It is also a very modern technology; some superconducting materials are ceramics.)

Ceramics can be made impermeable to water either by glazing or by a high-temperature process called vitrification—which literally means turning them to glass. Porcelain is a vitrified ceramic, while stoneware is not always so and often needs a glaze to prevent water from seeping through.

Household ceramics are typically made from clays, natural materials that are easily molded and shaped when wet, but hard when dry. Clays are composed of tiny crystals of minerals called kaolin clays, which contain aluminum, silicon, and oxygen. The minerals form in layers, with sheets of silica (silicon oxide) alternating with sheets of alumina (aluminum oxide). The sheets are linked to each other through oxygen atoms. When a kaolin clay gets wet, polar water molecules insinuate themselves between the layers. The hydrogen bonds that form between the water molecules and the oxygen atoms in the kaolin clay are strong enough to help the clay keep its shape, but flexible enough to allow the layers to slip past each other as the clay is pushed and pulled into shape. Firing a clay object at high temperatures drives off the water between the layers. Since the layers can no longer move past each other, the object becomes locked into whatever shape it has been given. It has turned back to stone.

Depending on the composition of the clay that is used and the temperature at which it is fired, different types of ceramics can be made. Modern stoneware is heated to between 1000 and  1300 °C. Pottery can be fired at lower temperatures, but the resulting earthenware is often softer and water permeable unless glazed. Think terracotta flowerpots. Porcelain is also fired at very high temperatures, sometimes higher than those used for stoneware. Porcelain is primarily distinguished from stoneware by its composition. In addition to the kaolin clay used in stoneware, minerals such as alabaster or quartz are added to the mix. Bone china actually has bone ash mixed in with the clay and other minerals. This results in a ceramic that is highly translucent and stronger than stoneware. The formation of needlelike mineral crystals within the porcelain are part of what gives it its strength.

One disadvantage to ceramic materials is their fragility. They can be very brittle and shatter when dropped. The polycrystalline nature of ceramics underlies their brittleness. Imagine trying to escape from a building where every level has a different floor plan and a different location for the stairs. Because there are so many different zones, with different ways to stack the atoms, there is no easy way for stress to move along a single plane through the material, what material scientists call “slip.” The stress has no way to escape through the material and it simply snaps. Ceramics, particularly porcelain, are resonant materials, in large part because of their brittleness. The sounds of tea gurgling within a pot, or of a spoon stirring a teacup, are generally pleasing as  a result.

Material scientists think about the ability of a material to retain heat in terms of its thermal conductivity—how much energy it can move through a given thickness of material at a given temperature—and its heat capacity—how much energy it takes to change the temperature of a particular amount of the material. When you pour something hot into a container with a high thermal conductivity, the heat transferred from the liquid inside it will make the cup hot as well, perhaps even too hot to touch. Ceramics are far less able to conduct heat than metals, by about a factor of 100, making them a good choice for teacups and teapots.

Heat capacity, which depends on the density of a material (among other more subtle factors) also plays a part. The ceramics used to make mugs include many small air pockets, making them a bit like a puffer jacket for the tea inside. We will see later in this chapter why that helps keep a cup warmer.

It is clear why ceramics have been used for so many centuries to make teaware—ceramics offer many of the qualities we expect in a teacup or teapot. Flexibility of shape and size, as well as the ability to glaze in an infinite array of colors, opens many aesthetic possibilities. And, like rocks, ceramics trump metal when it comes to many of the features desirable in teaware.

7.3.3 Glass

Glass and ceramics have similar properties at the macroscopic level. They are both brittle and they can be fragile. Despite their tendency to shatter when dropped, both glass and ceramics are durable, lasting centuries, as archaeologists can attest. Both glass and ceramics are impermeable to water and inert to many substances. They have roughly the same thermal conductivity. Yet glasses are quite different from ceramics at the molecular and atomic level. Rather than the patchwork of crystalline patterns that ceramics exhibit, glasses have no underlying structure—they are literally amorphous, lacking a form.

To a material scientist, the term glass encompasses a wide variety of amorphous solid††† materials, not just the transparent material commonly used to make drinking vessels. The everyday glass we use is made from sand, which is essentially silica (SiO₂). Most of the glass we use is soda-lime glass, silica to which sodium carbonate (washing soda) and other alkaline minerals, including lime (CaO) have been added to reduce the temperature at which the glass forms.

While transparent and easily formed, soda-lime glass is susceptible to thermal shock, which can lead it to fracture when exposed to a hot or cold liquid. Laboratory glass (borosilicate glass) is made from silica to which boron trioxide has been added. Borosilicate glasses are far more resistant to thermal shock, making them useful not only in the laboratory, but also in the kitchen.‡‡‡

7.3.4 Scissors (Metal)

Not every metal will be suitable for making teaware. Some will react with the compounds in tea, changing the taste or, in the case of lead, creating toxic compounds. Although we may cringe today, there are early Roman recipes that specifically call for the use of lead pots to cook in rather than bronze (a copper alloy). Some of the compounds that form when lead is used taste sweet—for example, lead acetate is also called sugar of lead—while copper compounds are less palatable. (The lead compounds are, of course, toxic.) The metal should have a melting point well above the boiling point of water. This last point may seem redundant. But while visions of furnaces and red-hot liquid iron are what come to mind when we think about the melting points of metals, not every metal melts at such high temperatures. Using a spoon made from gallium to stir your tea will result in a melted spoon and a small puddle of liquid gallium in the bottom of your cup.

Cast iron has been used to make teapots in Japan since roughly the middle of the 17th century. Called tetsukyūsu, these enamel- lined iron pots are used to steep tea leaves. Tetsubin, which are not lined with an enamel coating, are used to heat water for tea. Like pottery, the making of cast iron objects has a long history. Cast iron is not pure iron, but an iron alloy, often made by incorporating carbon and silicon. Cast iron is surprisingly brittle for a metal, for much the same reason that ceramics are brittle. Pockets of Fe₃C form throughout the cast iron and impede the distribution of stress through the material. It has a higher thermal conductivity than either stainless steel or ceramics.

Stainless steel—an alloy of iron with chromium—is also used to make teapots, tea kettles, and even teacups. Unlike ceramics or glasses, these materials are uniform throughout in terms of their atomic structure. This makes them far less brittle—in fact, metals are characteristically malleable. Some metals are so malleable that they would be unsuited to making teaware. The chemistry department at Bryn Mawr has a pure gold crucible, about the size of a cereal bowl. The crucible, about as thick as a sheet of cardboard, is so soft that I can deform it with my bare hands. I imagine I could crush it like a piece of paper, although undoubtedly my colleagues would be upset with me if I did so.

7.3.5 Paper

Practical considerations also come into play. If I get a cup of tea as a takeaway, when I’m done drinking it I don’t want to have to cart the cup around with me for the rest of the day. I prefer to be able to toss it away in a trash bin or, even better, to recycle or compost it. Enter the paper cup. Although we would probably consider paper cups to be a 20th century phenomenon, there is some evidence that they were used to serve tea in Imperial China. The modern paper cup was developed in the early part of the 20th century in response to the hygienic concerns of the period, replacing the shared cups for drinking water found in schools and other public areas.

To make the cups waterproof, they were originally coated with a thin layer of wax. Waxes are a broad class of materials, formed primarily from carbon and hydrogen. These non-polar substances do not interact strongly with the polar water molecules, keeping the water from wetting the paper below. Modern paper cups use polyethylene, a polymer made up of carbon and hydrogen alone, as a coating. Like waxes, polyethylene is hydrophobic, repelling the water molecules and keeping the paper underneath dry. Double-walled paper cups wrap the tea in an insulating blanket of air, keeping it warm on a walk from the campus cafe to  my desk.

