8D Yeasts and Wine Flavour 365
when an exogenous source of acetaldehyde is introduced into a culture (Stanley
et al. 1993). Exogenous acetaldehyde stimulates fermentation by coupled oxidation
of NADH, which is catalysed by alcoholdehydrogenase. Cofermentation of two
metabolically distinct strains not only affects the growth rate of each strain, but
also metabolite formation, since the production of many metabolites are potentially
modulated by cellular redox balance (Cheraiti et al. 2005; Eglinton et al. 2002)
(Fig 8D.2). In a cofermentation study of threeSaccharomyces cerevisiaestrains,
the final concentration of acetaldehyde was low compared to that produced by the
three strains when cultivated individually, pyruvate was intermediate and sulfite and
acetic acid were formed in much lower concentrations. Acetate esters were found
at intermediate concentration in mixed cultures compared to monocultures (Gross-
mann et al. 1996). This yeast preparation of three strains is produced commercially
by Siha as Varioferm.
Favale et al. (2007), who investigated cofermentation and sequential fermentation
of aSaccharomyces cerevisiaeand aSaccharomyces bayanushybrid by comprehen-
sive analysis of the volatile compounds formed, found somewhat different effects.
Ethyl esters and higher alcohols tended to be produced in concentrations similar to
the highest yeast producer in monoculture, whereas ethanol, glycerol and fatty acids
tended to be produced in intermediate concentrations. These differences suggest
that wine composition changes will depend on, at least in part, the choice of yeasts.
Furthermore, sequential culture fermentation introduces additional differences that
further modulate wine composition.
Howell et al. (2006) also carried out extensive metabolite profiling of the volatile
metabolites produced in wines made by cofermentation withSaccharomycesstrains.
They found that cofermentation not only produced a metabolite profile different
from wines made by monoculture fermentation, but that blends of the monocul-
ture fermentation wine were also notably different. These various finding therefore
strengthen the anecdotal observations of winemakers that cofermentation with two
or more strains can potentially increase the flavour complexity of wines. These stud-
ies also suggest that cofermentation of yeasts with greater metabolic dissimilarity
are likely to produce greater differencesin the profile of volatile and non-volatile
metabolites, as has been observed when non-Saccharomycesyeasts are cofermented
withSaccharomyces(Sect. 8D.6.4). Additional studies are needed to understand
better the complex metabolic interactions and to describe the major impacts on wine
composition and flavour.
Cofermentation with two or more strainshas recently been exploited to enhance
the aromatic profile of Sauvignon Blanc wines, in which a non-volatile
S-cysteinylated precursor (3-(hexan-1-ol)-L-cysteine) is first hydrolysed by a carbon-
sulfur lyase and subsequently esterified by alcohol acetyltransferase (Sect. 8D.5.2).
Strains ofSaccharomyces cerevisiaevary in ability to carry out the two reactions
(Dubourdieu et al. 2006; Swiegers and Pretorius 2007). Cofermentation with two
strains, one having higher hydrolytic function (release of 3-MH) and the other higher
esterification activity, substantially enhanced formation of 3-MHA, thereby increas-
ing the passion-fruit aroma when compared to monoculture wines. The authors
showed that interaction between the two strains produced more 3-MH and 3-MHA