488 N. Terrier et al.
Evidence in Wine
Copigmentation in wine is estimated from the effect of dilution on the absorbance
values in the visible range, at a given pH and ethanol content (Boulton R 2001).
Indeed, decreases in the absorbance values(reflecting the concentration of flavylium
ions) higher than the dilution factor are attributed to disruption of the copigmen-
tation complexes. On this basis, copigmentation has been reported to contribute
30–50% of the color of young red wines.
Factors Affecting the Interaction
Copigmentation, involving a shift from the colorless hemiketal to the flavylium, is
especially important in mildly acidic conditions such as the wine pH range where
hemiketal forms are prevalent. Association constants determined by spectropho-
tometry (89M-1; for epicatechin (Malien-Aubert et al. 2002), 101M-1 for catechin
(Mirabel et al. 1999b) indicate that flavanol monomers are poor copigments of
malvidin 3-glucoside compared to flavonols (> 103 /M; Malien-Aubert et al. 2002).
Association constants could not be accurately determined in the case of proantho-
cyanidins which were not available as pure compounds, but appeared even lower
(4/M for a fraction of procyanidin oligomers (Mirabel et al. 1999b).
Surprisingly, copigmentation was enhanced in the presence of 12% ethanol
(Mirabel et al. 1999b), reaching 157/M for epicatechin and 68/M for the procyanidin
fraction.
9B.4.2.2 Self Association and Aggregation
Flavanol self-association has been demonstrated by means of NMR (Dufour and
Bayonove 1999; Mirabel et al. 1999a) and massspectrometry (Sarni-Manchado and
Cheynier 2002) and their aggregation has been studied by means of dynamic light
scattering (Poncet-Legrand et al. 2003; Riou et al. 2002; Saucier et al. 1997a) and
cryo-transmission electron microscopy (Poncet-Legrand et al. 2003).
Actors
Self association of catechin and epicatechin was revealed by NMR (Dufour and
Bayonove 1999) but did not lead to aggregation (Poncet-Legrand et al. 2003). In
contrast, epicatechin gallate and, to a lesser extent, epigallocatechin gallate (Poncet-
Legrand et al. 2003), as well as procyanidins (Riou et al. 2002) and methyl-methine
linked catechin oligomers resulting from acetaldehyde condensation (Saucier et al.
1997a) aggregate into metastable colloidal particles in wine-like ethanolic solutions.
Aggregation and precipitation of flavonols is restricted to aglycones which exhibit
lower solubility than their glycosides.