120 M. Ugliano
due to the bifunctional nature of this enzyme, being involved in both formation and
hydrolysis of fatty acid esters (Saerens et al. 2006).
The studies on overexpression of ester-related genes, although carried out using
GMO organisms, have indicated that optimized expression levels of genes encod-
ing for ester synthase enzymes can assist in producing wines with specific ester
profiles. This desirable feature is certainly going to become a major focus of yeast
development research in the near future, either thorough the use of molecular biol-
ogy techniques or by extensive screening of natural yeasts. Nevertheless, it has
to be considered that the majority of the esters produced by yeast during fer-
mentation are largely hydrolyzed during the first two years of wine storage, the
hydrolysis rates being higher when the initial concentration is higher (Ramey and
Ough 1980; Moio et al. 2004). This does notapply however to ethyl acetate, the
most abundant ester produced during alcoholic fermentation, as this compound
is generally stable during aging. Becauseester profiles which are dominated by
high concentrations of ethyl acetate can potentially result in wines with unwanted
nail lacquer-like aromas, caution has to be used in choosing high ester producing
strains.
Recent work has also indicated that commercially available strains of LAB
are also able to synthesize acetates and ethyl fatty acid esters during winemaking
(Maicas et al. 1999; Ugliano and Moio 2005), which appear to be in contradiction
with early suggestions that the MLF could result in a decrease in wine esters (Davis
et al. 1988; Du Plessis et al.2002). More research is required in this area before
general conclusions can be drawn regarding the influence of MLF on the ester com-
position of wine.
4.7 Conclusion
The majority of wine sensorially-active compounds are formed or degraded during
winemaking though pathways involving the intervention of enzymes of different
origins, which underlines the importance of optimizing the extent of different enzy-
matic reactions to achieve wines with specific sensory attributes. Enzyme-driven
transformations also play a primary role in determining the technological efficiency
of various steps of the vinification process.
Addition of exogenous enzymes is currently the most commonly adopted prac-
tice in the wine industry to optimize therate of enzymatic reactions taking place
during winemaking. The large amount of research carried out on this topic has led
to the presence of a considerable number of commercial preparations on the market,
although in some areas, particularly protein stabilization of white wines, exogenous
enzyme-based approaches have been unsuccessful so far.
In addition, promising results are being obtained through strain selection as well
as by using molecular biology techniques to generate novel yeasts and bacteria
with optimized enzyme activity, particularly for applications related to wine aroma
enhancement.