Chemistry of Tea

Tea shoot, contains a full complement of enzymes, biochemical intermediates, carbohydrates, proteins and lipids. In addition, tea shoot is distinguished by its remarkable content of polyphenols and methyl xanthines (caffeine and other purines, such as theobromine and theophylline). Popularity of tea as a beverage may be due to the presence of these two groups of compounds which are mainly responsible for the unique taste of tea, in addition to various compounds associated with tea aroma. Chemical composition of tea shoot varies with agroclimatic condition, season, cultural practice and the type of material.

Flavanols, flavonols, flavonol glycosides, polyphenolic acids and depsides put together are referred to as total polyphenols and make up about 30% of the dry weight in a tea shoot. Flavanols or catechins are the major compounds that are oxidisable in the tea leaf. (-) epigallo catechin (EGC) and (-) epigallocatechin gallate (EGCG) are the predominant catechins present in tea leaf. The catechins are located in the cytoplasmic vacuoles and play a significant role during fermentation.

The enzyme polyphenol oxidase plays a key role in tea fermentation; it is present in chloroplasts. Polyphenol oxidase (PPO) has remarkable specificity for the ortho-dihydroxy functional group of the tea catechins. In intact plant, the enzyme is not in contact with the substrates, flavanols. The idea behind fermentation is to bring the enzyme and substrate together in the presence of oxygen by rupturing the membrane so that polyphenols can diffuse into the cytoplasm. As a first step during fermentation, the catechins are oxidised to highly reactive, transient orthoquinones by PPO. The quinones, thus derived from a simple catechin and a gallocatechin, dimerise to produce theaflavins which are orange-red substances that contribute significantly to astringency, briskness, brightness and colour of tea beverage. Theaflavins comprise 0.3 to 2.0% of the dry weight of black tea. Theaflavins of black tea comprise a number of fractions namely theaflavin, theaflavin monogallate and digallate, epitheaflavic acid and isotheaflavin. As the gallation increases the astringency also increases and the proportions of theaflavin fractions present in black tea depend upon the method and conditions of manufacture.

Quality of tea is determined by the presence or absence of chemical compounds which impart colour, briskness, brightness, strength and flavour in the infusion. Majority of the chemicals imparting quality are produced during processing of the tea leaves. Biogenesis of such precursors is influenced, on one hand by the genetic and environmental factors which cannot be controlled and on the other by the cultural practices adopted in the field as well as by the conditions of processing, which can be controlled. Though biochemical changes start immediately after plucking (the crop shoots) the precise changes required for quality start from withering onwards.

Theaflavins are orange red substances that contribute significantly to the astringency, briskness, brightness and colour of the tea beverage. TF of black tea comprise a number of fractions namely, theaflavin, theaflavin monogallate and digallate, epitheaflavic acid and iso theaflanin. Thearubigins are complete condensation products of oxidised catechins with theaflavins. Together with HPS, TR contributes to the colour, mouth feel and body of the tea liquor. Caffeine is relatively a stable molecule and is a direct stimulant of the central nervour system. Together with TF imparts briskness to the tea liquor. High levels of caffeine indicate a good leaf standard. Caffeine decreases with the maturity of the crop shoots. Volatile flavour constituents In addition to the biochemical constituents that give the tea its unique taste, black tea is also well known for its characteristic aroma. Terpenoids and amino acids degrade to produce linalool, phenyl acetaldehyde, phenyl ethanol and methyl salicylate which contribute to desirable aroma of tea.

Black tea also contains minerals such as calcium, phosphorus, iron, sodium potassium and vitamins (A, B1, B2, Niacin & C) besides the biochemical constituents such as amino acids and soluble sugars. Polyphenols work out to about 15-20% in the black tea and it comprises the catechin fractions. Compared to black tea, green tea has a higher content of polyphenols, almost around 25-30% on dry weight basis. All the above constituents may vary in their content according to the type of tea plants, environment, agronomic practices, pest & disease incidence and manufacturing practices. Table 1 provides list of chemicals present in black tea while Table 2 gives the names of compounds, commonly associated with the sweet aroma of black tea.