In steeped tea, polyphenols are largely responsible for astringency. The term polyphenol simply refers to a categorisation of compounds composed of many phenolic groups, hence the name poly-phenol. These compounds are plant metabolites produced as a defense against insects and other animals and are the most abundant compounds in tea comprising as much as 30-40% of both freshly plucked tea leaves and solids in tea liquor. The bud and first leaf have the highest concentration of polyphenols and polyphenol levels decrease in each leaf moving down the plant.The tender shoots consisting of two leaves and the bud are, therefore, considered ideal harvest of tea plant for quality tea production.
Among the polyphenols flavanols, flavanoids and phenolic acids (gallic, cumaric or caffeic) are present predominantly in tea leaf. Within the flavonoid group, flavanols (also known as flavan-3-ols) are the most prevalent. Flavanols are also referred to as tannins, and during oxidation are converted to theaflavins and thearubigins,the compounds responsible for the dark color and robust flavours notably present in black teas The primary polyphenols of green tea are devoid of tanning properties and lack of colour in tea infusions. These are bitter in taste and most of these are not found in other plants. The major flavanols in tea are catechin (C), epicatechin (EC), epicatechin gallate (ECG), gallocatechin (GC), epigallocatechin (EGC), and epigallocatechin gallate (EGCG). EGCG is the most active of these catechins. Tea flavanols are sometimes collectively referred to as catechins. Besides flavanols, tea flavonoids also include flavonols, flavones, isoflavones, and anthocyanins; all of which contribute to the colour of a tea’s infusion and its taste.