**3. Phenolic composition in different tea types**

The tea leaves are good source of several polyphenols and oxidative enzymes, hence they are selected for preparation of different types of the tea. These tea polyphenols are basically tea flavonoids, earlier known as tannins. The catechins of flavonoid group are the predominant polyphenols of fresh tea, which account for 12–24% of dry weight. Other than catechins, the tea leaves also contain other polyphenols such as phenolic acids, anthocyanidin and flavonols along with their glycosides [40, 41]. Depending on the harvesting season, cultivars, cultivation conditions and manufacturing process the polyphenolic content varies in different types of tea. The major catechins in tea leaves are (−)-epigallocatechin gallate (EGCG; 9170–14900 mmol/100 g leaf), (−)-epi-gallocatechin (ECC; 8060–17900 mmol/100 g leaf), (−)-epicatechin gallate (ECG; 1400– 2350 mmol/100 g leaf [42]. The catechin content in different tea types vary depending on fermentation process, green tea produced without fermentation contain highest amount of catechins among which EGCG is the major catechin found. Considering EGCG as an abundant catechin in all tea types, as the fermentation process is increased in different tea types the EGCG content decreased in different tea types; green tea (70.2 mg/100 g), oolong tea (34.48 mg/100 g), and black tea (9.36 mg/100 g) [43].

Flavonoids are phenolic compounds that are divided into several sub-classes: anthocyanidins, flavanones, flavanols, flavones, flavonols, and isoflavones. These sub-classes

#### *Phenolic Compounds in Tea: Phytochemical, Biological, and Therapeutic Applications DOI: http://dx.doi.org/10.5772/intechopen.98715*

share a basic structure of 15 carbons with a three-carbon bridge connecting two aromatic rings in the C6–C3–C6 configuration. Along with flavonoids, phenolic acids, which are divided into hydroxybenzoic acids and hydroxycinnamic acids, are an important group. Gallic acid, also known as 3,4,5-trihydroxybenzoic acid, has a relatively simple structure. This compound serves as the foundation for hydroxybenzoic acids and other derivatives with antioxidant activity, such as ellagic acid [44–46]. The hydroxycinnamic acid derivatives, on the other hand, have p-coumaric acid as the basic structure, which is formed by an aromatic ring with one hydroxy substitution and one propenoic acid.

In tea leaves almost 20 different flavonols and their glycosides have been detected which include quercetin, kaempferol, and myricetin which account for 2–3% of the water-soluble extractive in green tea [47, 48]. The main flavonol glycosides found in tea are rutin, quercetin glycoside and kaempferol glycoside with 0.05–0.15%, 0.2–0.5% and 0.16–0.35%, respectively of dry weight [47]. The other group of phenolic compounds found in tea are phenolic acids which account for 5% of tea leaf dry weight. The major phenolic acids found in tea are gallic acid, chlorogenic acid and theogallin which account for 0.5–1.4%, 0.3% and 1–2%, respectively of dry weight content. Whereas ellagic acid and m-digallic acid are found in trace amounts. These phenolic acids act as precursors of catechin gallate and in association with other polyphenols have an effect on the astringency of tea beverage [40, 41]. Anthocyanidins and leucoanthocyanidins are another group of phenolic compounds in fresh tea leaves with 2–3% of dry weight content. Anthocyanidins such as cyanidin, pelargonidin, delphinidin and tricetinidin are approximately 0.01% of dry weight in tea leaves, however they may reach up to 1.0% in processed tea which gives a purple colour to the tea preparation with some bitterness [40, 41].

Infusions of black tea contain relatively high levels of catechins, ranging from 102 to 418 mg of total catechins/L [49]. The four major tea catechins are enzymatically oxidised and converted to various oxidation products containing black tea polyphenols during tea fermentation. Characteristic pigments of these oxidation products are typically classified into two major groups: theaflavins and thearubigins [48, 50]. Theaflavin content of black tea leaves ranges between 0.8 and 2.8 percent, depending on fermentation conditions. Thearubigins, on the other hand, can account for up to 60% of the solids in black tea infusions [50]. In black tea, theaflavin, theaflavin 3-gallate, theaflavin 3′-gallate, and theaflavin 3,3′-digallate are the main theaflavins formed by the reaction of quinones derived from a simple catechin and a gallocatechin [48]. With relative molecular masses ranging from 700 to 40,000 Da, thearubigins remain ambiguous, and little is known about their chemical structures [48, 50]. Aside, from these unidentified colourless oxidation products, tea fermentation produces a plethora of other unidentified colourless oxidation products.

The polyphenols in green tea accounting for 40% of the dry leaf weight include different polyphenolic groups such as flavonols (quercetin, kaempferol and rutin), phenolic acids, leucoanthocyanins, caffeine and theanine [4]. Theanine is a nonprotein amino acid which account for 1.5–3.0% of dry weight of tea leaf and is almost 50% of the total amino acids content in tea. The natural theanine in tea is usually in L-form which significantly contributes to the sweet taste in green tea infusions [41].
