The type of tea drunk varies between geographical regions of the World. In the East, green tea and oolong tea is favoured. In the West, black tea is the prefered drink. Green tea is the unfermented leaves of the Camellia sinensis plant, while black tea is the fermented leaves of the same plant. Oolong tea is partially fermented and therefore fits halfway between green and black tea. Darjeeling tea is also partially fermented. The degree of fermentation of tea is important because it affects the chemical composition of the final decoction and this can affect the taste, aroma and health effects of the tea. Tea is classified as both astringent and bitter in taste, and this relates directly to the chemical composition of the leaves used in the decoction. A number of volatile compounds have been identified in tea including linalool, geraniol, and (Z)- 3-hexenol, which are responsible in part for the distinctive taste and smell of tea. To date around 600 constituents of tea have been identified, highlighting its complex and unique composition.
The flavonoid content of tea is one component that can change significantly during processing. Catechins are the main flavonoid found in tea and evidence suggests they possess beneficial health effects. In green tea the catechin content is similar to the catechin content of the original plant material as the processing in green tea is minimal. However fermentation can reduce the concentration of catechins significantly through oxidation of the catechin to larger molecular weight products called tannins. One groups tannins in black tea are the thearubigins, the most common being theaflavin. Black and oolong tea, both being fermented, therefore have much higher concentrations of tannins than green tea. As well as affecting the flavour, the catechin and tannin content of tea is important because both catechins and tannins may have important health effects. In particular, catechins may confer protection against a number of Western lifestyle diseases including cardiovascular disease, type 2 diabetes, cancer and obesity.
A number of studies have investigated the association between various chemicals in teas and the aroma and taste they confer. For example one study1 identified a number of volatile fractions of darjeeling tea including vanillin (vanilla-like), 4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel), 2-phenylethanol (flowery) and (E,E,Z)-2,4,6-nonatrienal (oat-flake-like). Of these (E,E,Z)-2,4,6-nonatrienal was identified as a key odorant of tea, along with linalool and geraniol, confirming previous studies. In black tea leaves 42 odorants were identified and these were the same as the odorants in the resulting decoctions, suggesting that the chemicals are transferred to the water. However, geraniol was particularly concentrated in the decoction compared to the original leaf concentration. Tea, like red wine, is therefore a highly complex mixture of chemicals, the exact composition of which varies between varieties. The chemical composition is responsible for both the health effects and the taste and aroma of tea.