444 M. Monagas and B. Bartolom ́e
Two different mechanisms have been postulated in order to explain the reactions of
anthocyanins with theseo-quinonic species (Cheynier et al. 1997; Sarni-Manchado
et al. 1997):
a)Coupled oxidation of anthocyanins to very unstable quinones regenerating the
initial caftaric acid.The first evidence of this mechanism was described when
the glycosides of cyanidin, pelargonidin and delphinidin were oxidized by enzy-
matically generatedo-quinones of catechol, (+)-catechin and chlorogenic acid,
producing discoloration of the anthocyanin solution (Peng and Markakis 1963;
Pifferi and Cultrera 1974; Wesche-Ebeling and Montgomery 1990).b)Condensation reactions ofanthocyanin in its hemiketal form (nucleophilic) to
the o-quinone (electrophilic) producing acolorless adduct which is then equi-
librated to the corresponding flavylium form(Fig. 9A.3a). In solutions contain-
ing malvidin-3-glucoside and caftaric acid in the presence of PPO, adducts of
caftaric acido-quinones with anthocyanins in hemiketal (colorless) or flavylium
(red) forms have been observed (Sarni-Manchado et al. 1997). Although no NMR
characterization of these adducts has been provided, the anthocyanin is probably
linked to the quinone by its C8 or C6 positions.Evidence in wine. The caftaric-malvidin-3-glucoside condensation product in
flavylium form has been detected in red wine after the oxidation of Gamay grapes,
together with the corresponding derivativeof peonidin-3-glucoside (Labarbe 2000).
Factors affecting the reaction. The type of reaction of anthocyanins witho-
quinones (coupled oxidation or condensation)depends on the chemical structure of
the anthocyanin.o-Diphenolic anthocyanins (cyanidin, delphinidin and petunidin-
3-glucosides) usually react rapidly by coupled oxidation, whereas peonidin and
malvidin-3-glucosides participate in condensation reactions with quinonic species at
a slower rate (Wesche-Ebeling and Montgomery 1990; Sarni-Manchado et al. 1997).
As expected, the nucleophilic addition of anthocyanins ono-quinones, which requi-
res the hemiketal form of the anthocyanin, was faster at pH 3.4 than at pH 1.7 (Sarni-
Manchado et al. 1997). In relation to the effect of oxygen, Cheynier et al. (1997)
concluded that in red vinification under oxidative conditions, the enzymatic oxi-
dation of tartaric esters of hydroxycinnamic acids and the coupled oxidation of
anthocyanins (delphinidin-3-glucoside) with enzymatically generated caftaric acid
o-quinones, could take place but only resulting in relatively low losses of these com-
pounds.
9A.2.2 Anthocyanin-Flavanol and Anthocyanin-Anthocyanin
Direct Condensation Reactions
Precursors.The precursors of these reactions are anthocyanins and flavanols, acting
both as electrophiles and nucleophiles (see also Chapter 9B).