The predominant phytochemicals of beverage tea are catechins, theaflavins, and thearubigins. Catechins are one form of a class of polyphenolic compounds termed 'flavonoids,' which are all 2-phenyl benzo-pyran-based compounds. The flavonoids include six major structurally related subclasses of compounds: flavones, flavonols, flavanones, anthocyanidins, isoflavones, and flavanols. Catechins belong to the flavanol subclass and are structurally defined as flavan-3-ols. The major catechins of the tea leaf are epicatechin, epicatechin gallate, epigallocatechin, and epigallocatechin gallate. These four catechins comprise approximately 25% of the weight of a dry tea leaf. Flavonols (kaempferol, quercetin, and myr-icitin glycosides) are minor flavonoid components of the tea leaf (approximately 3% of dry weight). Polysaccharides and cellulose (approximately 20% of dry weight) and protein (approximately 15% of dry weight) are other major components of tea leaf, and other nutrients and compounds comprise less than 5% of dry weight each.
Green, oolong, and black are the predominantly consumed types of tea. They are produced from the leaf of the same plant, Camellia sinseis. Green tea, which is consumed largely in Asia, is processed with the intent to minimize leaf damage and fermentation. Catechins constitute 30-40% of the green tea solids. To produce oolong and black tea, the leaves are rolled and fermented. This processing releases polyphenol oxidase that initiates polymerization and oxidation of the catechins to theaflavins and thearubigins. In contrast to the heterogeneous and unknown chemical structure of thearubigins, thea-flavins are chemically well defined. There are four primary theaflavins in black tea and they represent between 1 and 6% of the dry weight of the solids (theaflavin, theaflavin 3-gallate, theaflavin 3'-gallate, and theaflavin 3,3'-digallate). Thearubigins represent between 10 and 20% of the dry weight of black tea solids. Oolong tea is processed and fermented to a lesser degree than black tea, and therefore it contains a higher concentration of the theaflavins and lower concentration of thearubigins than black tea. Black tea is popular in Western countries, and oolong tea is sold commercially in the United States and is often served in Chinese restaurants. Theaflavins and thearubigins are responsible for the red-amber color of oolong and black tea and for its astringency.
Another important group of flavan-3-ol compounds are the procyanidin polymers. These compounds are catechin polymers linked by C4 to C6 and C4 to C8 bonds. The most predominant polymers are those ranging from dimers to decamers and are found in a variety of foods, including chocolate, cocoa, cereal grains, nuts, fruits, and vegetables.
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