112 M. Ugliano
Due to the occurrence of different chemical structures within the pool of grape
glycosides, the complete enzymatic hydrolysis of these components requires in the-
ory the action of multiple enzymatic activities. Studies with monoterpene glycosides
have shown that liberation of sugar-bound aglycones involves, in the case of disac-
charide glycosides, the preliminary action of an appropriate exo-glycosidase (arabi-
nofuranosidase, rhamnopyranosidase, or apiofuranosidase) to release the terminal
sugar before the -glucosidase is able to release the bound volatile component,
while for glucosidic precursors only -glucosidase is needed (G ̈unata et al. 1988).
Enzymatic activities potentially able to hydrolyze grape glycosides through this
sequential mechanism can be present in the grape or can be derived from microor-
ganisms involved in the winemaking process, such as fermentation yeasts and lactic
acid bacteria. Alternatively, commercial preparations with glycosidase activity can
be used to increase the degree of precursor hydrolysis and the corresponding release
of volatile compounds.
4.6.1.1 Grape Glycosidases
Grape berries have been shown to contain glycosidase activities which can, under
optimal conditions, catalyze the release of volatile compounds from grape glyco-
sides (G ̈unata et al. 1990b). Studies on grape -glucosidase have shown that this
enzyme is strongly inhibited by glucose and ethanol, and has optimal pH around 5
(Aryan et al. 1987), indicating that its activity is largely limited during winemaking.
Moreover, grape -glucosidase is mainly located in the hypodermic wall of berry
skins, and therefore its solubilisation in the juice during processing is likely to be
of little importance (Sarry and G ̈unata 2004). Based on these observations, grape
-glucosidase is considered to play a negligible role in flavor development during
winemaking (Aryan et al. 1987).
Interestingly, the occurrence of an endoglycosidase capable of hydrolysing mono-
terpene disaccharide glycosides has been reported (G ̈unata et al. 1988). This enzyme
operates by liberating of the aglycone moiety of disaccharide with a single step
mechanism instead of the sequential two step process proposed for other glycosi-
dases. The involvement of this enzymatic activity in the process of flavor develop-
ment during winemaking still needs to be clarified.
4.6.1.2 Microbial Glycosidases
The observation that a large part of the varietal character of wine develops during
fermentation suggests that microbial species responsible for the fermentation pro-
cess play a role in the release of varietal odor-active compounds from odorless pre-
cursors. This is particularly obvious in the case of wines obtained from non-aromatic
grape varieties (e.g. Chardonnay, Semillon, Pinot Gris, Trebbiano, and most of the
red varieties), in which specific varietal aroma attributes are often present at the end
of fermentation, in spite of the fact that grapes of these varieties lack any character-
istic aroma. Nevertheless, the ability ofS. cerevisiae, the main agent of alcoholic fer-
mentation, to hydrolyze glycosidic precursors during fermentation has been a topic