330 M. Ugliano and P.A. Henschke
and ethyl decanoate, which have an apple, fresh fruit aroma. Interestingly, while
deletion of one or both of the genes encoding for these enzymes resulted in reduced
formation of ethyl esters, their overexpression had limited or no effect on the final
concentration of these metabolites, however, depending on the yeast strain (Lilly
et al. 2006a; Saerens 2006). Several possible explanations are that these enzymes
might possess bifunctional synthetic and hydrolytic activities or that the levels of
fatty acid precursor are limiting (Saerens 2006, 2008).
8D.4.1.3 Modulating Factors
Yeasts vary widely, both between and within species, in their ability to produce
esters (Antonelli et al. 1999; Heard 1999;Houtman and Du Plessis 1986; Miller
et al. 2007; Plata et al. 2003; Rojas et al. 2001; Soles et al. 1982; Swiegers
et al. 2008c). Of the principal esters, ethyl acetate is produced in higher amounts
by non-Saccharomycesspecies, includingHanseniaspora uvarum (Kloeckera apic-
ulata), Hanseniaspora guilliermondii, Issatchenkia orientalis(Candida krusei),
Issatchenkia terricola, Metschnikowia pulcherrima,andPichia anomala,which
produce higher concentrations thanSaccharomyces cerevisiae, Torulaspora del-
brueckii, Kluyveromyces thermotoleransandCandida stellata,whereas more 3-
methylbutyl acetate is produced by Saccharomyces cerevisiae, Hanseniaspora
uvarum, Kluyveromyces thermotolerans and Pichia anomalathan the remaining
non-Saccharomycesspecies mentioned. This pattern of ester production tends to
preclude the use of many non-Saccharomycesstrains as key yeasts in wine produc-
tion; however cofermentation withSaccharomyces cerevisiaemoderates the produc-
tion of esters and other compounds, rendering such wines with a greater diversity of
acceptable flavours (Sect. 8D.6.4). Strainsvary in the pattern of acetates and ethyl
esters produced as well as in the overall concentration of esters (Soles et al. 1982),
which contribute to sensorially significant differences in young wines, with some
differences still apparent up to two years of storage (Dumont and Dulau 1997; Jane
et al. 1996).
Ester production is susceptible to variations in nutrient and fermentation condi-
tions; however, acetates and ethyl estersdo not always vary in the same way, reflect-
ing their different metabolic pathways. Variations in juice contents of sugar, oxygen,
lipids, and assimilable nitrogen, juice turbidity and fermentation temperature are
important factors, as observed under brewing conditions (Verstrepen et al. 2003a).
Grape maturity at harvest modulates the relative abundance of different ester
types produced during fermentation, suchthat acetate esters decrease whereas ethyl
esters increase with increasing maturity (Houtman et al. 1980). The addition of
sugar to de-aerated, clarified grape juice,however, stimulates both types of esters
in a similar manner, whereas added sugar to wort fermented with ale yeast stim-
ulates acetates production with little influence on ethyl esters (Plata et al. 2005;
Saerens et al. 2008). The level of grape solids, a source of lipids, in grape juice
affects yeast growth, fermentation rate and accumulation of esters; a small addition
of freshly settled juice solids (1–2 vol.%) to membrane-filtered juice stimulates ester