**1. Introduction**

Tea, which is prepared from the leaves of the *C. sinensis*, and is the second most popular beverage in the world, after water. Green tea, oolong tea, and black tea are made primarily from the young green shoots of the tea plant [1]. Only the young, top leaves and the unopened leaf bud are used in fine teas, which is one of their distinguishing characteristics. As a result, immature, light-green leaves are preferred for tea production, whereas mature and old leaves, which have a darker green colour, are unsuitable due to their unpleasant flavour. Tea beverages are classified as green (unfermented), white (lightly fermented), oolong (semi-fermented), or black (fermented) based on their manufacturing method (fully fermented) [2, 3]. It is cultivated in more than 50 countries of the world and the main producers of tea are China (2,473,443 t) and India (1,325,950 t) as per 2017 statistics (**Table 1**) [7].

Phenolic acids, terpenoids, flavonoids, alkaloids, tannins, and phytosterols are the major bioactive constituents in the plant leaves [8]. Tea chemical composition depends on the cultivars, environmental factors and different manufacturing process [9]. Flavonoids like flavanols (flavan-3-ols), flavonols, flavones, flavanones, and anthocyanidins are important components of tea leaves, accounting for up to 30% of the dry weight of the leaves [10, 11]. Catechins which are group of flavan-3-ols are the major bioactive compounds in fresh tea leaves, among them epicatechin (EC),


#### **Table 1.**

*Composition (%) of green, black and infusion [4–6].*

**Figure 1.** *The main phenolic compounds of tea polyphenols [12].* *Phenolic Compounds in Tea: Phytochemical, Biological, and Therapeutic Applications DOI: http://dx.doi.org/10.5772/intechopen.98715*

epigallocatechin (EGC), epicatechin gallate (ECG), epigallocatechin gallate (EGCG), catechin and gallocatechin are majorily found (**Figure 1**) [13, 14].

Tea is widely acknowledged to have numerous health benefits, including antioxidant activity, anti-inflammatory activity, anti-microbial effects, and anti-carcinogenic effects when consumed regularly. The phenolic compounds in tea are thought to be responsible for these effects. As a result, tea phenolics are thought to be have valuable phytochemicals and received a great deal of attention [15].

## **2. Different types of tea**

Type of tea produced depends on the various fermentation processes, white tea (sundried fresh leaves), green tea (heated or steamed fresh leaves), and black tea (fermented leaves) [16]. Among different tea types, black tea is produced highest about 76–78%, followed by green tea (20–22%) and oolong tea (2%) accounting for worldwide production [17].

#### **2.1 Black tea**

Black tea which is prepared from the young tender shoots of *C. sinensis* is consumed widely as a non-alcoholic beverage. It is most popular in India, Europe and North America [18]. The phenolic composition of black tea differs significantly from green tea due to the fermentation process resulting in the formation of condensation and oxidation products such as thearubigins and theaflavins, which might be due to the action of polyphenol oxidases (PPO) [19, 20]. In comparison to green tea, 85% of catechin in black tea is transformed into theaflavin-3-3′-digallate and thearubigin [21]. Astringency and brightness of tea is due to theaflavins, whereas colour and mouthfeel is because of thearubigins [18]. Theaflavins and thearubigins (2 to 6%) account for catechin content of 3 to 10% (w/w) of 10 to 20% (w/w) of the dry weight of black leaves (**Figure 2**) [18].

Rechner et al. [22] has reported numerous *in vitro* and *in vivo* effects of tea polyphenols, including antioxidant, anticarcinogenic, and hypolipidemic properties. The bioavailability and metabolism of individual polyphenolic constituents of black tea (flavan-3-ols, flavonols, hydroxybenzoic acids, hydroxycinnamates) in humans have been reported by Liebert et al. [23]. Rababah et al. [24] show that a cup of black tea, as it is traditionally brewed in the UK, is an excellent source of polyphenols, containing up to six different classes of polyphenols and having higher antioxidant activity than other dietary sources. Treatment of the black tea brew with simulated gastric juice resulted in a significant increase in the identified theaflavins, implying partial cleavage of thearubigins in the gastric lumen environment. As a result, black tea can be considered a good source of polyphenols and/or antioxidants [25].

