Phenolic Reactions during Winemaking and Aging

The reactivity of polyphenols is due to the position of the hydroxyl groups on their aromatic nuclei. Ortho-hydroxyl groups promote oxidation while meta-hydroxyl groups induce electrophilic aromatic substitution. Both hydroxylation patterns are encountered in flavonoid structures, on the B and A rings, respectively. In addition to oxidation and electrophilic aromatic substitution, flavonoids undergo nucleophilic addition on the central C ring when it is positively charged. Reactions of the A and C rings are pH-dependent. The A ring of flavonoids undergoes a polycondensation reaction mediated by an aldehyde. The products are anthocyanin and flavanol polymers and copolymers constituted of both. Flavanol polymers are not stable and rearrange into vinyl flavanols and xanthylium pigments. Vinyl flavanols can react with the positively charged C ring of anthocyanins, yielding pyranoanthocyanins, which can also be formed from components that have a reactive double bond, such as carbonyl and ethylene bonds. The positively charged C ring primarily undergoes direct reactions. Since the positive charge on the C ring of anthocyanins and flavanols is pH-dependent, their dehydration and interflavan bond cleavage reactions are also pH-dependent. This leads to flavanol–anthocyanin (F-A+) adducts at lower pH values and anthocyanin–flavanol (A+-F) adducts above pH 3.8. Temperature seems to favor formation of the latter.