Wine Chemistry and Biochemistry

(Steven Felgate) #1

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

Free download pdf