254 R. Baumes
Fig. 8A.3Grape phenolic
acid esters, and the chemical
bonds cleaved to release
odorous volatile phenols^2
2CoCouuttaaric ric aaciciddCCaaftftaaric ric aaciciddHO
COHOCO 2 HCO 2 HCO 2 HCO 2 HHO
COHOCH 3 Ofree acid forms only are precursors of wine volatiles, and little is known of the
distribution and generation of these free acid forms from their tartaric esters. In this
group, coumaric and ferulic acids only are precursors of odorous volatile phenols in
wine, coumaric acid generating (4-vinyl- and 4-ethyl)-phenols and ferulic acid, (4-
vinyl- and 4-ethyl)-guaiacols (Fig. 8A.3). The vinyl derivatives, 4-vinyl-phenol and
4-vinyl-guaiacol, are formed during thealcoholic fermentation by decarboxylation
of the free cinnamic acids with a cinnamate decarboxylase ofSaccharomyces cere-
visiaeyeasts (Albagnac 1975; Chatonnet et al. 1993a,b; Dugelay et al. 1992a, 1993).
However, as this enzyme is inhibited by catechins and catechic tannins, abundant in
red wines, the levels of volatile phenols formed in red wines are generally much
lower than those in white and ros ́e wines, although the contents in hydroxycinnamic
precursors in the corresponding red musts are higher (Chatonnet et al. 1993a,b).In
contrast, the cinnamate decarboxylase of contaminant yeasts of the genusBret-
tanomyces/Dekkerais not inhibited by these polyphenolic compounds of red wines;
furthermore, these yeasts can efficiently reduce the vinylphenols into ethylphenols
with a vinylphenol reductase thatSaccharomyces cerevisiaeyeasts or lactic acid
bacteria do not have, except for some strains of bacteria with very weak activity
(Cavin et al. 1993; Chatonnet et al. 1992a,b, 1995; Herensztyn 1986). Therefore,
red wines contain mostly very low levels of vinylphenols, and the formation of sig-
nificant quantities of ethyl phenols, higher than their olfactive perception thresholds,
occurs only in the presence of these contaminant yeasts, generally during aging prior
to bottling of wine, which always causes the sensory depreciation of wine (phenolic
off-flavor).
Thus, the vinylphenols may contribute to the aroma of white and ros ́ewines
only. However, 4-vinylphenol seems to depreciate the aroma of white wine as soon
as it is perceived by masking the fruity note, then at higher concentrations it is
responsible for phenolic off-flavors (Chatonnet et al. 1993a,b). That was observed in
winemaking, when enzymatic preparations containing high esterase activities, and
Saccharomyces cerevisiaeyeast strains with high cinnamate decarboxylase activi-
ties were used (Chatonnet et al. 1992c; Dugelay et al. 1992a, 1993). The influence of
4-vinylgaiacol on wine aroma would be less negative, as it was reported to play a
role in the varietal aroma of Gew ̈urztraminer from north of Italy (Versini 1985).
However, its limit preference threshold is not much higher than its perception