3.1. Anthocyanins

conditions such as pH [14] or oxidation-reduction potential [18], phenolics can act both as electrophiles (i.e., electron-loving molecules) and nucleophiles, thus readily reacting with

For the purpose of this chapter, occurrence, reactivity, chemical structure, and sensory properties of phenolic compounds are discussed exclusively in the context of Vitis vinifera L. grapes and wines. Also, only the most important polyphenols of the flavonoid group, namely anthocyanins, flavan-3-ols and tannins, as well as their reaction products, will be discussed. Phenolics belonging to the nonflavonoid class include benzoic (e.g., gallic, hydroxibenzoic, protocatechuic, vanillic, and syringic acids) and cinnamic acid derivatives (p-coumaric, cafeic, ferulic, and sinapic acids). Also, the hydroxylated stilbenes are included in the nonflavonoid class, of which trans- and cis-resveratrol (3,5,4-trihydroxystilbene), as well as their glucose derivatives (trans- and cis-piceids), have all been identified in grapes and wines [19, 20]. Although quantitatively much less important than the flavonoid class, nonflavonoids phenolics play a direct role in both chemical and coupled-enzymatic oxidation reactions in white and red wines [21, 22]. Cinnamic acids can act as copigments,2 inducing changes in color through the phenomenon of copigmentation3 [23] and can also impact wine aroma when metabolized by yeast of the genera Brettanomyces/Dekkera

Figure 2. (a) Basic flavonoid ring structure and numbering, (b and c) sites for electrophilic substitutions, and (d) sites for

Phenolic flavonoids possess a three-ring system, composed of 15 carbon atoms in the form C6-C3-C6 [12, 13]. The central C ring contains oxygen forming a pyran ring that can adopt various oxidation states, and it is fused to two aromatics rings, termed A and B (Figure 2a). In the flavonoid family, the B-ring is fused to the pyran ring at position 2. The A ring is derived from the phloroglucinol structure and is the most conserved portion of the C6-C3-C6 backbone. Furthermore, different substitutions in the B ring define different compounds within a family. The members of the flavonoid class found in grapes and wines all have the same substitution pattern of hydroxyl groups at carbons 5 and 7 of the A ring. On the other hand, differences in the oxidation state and substitution on the C ring define the different flavonoid classes [13]. Thus,

Copigments are typically noncolored phenolic and/or nonphenolic compounds able to engage in copigmentation reac-

As defined by Boulton [34], the phenomenon of copigmentation is due to molecular associations between pigments and other (usually non-colored) organic molecules in solution. These associations cause the pigments to exhibit far greater color than would be expected from their concentration. When anthocyanins engage in copigmentation, both hyperchromic (increase of absorbance) and bathochromic (shift of absorbance toward blue hues) are normally observed.

electron-rich or electron-deficient compounds, respectively (Figure 2).

to generate volatile ethyl-phenols [24].

2

3

tions with anthocyanins.

nucleophilic substitutions. Redrawn from Refs. [13, 25].

156 Phenolic Compounds - Natural Sources, Importance and Applications
