To gain a deeper insight into the physiological and pharmacological effects of tea, it is crucial to examine its chemical makeup and bioactive components. Tea comprises volatile and non-volatile compounds, with the former predominantly contributing to its aroma and the latter determining its color and taste.
Chemical composition of tea flush
Tea flush is the young shoots of tea consisting of the terminal bud and two adjacent leaves. It contains nonvolatile compounds such as polyphenols, flavonols, 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 chemical composition of tea leaves varies depending on leaf age, the clone, soil and climate conditions, and agronomic practices.
Tea polyphenols include mainly six groups of compounds, with flavonols (mainly the catechins) being the most important group. The total polyphenols in tea flush range from 20% to 35%. Four major catechins, namely (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG), and (-)-epicatechin (EC), constitute around 90% of the total catechin fraction, while (+)-catechin (C) and (+)-gallocatechin (GC) are present about 6% of the fraction. There are some minor catechins that constitute less than 2% of the total catechins. The catechins that are water-soluble, colorless compound contribute to astringency and bitterness in green tea.
There are three major flavonol aglycones in the fresh leaf, kaempferol, quercetin, and myricetin, which occur both as free flavonols and as flavonol glycosides. These compounds contribute to bitterness and astringency in green tea. The glycosidic group may be glucose, rhamnose, galactose, arabinose, or rutinose.
Tea contains amino acids, with theanine being the most abundant. Theanine is unique to tea and is mainly found in younger leaves. Theanine is synthesized from glutamic acid and ethylamine in the roots of the tea plant.
Tea contains various sugars such as glucose, fructose, sucrose, raffinose, and stachyose, which contribute to the sweet taste of tea infusion. Polysaccharides in tea can be separated into hemicellulose, cellulose, and other extractable polysaccharide fractions. Polysaccharides extracted from manufactured tea have been shown to have a decreasing effect on blood-glucose levels.
Caffeine is the primary alkaloid found in tea, with theobromine and theophylline found in small amounts. Other alkaloids such as xanthine, hypoxanthine, and tetramethyluric acid are present in trace amounts.
Many volatile compounds contribute to the aroma of tea, which can be divided into primary and secondary products. Primary products are synthesized by the tea plant and are present in the fresh green leaf, while secondary products are produced during tea manufacture. The aroma complex of tea varies with the country of origin, altitude, and season.
Read more: Chemical Composition of Tea Flush
Chemical composition of made tea
Tea manufacturing involves three basic types: green, semi-fermented (oolong), and black tea, which differ in the degree of fermentation. For green tea, the major steps are spreading out, fixing, rolling, and drying. For black tea, the major steps are withering, rolling, fermentation, and drying.
The polyphenol content decreases during green tea processing but undergoes marked changes during black tea processing. Enzymatic oxidation of catechins by polyphenol oxidase forms two groups of polyphenol compounds unique to black tea: theaflavins and thearubigins. Theaflavins account for 0.3-1.8% of the dry weight of black tea and contribute to the tea’s brightness and briskness. Thearubigins, which constitute 10-20% of the dry weight of black tea, have not been characterized yet. Amino acid content, particularly theanine, may increase during tea processing and affects tea quality.
Theanine is the most abundant amino acid found in tea and is important in the taste of green tea. Theanine has two enantiomers: L- and D-theanine. The ratio of D-theanine to L-theanine increases under high temperature of storage, which may indicate long-term storage or affect tea quality grading. Theanine has potential bioactivity, such as its ability to lower blood pressure and enhance the antitumor effect of doxorubicin.
The amount of caffeine in tea can change during processing. Withering, the early step in tea processing, can increase caffeine levels due to the loss of other components. However, the firing process of tea manufacturing can decrease caffeine content. Caffeine makes up 5-10% of the solid material extracted from tea when mixed with boiling water, while theobromine accounts for 0.3%. Contrary to common belief, ordinary tea does not contain theophylline. Caffeine has stimulatory effects on the central nervous system, and can be used therapeutically. However, excessive caffeine intake can have negative effects. The average daily caffeine intake per capita varies by country, with the no-effect dosage recommended as 40mg/kg/day. The caffeine content of tea varies by type and infusion time, and moderate tea consumption is generally below the recommended daily dosage level.
The aroma compounds in tea change during the tea manufacturing process. Some increase, while others decrease. Over 600 aroma compounds have been found in tea, including hydrocarbons, alcohols, aldehydes, ketones, acids, esters, lactones, phenols, nitrogenous compounds, sulfur compounds, and miscellaneous oxygen compounds. Most aroma compounds are likely derived from carotenes, amino acids, lipids, and terpene glycosides. Some aroma compounds have been found to have biological effects, but more research is needed.
Different kinds of tea can vary in composition and thus have different biological effects. There are over 250 different specialty tea products on the grocer price list, and over 200 varieties in China alone. These include green tea, black tea, semi-fermented tea, and miscellaneous teas.
Read more: Chemistry of Color and Taste of Made Tea