All about Tea

Do you know the chemistry of tea?

When you pour boiling water over black or oolong tea leaves you are releasing the results of some truly wonderful chemical developments that occur during the processing, which is known as fermentation. Green tea is not fermented.

The three most important chemical substances in the fresh tea leaf are caffeine, aromatic or essential oils, and polyphenols (popularly but incorrectly known as tannins). Caffeine, found in many plants in nature, will be discussed more fully in the following chapter. The essential oils are important constituents of the aroma of the beverage. These substances are sometimes known as volatiles, which means that they will totally evaporate in strong heat. When tea is kept a long time, these disappear, reducing aroma. Both tannins and oils aid digestion by stimulating peristalsis of the intestinal tract. There is some evidence that tea counteracts the effect of fats by emulsifying them in the digestive tract. The polyphenols are the most interesting elements and the ones which do the greater good for human health.

During manufacture, black and oolong tea undergo their fermentation process. Spread out in a cool place, the leaves absorb oxygen, which creates chemical changes. This process should correctly be called oxidation, for the leaves are worked on by oxygen rather than fermented by microorganisms.

The polyphenols, about thirty altogether, account for nearly a third of the soluble matter in the fresh tea leaf. During the fermentation process about a third of the total amount is oxidized into more complicated oxidized products such as theaflavin. Therefore, after this process the tea contains two kinds of polyphenols, oxidized and unoxidized (natural polyphenols). The latter, released in the beverage, create the astringent, "puckery" feeling in the month when you drink tea. This stimulates the salivary glands, which is why tea is a thirst quencher.

The unoxidized polyphenols provide the pungency, while the oxidized ones give the tea its color and flavor. The higher the degree of oxidation, the more color and less pungency a tea has. Green tea, which does not undergo oxidation, has more natural unoxidized polyphenols, and also more astringency. Black tea has more color but less astringency. Both oxidized and unoxidized polyphenols may be beneficial.

Now let's see what happens in the polyphenol oxidation process. At the tea factory, the freshly picked leaves sit until they have become soft and limp as a result of water evaporation. Then they are rolled to break down the membranes and bring the juices containing polyphenols in contact with the enzyme polyphenolase (polyphenol oxidase) which catalyzes the oxidation of the polyphenols by oxygen in the atmosphere. The product resulting from this process, together with other constituents, accounts for the unique flavor and rich, deeper color of black and oolong teas.

The action of the enzyme, and therefore the oxidation process, is eventually stopped through heating, but these compounds remain in the dried prepared leaves waiting for the boiling water to dissolve them.

MILK, SUGAR, LEMON

Milk, sugar, or lemon are frequently added by black tea drinkers of the Western world. For those accustomed to it, milk seems to add to the comfort of a cup of tea.

However, the polyphenols in tea also bind with milk protein. Therefore, does putting milk into black tea cut down the effects of its polyphenols? Not substantially, says Professor Chi-tang Ho of Rutgers University, who has done considerable research on tea chemistry. Even in black tea, which has been fermented, about 30 percent of the polyphenols remain unoxydized, and the milk combines first with the 70 percent of oxidized polyphenols, and the chief effect is to ease their harshness to your stomach. So if you prefer the taste of black tea, milk is probably a good thing. However, if you put in too much milk, when it has finished with the oxydized polyphenols, it will go on to the unoxydized ones.

Neither sugar nor lemon juice had until recently been thought to have any effect on chemistry of tea. However, researchers at the University of Wisconsin found that adding the enzyme tannase, or lemon juice, to black tea increased iron and calcium solubility, and therefore absorption by the body. The polyphenols can bind with iron and calcium, preventing there minerals from being absorbed, but lemon juice inhibits this binding, keeping the minerals available.

In the 1950s it was learned that when lemon tea was served in Styrofoam containers the polystyrene dissolved into the beverage. It should never be served in these containers.

Artificial sweeteners, or sugar substitutes, are safe with tea, according to Professor Ho. They do not create any chemical reaction.

VITAMIN C AND OTHER NUTRIENTS

In China it is widely stated that tea (green) is a source of vitamin C. Since this vitamin is destroyed by heat and tea is made in hot water, this statement seems contradictory. Recent tests in China found that heat destruction does occur, but not in tea. Something in tea, as yet undetermined, apparently helps stabilize vitamin C.

The amount of vitamin C varies greatly depending on growing conditions, the age of the leaves at picking, and how long they have been stored. Japanese tests found that tea stored three years had lost all its vitamin C. according to Chinese calculations, typical green tea made with three grams (one teaspoonful) of dry leaves to a cup should yield about six milligrams of the total content of vitamin C in three infusions in water at 158 to 212 ^oF (70 to 100 ^oC). Eighty-five percent of vitamin C is released in one five minute infusion at 176 ^oF (80 ^oC). As for black tea, authorities have long believed that almost all its vitamin C is destroyed during fermentation.

Tea also contains vitamin B₁, B₂, K and P, and niacin, folic acid, and manganese, but in such small amounts as to be negligible. a cup of black tea has 58 milligrams of potassium.

[ Top ]   [ Back to All about Tea ]