7.4 Getting Into Hot Water

To make tea, you need hot water. To heat water up requires energy. The first law of thermodynamics says that energy can be neither created nor destroyed, just moved around. (Unless, of course, you are doing nuclear fission, in which case Einstein’s famous equation E = mc² comes into play.§§§ ) Where can we find the energy needed to make a cup of hot tea? And just how much energy do we need?

7.4.1 Calorie Counts

Whatever the source of energy, whether it comes from eating food or from electricity or chemical reactions or cellular processes, scientists measure it in units called joules. The standard abbreviation for a joule is J. An older unit, called the calorie, is still in use in some parts of the world, including the US. A joule is about a quarter of a calorie.

If you turn over a candy bar, the nutritional information will tell you how many thousands of joules (kJ) of energy your body can extract by eating it. The calories given on food packages in the US are nutritional calories, each equal to a thousand of scientists’ calories, or a kilocalorie (kcal). Nutritionists will say that there are 16 calories in one teaspoon of sugar, but a chemist would say there are 16 kcal of energy. If this sounds awkward and confusing, it is. I will stick to joules, despite my US citizenship.

Heating 1 g of water by 1 °C requires about 4 J¶¶¶ of energy (or one of the old-style calories). Different materials respond to heat differently depending on their underlying molecular and atomic structure. It takes 4000 times as much energy to heat a liter of water up by 1 °C than it does to heat a liter of air in a room. Gases are much less dense than liquids, so there are fewer molecules to take up the energy in air than in water. This is part of why water is so good at cooling things down. Solid materials, such as steel or glass, can soak up roughly the same amount of heat as the same volume of water.

7.4.2 Burning Questions

For centuries, water was heated by placing it over a fire. Combustion, the burning of a fuel in the presence of oxygen, provided the energy. Most of the fuels were hydrocarbon fuels, such as wood or peat. Charcoal, essentially pure carbon, or coal, was also used. Fossil fuels, such as propane or methane, are modern options. When any hydrocarbon is burned under optimal conditions it produces only carbon dioxide and water—and energy. Where exactly does that energy come from?

At the molecular level, combustion energy comes from the making and breaking of bonds. It costs energy to break a bond, but energy is released when bonds are made. If more energy is released than is used in a reaction, it will produce energy, usually in the form of heat. Let’s take a look at the energy balance when you burn propane. Propane is a gas at room temperature and pressure with the formula C₃H₈. We can look at its structure (Figure 7.8) and see that it has two carbon–carbon single bonds and eight carbon–hydrogen single bonds. When propane burns the reaction is

C₃H8+5O₂→3CO₂+4H₂O

Figure 7.8 Molecular structure of propane showing all the C–H bonds.

One way to read this reaction is as one molecule of propane reacting with five molecules of oxygen to produce three molecules of carbon dioxide and four molecules of water. But since molecules are so tiny, chemists prefer to work in the chemical version of dozens: moles. However, instead of packaging up 12 molecules at a time, we bunch up 6.02 × 10²³ molecules to make a mole. (This is how many molecules are in just over a tablespoon of water or roughly the same number of stars there are in the universe.) It is a lot of molecules! The chemical recipe easily scales up from molecules to moles: one mole of propane reacts with five moles of oxygen to produce three moles of carbon dioxide and four moles of water.

The first step in figuring out the energy released in this combustion reaction is to compute how much energy is required to break down the propane and oxygen into their basic building blocks: atoms.‖‖‖ To do this, you must first break two moles of carbon–carbon single bonds and eight moles of carbon–hydrogen single bonds. It costs 350 kJ to break each mole of carbon–carbon bonds and 420 kJ for each mole of carbon–hydrogen bonds. You also have to dismantle those five moles of oxygen, which costs 500 kJ for each mole of oxygen that reacts. In total, dismantling the mole of propane and the five moles of oxygen into atoms requires just over 6500 kJ of energy.

Reassembling the atoms into carbon dioxide and water will return some energy. Each mole of carbon–oxygen double bonds gives us 800 kJ back, while each mole of hydrogen–oxygen bonds returns 460 kJ. In total, we release about 8500 kJ from creating three moles of carbon dioxide and four moles of water from the atoms we got from the propane and oxygen. Since more energy is given off than we used to break up the molecules, for every mole of propane that we burn we net 2000****** kJ of energy, which we can use to heat the water for tea—and, of course, the kettle that contains it.

Perhaps to the despair of chemists, most people don’t measure quantities in moles, but rather in mass or volume, as kilograms or liters (or, in the US, pounds and gallons). To get a sense of how much energy that 2000 kJ is, consider a propane tank of the size used for a backyard grill. It holds about 9 kg (20 pounds or 200 moles) of propane. Burning that much propane will produce 400 million joules of energy, compared with about 100 million joules of energy to meet the electrical needs of the average US house for one day.

We know that it takes 4.184 J to raise the temperature of 1 g of water 1 °C. So, to heat 250 mL of water for a mug of tea from 20 °C to the boiling point of 100 °C requires about 80 kJ. It may seem obvious, but to heat the water it must be in a container, which is being heated as well. My stainless-steel tea kettle weighs 1400 g. It takes about 60 kJ to heat that up to 100 °C. (And, of course, I’m spilling energy to the room as a whole, as well as heating up the grate on the stove, but to a first approximation the heat of combustion goes to heating the water and the kettle.) In total, I need 140 kJ, or what I would get from burning about 3 g of propane.

7.4.3 The Nuclear Option

One winter I spent 30 days in silence in a monastic retreat house on the Atlantic coast of the US. Coffee was available from five in the morning thanks to an early rising priest, but tea-drinkers were left to their own devices, literally. I and the woman in the room next door to me had, unbeknownst to each other, brought our own electric kettles to make our morning cup of tea. Our only other option was the microwave oven in the dining room, which, as we learned in Chapter 5, often leads to the development of scum on your tea. As monastic as we were being, that was one sacrifice neither of us was willing to make. Or perhaps we were practicing patience by using the slower-to-boil electric kettles?

Still, water boiled in a microwave is better than nothing at all and certainly better than the lukewarm water set out in a thermos alongside the coffee in many places. And I freely admit to putting up with the development of scum on my tea to reheat a cup grown cold. How does a microwave gather up energy and push it into your tea and apparently nowhere else? It is another example of the relationship between structure and function.

We often say when we heat something up in the microwave that we are going to “nuke it.” Even though a microwave oven has nothing to do with nuclear fission (or fusion), it does have something to do with the nuclei of the molecules in the food. The structure of the molecules—that is, the location of their nuclei—is what makes them heat up when exposed to microwave radiation. Recall that heat is really the result of molecular motion: the faster molecules are moving, the hotter the material. Polar molecules can be made to tumble faster when they are exposed to microwaves.†††† Water, as we have seen, is polar and most food contains water, while the containers do not. As the polar molecules tumble faster and faster, they heat up and that heat is then transferred to the rest of the food. Sugars are also polar and will also heat up in the microwave.

7.4.4 Climate Change

Living where and when I do, it is tempting to treat energy as an infinite resource. I turn on the stove, hit the start button on the microwave, plug in the kettle in my office and energy flows. Soon I will have a hot cup of tea. But I’m reminded of a visit over a decade ago to Kamikatsu, a small town on the island of Shikoku in Japan. Kamikatsu has been working toward being a zero-waste village, generating no trash. Atsuko Watanabe, one of the leaders of the movement, served me tea one afternoon and I noted that she was careful to measure the water so as not to heat more than she needed for the pot and waste energy.

