532 C. Santos-Buelga and V. de Freitas
ACACCFig. 9D.2Verticalπ–πstacking between an anthocyanin (A) and a copigment (C)
(i.e., copigments). Copigmentation complexes adopt a sandwich configuration
(Fig. 9D.2) that protects the flavylium chromophore from the nucleophilic attack of
water, thus preventing at least in part the formation of colorless hemiketal and chal-
cone forms. The final result is that the anthocyanin solutions show a more intense
color that theoretically could be expected according to the pH value of the media.
Occasionally a bathochromic effect can also be produced due to the proton transfer
equilibrium between the flavylium cation and the quinonoidal base and/or preferent
association between quinonoidal forms and copigments. Depending on the type of
anthocyanins and copigments and their concentrations there will be variations in
color hue and intensity. Thus at the same time color stabilization and variation can
be obtained (Brouillard et al. 2003).
It is well known that the copigmentation phenomenon occurs in plant tissues
offering an explanation for the large color variations in flowers and fruits; excellent
reviews on the subject have been published by Goto and Kondo (1991) and Brouil-
lard and Dangles (1993). However, the influence of copigmentation in the color of
red wines is subjected to more controversy, due to the lower anthocyanin concentra-
tions existing in wine compared to those in plant vacuoles and the dissociating effect
of the ethanol on the copigmentation complexes (Dangles and Brouillard 1992).
Despite this, it is assumed that it must also constitute a relevant process for the
definition of the color in young red wines (Boulton 2001).
Many of the studies about copigmentation have been carried out in model sys-
tems and they are not always strictly applicable to wine. A method for the esti-
mation of the copigmentation effect in red wines, based on the comparison of
the absorbance at 520 nm before and after disruption of the copigmentation com-
plexes by dilution with a wine-like solution, was proposed by Boulton (1996). That
method has the inconvenient that only takes into account the variation atλmaxof the
flavylium ion in the visible region, thus ruling out the modifications that the copig-
mentation induces in other regions of the visible spectrum and that logically have
also qualitative and quantitative influence on wine color. Gonnet (1998) proposed
another approach based on the colorimetric analysis in the CIELAB color space
that consider the spectral changes over the complete range of visible wavelengths.