Tea and Its Chemical Constituents
Tea from the leaves of Camellia sinensis, a plant of the Theaceae family, is consumed by more than two thirds of the word’s population and is the most popular beverage next only to water.
The tea plant is cultivated in more than 30 countries. Approximately 2.5 million metric tons of dried tea are produced annually.
Production involves a series of drying and fermenting steps.
Green tea is consumed primarily in some Asian countries, such as Japan, China, Korea and India and a few countries in North Africa and Middle East.
Black tea is consumed in some Asian countries and Western nations. Oolong is consumed in southeastern China and Taiwan. There are also many products sold in the market as herbal tea, which are not derived form the plant camellia sinensis. They are extracts of several herbs.
Teas differ regarding how they are produced. Green tea production involves steaming fresh leaves at elevated temperatures, followed by a series of drying and rolling so that chemical composition essentially remains to that of the fresh leaves.
Black tea production involves withering plucked leaves, followed by extended fermentation. Thus, depending on the extent of fermentation, the chemical composition of most black teas is different.
Oolong tea is made by solar withering of tea leaves followed by partial fermentation.
Tea leaves are unique because they are rich source of catechins and theanine. These constituents impart flavor and taste to tea beverages. Green tea also conations caffeine, theophylline and theobromine, the principle alkaloids, and gallic and theanine, the phenolic acids.
Black tea in addition to the catechins, also contain thearubigins, theaflavins and caffeine.
Oolong tea contains monomeric catechins, thearubigins, and theaflavins.
Caffeine is a natural component of all teas. Although a serving tea usually contains less than half the caffeine of coffee, actual caffeine levels are dependent on specific blends and the brew strength.
In general green tea contains 3 – 6% and black tea contains 2- 4% of dry eight caffeine.
Tea contains phenolic acids, mainly caffeic, quinic and gallic. Theanine is an amino acid found only in tea leaves, which imparts a pleasantly sweet taste to tea. It is degraded to Glutamic acid and has relaxation effects in humans. Up to one third of the dry weight of tea comprises catechins and other polyphenols such as quercitin, myricitin and kaempferol.
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