I think of Atsuko every time I make a pot of tea in my own kitchen. Much of the energy we use in the US and the UK is generated in ways that add additional carbon dioxide to the atmosphere, exacerbating the global climate crisis. It is a small thing to use only what I need in terms of energy to heat my tea, but that small thing reminds me of the larger issue day in and day out. Whether we use a microwave, an electric kettle, or the stove to heat the water for a cup of tea, we all have a responsibility to each other and to the planet that we would be wise to bear in mind.

7.5 Keeping Tea Cozy

The changes in temperature when two bodies are placed in contact with each other, such as when hot water is poured into a teapot, depend on the temperature difference between the two objects. This is why many instructions for making tea suggest warming the pot ahead of time, either by letting hot water stand in the pot or rinsing it with hot water. Pre-warming a teapot helps it to maintain a higher temperature while steeping, but it will also help the contents of a teapot or teacup stay warm longer once the infusion is complete.

Mass and composition also make a difference. The cast iron teapot in Figure 7.6 weighs 1200 g, while the glass pot weighs only 400 g. I filled each pot with 500 g of water at 100 °C and monitored the temperature. The temperature in the cast iron teapot quickly dropped from the 100 °C of the boiling water to only 80 °C, well below the optimal steeping temperature for black tea. Four minutes later the temperature had continued to drop, reaching 76 °C. By contrast, the water in the glass teapot dropped to 90 °C at the beginning, reaching 82 °C after four minutes. A porcelain teapot behaved similarly, cooling by 7 °C in four minutes.

I repeated the experiment, this time pre-warming the teapots by filling them with boiling water and letting them sit for five minutes. The temperature of the water in the cast iron teapot only dropped to 90 °C when the water was first added and ultimately fell by 5 °C during the four-minute course of the infusion. Overall, the temperature was much closer to the optimal temperature for steeping. The effects were not as pronounced with the glass teapot; both the initial and final temperatures were roughly the same as with the room temperature teapot. If you are using a heavy pot, such as stoneware or cast iron, taking the time to pre-warm the pot is well worth it.

If you want to steep your tea at an optimal temperature and don’t have access to a constant temperature bath, a well- insulated teapot, such as a double-walled glass teapot, is an excellent choice. When I poured hot water into the cold teapot, the temperature only dropped to about 93 °C, reaching 89 °C at the end of four minutes. Pre-warming the teapot kept it solidly in the optimal steeping temperature range, starting at 95 °C and ending at 91 °C. I admit the teapot is not the one I reach for first when making my morning tea, but it should be. What makes this teapot so good at holding the heat? The answer is nothing. Literally nothing, as we shall shortly see.

“Tea grown cold is worse than death” lamented the Iraqi poet Dunya Mikhail in her poem The Iraqi Nights. As far as I am concerned, even lukewarm tea is unappetizing. I prefer my tea either piping hot (65 °C) or ice-cold. Keeping it at my preferred temperature is an exercise in the management of molecular motion.

Heat is energy that is manifested as random molecular motion. The temperature of an object is a measure of the average speed that the molecules in the object are moving: the faster they move, the hotter the object. Molecules can be moving about within the material; think of the molecules in the air as caroming about like bumper cars. Molecules in a solid may not be able to move from their places, but they can nonetheless rock and roll. When energy is moved from one object to another, the objects change temperature. To keep my tea hot I need to prevent the energy that the molecules in my tea have from transferring to other objects they are in contact with, such as the teacup, teapot, or the air above the cup.

A hot object can transfer heat to its surroundings by convection, conduction, or radiation. One of the things I love about my first cup of tea for the day is watching the steam twirl and dance in the early morning light above the cup. I take a deep breath of the warm damp air, with its faint scent of flowers. But I am also aware that, in addition to carrying the scent to my olfactory receptors, the steam is also carrying off some of the heat energy from my tea. This is the process of convection,‡‡‡‡ where fast-moving molecules move away from a hot object, taking some of its energy with them. This is one reason to keep the lid on the teapot while it steeps, to keep the temperature as high as possible during the extraction process. If you want to keep your tea warmer longer, cover it, or use a travel mug with a lid. The lid not only prevents spills of the liquid tea inside, but also keeps the tea from spilling energy to the room at large via convection.

Tea cools principally by conduction. Thermal conduction is much like a molecular bucket brigade. A molecule in the tea collides with a molecule in the cup, handing off some of its energy in the process. The molecule or atom in the cup now hands off some of that energy to a molecule next to it and the process continues, carrying heat energy away from the tea into the cup and, eventually, into the surrounding air—or your hands if you’re holding the cup. How effective this bucket brigade is depends on the structure of the materials involved and the temperature difference between them. Materials that are very dense, where the molecular buckets are close together, generally conduct heat energy well. Metal and ceramics are dense materials that easily move energy around.

Surrounding a cup of tea with materials that are not very dense—slowing down the transfer of energy from one molecule to another—is one strategy for keeping tea (or anything) warm. A tea cozy, made of a quilted fabric or felt, makes use of this strategy, as do puffer coats and duvets. Cozies rely on trapped air to slow heat transfer. Air doesn’t conduct heat very well and trapping it reduces the heat loss by convection as well as conduction. Using a quilted tea cozy slows the cooling of the porcelain teapot by factor of two. The felt-lined stainless steel cozy that fits over the black ceramic teapot in is even more effective, slowing the cooling by a factor of three. Practically, this means the tea in that teapot is still above my preferred drinking temperature half an hour after I steeped it—no need to resort to re-heating my second cup of the morning in the microwave and enduring the resultant scum.

If you could envelop a cup of tea in a bubble of nothingness—a vacuum—you could keep it from cooling by conduction. Of course, it is not practical to do this—a ball of liquid in a vacuum would quickly boil away. A convenient way to do this is to use a vacuum flask or thermos, where a layer of vacuum is sandwiched between two walls. Originally, glass walls were used. If you are my age, you might remember dropping your lunch box and then opening your thermos at lunch to find shards of glass inside. Sadly, you were then forced to buy chocolate milk from the lunch lady. These days, doubled-walled stainless-steel containers based on physicist William Stanley’s 1912 design§§§§ make lugging your milk to school—or your tea to work—a less chancy proposition.

Chemists still use the glass-walled version of a thermos, as do tea enthusiasts. Chemists call it a dewar (for James Dewar, the  Scottish chemist who invented the contraption in 1892). The double-walled teapot that so ably kept the temperature of  the water high is a glass dewar, wrapping the tea in a vacuum, a sort of nothingness. Vacuum doesn’t quite mean a complete absence of molecules—practically, it means a very low pressure gas. Even interstellar space has roughly 10 atoms per cubic meter. A chemical dewar has a pressure of about 0.01% of the air pressure at sea level, containing on the order of a billion trillion (10²¹) atoms per cubic meter.

Exploring the thermal properties of my teacups and teapots has given me a new set of favorites, cups and pots that inarguably give me a better cup of tea. I cleaned out my collection of tea infusers and kept only the largest of my tea baskets. And while I appreciate the convenience of a teabag, I am more convinced than ever that my tea caddies filled with whole tea leaves are the gateway to a superb cup of tea. More than 50 years after my mother held out that steaming, sweet mug of Lipton tea, I am still learning to brew a better cup. I hope that you, too, have found a few ways to improve your cup, whether made from a teabag, or whole leaves, or reheated in the microwave.

7.6 Brewing a Better Cup

Don’t let anything come between you and your tea. Use loose-leaf tea or a large brewing basket, preferably made from stainless steel.

Warm the teacup or teapot before steeping.

