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Some Tea Chemistry

Release date:2014-10-06

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, in moderate quantities, stimulate the central nervous system and promotes blood circulation. It stimulates the process of elimination and act as a diuretic promoting better kidney function. There’s some evidence that regular tea drinkers have a lower incidence of kidney ailments and gallstone. Some researchers have even claimed that it also helps the body excrete radioactive strontium 90, the element which entered the food chain from atmospheric fallout from nuclear bomb testing. 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 greatest 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 (some say half) 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 mouth 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.

Tea polyphenols, though popularly known as tannins, are not, as widely believed, the tannic acid used in leather preparation. In fact, they are not tannins at all. With some chemical and functional similarity, they became known by this name long before modern methods of chemical analysis made it possible to distinguish the two.

Here we have run into another of those name confusions that seem to haunt tea. There's more. Three-fourths of the tea polyphenols are catechins which are part of the chemical group flavanols (they do indeed have something to do with flavor, as distinct from either aroma or astringency). At least six catechins have been isolated. 

We might make a summary outline like this:

Polyphenols (compounds with two or more phenolic hydroxyl groups)

Flavanols (one kind of polyphend )

Catechins (the name for flavanols in tea )

Reports on the health benefits of these substances may use any of the three. Often no distinction is made between the terms "catechins "and "polyphenols," and in many cases the two are used inter-changeably.