Tea As A Healthpromoting Beverage

Tea is a frequently used beverage worldwide. Tea is a hot-water extract of the leaves of the plant Camellia sinensis. Upon harvest, the best teas are obtained by collecting the top two leaves and the bud of the tea bush (10,11). The leaves contain the polyphenol epigallocatechin gallate and an enzyme, polyphenol oxidase. When the leaves are withered and steamed, the polyphenol oxidase is inactivated, yielding green tea upon drying. If the withering step is omitted, and the leaves are steamed, the ground product is white tea, commercially available but not used frequently. Upon more elaborate processing, crushing the leaves and incubating for about 60 min, the polyphenol oxidase converts the polyphenol to other polyphenols, such as theaflavin and thearubigin, typical of black tea (Fig. 1) (Table 1). With a lesser time of incubation, such as

OH

(-)-Epigallocatechin gallate

Figure 1 The main polyphenol in green tea is (-)-epigallocatechin gallate (EgCg) (right), which accounts usually for about 30% of the dry weight of the tea leaf. The tea leaf contains an enzyme, polyphenol oxidase, that is deactivated when the withered, macerated tea leaves are heated by steam or in a pan, yielding a product containing mainly EgCg. However, when the cut, macerated, or ''rolled'' leaves are transported in a moving band surrounded by warm 40oC air, the enzyme-mediated oxidation (wrongly called fermentation) occurs. One popular method of production is ''crushing, tearing, curling, named CTC.'' Partial oxidation for about 30 min yields oolong tea. Full biochemical oxidation, which requires 90-120 min, yields black tea, containing some theaflavin (left), more complex thearubigins, and also theanine, which account, in part, for the flavor of black tea. Since the original tea leaf contains caffeine, green, oolong, and black teas contain caffeine in the amount of 40-50 mg in a 125-150-mL tea cup, made with a 2.25-g tea bag.

Theaflavin gallates

(-)-Epigallocatechin gallate

Figure 1 The main polyphenol in green tea is (-)-epigallocatechin gallate (EgCg) (right), which accounts usually for about 30% of the dry weight of the tea leaf. The tea leaf contains an enzyme, polyphenol oxidase, that is deactivated when the withered, macerated tea leaves are heated by steam or in a pan, yielding a product containing mainly EgCg. However, when the cut, macerated, or ''rolled'' leaves are transported in a moving band surrounded by warm 40oC air, the enzyme-mediated oxidation (wrongly called fermentation) occurs. One popular method of production is ''crushing, tearing, curling, named CTC.'' Partial oxidation for about 30 min yields oolong tea. Full biochemical oxidation, which requires 90-120 min, yields black tea, containing some theaflavin (left), more complex thearubigins, and also theanine, which account, in part, for the flavor of black tea. Since the original tea leaf contains caffeine, green, oolong, and black teas contain caffeine in the amount of 40-50 mg in a 125-150-mL tea cup, made with a 2.25-g tea bag.

TABLE 1 Composition of Catechins in Polyphenols

Percent by weight

Polyphenon Polyphenon Polyphenon

(—)-Epigallocatechin gallate (EGCg)

Total catechins

Caffeine

Theaflavin

Thearubigins

about 25-35 min, an intermediate product, oolong tea, popular in southern China and in Taiwan, is obtained.

We have described the history of tea and its use worldwide, including the original discovery of tea in China some 4000 years ago, in the form of green tea, and later of black tea in northern India. Currently, green tea is used mainly in China, Japan, and North Africa (11).

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