9D Influence of Phenolics on Wine Organoleptic Properties 547
Mv3glc Catechin-ethyl-Mv3glcCatechin-ethyl-Mv3glcFig. 9D.8Absorption spectra in the visible region of malvidin 3-glucoside (Mv3glc) and catechin-
ethyl-Mv3glc as a function of pH (adapted from Escribano-Bailon et al. 2001)
constant (3.44), meaning that it is mostly present in wine as the blue quinonoidal
base turning more blue as pH increases (Cheynier et al. 2006). This increased ten-
dency of the anthocyanin-ethyl-flavanol pigments to stabilize by deprotonation to
their quinonoidal bases had already been noticed by Timberlake and Bridle (1976).
The greater bathochromic displacement in maximum wavelength observed in their
absorption spectra with pH increase and the presence of an isosbestic point around
580 nm (Fig. 9D.8) is proof of the shift of the equilibria in that direction.
The extinction of catechin-ethyl-malvidin 3-glucoside was calculated to be
17,100/M at pH 0.5 in 10% ethanol by Escribano-Bailon et al. (2001) slightly lower
than that of malvidin 3-glucoside (20,200/M). In spite of their smaller extinction
as compared with anthocyanins, the increased resistance to pH-induced and sul-
fite discoloration makes these pigments express more color than free anthocyanins
in wine conditions. However, the pigments of this family do not seem to have a
relevant direct contribution to the color of red wines in a great extent, as they
are usually present in very low amounts. According to Alcalde-Eon et al. (2006)
and Boido et al. (2006) they would not constitute more than 1% of total pigments
either in young or aged red wines and their relative contribution to pigment com-
position would even decrease with aging. This scarce presence may be explained
by their instability, which would also be the reason why only pigments consti-
tuted by an anthocyanin residue linked to a flavanol monomer or dimer are usu-
ally found in red wines. When sufficient acetaldehyde is available and a relatively
low pH value exists, they rapidly progress to larger structures by the incorpora-
tion of new ethyl-flavanol units until a critical size is reached, after which they
precipitate (Es-Safi et al. 1999; Rivas-Gonzalo et al. 1995). In addition, soluble
ethyl-linked pigments are quite instable and suffer an easy cleavage at the level of
the ethyl bridge releasing the anthocyanin moiety and ethyl-flavanols (Escribano-
Bailon et al. 2001) that can either react with anthocyanins (or flavanols) to give new
ethyl-bridged derivates or alternatively the more stable flavan-pyranoanthocyanins
(Mateus et al. 2002b). Important amounts of acetaldehyde are formed during the