116 M. Ugliano
4.6.1.3 Exogenous Glycosidases
Due to the limited glycosidase activities associated with grape, yeast, and bacte-
ria metabolism, exogenous enzymatic preparations with glycosidase activities can
be added during winemaking to enhance the liberation of glycosidically-bound
volatile compounds. Particularly, commercially available pectinase and hemicel-
lulase preparations obtained from the fungusAspegillus nigerhave side activities
that can catalyze the release of volatilecompounds from glycoconjugates. Addition
of enzymatic preparationduring winemaking generally results in higher concen-
trations of different classes of volatilecompounds, particularly monoterpene alco-
hols, monoterpene polyols, norisoprenoids and benzenoids (G ̈unata et al. 1990b,
1993). Experiments with monoterpene-dependant grape varieties have shown that
the enzyme treated wines are characterisedby increased floral attributes, due to the
large release of monoterpene alcohols (G ̈unata et al. 1993; Bayonove et al. 1996).
In the case of non-terpene dependant grape varieties, the sensory impact of enzyme
treatment on wine sensory characteristics is less predictable, and depends largely on
the composition of the pool of glycosidically-bound volatiles of individual varieties.
For example, some volatiles released with the enzymatic treatments, particularly
those belonging to the class of norisoprenoids, are odorless, although they can later
generate other powerful odorants during wine aging (Kotseridis et al. 1999). Also,
when the enzyme treatment is performed before alcoholic fermentation on a grape
juice rich in bound geraniol, increased concentrations of citronellol are then found in
the final wine, due to the yeast-catalyzed formation of citronellol from the geraniol
released from precursors (Ugliano et al. 2006).
Although the use of enzymatic preparationswith glycosidase activities represents
an effective way of accelerating the otherwise slow release of aroma compounds
from glycosides during winemaking, several implications should be considered in
relationship to the impact of this practice on wine sensory properties. First, some
enzymatic preparations contain significant levels of cinnamate esterase activity
which, in conjunction with the decarboxylase activity ofS. cerevisiae, can lead
to the formation of volatile phenols that are detrimental to wine quality (G ̈unata
et al. 1993). Second, as red wine color is mainly dependent on glycosylated antho-
cyanins, preparations containing non-specific -glucosidase activity could cause the
hydrolysis of these colored compounds, resulting in a general loss of wine colour
(Le Traon-Masson and Pellerin 1998). Finally, the presence of esterase activities
in some of the commercially available glycosidase preparations has been observed
(Tamborra et al. 2004). These side activities have been shown to cause a decrease in
the concentration of fermentation-derived esters such as isoamyl acetate and ethyl
hexanoate, which are well known contributors to the fermentation bouquet of young
wines. Careful attention has to be paid therefore to the selection of glycosidase
preparations that are largely free of side activities detrimental to wine quality.
Moreover, altering the balance between free and bound forms of the pool of
grape-derived volatile compounds of wine through addition of an exogenous gly-
cosidase has to be considered in conjunction with the role played by both glycosides
and aglycons as precursors to aroma compounds during wine aging. For example,