4 Enzymes in Winemaking 119
formed through a two step mechanism involving liberation of 3-MH from cystein-
S-conjugates by the yeast and subsequentesterification withacetate (Dubourdieu
et al. 2006). King et al. (2007) demonstrated that increases in the concentration
of this powerful odorant can be achieved through the use of mixed yeast starter
cultures containing a blend of two differentstrains characterised respectively by
high S-cysteine lyase and high esterase activities.
4.6.3 Esterases
Volatile compounds synthesized by yeastduring alcoholic fermentation play an
important role in the aroma characteristics of wine. Among these, esters are con-
sidered the major contributors to the fruity character of wine. Two main groups of
esters are synthetized by yeast, namelyacetate esters and ethyl fatty acid esters.
Acetate esters, often simply referred to as acetates, are obtained from condensation
of yeast-derived higher alcohols with acetyl-CoA, catalysed in the cell by alco-
hol acyl transferase enzymes (Mason and Dufour 2000). It has been shown that
the final concentration of these compounds is the result of the balance between
alcohol acyl transferase enzymes promoting their synthesis, and esterase enzymes
promoting their hydrolysis (Fukuda et al. 1998). The feasibility of improving
acetates production by overexpressing one of the endogenous genes that con-
trol their production during fermentation has been recently explored in a num-
ber of studies. Experimental work on the alcohol acyl transferase gene ATF1
has shown that overexpression of this gene can result in 2- to 12-fold increases
in the concentration of different acetates (Lilly et al. 2000), while overexpres-
sion of a second alcohol acetyl transferase, encoded by theATF2gene, affected
acetate production to a lesser degree (Lilly etal. 2006a). Interestingly, manipulation
of genes not strictly related to ester biosynthesis but involved in the production
of ester precursors in the cell can also affect formation of acetates. Lilly et al.
(2006b) demonstrated that overexpression of abranched-chain amino acid trans-
ferase geneBAT1enhanced the production of isoamyl acetate, which appeared
to be linked to increased production of isoamyl alcohol, the precursor of isoamyl
acetate.
Ethyl fatty acid esters are produced by fermentation yeasts through esterification
of short and medium chain (C 4 -C 12 ) fatty acids coming from lipid metabolism (??).
Only recently, researchers have started to investigate the mechanisms controlling
formation of ethyl fatty acid esters during fermentation, in spite of the fact that
compounds such as ethyl hexanoate and ethyl octanoate occur in young wines in
concentration largely above their odour threshold, and are therefore likely to have a
significant influence on the aroma characteristics of wine. A possible alcohol acyl
transferase, designated Eht1p (ethanol hexanoyl transferase), has been suggested as
a possible candidate for ethyl ester production (Mason and Dufour 2000), which was
later confirmed by Saerens et al. (2006). Interestingly, overexpression of theETH1
gene did not result in increased concentration of ethyl fatty acid esters, possibly