260 R. Baumes
be present at high levels (5–10 mM) in the corresponding must and wine, which
completely inhibits -glucosidases, and prevents the enzymatic hydrolysis of grape
glycoconjugates (G ̈unata et al. 1993).
The increase in the rate of glycoconjugates hydrolysis by exogenous fungal
enzymes in the preparation of dry wines, does not lead to a simple increase in
the production rate of odorants from glycoconjugates. Indeed, it alters the whole
dynamic release of odorants from glycoconjugates in wine, as well as the chemical
changes that certain aglycons undergo (Skouroumounis et al. 1992; Winterhalter
and Skouroumounis 1997). Furthermore, the structural change of aglycons released
during the alcoholic fermentation could also be carried out by yeast, in much the
same way as free volatiles, e.g. the reduction of geraniol to citronellol (Dugelay
et al. 1992b).
As mentioned above, grape andSaccharomyces cerevisiaeyeasthavemostgly-
cosidases appropriate to hydrolyze grape glyconjugates, but they become effective
only under conditions (pH, sugar content) quite different from those found in wine-
making (Darriet et al. 1988; Delcroix et al. 1994; G ̈unata et al. 1986, 1989a, 1993;
Lecas et al. 1991; Sarry and G ̈unata 2004). However, it was recently reported that
some yeast strains hydrolyzed to the extent of 40%, glycoconjugates of Muscat
grape during the fermentation of a model must at pH 3.2, and up to 70%, linalool
- D-glucoside, known to be not easily hydrolyzed by fungal - D-glucosidases, but
no data were reported for grape must (Ugliano et al. 2006). Finally, recent studies
showed that enological strains of lactic bacteria have a slight influence only on the
hydrolysis of grape glycoconjugates under the conditions of malo-lactic fermenta-
tion (D’Incecco et al. 2004; Grimaldi et al. 2005; Sarry and G ̈unata 2004; Ugliano
et al. 2003).
8A.6S-Cysteine Conjugates
S-(L-cysteine) conjugates, odourless nonvolatile cysteinylated precursors of very
odorous volatile thiols, were evidenced and identified in grape much later than
the afore-mentioned classes of grape aroma precursors (Darriet 1993; Darriet et al.
1993; Tominaga 1998; Tominaga et al. 1995, 1998b). Chemically, they are
S-substituted derivatives ofL-cysteine, different by the nature of the moiety on
the cysteine sulfur atom, and to date only three of these precursors were identified
in grape:S-(1-hydroxyhex-3-yl)-L-cysteine (P3MH),S-(4-methyl-2-oxopent-4-yl)-
L-cysteine (P4MMP) andS-(4-methyl-2-hydroxypent-4-yl)-L-cysteine (P4MMPOH
(Fig. 8A.6) (Tominaga 1998; Tominaga et al. 1995, 1998b). However, an
S-substituted tripeptide related to P3MH,S-(1-hydroxyhex-3-yl)-glutathion, was
identified later in Sauvignon Blanc must, and probably in Gros Manseng must
(Peyrot des Gachons et al. 2002b). ThisS-glutathione conjugate could be a bio-
genetic precursor of P3MH, as it could be cleaved by a-glutamyltransferase, which
eliminates glutamic acid, and by a carboxypeptidase, which eliminates glycine,
to give rise to P3MH (Peyrot des Gachons et al. 2002b). However, no other