Don’t use teabags made from plastic, natural or not, if you are concerned about microplastics. Choose paper.

Cover your teapot or teacup to keep it warm.

Be nice to the planet and don’t heat more water than you need for a cup or pot. Compost your tea leaves and teabags.

References

1. R. Lawson and M. McLaren, US Pat., 723287A, 1903.

2. M. Spiro and D. Jaganyi, Food Chem., 2000, 69, 119–124.

3. D. Jaganyi and S. Mdletshe, Food Chem., 2000, 70, 163–165.

4. C. Astill, M. R. Birch, C. Dacombe, P. G. Humphrey and P. T. Martin, J. Agric. Food Chem., 2001, 49, 5340–5347.

5. D. K. Jha, P. P. Dhekne and A. W. Patwardhan, J. Food Sci. Technol., 2020, 57, 3060–3070.

6. L. Ryan and S. Petit, Nutr. Res., 2010, 30, 14–20.

7. V. Lo Turco, G. Di Bella, A. G. Potorti, M. R. Fede and G. Dugo, Eur. Food Res. Technol., 2015, 240, 451–458.

8. J. Larsen, In Search of Stardust: Amazing Micrometeorites and Their Terrestrial Imposters, Voyageur Press, Minneapolis, Minnesota, 2nd edn, 2017.

9. D. Jaganyi and T. Ndlovu, Food Chem., 2001, 75, 63–66.

10. G. J. Consolmagno S.J. and D. T. Britt, Meteorit. Planet. Sci., 1998, 33, 1231–1241.

11. H . Hosack, N. Marler and D. MacIsaac, Phys. Teach., 2002, 40, 264–266.

12. K. Vonnegut, Cat’s Cradle, Gateway, 2010.

13. S. J. La Placa, W. C. Hamilton, B. Kamb and A. Prakash, J. Chem. Phys., 1973, 58, 567–580.

14. O. Gulbiten, J. C. Mauro, X. Guo and O. N. Boratav, J. Am. Ceram. Soc., 2018, 101, 5–11.

15. R. S. Treptow, J. Chem. Educ., 2005, 82, 1636.

†Lint, for example, would be a most unwelcome addition to a cup of tea.

‡Glacial acetic acid is highly concentrated acetic acid. This is not the dilute acetic acid found on pantry shelves as vinegar. Although its name suggests it might be extracted from a glacier, it takes its name from the crystals resembling ice that frequently form in highly concentrated acetic acid solutions at cool room temperatures.

§Today, chemists use chromatographic techniques to separate and analyze an enormous array of compounds using many different materials to carry out the separation.

¶On average, about 200 micrometeorites impact each square meter of the Earth each year. See Jon Larsen’s book, In Search of Stardust, for beautiful photographs of what is on your roof (and garden).⁸

‖The Specola Vaticana, the Vatican Observatory, has one of the largest research collections of meteorites in the world.

**Yixing-style teapots have unglazed interiors and are meant to be used with a single tea. They are never washed with soap and, over time, absorb compounds from the tea.

††The song, written by George Harold Sanders and Clarence Kelley, begins, “I’m a little teapot, short and stout, here’s my handle, here’s my spout. When I get all steamed up hear me shout, ‘tip me over and pour me out’.” It is often accompanied by a dance, called the Teapot Tip, which mimes the pouring of tea.

‡‡For the chemists and physicists in the crowd, a near-spherical top where all the moments of inertia are approximately equal is preferred to either oblate or prolate tops.

§§This isn’t a book about the physics of tea, but a water xylophone is another familiar example of how the resonant frequency of a container changes with its volume.

¶¶Rock, paper, scissors is a hand game, versions of which have been traced to the Han dynasty almost 2000 years ago. Like tea, its origins are lost in the mists of history. Players simultaneously make one of three hand signs corresponding to rock, paper, or scissors. Rock beats scissors, paper beats rock, scissors beats paper.

‖‖While a plasma may seem to be an esoteric phase of matter, found in stars or tokamaks, it is entirely possible to create a ball of plasma using a cup of tea and a microwave oven. Although I hasten to add that the microwave oven may not be useful for much else afterwards. You can make a tinier ball of plasma using a grape, with less risk to household appliances.¹¹

****Vonnegut’s fictional ice IX froze any liquid water it came into contact with. Real ice IX exists only at low temperatures, at about −135 °C, and pressures 2000–4000 times that of the Earth’s atmosphere.

†††It is an urban myth that glass is really an incredibly viscous liquid. While some will point to old windows where the glass is thicker at the bottom as evidence that glass flows very slowly over time, the method used to produce the glass led to these discernible variations in thickness. Using measurements made on 13th century glass from Westminster Abbey in London, researchers have suggested that the flow rate is no more than 1 nm per billion years.¹⁴

‡‡‡Pyrex is the trade name given to borosilicate glass made by the Corning company in the US. The glass was marketed to consumers first as pie plates, hence the name.

§§§It is actually always in play, even in a chemical reaction, but the changes in mass are too small to detect.¹⁵

¶¶¶4.184 joules to be exact. This is the so-called specific heat of water.

‖‖‖To be clear, this is not how a combustion reaction actually happens. But Hess’s law tells us that the overall energy released in a reaction does not depend on the path taken from the reactants to the products. Combustion is a complicated process.

******Chemists will recognize this as a very rough calculation. The US National Institute of Standards and Technology reports that the combustion of one mole of propane release a net 2219 kJ.

††††This is dielectric heating, not the excitation of molecular rotation frequencies in water.

‡‡‡‡Technically, convection combines conduction—the diffusion of heat through a material—and advection—the wholesale movement of a liquid or gas.

§§§§Stanley’s original design used an inert insulating solid material such as carbon or silica between the walls to achieve the same degree of insulation at a higher, more easily achieved pressure. Modern doubled-walled stainless-steel containers generally have a near-vacuum cavity.

Epilogue

I hope that after reading this book you have a deeper understanding of what is in your cup of tea and how to tweak your brewing process to get what you want out of the tea leaves and into your cup more consistently. If you want a strong dose of caffeine, use the hottest water you can, and never reuse a teabag. In a hurry? Know that the caffeine levels in a cup of tea go up quickly; even 30 seconds of steeping a teabag can be enough. If it is the antioxidants you desire, let it steep a little longer and cut the resulting bitterness with a bit of sweetener or a pinch of salt. Be willing to experiment and find the conditions that give you what you consider to be the best cup of tea.

If you are not a chemist, I also hope that this book has given you some sense of how chemists understand the world. The same four big ideas of chemistry—the connection between molecular structure and function, how opposite charges attract one another, proximity, and the ubiquitous building blocks that are atoms—that we used to understand infusions of tea can be applied to answer so many questions about how the universe works.

