Condensation between anthocyanins and flavanols mediated by aldehydes

One of the first reactions described in red wines was the polymerization reaction between anthocyanins and flavanols (catechins and condensed tannins) mediated by acetaldehyde (Timberlake Bridle, 1976). This reaction involves the protonation of the aldehyde, followed by addition of the respective carbocation to the nucleophilic position C-8 and less likely C-6 of the phloroglucinol ring of the flavanol (see Fig. 3.6). Further dehydration and a nucleophilic attack from the anthocyanin give rise to the formation of two new pigments that differ in the stereochemistry of the asymmetric carbon of the interflavonoid linkage [catechin-HC (CH3)-anthocyanin], as demonstrated by several authors (Rivas-Gonzalo etal, 1995 Escribano-Bailonefa/., 1996 Lee etal, 2004). 

Regarding the coloring capacity of these adducts, previous studies have shown that the molar extinction coefficient (e) of catechin-HC(CH3)-mv3glc (isomer S) was lower than that of mv3glc at its X in the visible region [17,100 and 20,200 cm, respectively, in 

10% ethanol solution at pH 0.5 (Escribano-Bailon effl/., 1996 Heredia etal, 1998) 10,870 and 23,050 cm , respectively, in 0.1 M citric acid at pH 1 (Duenas etal, 2006)]. 

Acetaldehyde may also mediate the self-condensation of anthocyanins, leading to the formation of oligomeric methylmethine-linked anthocyanins (Atanasova et al, 2002b Salas et al, 2005). These adducts are described to possess a (525 nm) hypsochromi-cally shifted from that of mvSglc (530 nm), as obtained by LC-DAD, and to have one of the anthocyanin moieties of the dimer mv3glc-HC(CH3)-mv3glc in their hydrated form at wine pH (Salas et al, 2005). 

These compounds are more stable than anthocyanins toward pH variations, probably as a result of a greater protection of the anthocyanin moiety against the nucleophilic attack by water (Escribano-Bailon et al, 2001 Pissarra et al, 2004). Effectively, the hydration constant (p. Tij) for mv3glc was found to be between 2.6 and 2.3, whereas for an isomer of catechin-HC(CH3)-mv3glc it was 4.17 (Duenas et al, 2006). Furthermore, these adducts were shown to be much more resistant to bisulfite-induced discoloration than the respective anthocyanin. 

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