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 be of health contributing potential.[1][2][3][4][5]
Tea has one of the highest contents of flavonoids among common food and beverage products.[6] A group of flavonoids called catechins is responsible for the majority of flavonoids in the growing tea leaves.[7] Most of them are retained in processed green tea.[7] Epigallocatechin 3-gallate (EGCg), aka EGCG, is the single most studied catechin in relation to health contributing potential.[4]
Other flavonoids in tea, in much lower concentrations, include kaempferol, myricetin, quercetin, and minute quantities of apigenin and luteolin.[6][7]
According to a report released by USDA, in a cup of tea of 200 ml, the mean total content of flavonoids is 266.68 mg for green tea, and 233.12 mg for black tea.[6] The tea is to be prepared with 1 gram of tealeaves to 100 ml of hot water. The mean averages are much lower for instant tea mixes, decaf, flavored, or ready-to-drink tea products.[6]
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Tea catechins is a most researched subject concerning the health potential of tea.[1][4] The catechins in green tea are epicatechin (EC), epicatechin 3-gallate (ECg), epigallocatechin (EGC), epigallocatechin 3-gallate (EGCg), catechin, and gallocatechin (GC).
Of all the catechins, EGCg has the most scientific attention, being singled out in a number of them as a key contributive element to the possible health effects of tea.[1][2][3][4][5]
The catechins are turned from the monomer structure to become the dimers that are the theaflavins and the oligomers that are thearubigins.[7]
There are chiefly three types of theaflavins in black tea, namely theaflavin (TF-1), theaflavin-3-gallate (TF-2), and theaflavin-3,3-digallate (TF-3). A number of studies have been done on their possible health effects with positive results.[8][9]
The mean amount of theaflavins in a cup of black tea (200ml) is 12.18 mg.[6]
Thearubigins are of unknown structure[7] and no specific studies have been focused on them for health effects.
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, tea does not contain tannic acid.[10] Tannic acid is not an appropriate standard for any type of tannin analysis because of its poorly defined composition.[11] Catechins constitute about 25% of the dry weight of fresh tea leaf,[12] although total catechin content varies widely depending on clonal variation, growing location, seasonal/ 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 post-fermented, contain very little -if at all- catechin as a result of long time fermentation. [13]
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.[14]