Subject Index

References to figures are given in italic type. References to tables are given in bold type. Footnotes are indicated by a 'n' following the page number.

absorption, 61, 64

acetic acid (vinegar), 13, 19, 112

acetone, 37

acid reflux, 67

acrylamide, 40, 40–41

addictions, 68–69

adenine, 50, 52–53, 70

adenosine, 57–59, 60

molecular on/off switch, 57–59

receptors, 57, 67–68, 90

adenosine triphosphate (ATP), 57, 152

advantame, 157

Advertising Standards  Authority, 182

agave, 154–155

nectar, 152, 154

agitation, 134–136, 178

agony of the leaves, 118–145

agitation, 134–136

brewing a better cup, 143–144

exhausted tea leaves, 140–141

extracting what you want, 119–120

further reading, 144–145

hotness levels, 128–134, 207

instant tea, 141–143

making a cup of tea, 120–121

tea as mostly oxidane, 121–125

tea grown cold,  136–140

tea pairing, 118

time and temperature, 126–129

alanine, 157

albuterol, 27

alcohol, 7, 41, 46, 64

alertness, 62

alitame, 157

alkaloids, 48–50, 51, 83, 86, 104–106, 149

alumina (aluminum oxide), 193

aluminum, 83, 109–110

ions, 138

aluminum oxide see alumina

Alzheimer's disease, 63

amino acids, 27, 39, 43, 54,  55, 83, 93–95

d-amino acids, 93n

ammonium chloride, 13

amorphous solid materials, 195

amoxicillin, 49, 49

antioxidants, 2, 31, 97,  100–101, 148–149, 178

anxiety, 81, 96

Apiaceae family, 66

Apostolides, Zeno, 76

aroma, 33, 36

citrus-scented, 163–166, 166

floral-scented, 142,  163–164, 165

artificial sweeteners, 155–157

ascorbic acid (vitamin C),  112, 113, 169

aspartame, 92, 95, 152–153, 156–157

molecular structure of, 155

potentially carcinogenic, 153

aspartic acid, 95, 157

aspergillum, 42n

Aspergillus niger, 42

Aspergillus sydowai, 42

Assam Pekoe-Souchong  tea, 1

Assam tea, 1, 126, 189

Astill, Conrad, 136

astringency, 87, 87n, 98

atomic tweezers, 71, 71n

atoms, 8, 10–11, 15–18

ATP see adenosine triphosphate

Atropa belladonna see nightshade

atropine, 51

Avci, Elif, 148

Axel, Richard, 89–90

Ba see barium

baking powder, 112

ball and stick models, 17, 17–18

balls, 183–187

ban-cha white tea, 30

barium (Ba), 109

baskets, 183–187

Battuta, Ibn, 6

beer, 41

bees, 114

benzene (C₆H₆), 9, 15–16,  17, 18

benzoic sulfimide, 155–156

benzyl acetate, 163

benzyl alcohol, 164, 165

bergamol, 166

bergamots, 165

bergapten, 165, 166

beta-carotene, 13, 36, 36, 69

beta-damascenone, 36

beta-ionone, 36, 36

bicarbonate, 125

bitterness, 86, 125, 168

black pepper, 161–162

black tea, 32, 35, 41, 44

catechins in, 148

Halmari tea garden in Assam, from, 45, 94

loose-leaf, 120

l-theanine in, 149

most tea consumed daily, 119

whole-leaf, 127

bladder cancer, 156

blood–brain barrier, 61,  64, 96

blood pressure, reducing, 96

Bodhidharma, 6

boiling point, 129–130

bone density, 67

BOP see Broken Orange Pekoe

Bordeaux–Paris race, 53–54

borosilicate glass, 195

Boston Tea Party, 140n

Botero, Giovanni, 7

Brassica family, 66

Brazil nuts, 109

Bristol, Blanche Muriel, 150

British Standard, 151

Broken Orange Pekoe (BOP), 135

bromelain, 161

bromotheophylline, 52, 53

bronze, 195

bruising, 34–37

Buck, Linda, 89–90

Buddhism, 6

butter, 146–147

C see carbon

C₄H₈O₂see ethyl acetate

C₆H₆see benzene

C₆H₁₂see cyclohexane

Ca see calcium

cadmium (Cd), 109

caffeine (trimethylxanthine), 3–4, 31, 45–47, 54, 91–92. see also drug in the cup

