8D Yeasts and Wine Flavour 329
example, the ethyl esters ethyl hexanoate, ethyl octanoate, ethyl decanoate, the
branched-chain esters ethyl 2-methyl propanoate, ethyl 2-methyl butanoate, ethyl
3-methyl butanoate, and the acetate esters hexyl acetate, 2-methylbutyl acetate
and 3-methylbutyl acetate were likely to be important. Conversely, in the case
of Riesling, ethyl-2-methyl propanoate, ethyl-2-methyl butanoate, ethyl-3-methyl
butanoate, ethyl octanoate, ethyl decanote, hexyl acetate,2-methylbutyl acetate and
3-methylbutyl acetate were found to be important.
8D.4.1.2 Metabolism
Acetate esters are formed through the condensation of higher alcohols with acetyl-
CoA, catalysed in the cell by alcohol acetyltransferase enzymes (Malcorps et al.
1991; Mason and Dufour 2000). The final concentration of these compounds is the
result of the balance between alcohol acetyltransferase enzymes promoting their
synthesis, and esterase enzymes promoting their hydrolysis (Fig. 8D.4) (Fukuda
et al. 1998; Plata et al. 2005). TheATF1andATF2genes encode for alcohol
acetyltransferase enzymes (Atf1p and Atf2p) that are responsible for the syn-
thesis of acetate esters bySaccharomycesyeast (Yoshimoto et al. 1998; Lilly
et al. 2006a). In gene over-expression and deletion studiesATF1,with a minor
role byATF2,contributed to the formation of ethyl acetate, 3-methylbutyl acetate,
and 2-phenylethyl acetate, which are associated with fruity and floral characters
in wine (Lilly et al. 2000, 2006a; Verstrepen et al. 2003b). Deletion of bothATF
genes eliminated 3-methylbutyl acetate but not ethyl acetate formation indicating
that another biosynthetic gene exists for an ethanol acetyltransferase. On the other
hand, overexpression of the esterase geneIAH1lowered formation of ethyl acetate,
3-methylbutyl acetate, 2-phenylethyl acetate and hexylacetate (Fukuda et al. 1998).
Variations in the expression profile of these ester synthetic and hydrolytic genes are
believed to provide the basis for modulated production of acetate esters by various
yeasts strains under different fermentation conditions.
In contrast with the considerable knowledge available on the formation of acetate
esters during fermentation, the enzymology and genetics of ethyl fatty acid esters
formation is still poorly understood. The lowmolecular weight ester ethyl propanoate
is formed from propanoic acid derived from -ketobutyrate (Eden et al. 2001).
Ethyl butanoate is also formed, in part,from butanoic acid derived from the same
pathway. However, the largest portion of ethyl fatty acid esters is thought to be
formed enzymatically through esterification of the activated fatty acids (acyl-CoA)
formed during the early stages of lipid biosynthesis (Suomalainen 1981). Recently,
two enzymes (hexanoyl transferase or acyl-coenzymeA:ethanolO-acyltransferase)
responsible for the formation of ethyl esters of medium chain fatty acids, namely
Eht1p and Eeb1p, have been identified, although others are predicted to exist (Mason
and Dufour 2000; Lilly et al. 2006a; Saerens 2006). In vitro studies show that Eht1p
has preference for short-chain substrates whereas Eeb1p accepts longer-chain sub-
strates, although no preference was observed by in vivo studies. In wine strains
Eht1p has been shown to catalyse formation of ethyl hexanoate, ethyl octanoate