322 M. Ugliano and P.A. Henschke
for sugar to alcohol is: 16.83 g/L sugar produces 1 vol. %alcohol (Rib ́ereau-Gayon
et al. 2000).
The question of strain variability in ethanol yield has assumed high interest in
recent years as a potential tool for controlling the alcohol content of wine. How-
ever, published data show that the ethanol production by 56 commercial wine yeast
strains in a simulated white wine fermentation of 200 g/L sugar only ranged from
11.75 vol.% to 12.09 vol.%, that is they only varied by 0.34% v/v alcohol (Palacios
et al. 2007). This variation between strains is of limited commercial significance.
Larger reductions in ethanol yield are likely to depend on genetic strategies that
divert sugar-carbon away from ethanol production (de Barros Lopes et al. 2003).
8D.3.1.3 Modulating Factors
Few studies on the impact of fermentation conditions on ethanol yield have been
reported. Regarding nutrients, the choice of nitrogen source can significantly modu-
late ethanol yield by altering metabolite formation, especially glycerol and organic
acids. Growth of yeast on amino acids, as the main nitrogen source, limits the need
for amino acid biosynthesis, and hence little associated NADH is generated. This
limits the need for NADH reoxidation, which is coupled to glycerol production. On
the other hand, growth on ammonium salts requires de novo amino acid synthesis,
which results in significant glycerol production in order to maintain redox balance.
The extra glycerol produced results in a 14% decrease in ethanol yield in chemi-
cally defined media (Albers et al. 1996). Conflicting results have been reported in
grape juice-like media (Hern ́andez-Orte et al. 2006; Vilanova et al. 2007) indicating
further work should be carried out with authentic grape musts.
Fermentation temperature and fermentor design can induce evaporative alcohol
losses but these are relatively small. Based on current state of knowledge, choice of
yeast and fermentation conditions constrain wine style and do not provide a reliable
approach for achieving at least a one percent or greater loss of potential alcohol con-
tent of wine (Boulton et al. 1998; de Barros Lopes et al. 2003; Palacios et al. 2007).
Several physical techniques, such as spinning-cone and reverse osmosis, and nano-
filtration can be used to remove sufficient sugar or alcohol for the production of
reduced or low alcohol products (Ferrarini et al. 2008; Grosser 2008).
8D.3.2 Glycerol and Other Polyols
8D.3.2.1 Significance
Glycerol is the major polyol produced by fermentation yeast, with a smaller contri-
bution by 2,3-butanediol. Glycerol ranges from 5 g/L to 14 g/L in dry and semi-dry
wines, with less in flor Sherry wines and up to 25 g/L in botrytised wines (Rankine
and Bridson 1971; Nieuwoudt et al. 2002). Red wines contain higher concentra-
tions than white wines, with mean values of 10.49 g/L and 6.82 g/L, respectively