110 M. Ugliano
activity responsible for increased production of volatile phenols can be also present
in some pectinase preparation (Gerbaux et al. 2002). Great efforts are currently
being made by enzyme producers to achieve a high degree of specificity for com-
mercial pectinase preparations.
Because S. cerevisiaehas limited pectinase and polysaccharide activities,
expression of genes encoding for polysaccharide degrading activities have been
investigated as an alternative strategy to improve skin cell degradation and colour
extraction during maceration. Louw et al. (2006) reported that heterologous expres-
sion of endo- -1,4-xylanase and endo- -1,4-glucanase in a commercialS. cerevisiae
strain resulted in increased pressing yield, lower hue, and increased color intensity,
although the latter effect was only detectable after six months of bottle storage.
The wines made with the recombinant strains were also preferred during sensory
analysis. In theory, one attractive characteristic of recombinant strains compared
to the addition of exogenous enzymes is that, since the strains will only produce
the required enzymes, there is no risk of side activities which can be potentially
detrimental to wine quality. Nevertheless, the strategies employed for the develop-
ment of these recombinant strains appearedto affect other pathways related to the
production of volatile compounds during fermentation (Perez-Gonzalez et al. 1993;
Louw et al. 2006). Although major improvements are expected in this area in the
next few years, the use of the recombinant yeast strains developed so far may result
in unpredictable repercussions on wine flavor.
4.6 Aroma Enhancing Enzymes
4.6.1 Glycosidases
Since the occurrence of glycosylated precursors of volatile compounds in grapes
was first reported (Cordonnier and Bayonove 1974), a considerable amount of
research has been devoted to the understanding of the factors that determine their
hydrolysis during winemaking. The ultimate goal has been to provide practical
tools to modulate the release of the odor-active compounds contained in this type
of precursor. Glycosides of volatile compounds are mainlyO- -D-glucosides or
O-diglycosides, in which the aglycone moiety is always linked to the
-D-glucopyranose unit. In diglycoside structures, the glucose moiety is further
substituted with a second monosacharide unit, which can either be -L-arabinose,
-L-rhamnose, or -D-apiose (Williams et al. 1982; G ̈unata et al. 1988; Voirin
et al. 1990, 1992). In grapes and wines, monoterpenes, C 13 norisoprenoids, aliphatic
alcohols, and benzene derivatives have all been identified as grape-derived volatile
compounds that are largely present in the form of glycosidic aroma precursors
(Fig. 4.3). Particularly, several powerful aroma compounds that have been shown
to play an important role in the varietal aroma character of wine, such as linalool,
geraniol, -damascenone, 1,1,6-trimethyl-1,2-dihydronaphtalene (TDN), vinyl gua-
iacols, have been identified in the hydrolysates of precursors extracted from various