8A Wine Aroma Precursors 259
et al. 1992; Voirin 1990; Winterhalter 1993). It is an acido-catalyzed process, which
involves hydrolysis of the glycosides, and depending on the aglycon structure, its
chemical transformation. The hydrolysisstep release the volatile aglycon, i.e. a
compound with an hydroxyl functional group (alcohol, phenol, acid), which can
be odorous or not, but which can undergo a sequence of changes under wine
aging conditions, susceptible to the generation of odorants from inodorous agly-
cons, and conversely (Cox et al. 2005; Francis et al. 1992, 1996; Rapp et al. 1985;
Schneider 2001; Sefton 1998, 1993; S ́egurel 2005; Versini et al. 1996; Voirin 1990;
Williams et al.1980, 1982b). Thus, in the first part of wine aging, the hydrolytic
release of volatiles from glycosides and their possible chemical changes are con-
comitant. Then, when the corresponding glycosides are entirely hydrolyzed, the
chemical changes of the volatiles only occur, so that their contents go down grad-
ually, more or less sharply, depending on their chemical stability under wine aging
conditions. More than a hundred volatiles generated from glycosides have so far
been identified in wine, but only a few could reach levels higher than their olfactive
perception thresholds. Such odorants mostly listed are linalool, geraniol, roseoxyde,
1,8-cineole and wine-lactone, as monoterpenoidic derivatives, eugenol, guaiacol,
zingerone, methyl salicylate, as volatile phenolic derivatives, -damascenone, 1,1,6-
trimethyl-1,2-dihydronaphtalene (TDN) and (E)-1-(2,3,6-trimethylphenyl)buta-1,3-
diene (TPB), as norisoprenoidic derivatives (Bonnl ̈ander et al. 1998; Eti ́evant 1991;
Farina et al. 2005; Francis et al. 1992, 1996; Guth 1997; Janusz et al. 2003;
Kotseridis 1999; Kotseridis et al. 1999a; Schneider 2001; Sefton 1998, 1993;
S ́egurel 2005; Simpson 1978; Skouroumounis et al. 1992; Winterhalter 1992;
Winterhalter and Skouroumounis 1997). It must be noted that linalool, geraniol,
eugenol, guaiacol, zingerone and methyl salicylate are genuine aglycons of grape
glycosides, whereas the other odorants in this list arise from chemical changes of
aglycons.
This general chemical reaction pathway can be modified when enzymatic prepa-
ration containing appropriate glycosidases are used in winemaking. The enzymatic
hydrolysis of grape diglycosides is a sequential process. The first step consists in
the hydrolysis of the terminal oses by the corresponding glycosidases, releasing
- D-glucosides similar to those occurring in grape. All these - D-glucosides are
hydrolyzed in the second step by a - D-glucosidase to release the aglycons (G ̈unata
et al. 1989a). Filamentous fungi glycosidases are appropriate to hydrolyze grape
glycoconjugates, and they are much more stable than those from grape orSaccha-
romyces cerevisiaeyeast at wine acidic pH. Thus, despite the inhibition of their - D-
glucosidase by must glucose higher than that of grape orSaccharomyces cerevisiae
yeast, they can be used during dry wine preparation to hydrolyze grape glycocon-
jugates, as they are still active at the end of fermentation, when the sugar content
in must is residual (G ̈unata et al. 1989a, 1990, 1993). Some non-Saccharomyces
yeasts, with - D-glucosidase weakly inhibited by glucose, were also used in wine-
making (Gueguen et al. 1997; Belancic et al. 2003). However, gluconolactone was
also reported as inhibitor of these - D-glucosidases from grape, yeast and filamen-
tous fungi, used in winemaking. This compound, produced in grapes infected by
Botrytis cinereafungi (Heyworth and Walker 1962; G ̈unata et al. 1989b), may