118 M. Ugliano
quickly (Fig. 4.6b) (Skouroumounis et al. 1993). These observations were con-
firmed in a study on -damascenone in Merlot wines submitted to accelerated aging,
where it was found that glycosidase treated samples generated, during aging, about
half the amount of -damascenone observed in the non-glycosidase-treated control
(Kotseridis et al. 1999). In this case, therefore, extensive hydrolysis of the pool of
wine glycosidic precursors by exogenous enzymes appeared to reduce the potential
for generating grape derived aroma compounds through acid-catalyzed reactions
during aging. More research in required to understand the existence of this type of
competing reactions for precursors of other key aroma compounds, in order to estab-
lish the potential benefits of enzyme treatments to the expression of wine varietal
volatile character.
4.6.2 Cysteine-S-Conjugate -Lyases
Volatile thiols are important aroma constituents of several food and beverages,
including wines obtained from certain grape varieties such as Sauvignon Blanc,
Scheurebe and Verdelho (Darriet et al. 1995; Guth 1997; Tominaga et al. 2000;
Lopez et al. 2003). Three compounds in this group, namely 4-methy-4-
mercaptopentan-2-one, 4-methyl-4-mercaptopentan-2-ol, and 3-mercaptohexanol,
have been found in grapes only in the form of non-volatile odourless cysteine-
conjugated precursors, and their concentration has been shown to increase with the
fermentation process (Tominaga et al. 1998). It has been proposed thatS. cerevisiae
can hydrolyze the cysteine-conjugated precursor though a -elimination reaction
catalyzed by a cysteine-S-conjugate -lyase (Tominaga et al. 1998; Howell et al.
2005).
Since a significant portion of the cysteine conjugates survives the fermentation
process, as observed in the case of glycosylated aroma precursors, researchers have
investigated possible strategies to enhance the cleavage of the conjugated thiols.
Based on the observation that the concentration of volatile thiols in the wine at the
end of fermentation is strictly linked to theS. cerevisiaestrain used for fermenta-
tion, identification of strains with enhanced -lyase activity has been indicated as a
possible strategy to increase the concentrations of volatile thiols in Sauvignon blanc
(Howell et al. 2005; Swiegers et al. 2006). A cysteine-S-conjugate -lyase has also
been isolated from various bacteria (Tominaga et al. 1998), but the fact that the opti-
mal pH for these enzymes is around 8 prevents their possible utilization as exoge-
nous enzymes during winemaking. Recently, the construction of a genetically mod-
ifiedS. cerevisiaecommercial strain overexpressing anE. coligene encoding for a
tryptophanase with strong -lyase activity has been reported (Swiegers et al. 2007).
This strain was shown capable of releasing increased amounts of 4-mercapto-4-
methylpentan-2-one and 3-mercaptohexanol during laboratory scale fermentations.
Current legislation in the majority of wine producing countries does not allow the
use of genetically modified organisms in the wine industry.
3-Mercaptohexyl acetate (3-MHA) is another powerful thiol that contributes the
passion fruit-like character of Sauvignon Blanc wines (Tominaga et al. 2000). It is