Read What Einstein Kept Under His Hat: Secrets of Science in the Kitchen Online
Authors: Robert L. Wolke
In the case of black and oolong teas, the withered leaves are rolled in a large rolling machine that twists them and breaks open their cells, simultaneously exposing the insides to oxygen and releasing an enzyme (polyphenol oxidase) that oxidizes the polyphenol tannins in the leaves. Among the products of the oxidation reactions are orange, red, and yellow compounds called theaflavins and thearubigins, which give the tea briskness and color.
This oxidation process is almost universally but mistakenly referred to as fermentation, but yeasts and bacteria have nothing to do with it; it’s purely a chemical, not a biological, process. The difference between oolong and black tea lies in how long the oxidation process is permitted to continue: a few hours for black tea and only about half as long for oolong. The time and temperature must be strictly controlled to produce a tea of superior flavor. The oxidation process is stopped by deactivating the enabling enzyme with hot air, as in making green tea. That’s why green tea brews a lighter-colored beverage than black tea: fewer theaflavins and thearubigins.
The ultimate flavor properties of a tea will depend not only on how the leaves are processed but on how and where the bush grows, the local climate, the season during which the leaves are picked, and the position of the leaves on the plant.
By the way, if you expect me to expound on the reputed health benefits of drinking green tea, I must disappoint you. All I know is what I read, and my take on what I read is that the outlook is encouraging but that the jury has not yet come to a unanimous verdict. Presumably, any healthful benefits of green tea must have something to do with the fact that its polyphenols have not been oxidized, and polyphenols are antioxidants: they gobble up age- and illness-causing free radicals in the body.
I drink it every morning instead of coffee.
Sidebar Science:
What’s an enzyme?
ENZYMES HAVE
been misunderstood almost as much as instruction manuals for VCRs. Everyone know that enzymes play essential roles in all living things, but what exactly are they?
Are they alive, like bacteria? No. They’re chemicals, almost all of them proteins, that accelerate the complex chemical reactions essential to living things, both plant and animal. In other words, they are
catalysts
, substances that help chemical reactions go faster but are not used up in the process. Without enzymes, the chemistry of life would be impossibly slow, if it proceeded at all.
An enzyme molecule does its catalyzing job when a specific part of it, called its
active site
, reacts with a specific chemical, called its
substrate
, enabling that substrate to take part in vital chemical processes thousands or millions of times faster than it ordinarily would. The molecules of each type of enzyme have a unique shape that can react with only one specific substrate, thereby catalyzing only one specific chemical reaction. There is a unique enzyme for each of the hundreds of chemical reactions essential to the lives of all plants and animals.
For example, the dissolving of waste carbon dioxide from our bodily tissues into our bloodstreams and the “undissolving” of it back to gas for exhalation from our lungs are absolutely essential life processes. But if it weren’t for the enzyme
carbonic anhydrase
, these processes would take place so slowly that we couldn’t survive. Carbonic anhydrase makes the processes happen ten million times faster. Each carbonic anhydrase molecule can perform its speed-up act on a million carbon dioxide molecules per second.
An enzyme is named by tacking the suffix -
ase
onto a brief description of what it does. The tea enzyme is named polyphenol oxidase because it oxidizes polyphenols. If there were such a thing as an enzyme that speeds up the glazing of pottery, it might be called a vase glazease.
| | LITMUS TEA | |
Why does my tea turn a lighter color when I add lemon? On the other hand, my grandmother used to add a pinch of baking soda to her tea, and it turned as dark as brandy. What did she know that I don’t know?
....
A
re you sure it
wasn’t
brandy? Might Granny perhaps have been engaging in a bit of teacup tippling?
But okay, I’ll take her word for it. Here’s what was going on in both your cups.
You’ve heard people talk about a “litmus test,” to show whether a politician is on one side of an issue or the other? Well, litmus is a dye obtained from lichens; it is pink when in an acidic environment and blue when in an alkaline environment. Unlike politicians, litmus gives a straight yes or no answer: either acidic or not acidic (alkaline).
