4 Enzymes in Winemaking 107
into its corresponding 9- and 13-hydroperoxide. The subsequent action of a hydro-
xyperoxide-cleaving enzyme on the 13-hyroperoxide results in the formation of
cis-3-hexanal, which is partially isomerized totrans-2-hexanal. The aldehydes are
then transformed into their corresponding alcohol by alcohol dehydrogenase. A sim-
ilar mechanism leads to the formation of hexanal and hexanol from linoleic acid
(Crouzet 1986).
Control of C 6 compounds formation is normally achieved in the winery through
careful clarification prior to alcoholic fermentation (Dubourdieu et al. 1986), as well
as through removal of stalks and leaves, which have been shown to be a major source
of these compounds (Cordonnier Bayonove 1981).
4.4 Wine Stabilization Enzymes
4.4.1 Proteases
Aggregation and subsequent precipitation of unstable proteins is the main cause of
post-bottling haze formation in white wines (Ferreira et al. 2002). Although for-
mation of haze is unlikely to affect wine taste and flavor, cloudy wines are nor-
mally rejected by consumers, resulting in significant economic losses for the wine
industry. Wine proteins can originate from the grapes or from the metabolism of the
various microorganisms involved in the winemaking process. Various studies have
shown that the total protein content of a wine is a poor indicator of its haze insta-
bility, because only a fraction of the proteinpool is responsible for the formation of
haze (Paetzold et al. 1990; Waters 1991). Further investigations have indicated that
this protein fraction is synthesized in grape berries, and appears to be associated
with the occurrence of fungal diseases (Waters et al. 1996).
The occurrence of protease activity both in grape berries (Cordonnier and Dugal
- and wine yeast (Charoenchai et al. 1997; Dizy and Bisson2000) has been
reported. Nevertheless, these proteases have low activity towards haze-forming pro-
teins, which therefore survive the winemaking process. Pocock et al. (2003) have
shown that heat treatment combined with the addition of proteolytic enzyme can
reduce the incidence of haze formation, although the low specificity of commer-
cially available proteases towards haze-forming proteins appeared to significantly
reduce the possibilities offered by this strategy. At the moment, removal of wine
proteins by means of bentonite fining remains the only effective method to control
protein haze in white wine.
4.4.2 Lysozyme
Control of bacteria development is essential for the production of wines with con-
sistent compositional and sensory characteristics. Although malolactic fermentation
(MLF) is frequently carried out in the winery due to its beneficial effects on wine
aroma and palate, for certain grape varieties and wine styles MLF is considered
detrimental. Moreover, even in the cases where MLF is desirable, the wine might