alkaloid, 104, 104–106

effects of agitation, 136

extraction, 127, 127–128, 207

infrared spectrum of, 85, 85

lethal dose of, 67

molecular structure of, 48, 49, 52

naming of, 47

pesticide, as, 47–48

synthesis in tea plants, 70, 71

transfer from teabags, 178, 182

world's most widely  used psychoactive  substance, 46

calcium (Ca), 139, 148

ions, 125, 138

calcium carbonate, 12, 137

calcium oxalate, 113

Camellia family, 106

Camellia ptilopylla, 75

Camellia sinensis, 4–6, 46, 75–76, 107, 109

cancer, 2, 81

bladder, 156

esophageal, 81

cannabis, 163

CaO see lime

capsaicin, 166

caraway oil, 25, 25, 27

carbohydrates, 41

carbon (C), 15–16, 196–198

atoms, 15, 16

nucleus, 18

carbon-14, 110

carbonate, 138

carbon dioxide, 41, 74, 133, 138, 198–199

carbon disulfide, 13

carbonyl group, 39, 39

cardamom, 146, 161

carotenoids, 36, 83

carrots, 13–14, 36

carvone, 25, 26

caseins, 148

cast iron, 196

catechins, 35, 44, 98–101, 99, 133–134, 148–149

Caventou, Joseph Bienaimé, 47

cavitation, 142

Cd see cadmium

Celestial Tribute, 29

cellulose, 177, 177

acetate, 176

central nervous system, 46, 96

ceramics, 191–194

CH₂Cl₂see methylene  chloride

CH₃CH₂OH see ethanol

chai

masala, 146, 159

tea, 159–163

Chambers, Delores, 88

Chambers, Edgar, 88

chamomile, 163

Chang, Hasok, 129

chanoyu (Japanese tea  ceremony), 119

charcoal, 198

chemical

extractions, 119–120

structure, how to read a, 14–18

chemistry of tea

agony of the leaves, 118–145

conclusions on, 207

drug in the cup, 45–78

earth, wind, fire, and water, 172–206

overview of chemistry, 1–28

reading the tea leaves, 29–44

sugar and spice, 146–171

taste of Zen, 79–117

Chen, Liang, 75

chichona bark, 48

Chinese tea, 7, 75

chlorine (Cl), 156

chlorofluorocarbons, 101

chlorogenic acid, 112, 113

chlorophyll, 43

chlorotheophylline, 52, 53

chocolate, 105

chromatography, 84–85

chromium (Cr), 109, 196

Cicuta maculata, 66

cinnamaldehyde, 160,  161–162

Cinnamomum cassia trees, 161–162

Cinnamomum verum trees, 161–162

cinnamon, 146, 160, 161

citrate, 168, 169

citric acid, 24, 112, 138, 168

citrus, 146

flavoring, 164

Cl see chlorine

clays, 193

climate change, 201

cloves, 146, 160, 162

coal, 198

tar dyes, 13–14

cocaine, 50

coconut milk, 147

Coffea see coffee plant

Coffea humblotiana, 75

coffee, 7, 46, 59

coffee plant (Coffea), 46, 50

cognition, 96

Cola acuminata see kola

cold-brewed teas, 127, 131–132

Collings, Virginia, 86

colloids, 139, 147

combustion, 198

energy, 198

commercial decaffeination, 74

common names, 18

compost, 181, 205

Concord grape juice, 88n

conduction, 203–204

Consolmagno, Guy, 186

convection, 203, 203n

Cooke, Maud, 189

copper (Cu), 35, 191, 195

copper sulfide, 84

copper tannate, 84

Cottrell, Stephen, 132

Coulto, Richard, 147

coumarin, 160, 162

covalent bonds, 16

cozies, 204

Cr see chromium

creaming, 138

cream of tartar, 112

crush, tear, curl teas see CTC teas

CTC teas (crush, tear, curl teas), 30, 30, 34, 108, 135

Cu see copper

cyclamate, 155, 156

cyclohexane (C₆H₁₂), 9,  15–16, 17

cyclohexylamine, 156

CYP1A2, 65–66

cytochrome P450, 65–66

D₂O, 124–125

Darjeeling tea, 94

black tea, 88–89

whole-leaf, 118

Davy, Humphry, 98n

decaffeinated tea, 70–76

Decaffito, 76

decoction, 119, 159

de Hevesy, George, 2, 11

Del Mármol, Josefina, 90

deuterium, 124

dewar, 205

Dewar, James, 205

diabetes, 81

dielectric heating, 201n

diffusion, 142

dihydrogen monoxide see water

dispersion, 139

diterpene glycosides, 75

diuretic, 46, 52, 67

DNA, 18, 27, 50, 54, 70, 93

dogs

caffeine toxic to, 106

chocolate toxic to, 105–106

odor detection in, 90,  90n

doping, 52–54

double helix structure, 18

drug in the cup, 45–78

addictions, 68–69

adenosine, 57–59

atomic view of caffeine, 48–54

benefits to plants,  69–70

beyond caffeine, 76–77

blocking the signal to sleep, 59–62

brewing a better cup, 77

caffeine and humans, 62–63

cup to receptor, from, 63–64

further reading, 77

making decaffeinated tea, 70–76

molecular locks and keys, 54–57

pharmacokinetics,  65–66

side effects, 66–68

stimulating beverage, 45–48

tea pairings, 45

Dupont, 86

Earl Grey tea, 10, 141, 146, 163–167

earth, wind, fire, and water, 172–206

brewing a better cup,  205

hot water, 197–201

calorie counts, 197–198

climate change,  201

combustion energy, 198–200

nuclear option, 200–201

keeping tea cozy,  202–205

paper cups, 196–197

teabags, balls, and  baskets, 173–174

brief history of  teabags, 174–176

infusers: balls and baskets, 183–187

paper teabags, 176–178

plastic teabags, 179–181

spent teabags, 181

tea pairing, 172

teapot preference, 187–190

ceramics, 190–194

glass, 194–195

metal, 195–196

technology of tea, 172–173

Eder, Josef, 84n

EGC see epigallocatechin

EGCG see epigallocatechin gallate

Einstein, Albert, 197

Eisai, 6

electric kettle, 200–201

electrons, 11

electrostatic forces, 139

enzymatic oxidation, 34–38

enzymes, 37–38, 54, 56–57

epigallocatechin (EGC), 98,  99, 102

epigallocatechin gallate (EGCG), 98, 99, 101–102, 125

Erdem, Yasar Kemal, 148

Erdös, Paul, 126, 126n

ergot, 50

ergotamine, 50

Erythroxylum coca, 50

esomeprazole, 27

esophageal cancer, 81

ethanol (CH₃CH₂OH), 19

ethyl acetate (C₄H₈O₂), 74

ethylamine, 94

eugenol, 160, 162, 164

European Food Safety  Authority, 66, 101, 162

evergreens, 31

exhausted tea leaves,  140–141

exponential kinetics, 127

F see fluorine

Fahlberg, Constantin, 155

fairy flies, 114

farnesene, 163, 165

fats, 147

Fe see iron

femtometer (fm), 17

fertilizers, 107

Fielding, Henry, 146

Finnie, Mike, 149

fired teas, 40–41

first flush, 31

first law of thermodynamics, 197

first leaves, 31

first-order kinetics, 127, 127

Fischer, Emil, 2, 50, 56

Fisher, Ronald, 3, 150

Fleming, Alexander, 49

Flo-Thru teabag, 182

flowers, 146

fluoride, 83, 106–109

fluorine (F), 35, 106–107

fm see femtometer

Formosa oolong tea, 172

fossil fuels, 198

Fox, Arthur, 86

frogs, 49

fructose, 152–153, 154

fungi, 41–42

furan, 40, 40

Ga see gallium

gallic acid, 112, 113

gallium (Ga), 196

gallocatechins, 100

gas phase, 12, 18, 75, 89

gelatins, 161

generally recognized as safe (GRAS), 42, 66, 96

genetic mutations, 76

GF see golden flowery

ginger, 160, 161

plant, 159

root, 159

gingerols, 159, 160, 161, 163

glacial acetic acid, 176, 176n

glass, 184, 191, 194–195

glazing, 193

glucose, 153–154, 154

glutamates, 95–96

glutamic acid, 54, 94–95, 95

glyceraldehyde, 26

von Goethe, Johann  Wolfgang, 47

golden flowery (GF), 135

gout, 50

GPCRs see G protein-coupled receptors

G protein-coupled receptors (GPCRs), 57, 90, 158

GPS system, 19

GRAS see generally recognized as safe

Green, Wilhelmina, 1–3, 81–83, 138

green snail tea, 41

Yunnan, 79

green tea, 32, 120

amount consumed  daily, 119

steeping, 130

Greitemeyer, Tobias, 87

guanine, 50, 52–53, 70

H see hydrogen

H₂O see water

half-life of caffeine, 65

Hạnh, Thích Nhất, 123

HDL cholesterol see high- density lipoprotein cholesterol