Litmus is what chemists call an
acid-base indicator
. Some of the tannins in tea are also acid-base indicators; they are one color in an acidic environment and another color in an alkaline environment. Your acidic lemon juice turns some of the tea’s tannins yellow, and Granny’s alkaline baking soda turns them reddish-brown.
Another example of an acid-base indicator is the pigment in red cabbage, one of the class of colored food chemicals called anthocyanins. Anthocyanins are responsible for the colors of many flowers and fruits, including apples, plums, and grapes.
The color of cabbage’s main anthocyanin changes with the acidity or alkalinity of its surroundings. It ranges from red in a strongly acidic environment to purple in a neutral (neither acidic nor alkaline) environment and goes from blue to greenish-yellow in increasingly alkaline environments. Cabbage looks more appetizing when toward the reddish end of the spectrum, so it is often cooked with (acidic) apples, whose sweetness can be balanced by a little red-enhancing vinegar added before serving.
Sidebar Science:
A litmus quest
WHAT MAKES
acid-base indicators change color?
Tannic acid, as one example, is what chemists call a weak acid, meaning that it is, well, not a strong acid. (See how easy chemistry is?) A weak acid’s molecules consist of two parts, a
hydrogen ion
(a positively charged hydrogen atom) and an
anion
(AN-eye-on), a negatively charged atom or group of atoms. We’ll call the hydrogen ion H and the anion A; when they’re together in the whole acid molecule, we’ll call it HA.
It’s the A part of the tannic acid molecule that is colored. When Grandma adds her alkaline baking soda, it gobbles up some of the acid’s H’s, leaving an excess of free A’s and therefore a darker color. On the other hand, when you add your acidic lemon juice, it contributes lots of H’s of its own, which combine with many of the A’s, in effect tying them up and curtailing their effects. Hence, the color becomes weaker; the brown turns to yellow.
Tannins have historically been used as dyes, as I learned early in life from my grandfather, a Russian immigrant. He had a magnificent white beard, but his mustache was permanently stained yellow from his habit of drinking tea from a glass.
| | NO CAF-FIEND | |
I’m a tea drinker, but I’m trying to cut down on caffeine. What if I used smaller cups? That is, what if I put one tea bag in a small cup and another tea bag in a large cup, filled them both with boiling water, and let them steep for five minutes? Would the smaller cup contain less caffeine?
....
I
nteresting thought, but no cigar.
Caffeine is very soluble in water; as much as 150 grams of caffeine could dissolve in a cup of boiling-hot water. But there isn’t anywhere near that much caffeine in a tea bag. Typically, it’s less than a thousandth of that.
What caffeine there is is extracted out of the tea bag almost completely in the first minute or so of steeping. So either way, big cup or small cup, all the bag’s caffeine is in your tea. You might as well use the big cup and have a longer-lasting drink at no increase in caffeine.
| | TEA (AND COFFEE) FOR TWO | |
I drink tea and my boyfriend drinks coffee. As soon as the kettle begins to whistle, he wants to take it off the burner and pour the water into his filter. I want to leave the kettle on longer, because I think water for tea should be hotter. But he says it will never get any hotter, no matter how long we leave it on. Who’s right?
....
Y
ou’re right about the tea and he’s both right and wrong about the water. I think we can work it out without your having to resort to separate kettles.
Most connoisseurs agree with you that in order to extract the proper amount of flavor from black or oolong tea leaves, the water must be as hot as possible. But no matter how much you heat it, water will never get hotter than boiling: 212°F (100°C), minus a degree or two, depending on the altitude and weather. That’s because water boils—turns to steam—when its molecules acquire just enough energy to overcome the atmosphere’s pressure on the water’s surface and break away into the air. If a molecule happens to acquire more than that amount of energy, it takes its excess energy along with it as it flies out. That extra energy is therefore lost from the water in the kettle and isn’t available to raise its temperature. So your boyfriend scores a point on that one.
