332 M. Ugliano and P.A. Henschke
Empirically, it has long been known thatlower fermentation temperature pre-
serves the fruity characters typical of young white wines. Temperature affects the
accumulation of acetate and ethyl fatty acid esters during fermentation (Killian and
Ough 1979), although the response of individual esters appears to be linked to both
evaporative losses and changes in yeast metabolism. Altered fatty acid biosynthesis,
as reflected by differences in membrane fatty acyl composition, is involved (Boulton
et al. 1998; Torija et al. 2003; Watson 1987). In a model study, lower fermentation
temperature (15◦C) favoured ethyl esters whereas higher temperature (28◦C), typ-
ical of red wine fermentation, favoured 2-methyl acetate, ethyl 2-methylbutanoate,
2-phenyl ethanol and 2-phenylethyl acetate (Molina et al. 2007). Similar results
were obtained in grape juice (Beltran et al. 2008). These ester profiles could be
linked to different expression profiles of ester synthase genesATF1andEHT1,and
the ester degradation geneIAH1under different temperature conditions (Molina
et al. 2007).
The nitrogen content and composition of must can strongly affect the accumula-
tion of volatile esters during fermentation (Bell and Henschke 2005; Carrau et al.
2008; Garde-Cerd ́an and Anc ́ın-Azpilicueta 2008; Guitart et al. 1999; Hern ́andez-
Orte et al. 2002; Vilanova et al. 2007). Total and individual esters tend to increase
with increasing must amino nitrogen, although the responses of the various esters
to individual amino acids is not yet clear (Hern ́andez-Orte et al. 2002). Addition
of ammonium salts, a common practice in the wine industry, strongly stimulates
the production of esters. The stimulation of ester biosynthesis with nitrogen sup-
plements, which is observed under various winemaking conditions, appears to con-
firm the link between increased nitrogen availability and expression ofATF1gene
reported by some authors in model fermentations (Yoshimoto et al. 1998). Stimu-
lated production of medium chain fatty acids by assimilable nitrogen might account
for large increases in corresponding esters.
The metabolism of branched-chain esters is largely unknown. Based on their
structure, it is reasonable to assume that they are formed through esterification
of the branched-chain acids formed during amino acid metabolism. It has been
reported that nitrogen supplementation of low nitrogen must with ammonium salts
has a negative impact on the formation of these compounds during fermentation
(Hern ́andez-Orte et al. 2006; Vilanova et al. 2007).
Alcohol acetyltransferases
Atf1p, Atf2p, Eht1p, Eeb1p
Acyl-CoA+Alcohol Ester
Iah1p, Eht1p, Eeb1p
EsterasesFig. 8D.4The biosynthetic and degradation pathway for esters
Fatty acids, following activation with coenzyme A, condense with alcohols catalysed by alco-
hol acetyltransferases. Esterases hydrolyse esters to their constituent acid and alcohol. Several
enzymes have dual synthetic and esterase activity