THE CHEMISTRY OF TEA

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What is tea?

Tea (Camellia sinensis) is one of the oldest and most consumed beverage in the whole world. Tea is grown in about 30 countries but is consumed worldwide. Traditionally, tea has been known to have potentially beneficial effects were not documented by laboratory studies until 1970. However, current studies have revealed the chemical composition and biological effects of tea at a molecular level.

According to the different ways of processing, tea is usually divided into four basic types; green tea, black tea, oolong tea and white tea. Shoots, young leaves, mature leaves and stems of tea plant are usually used as materials for processing various tea products.




Main composition of tea

There are approximately 600 traces of aroma compounds in tea leaves, some of which are lost and some that are released during the manufacturing process.  Once plucked, tea leaves begin to wither, their cell walls begin to break down and chemical compounds begin to form new chemical compounds, not all of which are water soluble.

The main constituents of tea leaves belong to the polyphenol group accounting for 25 to 35% on a dry weight basis (Balentine, 1997; Hara et al., 1995d). The polyphenols in tea mainly include the following six groups of compounds: flavanols, hydroxyl-4-flavanols, anthocyanins, flavones, flavonols and phenolic acids.

  Theaflavin.

Catechin


Polyphenols

Important and characteristic tea polyphenols are the flavanols of which catechins (flavan-3-ols) are pre-dominant and the major ones are: (-)-epicatechin (EC),  (-)-epicatechin gallate (ECG),  (-)-epigallocatechin (EGC), (-)-epigallocatechin gallate (EGCG), (+)-catechin (C), and (+)-gallocatechin (GC) (Hara et al., 1995a; Liang et al., 2003). These compounds contribute to the bitterness, astringency and sweet aftertaste of tea beverages (Hara et al., 1995b).

Minerals

Mineral constitutes about 4 to 9% of the inorganic matter of tea (fluorine, potassium, aluminum, iodine, selenium, nickel, and manganese.

Aroma compounds

Aroma or volatile organic compounds (VOCs) are important factors of tea quality, especially for the sensory traits of each tea type. To obtain the VOCs, usually, there are several extraction methods, such as simultaneous distillation extraction, solvent-assisted flavor evaporation, solid-phase micro-extraction, and analysis of VOCs by means of headspace measurements.

Taste and color of tea

A cup of infusion of made tea is completely different from the infusion of fresh tea flushes in color, taste and aroma. These characteristics are developed during the manufacturing process after the harvesting of tea flushes. Tea flush is generally a reference to young shoots of tea that consists of terminal bud and two adjacent leaves. In fresh tea flush there exists a wide variety of non-volatile compounds; polyphenols, flavonols and flavonol glycosides, flavones, phenolic acids and depsides, amino acids, chlorophyll and other pigments, carbohydrates, organic acids, caffeine and other alkaloids, minerals, vitamins and enzymes. The total phenols in tea flush ranges from 20 to 35% . A series of changes occur in the process of manufacturing tea (made tea). The three basic types of manufacturing tea are green, semi-fermented and black tea. They differ mainly in the degree of fermentation. Green tea undergoes little or no fermentation, and black tea is produced as a result of full fermentation. Semi-fermented (oolong tea) is a product of partial fermented tea. The difference in color, taste and aroma of various teas are caused by the manufacturing.




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