**4.2 Substitution pattern of flavonoids**

The main flavonoid classes and some of their discovered structural variants are shown in **Table 2**. Within the primary classes, flavonoids' structures differ significantly by substitutions such as hydroxylation, glycosylation, hydrogenation, methylation, malonylation, and sulphation etc. Many flavonoids are found in nature


#### **Table 2.**

*Some example of substitution pattern of flavonoids [9, 49, 59].*

#### *Chemistry and Role of Flavonoids in Agriculture: A Recent Update DOI: http://dx.doi.org/10.5772/intechopen.106571*

as flavonoid glycosides, and D-glucose, L-rhamnose, glucorhamnose, galactose, lignin, and arabinose are some examples of carbohydrate substitutes [9, 49, 50]. The most prevalent flavonoid glycosides in the diet are quercitrin, rutin, and robinin. Intestinal flora hydrolyzes them to create the physiologically active aglycone (sugarfree flavonoid). Due to its prominence as the primary flavonoid present in foods, quercetin has been the focus of numerous studies examining the biological impacts of flavonoids [9, 49, 50].

#### **4.3 Polymerization of flavonoids**

In term of units of flavonoids molecules there are three types of flavonoids namely monomers, dimers, and oligomers. There are huge differences between the molecular weights of different monomers. Polymers of flavonoids make up condensed tannins. Epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin gallate are the four primary catechin components found in tea tannins. The main catechin in tea, epigallocatechin gallate, accounts for more than half of the total catechin content. The dimeric theaflavins and polymeric thearubigins of black Indian tea, which generate brightness and astringency, respectively, are produced by enzymatic oxidation of tea catechins during fermentation of macerated tea leaves [56, 57]. Thearubigins come in a wide variety of sizes, from molecules with up to 100 flavonoid units to oligomers of four or five units [56]. Green "Chinese" tea does not undergo fermentation during processing, in contrast to black tea, hence its flavonoids largely exist as monomers. The anthocyanins and other flavonoids in red wine polymerize to create tannins, which give the wine its distinctive hues, tastes, and astringency [58, 59].
