The Chemistry of Tea
From: Tea and Cancer
Chung S. Yang, Zhi-Yuan Wang
Steaming or drying fresh tea leaves at elevated temperatures makes commercial green tea. Its chemical composition is similar to that of fresh tea leaves. Green tea contains polyphenols, which include flavanols, flavandiols, flavonoids, and phenolic acids; these compounds may account for up to 30% of the dry weight. Most of the green tea polyphenols are flavonols, commonly known as catechins. Some major green tea catechins are (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG), (-)-epicatechin (EC). (+)-gallocatechin, and(+)-catechin (5). The chemical structures of some of these compounds are shown in Fig. 1. Caffeine, theobromine, and theophylline, the principal alkaloids, account for about 4% of the dry weight. In addition, there are phenolic acids such gallic acids and characteristic amino acids such as theanine. A cup (200 mL) of green tea (Gun Powder, Hangzhou, China) contains about 142 mg EGCG, 65 mg EGC, 28 mg ECG, 17 mg EC, and 76 mg caffeine.
In the manufacture of black tea, the monomeric flavan-3-ols undergo polyphenol oxidase-dependent oxidative polymerization leading to the formation of bisflavanols, thea-flavins, thearubigins, and other oligomers in a process commonly known as "oxidation." Theaflavins (about 1%-2% of the total dry matter of black tea), including theaflavin, theaflavin-3-O-gallate, theaflavin-3-O-gallate, and theaflavin-3,3-O-digallate, possess benzotropolone rings with dihydroxy or trihydroxy substitution systems, which give the characteristic color and taste of black tea. About 10%-20% of the dry weight of black tea is due to thearubigens, which are even more extensively oxidized and polymerized, have a wide range of molecular weights, and are less well characterized. The structure of theaflavins and thearubigins are shown in Fig. 1.
Oolong tea, a partially oxidized tea, contains monomeric catechins, theaflavins, and thearubigins. Some characteristic components, such as epigallocatechin esters, theasinensins, dimeric catechins, and dimeric proanthocyanidins, are also found in oolong tea.
The flavanols are easily oxidized to the corresponding O-quinones. These flavanols and quinones can function as either hydrogen acceptors or hydrogen donors. In addition, tea polyphenols effectively interact with reactive oxygen species. In the flavanol structure, the 5- and 7-dihydroxy groups and 1-oxygen make the carbons at positions six and eight strongly nucleophilic. During enzyme oxidation or non-enzyme oxidation, including autoxidation or coupled oxidation, tea flavanols may undergo oxidative condensation via either C-O or C-C bond formation in oxidative polymerization reactions. Tea polyphenols also have high complexation affinity to metals, alkaloids, and biologic macromolecules such as lipids, carbohydrates, proteins, and nucleic acids.
The term "tannins" has been used by many to describe certain tea constituents. In industrial and botanic literatures, tannins are characterized as plant materials that give a blue color with ferric salts and produce leather from hides. Thus, tannins are a group of chemicals usually with large molecular weights and diverse structures. Monomeric flavanols, the major components in green tea, are precursors of condensed tannins. It would be more appropriate to use the term "tea polyphenols" or "tea flavanols" because they are quite distinct from commercial tannins and tannic acid.
"Herbal teas," which contain various known or unknown components, are quite different from tea (Camellia sinensis) and, as is my practice, will not be discussed in this section or in any section of The Tea Mans Tea Talk.
The Tea Man
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Copyright © February 1996