8D Yeasts and Wine Flavour 335
8D.4.2.3 Modulating Factors
Many factors affect formation of higher alcohols during fermentation, includ-
ing yeast species and strain, initial sugar, fermentation temperature, the pH and
composition of grape juice, assimilable nitrogen, aeration, level of solids, grape
variety and skin contact time (Fleet and Heard 1993; Houtman and Du Plessis 1981;
Houtman et al. 1980). Choice of yeast has a considerable impact on higher alcohol
content of wine due to wide variability amongst strains ofSaccharomycesspecies
withSaccharomyces cerevisiaegenerally producing lesser concentrations of higher
alcohols than cryotolerantSaccharomyces bayanus/uvarumstrains (Antonelli et al.
1999; Massoutier et al. 1998). Most non-Saccharomycesspecies are reported to
produce lesser concentrations of higher alcohols thanSaccharomyces cerevisiae
although in fermentation with mixed yeast species higher concentrations of higher
alcohols can be produced (Heard 1999).Metschnikowia pulcherrimahas a notably
high production of 2-phenyl ethanol (Clemente-Jimenez et al. 2004).
The assimilable nitrogen composition of grape juice has a strong influence on
the production of higher alcohols during fermentation. When initial nitrogen con-
centrations are very low, increasing nitrogen results in greater concentrations of
higher alcohols (Ayr ̈ ̈ ap ̈a ̈a 1971; Carrau et al. 2008; Garde-Cerd ́an and Anc ́ın-
Azpilicueta 2008; Vilanova et al. 2007). According to Oshita et al. (1995), under
these conditions, surplus keto acids cannot be converted into amino acids, due to the
lack of available nitrogen, and are therefore excreted as higher alcohols. Conversely,
at high initial juice nitrogen concentrations, increased availability of nitrogen causes
a reduction in higher alcohol production because most of the keto acids produced
are directly converted into the corresponding amino acids.
The degree of juice turbidity increases the production of higher alcohols, which is
linked to biomass production (Houtman and Du Plessis 1981; Klingshirn et al. 1987);
red wines, produced by fermentation on grape solids typically contain more higher
alcohols. Similarly, chemically inert solids stimulate higher alcohols production,
although the mechanism of action is not clear. Aeration and increased fermentation
temperature are believed to stimulate the production of higher alcohols by stimulat-
ing nutrient (nitrogen) uptake and growth.
8D.4.3 Volatile Fatty Acids
8D.4.3.1 Significance
Wine contains a mixture of straight chain fatty acids, usually referred to as short
chain (C 2 –C 4 ), medium chain (C 6 –C 10 ), long chain (C 12 –C 18 ), and a group of
branched-chain fatty acids that include 2-methyl propanoic, 2-methyl butanoic,
and 3-methyl butanoic acids. Acetic acid is quantitatively and sensorially the most
important volatile fatty acid produced during alcoholic fermentation. Accounting for
more that 90% of the total wine volatile acidity, acetic acid plays the most important