#### **2.2 Green tea**

Green tea is prepared by leaves of *C. sinensis* which after harvest are heated with rolling immediately for inactivating the polyphenol oxidase (PPO) enzyme which is responsible for oxidation of tea catechins into theaflavins and thearubigins. Green tea is prepared by steaming of fresh leaves and drying at higher temperature to avoid the polymerisation and oxidation of polyphenols [26].

Green tea is a popular tea that is usually consumed as an infusion with a pleasant taste and is thought to have a positive effect on general health even at high doses of 8 to 16 cups per day [27]. Green tea leaves are high in bioactive compounds, especially

**Figure 2.** *Mean percentile composition of different classes of polyphenols in consumer brews of different black teas [22].*

phenolic compounds with antioxidant activity (**Figure 3**). Although recent studies have identified several other phenolic compounds at lower concentrations, particularly flavonols and phenolic acids, the increased proportion of catechins is related to biological functionality [29]. The stability of green tea flavanols depend on the intactness of the plant cell [16]. Green tea consumption has been shown in scientific studies to improve general health and reduce the risk of severe diseases. This is a trend with promising and positive results to help with body weight control [30], UV radiation protection, physical functional performance [27], oral health, bone health, and other physiological effects [23]. Specific diseases, including those with severe consequences, such as neurodegenerative and cardiovascular diseases, have received special attention.

Green tea health benefits are linked to polyphenolic compounds, which have piqued the interest of the food industry and researchers [31]. Green tea can be used in the formulation of some products to boost antioxidant activity for nutritional or technological purposes. Several mechanisms, similar to those seen in biological structures, can be used to prevent lipid oxidation in food (e.g., free radical scavenging and metal-chelating activity). Lipid oxidation can change physical–chemical and sensory properties like colour, flavour, and taste. Among the many foods that require the use of antioxidants, meat and muscle products are particularly vulnerable to lipid oxidation, necessitating the addition of antioxidants to extend shelf life [23, 29, 32].

#### **2.3 Oolong tea**

Oolong tea is a semifermented tea, which is less fermented than black tea. Young green shoots (usually the top three leaves of each branch) are freshly harvested in the early morning and allowed to wither in the sunlight for a few hours before undergoing the semifermentation process, in which tea leaves are oxidised, pan fired at 200°C, rolled into a ball shape, and then dried in a specialised oven at various desired temperatures [33].

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

#### **Figure 3.**

*Major green tea polyphenols [28].*

The partial fermentation of oolong tea produces polymerised polyphenols such as procyanidins (condensed tannins) which is unique to the limited time of oxidation process. Oolong tea contain both properties of green tea and black tea with catechins and theaflavins, however it contains half the content epigallocatechin-3-gallate in comparison to green tea [19, 34]. The components of oolong tea are classified in **Table 2**.

Oolong tea, which has a taste and colour between green and black tea, is primarily produced in China's Fujian [33] and Guangdong provinces [37], as well as in Taiwan [38]. Oolong tea absorbs a lot of moisture from the air after a long time in storage, so it needs to be refined by drying on a regular basis. In general, old oolong tea refers to oolong teas that have been stored for more than five years and refined annually through a professional drying process. Experientially, the longer oolong tea is stored and gradually oxidised, the better it tastes and has beneficial effects on human health. As a result, fermentation (oxidation) and drying are two critical steps in the production of oolong tea [39].

Wang et al. [37] discovered that the main bioactive compounds in oolong tea, such as phenolics and flavonoids, have remarkable antioxidant activity and inhibitory


*Phenolic Compounds - Chemistry, Synthesis, Diversity, Non-Conventional Industrial...*

#### **Table 2.**

*Components of oolong and green tea [35, 36].*

potential on the growth of 4 T1 murine breast cancer cells in vitro. Thus, after further research, the phenolic enriched extracts of Tieguanyin tea are expected to have a potential application in the food and pharmaceutical industries. More alike, it has similar constituent of green tea.
