4 Enzymes in Winemaking 113
of great controversy for many years. Early studies had indicated thatS. cerevisiae
possess -glucosidase activity (Darriet et al. 1988), although in levels that were
lower than found in other non-Saccharomycesyeast (Rosi et al. 1994). Typical
winemaking conditions were shown to inhibit yeast -glucosidase, -arabinosidase
-rhamnosidase activities (Delcroix et al.1994), suggesting that most of glycosides
present in the juice remain virtually unchanged during fermentation. However, as
the majority of these studies were carried out with non-fermenting yeast cells and
using model substrates instead of native grape glycosides, the question remained
open as to whetherS. cerevisiaeyeast were able to hydrolyze grape glycosides
during fermentation. Also, in several studies where low stability ofS. cerevisiae
glycosidases were observed, quantification of yeast enzyme activities was carried
out on cell enzyme extracts, regardless of the fact that glycosidases are located in
the cell periplasmic space (Darriet et al. 1988; Mateo and Di Stefano 1997) and
therefore not directly exposed to the action of potential inhibitors such as pH or
ethanol. Recently, a series of studies haveprovided clear evidences for the major
role played by yeast in the hydrolysis of glycosides during fermentation (Delfini
et al. 2001; Ugliano 2006; Loscos et al. 2007). Under typical winemaking con-
ditions, it was proven that the hydrolytic activity of yeast on various glycosides
was responsible for the liberation of several bound volatile compounds mainly from
glucosides, rhamnosides, and arabinosides, with concomitant acid hydrolysis play-
ing a much less important role (Fig. 4.4). These experimental results are likely
to renew the interest in yeast strain selection as a tool to modulate wine varietal
character.
AlthoughS. cerevisiaepossesses enzymatic activities that can catalyze the hydrol-
ysis of glycosides during fermentation, a large portion of the glycosides originally
present in the must survive the fermentation process and are still present in the
finished wine (Williams et al. 1996; Zoecklein et al. 1997a, b). This suggests that
S. cerevisiaeglycoside hydrolases, although capable of catalyzing the release of
a significant portion of glycosidically-bound aroma compounds, are insufficient to
induce complete hydrolysis of the whole glycosidic pool of grape juice. Based on
this observation, other yeast species, particularly non-Saccharomycesyeast, have
been largely studied as a possible source of additional glycosidase activities dur-
ing fermentation (G ̈unata et al. 1990a; McMahon et al. 1999; Arevalo Villena
et al. 2006). Several species of non-Saccharomycesyeast, some of which are present
during fermentation, were found to possess significant levels of glycosidase activi-
ties, suggesting that liberation of aroma compounds from glycosides during fermen-
tation can partially be linked to the activity of non-Saccharomycesyeasts.
More recent research has focused on the construction of genetically modified
organisms which over-express enzymatic activities that can catalyze the liberation
of aroma compound from glycosidic precursors (Van Rensburg et al. 2005; Gil
et al. 2005). However, the results obtained so far are still of limited practical interest
compared to those achievable throughthe addition of exogenous enzymes, manly
due the fact that the recombinant strains generated exhibit unpredicted patterns of
production of volatile compounds, particularlyfermentation esters. Interestingly, in
the study of Gil et al. (2005), overexpression of theS. cerevisiaegeneEXG1gene