8D Yeasts and Wine Flavour 331
production which however decreases at higher addition levels (>5 vol.%) (Houtman
and Du Plessis 1981).
Lipid composition of fermentation media markedly affects yeast cellular compo-
sition due to the anaerobically-induced requirement ofSaccharomyces cerevisiaefor
sterol and unsaturated fatty acid necessary for growth (Henschke and Jiranek 1991).
Lipid availability also influences the production of esters and other metabolites of
sensory importance (Anderson and Kirsop 1974; Rossi and Bertuccioli 1992). Addi-
tion of fatty acids and triglyceride mixtures to a fermentation medium promotes
increase in the yeast intracellular concentration of C 8 -C 16 fatty acids, which results
in increased concentrations of acetates and ethyl fatty acid esters. Grape-derived
short to long chain fatty acids vary according to grape variety and cultural con-
ditions, and influence the formation of their respective esters during fermentation,
although higher levels of linoleic acid (C18:2) in grape must are associated with
lower formation of total fatty acid esters by yeast (Yunoki et al. 2007). One inter-
esting observation can be made regarding the regulation of ethyl fatty acid esters
compared to acetates. Whereas the final concentration of acetates does not always
depend on the concentration of their corresponding higher alcohol, formation of
ethyl fatty acid esters is closely linked to the availability of their precursor fatty
acids, so that the conditions promoting formation of esters are usually also favorable
for the respective fatty acid (Saerens et al. 2008).
Addition of ergosterol, the principal yeast sterol, to clarified grape juice markedly
stimulates synthesis of acetate esters, especially 3-methylbutyl, 2-phenylethyl andn-
hexyl acetates but ethyl acetate to a lesser extent. Ergosterol is believed to stimulate
acetate production by stimulating acetyl-CoA synthesis (Houtman et al. 1980). In
winemaking, fatty acids are derived from grape pulp and waxes present on the grape
berry cuticle and grapes contain a variety of phytosterol that yeast can accumulate
(Jackson 2000; Luparia et al. 2004). Therefore, clarification methods can be used
to lower or modulate concentration of lipids in grape juices (Delfini et al. 1992;
Delfini and Costa 1993). Exogenous sources of lipids, such as inactive dry yeast
additives, which can be added during yeast rehydration, could provide another level
of control of flavour-active compounds (Belviso et al. 2004; Swiegers et al. 2008b;
Trioli 1996).
Oxygen availability is an important factor affecting the formation of esters during
fermentation (Fujii 1997; Houtman et al. 1980; Mauricio et al. 1995). Aeration of
clarified grape juice stimulates yeast growth and ester production whereas in slightly
turbid, settled juice esters are decreased, as is observed in wort fermentation. In the
case of acetate esters, aeration of the fermentation medium repressesATF1gene
expression and consequently decreases the formation of acetates (Fujii 1997; Plata
et al. 2005). According to Bardi et al. (1998), the inverse relationship between
production of ethyl fatty acid esters and oxygen availability can be considered to
be a consequence of the arrest of lipidmetabolism due to lack of oxygen. Under
these conditions, an excess of acyl-CoA accumulates in the cell, and esters are then
formed in order to recover free coenzyme A. On the other hand, aerated fermentation
tends to stimulate formation of acetates by several non-Saccharomycesyeasts (Rojas
et al. 2001).