headaches, 68

health benefits, 81

heart disease, 81

heat, 203

heating water, 197–200

hemoglobin, 54, 100–101

Hendon, Chris, 168

herbicides, 104

Hermanson, William, 176

HF see hydrofluoric acid

high-density lipoprotein (HDL) cholesterol, 106

Hildegard of Bingen, 124

history

tea, of, 5–7

teabags, of, 174–176

Hofmann, Thomas, 88, 94

honey, 146, 151–153

honyacha, 75

hotness levels, 128–134, 207

hydrangeas, 110n

hydrocarbon, 198

hydrofluoric acid (HF), 106n

hydrogen (H), 15–16, 197

atoms, 15–16

bonds, 72–73, 73, 123

gas, 84

hydrophilic, 72

hydrophobic, 148, 197

hydroxides, 137

4-hydroxy-3-methoxybenzaldehyde see vanillin

hyperkalemia, 27

ibuprofen, 62

ice IX, 191, 191n

Ilex paraguariensis see yerba-maté

Ilex vomitoria see yaupon holly plant

infrared spectrum of caffeine, 85, 85

infusers, 173, 183, 183–187, 186

expansion of tea leaves, 185, 185–186

glass, 184

stainless steel, 184

insects, 114–115

instant tea, 141–143

International Organisation for Standardisation (ISO), 119, 121, 151

International Space Station,  7, 123n, 141

International Union of Pure and Applied Chemistry (IUPAC), 15, 19, 121

intravenous, 64

ionic bonds, 11–12

iron (Fe), 100, 109–110, 196

ISO see International Organisation for Standardisation

IUPAC see International Union of Pure and Applied Chemistry

Jaganyi, Deogratius, 137–138, 178

Japanese tea ceremony see chanoyu

jasmine, 163

flowers, 163

tea, 164

Jones, Paul, 149

joules (J), 197

Juenong, Wu, 124

Juko, Murata, 87

K see potassium

Kaffebase, 47

kaolin clays, 193

Kato, Misako, 70

Keenan, Emma, 149

Keller, Helen, 89, 91

Kelley, Clarence

"Tea Pot Song," 189n

Kelvin scale, 129

ketones, 36, 36–37

Keurig machines, 119, 135, 143

kidney stones, 112, 114

kilocalorie (kcal), 197

kola (Cola acuminata), 46

Kolbe, Hermann, 13

kombucha, 41

Kong, Joon Mo, 14–15

kucha, 75

Kuhnert, Nikolai, 102, 104

Lawson, Roberta, 176

lead (Pb), 109

lead acetate, 195

leafhoppers, 34n, 114

Lee, Jeehyun, 88

lemon, 146, 168–170

juice, 138, 168–169

lime (CaO), 195

limes, 166

limonene, 165, 166

linalool, 163, 165–166, 165–166

linalyl acetate, 166

Linton, Arthur, 53–54

Lipton Green Tea, 45

Lipton tea, 205

company, 182

liquid phase, 12, 18

liver, 65, 101

damage, 162

Lonsdale, Kathleen, 16

loose-leaf

black tea, 120

pu'erh tea, 120

tea, 29–30, 135, 175

Lopez-Alt, Kenji, 168

Louyet, Paulin, 106n

LSD (lysergic acid  diethylamide), 50, 51

Luft, Robert, 89

lugduname, 92, 95

lysergic acid diethylamide  see LSD

magnesium (Mg), 125, 138

Maillard reaction, 38–41, 40

malaria, 49

malic acid, 112, 113

manganese (Mn), 109, 138

masala chai, 119, 146–147, 159

mass spectrometry, 85

Masters of Reality, 118, 120, 131

matcha tea, 30, 141

McLaren, Mary, 176

metal carbonates, 137

metals, 108–110, 168,  195–196

methane, 198

methanol, 157

methylene chloride  (CH₂Cl₂), 74

methyl groups, 42, 66, 70

Mg see magnesium

mice

studies with capsaicin, 166

Micheli, Pier Antonio, 42n

microplastics, 179–181, 205

microwave, 136–138, 144, 200–201

Midol, 52

migraines, 46

Mikhail, Dunya, 136, 203

milk, 138, 146–150

plant-based milk, 147

skimmed, 148

tea first, or?, 150–151

minerals, 83, 108–110

Mn see manganese

modern decaffeination  processes, 74–75

molds, 42

molecular chirality, 24–27

molecules, 8

structure of, 14–18

monosodium glutamate (MSG), 24, 95, 95–96

Moseley, Henry, 2

MSG see monosodium glutamate

N see nitrogen

Na see sodium

NaCl see sodium chloride

nalaxone (Narcan), 166

Narcan (nalaxone), 166

natural gas, 40

natural sweeteners,  152–155

neotame, 155

neoxanthin, 36, 36

Nespresso machines, 119,  135, 143

neurodegenerative diseases, 61, 63

neutrons, 11

Nicklès, Jérôme, 106n

nicotine, 64

nightshade (Atropa belladonna), 49

nitrogen (N), 17, 39, 39

NMR see nuclear magnetic resonance

non-enzymatic oxidation, 43

Northrup, John, 56

nuclear fission, 197, 201

nuclear magnetic resonance (NMR), 56, 56n

nucleus, 17–18

nylon, 180, 181

nylon 6, 180

O see oxygen

O₂ gas, 97

oat milk, 147

octopuses, 49

odor detection

dogs, in, 90, 90n

elephants, in, 90, 90n

mice, in, 90

oil of bergamot, 164–167, 166

oils, 72

Okakura, Kakuzō, 29, 31, 46

omeprazole, 27

onions, 146, 149n

oolong tea, 35, 37, 44

amount consumed daily, 119

steeping, 130

OP see Orange Pekoe

opium, 7

Orange Pekoe (OP), 135

organic acids, 111–114

Oriental Beauty, 34n

Orwell, George, 119

Osborne, Fay, 176

osteoporosis, 67

Oudry, M., 47

over brewed, 72

overview of chemistry

big ideas about tea, 27

chemistry lessons, 7

chemistry's big ideas, 10–14

chemistry TL;DR, 8

decoding chemical names, 18–24

hints for, 21–24, 23

further reading, 27–28

how to read a chemical structure, 14–18

incredibly brief history  of tea, 5–7

mirrored molecules, 24–27

mysterious Wilhelmina Green, 1–3

slightly longer course in chemistry, 8–10

tea pairing, 1

what book is about, 3–4

what book is not about, 4–5

oxalate poisoning, 113

oxalic acid, 112, 113

oxidane, 19, 121–125. see also water

oxidation, 8, 34–35, 43, 102

enzymatic, 34–38

non-enzymatic, 43

oxygen (O), 97, 133–134, 198–199

atoms, 50

ozone layer, 101

Pacific yew tree, 49

paper cups, 131, 196–197

Parkinson's disease, 61, 63

Pasteur, Louis, 24, 26

Pb see lead

pekoe, 43

Pelletier, Pierre-Joseph, 47

penguinone, 20

penicillin, 49, 49

penicillin F, 49

Penicillium camemberti, 42

Penicillium roqueforti, 42

percolation, 119

periodic table, 2

pesticides, 47–48, 69–70, 104

PET see polyethylene terephthalate

Petit, Sébastien, 148, 178

PG Tips, 30, 30, 172

pH, 133–134, 138

Phadnis, Shashikant, 156

pharmacokinetics, 65–66

phenethyl alcohol, 164, 165

phenolphthalein, 102, 102n

phenylalanine, 95, 157

phenylketonuria (PKU), 157n

phenyl thiocarbamide (PTC), 86

phosphodiesterase pathway, 61

phosphorus, 148

photographic prints, 84n

phototoxicity, 166

phytochemicals, 48

Piere, Emma Elizabeth, 185

piperine, 160, 161–163

PKU see phenylketonuria

plasma, 191, 191n

polar, 72, 123

poly(1,1,2,2-tetrafluororoethylene), 19

polyethylene, 179, 180, 197

polyethylene terephthalate (PET), 179, 180

polylactic acid, 181

polyphenol oxidases, 34–35, 38

polyphenols, 42, 44, 98, 136, 140, 163, 178

polystyrene, 180

porcelain, 193

potassium (K), 110n, 111, 148

potassium-40, 110, 110n

potassium cyanide, 13

potassium hydrogen tartrate, 112

premature infants, 46

Priestley, Caroline M., 149

processing of tea leaves, 33

propane, 198–200, 199

proteins, 43, 54, 56, 93, 138

milk, in, 147–148

protons, 11

Proust, Marcel, 80, 91

proximity, 12

PSB36 (super-caffeine), 61, 61

psilocin, 50, 51

psychedelic mushrooms, 50

PTC see phenyl thiocarbamide

pu'erh tea, 29, 41–42

loose-leaf, 120

purines, 50, 52, 56, 104

Pyrex, 195n

pyrrole, 40

quercetin, 149n

quinine, 48, 51, 83

Ra see radium

radiation, 203

radioactivity, 110–111

radioisotopes, 110

radium (Ra), 109, 111

rats

bladder cancer research in, 156

caffeine studies with, 68

