328 M. Ugliano and P.A. Henschke
volatile compounds are also released from non-volatile grape flavour precursor com-
pounds by yeast enzymes. Examples are monoterpenes and C 13 -norisoprenoids,
which are released from glycosidic precursors, and long-chain polyfunctional thi-
ols, which are derived fromS-cysteinyl conjugates. Some phenolic compounds,
which undergo transformation reactions orreact with yeast metabolites, affect the
colour, aroma and flavour of wine. The significance of these compounds to wine
flavour, their metabolism, and factors that affect their formation are discussed in
Sect. 8D.5
8D.4.1 Esters
8D.4.1.1 Significance
Fermentation-derived esters are largely responsible for wine fruitiness, and there-
fore they play an important role in the sensory composition of young red and
white wines. Although at wine pH a significant portion of esters hydrolyses dur-
ing aging, the major fermentation-derived esters are still present in concentration
higher than their odour threshold in 1–2 years old wines (Escudero et al. 2007; Moio
et al. 2004; Rapp and Marais 1993), which suggests their involvement in the sensory
composition of aged wines as well. The two main groups of fermentation-derived
esters that have been long associated with wine fruitiness are acetate esters [ethyl
acetate, 2-methylpropyl acetate (isobutyl acetate), 2- and 3-methylbutyl acetate
(active amyl and isoamyl acetate, respectively), hexyl acetate, and 2-phenylethyl
acetate], and ethyl fatty acid esters (ethyl C 3 - ethyl C 12 ). The organoleptic synergy
existing between different esters determines the overall sensory characteristics of
esters mixtures. Van der Merwe and vanWyk (1981) demonstrated that acetates are
more important than ethyl fatty acid esters for the perceived aroma of wine, and
that the odour intensity of a mixture of esters is higher that that of the individual
components. More recently, the role playedby branched-chain esters, namely ethyl
2-methylpropanoate, ethyl 2-methylbutanoate, and ethyl 3-methylbutanoate, in the
aroma of wine has been investigated. Theseesters are characterized by strawberry-
like aromas, and probably contribute to the red-fruits notes of some red wines
(Piombino et al. 2004). Odor threshold values fall between 3 g/L for ethyl 3-
methylbutanoate (Guth 1997) and 18 g/L for ethyl 2-methylbutanoate (Ferreira
et al. 2000), indicating that these compounds are among the most powerful odorants
in the class of esters. However, as they occur in much lower concentrations com-
pared to ethyl fatty acid esters and acetates, their impact on wine aroma is still to be
established (Escudero et al. 2007).
In a recent study, using multivariate statistical analysis of quantitative sensory
descriptive analysis and precise chemical compositional data, Smyth et al. (2005)
found that the importance of individual yeast esters to the aroma profile of wine
can vary with the type of wine. In the case of unwooded Chardonnay wines, for