# Polypphenols: Natural Antioxidants to Be Used as a Quality Tool in Wine Authenticity
## 2 Structural Information
### Flavonoid Compounds
#### 2.2.4 Flavan-3-ols  
Flavan-3-ols, also named as flavanols or catechins, are mainly located in the solid parts of the berry (stem, skin, seeds), with seeds presenting the highest concentration [62]. Structurally monomeric flavan-3-ols are benzopyrans displaying a carbon chain between C\({}_{2}\) and C\({}_{3}\), a hydroxyl group at position three of the central ring, and absence of carbonyl moiety in position 4 [17]. Flavonols possess two chiral carbons, able to form four diastereoisomers for each of them. In _Vitis vinifera_ grape varieties, (+)-catechin and (-)-epicatechin are the principal compounds followed by (+)-gallocatechin, (-)-epigallocatechin, and the gallbladder form of (-)-epicatechin (i.e., (-)-epicatechin gallate), which is found at lower concentrations [63; 18] (Figure 6). Flavan-3-ols are responsible for influencing the mouthfeel of wines imparting astringency. This tactile sensation results from fl  
Figure 5: Structure of the six main flavonol aglycons. salivary proteins and glycoproteins causing a loss of lubricating power loss of the saliva or with the glycoproteins of the mouth epithylium [64]. It has been reported that flavanol bitterness decreases from the monomer to trimer flavan-3-ols. Oligomeric and polymeric forms can occur by condensation of the monomeric flavan-3-ols, which are called proanthocyanidins or condensed tannins. Proanthocyanidins have the ability to release anthocyanin under heated acidic conditions resulting from the cleavage of the interflavanic bond. In _Vitis vinifera_ varieties, proanthocyanidins are further separated into procyanidins and prodelphinidins that comprise of (epi)catechin and (epi)gallocatedchin monomeric units, respectively, as well as (\(-\))-epicatechin-3-O-gallate subunits. Both groups are present in the skins and wines while procyanidins are localized only in grape seeds [18]. Proanthocyanidins are further distinguished by the length of the proanthocyanidin chain, into oligomers with mean degree of polymerization (mDP ) lower than 5 and polymers (mDP \(>\) 5) and by the type of the interflavanic bond into A or B type procyanidins as extensively described in past studies [17; 18; 64]. The mDP can vary from two or three subunits to over 30, meaning that compound with a mDP of 10 contains 10 flavan-3-ol subunits in it. The mDP values of skin, seed, and wine flavan-3-ols can vary greatly and are mostly influenced by maturity stage and seasonal variation thus they cannot be considered as a marker for varietal discrimination [65; 66]. Unlike the mDP index, that subunit composition of skin and wine proanthocyanidins exhibit a strong correlation possibly due to the presence of prodelphinidin subunits [65]. Taking into consideration that the amount of skin and seed proanthocyanidins extracted in the wine is greatly influenced by grape variety, it can be suggested that wine proanthocyanidin composition can be grape variety-dependent [66]. [PERSON] et al. (2017) conducted a comparative study on the proanthocyanidin content of cv. Agiorgitiko from various vineyards in the Nemea PDO (Protected Designation of Origin) region in Greece, providing useful insight on the diversity of this grape variety [65]. Browning of white wines and flavanol content are closely linked. Flavanol auto-oxidation and condensation with glyoxylic acid formed from the oxidative cleavage of tartaric acid have been shown to contribute to the browning process [64; 67]. The latter process gives rise to new xanthylium derivatives (ranging in color from yellow to orange), which further degrade during storage in various conditions. [PERSON] et al. (2017) report that despite the low perception threshold observed, xanthylium derivatives might have no direct impact on white wine color, however, they may influence color formation as intermediate products in polymerization and browning [67]. Figure 6: Flavan-3-ol monomeric forms. Glycosylated flavanol derivatives (monoglycos and diglycosides) have been also identified in grape skins and seeds [63; 68]. [PERSON] et al.