Phenolic Content in Tea
Natural phenols and polyphenols are compounds found naturally in plants. A group of natural phenols called the flavonoids are of most popular interest because researchers have found them to have the potential to contribute to better health.
Tea has one of the highest contents of flavonoids among common food and beverage products. A group of flavonoids called catechins is responsible for the majority of flavonoids in the growing tea leaves. Most of them are retained in processed green tea. The green tea catechin epigallocatechin-3-gallate (EGCG) is studied in great detail in relation to its health contributing potential.
According to a report released by USDA, in a 200-ml cup of tea, the mean total content of flavonoids is 266.68 mg for green tea, and 233.12 mg for black tea. The tea is to be prepared with 1 gram of tea leaves to 100 ml of hot water. The mean averages are much lower for instant tea mixes, decaffeinated, flavored, or ready-to-drink tea products.
Catechins: the main phenolics in green tea
Tea catechin is a most researched subject concerning the health potential of tea. The catechins in green tea are epicatechin (EC), epicatechin-3-gallate (ECg), epigallocatechin (EGC), epigallocatechin-3-gallate (EGCG), catechin, and gallocatechin (GC).
Catechins constitute about 25% of the dry weight of fresh tea leaf, although total catechin content varies widely depending on clonal variation, growing location, season, light variation, and altitude. They are present in nearly all teas made from Camellia sinensis, including white tea, green tea, black tea and oolong tea. Nevertheless, the so-called Pu-Erh teas, which are fermented with microbes, contain very little catechins due to their unique microbial breakdown process, whilst black tea contains very little catechins due to the full oxidization process which converts catechins to Theaflavins. 
Theaflavins: the polyphenol in black tea associated with health effects
The catechin monomer structures breakdown and become dimers theaflavins and oligomers thearubigins with increasing degrees of oxidation of the tea leaves.. Theaflavins directly contribute to the bitterness and astringency of steeped black tea.
Three main types of theaflavins are found in black tea, namely theaflavin (TF-1), theaflavin-3-gallate (TF-2), and theaflavin-3,3-digallate (TF-3). A number of studies on their possible health effects have shown positive results.
The mean amount of theaflavins in a cup of black tea (200 ml) is 12.18 mg.
About tannins in tea
Tea (Camellia sinensis) is a source of dietary polyphenols, notably tannin, which is an astringent, bitter polyphenolic compound, also found in many other plants. Those in green tea are mainly flavan-3ols (catechins). Although tea contains various types of polyphenols and tannin, it does not contain tannic acid. Tannic acid is not an appropriate standard for any type of tannin analysis because of its poorly defined composition.
4-Hydroxybenzoic acid, 3,4-dihydroxybenzoic acid (protocatechuic acid), 3-methoxy-4-hydroxy-hippuric acid and 3-methoxy-4-hydroxybenzoic acid (vanillic acid) are the main catechins metabolites found in humans after consumption of green tea infusions.
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- ^ Tannin Chemistry pg 11
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