But whistling tea kettles can be deceptive. When your kettle emits its first weak peeps, only a few of the more robust bubbles will have succeeded in making it all the way to the surface to release their steam and make the whistle whistle. The water is not yet fully boiling. For your black or oolong tea, then, you have to keep heating until all the water is bubbling furiously, the whistle has been screaming at maximum pitch and volume for at least several seconds, and your kitchen is beginning to fill up with stray dogs.
Green tea, however, follows different rules. Experts say that it should be brewed at a lower temperature of about 165 to 180°F (74 to 82°C), presumably because higher temperatures could promote oxidation of its valuable polyphenols (see p. 11).
Coffee is an entirely different cup of tea, so to speak. Water that is boiling vigorously isn’t desirable for making coffee because its steam would carry off too many of the volatile, aromatic flavor components, of which coffee has many more than tea. (Nobody has ever said, “Wake up and smell the tea.”) That’s why the crudest and most forthright method of making coffee—boiling the grounds in a pot of water—makes a brew better suited to the inside of an automobile battery than a breakfast cup.
The best ways to make coffee, in my opinion, are the filter method, in which hot water is poured over freshly ground beans in a filter-paper cone and drips through by gravity, and the French press or piston-and-cylinder method, in which hot water is poured over the grounds in the bottom of a tall vessel and allowed to steep for about three minutes, after which a perforated plunger is pushed down to press the “mud” to the bottom.
Whatever the method, water that isn’t hot enough won’t extract enough of the hundreds of chemical constituents that have been identified in coffee, all of which are sensitive to heat, air, and interactions with one another. Which ones and how much of each wind up in your cup depend on such things as the type of coffee, the relative amounts of coffee and water, the particle size of the grind, the mixing action in the brewing apparatus, the temperature of the water, and how long it is left in contact with the grounds. All in all, though, the optimum temperature for coffee water is around 190 to 200°F (88 to 93° C), or “just off the boil.”
To settle your domestic dispute, then, I recommend that you get the water to a full, roaring boil, turn off the heat, and quickly pour some onto your tea leaves or bags in a preheated pot. Then count to ten, during which time the water will cool just enough, and hand the kettle to your boyfriend, who may then proceed with his coffee-making.
Could Solomon have done better?
| | YOU WANT CREAM IN THAT? | |
I like my coffee with cream, but I also like it to be as hot as possible when I drink it. I know that the cream will cool it off, but when should I add it? As soon as I pour the coffee, or only when I’m ready to drink it? In which case will the coffee be hotter at drinking time? Or does it make any difference?
....
I
doubt that the ancient Greek philosophers spent much time on this (especially since they didn’t have coffee), but it’s a challenging question, if not an earthshaking one.
You could settle it with an accurate thermometer, but you’d have to measure out exactly the same amounts of coffee and cream into exactly the same type of cup, all at precisely the same initial temperature, etc., etc. But doing a carefully controlled scientific experiment in a kitchen has its problems, so let’s just think it out.
All other things being equal, you’d think that both methods would lead to the same temperature of the final mixture, because you’re combining
x
calories of heat in the coffee with
y
calories of heat in the cream, for a total of
x + y
calories in the mixture either way. (Regarding the use of the word
calorie
, see the box on p. 21.)
Unfortunately, according to Wolke’s Law of Pervasive Perversity, all other things are never equal. Whether it’s black coffee or creamed coffee, it must sit around until you’re ready to drink it. Meanwhile, it has been cooling off, because the air is cooler than the liquid in the cup and heat is therefore flowing from the liquid into the air. Heat will always flow from a substance at a higher temperature to an adjoining substance that is at a lower temperature.
But there are two important differences between the creamed coffee and the black coffee: (1) the cup of creamed coffee contains slightly more liquid because of the added volume of the cream, and (2) the creamed coffee is cooler than the black coffee.