studies with, 101

reading the tea leaves, 29–44

brewing a better cup, 44

bruising enzymatic  oxidation, 34–37

further reading, 44

killing the green, 37–38

leaf to cup, from, 43–44

Maillard's reaction, 38–41, 40

making fermented pu'erh teas, 41–42

shaped and dried, 41

tea as work of art,  29–32

tea pairing, 29

white and yellow teas, 42–43

withering, 32–33, 43

receptors, 63–65

olfactory, 90

taste, 86, 94

reduction, 35

Revolutionary War, 140n

rhubarb, 113

Ricinus communis, 66

RNA, 54, 70

Roach, Mary, 14

Roach, William, 150

Robiquet, Pierre Jean, 47

robots, 32

rock, paper, scissors (hand game), 190–191, 190n

Rogers, Peter, 149

Rose Congou tea, 126, 146, 164

roses, 164

Rowland, Francis Sherwood, 101

Runge, Friedlieb Ferdinand, 47, 70

Ruta, Vanessa, 90

Ryan, Lisa, 148, 178

S see sulfur

saccharin, 152–153, 155, 155–156

Saccharomyces cerevisiae, 41

saffron, 146

Sagioglou, Christina, 87

Sa'id of Mogadishu, 6

Sakato, Yajiro, 92

Sakurada, Shinobu, 166

saliva, 87, 100

salt, 168, 207

salty, 86

Sanders, George Harold

"Tea Pot Song," 189n

Scharbert, Susanne, 88, 94

scurvy, 169

seltzer, 74

separation chromatography, 176–177

Serrin, James, 129, 129n

Shi, Siquan, 142

shikimic acid, 20, 113

shogaol, 160, 161

shot-hole borer beetles (Xyleborou fornicatus), 69, 69n

sickle cell anemia, 54

Siddique, Shama, 127

silica (silicon oxide), 193, 195

silicon, 184, 192

silicones, 184–185

silicon oxide see silica

sleep

blocking the signal to, 59–62

improving quality of, 96

smell, potent, 89–91

snoutane, 20

Snow, John, 124

snow melt, 124

soda-lime glass, 195

sodas, 47, 59

decaffeinated, 47

sodium (Na), 24, 168

sodium benzoate, 24

sodium carbonate (washing soda), 195

sodium chloride (NaCl), 168

sodium ions (Na⁺), 11

solid phase, 18

Sommerhalter, Monika, 125

sour, 86

soy milk, 147

spearmint, 91

oil, 25, 25, 27

Special Temple of Heaven Gunpowder tea, 30

spices, 146–147

Spiro, Michael, 127, 137–138, 178

sports, 76

stainless steel, 184, 191, 196, 205

Stanley, Wendell, 56

Stanley, William, 204

Stanley cup, 204

steeping tea, 119, 127–128

black tea, 130

stevia, 154

Stevia rebaudiana, 154

stevioside, 154, 155

stimulant, 62

stroke, 81

strychnine, 53, 53n

sucralose, 152, 155, 156

sucrose, 25, 26, 152–153, 154. see also table sugar

sugar and spice, 146–171

adding honey, 151–153

artificial sweeteners, 155–157

brewing a better cup,  170

chai, 159–163

Earl Grey tea, 163–167

further reading, 170

lemon, 168–170

milk, 147–150

milk or tea first?,  150–151

natural sugars, 153–155

salt, 168

spicing up a cup,  146–147

sweetness triangles,  158, 158–159

tea pairing, 146

sugar(s), 83, 147

natural, 153–155, 154

shortages during World War I, 156

sulfate, 125

sulfur (S), 40, 84

Sumner, James, 56

sun tea, 127, 131–132

super-caffeines

PSB36, 61, 61

superconductors, 192

supercritical fluid, 74–75

Sveda, Michael, 156

sweetness, 86, 94–95

syrups, 152

systematic names, 18

table salt, 11–12, 168

table sugar, 20, 25, 152–153. see also sucrose

tannic acid, 82, 82

tannins, 82–83, 98

taste of Zen, 79–117

brewing a better cup,  115

catechins, 98–101, 99

fluoride, 106–108

further reading, 115–116

insects as feature, 114–115

joy and delight, 79–82

l-theanine, 91–96, 92

metals and minerals, 108–110

molecular chameleons, 101–104

molecular inventory, 82–83

organic acids and vitamin C, 111–114

other alkaloids, 104–106

oxygen, 97

potent smell, 89–91

radioactivity, 110–111

reading the tea leaves, 83–85

taste of tea, 86–89

tea pairing, 79

tastes, five basic, 86

taxol, 49

tea

citrus-scented, 163–166, 166

cream, 138–140, 142,  168

dust, 134n, 135

fannings, 135, 174

floral-scented, 163–164, 165

most popular beverage in world, 2

plants, 70, 94

scum, 137, 137–139, 144, 168

teabags, 121, 173–174

brief history of, 174–176

flow-through, 174, 182

paper, 176–178

percentage using, 174–175

plastic, 179–181

pyramidal, 182

reusing, 140, 207

round, 174, 182

size matters, 182–183

spent, 181

tetrahedral, 173, 174

teapots

ceramics, 190–194

glass, 194–195

heat retention, 188–189

metal, 195–196

preference, 187–190, 188

pre-warming, 202–203, 205

"Tea Pot Song," 189, 189n

Teapot Tip, 189n

tea-tasters, 142

technology of tea, 172–173

teflon, 19

temperature, 126–129, 132, 143, 189

terpenes, 161, 163

Tetley, 182

black tea, 29, 79

green tea, 79

tetrafluoroethylene, 19

tetrahedrons, 173

tetrodotoxin, 49, 51

tetsubin, 196

tetsukyūsu, 196

TG see tippy golden

Th see thorium

theabrownins, 35, 42, 100

theacrine, 104, 104, 106

theaflavins, 102, 103, 122, 138

theanine, 93–96, 132, 149

d-theanine, 93n

l-theanine, 26, 79, 91–96, 92, 125, 149

thearubigins, 20, 35, 102, 103, 122, 138

theaspirone, 36, 36

theobromine, 52, 104,  104–106, 113

theophylline, 104, 104–106

thermal conductivity, 194

thermometer, 129, 129n

thermos, 205

thiazole, 40, 40

thiophene, 40, 40

thorium (Th), 109, 111

time, 126–128, 143

tippy, 31

tippy golden (TG), 135

tips of tea plant, 31

TL;DR (too long; didn't read), 8n

Tong, Lu, 79

tonic water, 48

too long; didn't read see  TL;DR

tooth-etch, 88

Toxicarium family, 106

trade names, 18

trichomes, 43

trimethylxanthine (caffeine), 54

Twinings Irish Breakfast  Tea, 1

U see uranium

ultrasonic extraction, 142

umami, 86, 125

uranium (U), 109, 110n, 111

urea, 13

Urey, Harold, 2

uric acid, 50, 52, 66

urine, 13, 50, 66–67

US Department of Agriculture, 113

US Environmental Protection Agency, 111

US Food and Drug Administration, 42, 66

vanilla extract, 21, 21

vanillin (4-hydroxy-3- methoxybenzaldehyde),  21, 21

vinegar (acetic acid), 13, 19, 112

vitamin C (ascorbic acid), 112, 113, 169

vitamins, 83

vitrification, 193

Vonnegut, Kurt, 191

Wan, Xiachun, 142

Warburton, "Choppy,"  52–54

Watanabe, Atsuko, 201

water (H₂O), 19, 121. see also oxidane

amorphous solid, 191, 192

boiling point of,  129–130

distilled, 138

hardness of, 125

heating, 197–200

hot, 197–201

lethal dose of, 67, 81

molecular structure of, 121, 122

molecules, 11

water bath, 132–133, 133

wax, 196–197

Weeks, Mary Elvira, 106n

White, Florence, 120

white tea, 42–43, 120

wine-making, 24

withdrawal, 68

withering, 32–33, 43

Wöhler, Friedrich, 13

World Health Organisation, 153

xanthine, 50–51, 66

xanthosine, 70

Xerocomius badius, 92

Xia, Tao, 142

X-ray crystallography, 56

X-ray diffraction, 18

Xyleborou fornicatus see shot-hole borer beetles

yak butter, 147

yaupon holly plant (Ilex vomitoria), 47n, 92–93

yeasts, 42

Yedlin, Mackenzie, 90

yellow tea, 42–43

yerba-maté (Ilex paraguariensis), 46

Yin Zhen white tea, 30, 30

Yu, Lu

adding chives to tea, 159

adding salt to tea, 146, 168

benefits of tea, 45–46

bitterness of tea, 86

boiling point of water, 130

choice of teacup, 190

improved cognition, 62

steeping tea, 119

tea as medicinal beverage, 81

Yuksel, Zerrin, 148

Yunnan green snail tea, 79

zinc (Zn), 168

zinc lactate salt, 168

zingibain, 161

Zn see zinc