9D Influence of Phenolics on Wine Organoleptic Properties 531
Fig. 9D.1Equilibrium distribution at 25◦C among structural forms of malvidin 3-glucoside as
a function of pH. AH+, flavylium cation; A, quinonoidal base; B, hemiketal; C, chalcone (from
Brouillard1982, with permission from Elsevier)
value, and the flavylium ions predominate only in very acidic solutions (Brouil-
lard et al. 1977). Weakly acidic aqueous or hydroalcoholic solutions of pure antho-
cyanins show a poor color or are practically colorless depending on the pH value,
due to the predominant presence of colorless hydrated hemiketal forms (Fig. 9D.1).
In red wines the usual presence of sulfites, which form adducts with the anthocyanin
flavylium in the same way as water to give colorless products, may provide further
discoloration. Even in these circumstances, red wines continue showing an intense
red color, indicating that pigment stabilizing mechanisms exist that provide a suffi-
cient presence of anthocyanin chromophores to ensure the definition and the main-
tenance of the color even over years. It is generally assumed that such stabilization
is achieved through two major mechanisms: (1) the non-covalent associations of
anthocyanin chromophores with other wine compounds, process called copigmen-
tation, and (2) the progressive substitution of grape anthocyanins by more stable
pigments (Cheynier et al. 2006). The first process would have particular importance
in young red wines, whereas the second one mainly participates in the color of aged
red wines.
9D.2.2 Copigmentation
9D.2.2.1 Influence in the Color of Red Wines
The copigmentation phenomenon consists of hydrophobic interactions ( - stack-
ing) between the planar polarizable nuclei of the colored forms of the antho-
cyanins (i.e., flavylium ion and quinonoidal forms) with other organic molecules