8D Yeasts and Wine Flavour 355
appear to have an especially notable impact on red wine properties, which is appar-
ent immediately after fermentation and remains evident after maturation for at least
one year (Eglinton et al. 2005; Hayasaka et al. 2007). Variations in the produc-
tion of the various vinyl and carbonyl metabolites are likely to explain, at least in
part, differences between strains (Caridi et al. 2004; Hayasaka et al. 2007; Medina
et al. 2005; Morata et al. 2006). Fermentation conditions, which alter the maceration
time on grape skins, act by not only affecting extraction of anthocyanins and tannins,
but also the production of important yeast metabolites. Fermentation temperature,
aeration, pH, nutrients, use of SO 2 in the case of carbonyls, all affect yeast metabo-
lite formation and modulate wine colour and stability. These factors are also likely
to explain changes in the intensity and mouth-feel properties of astringency.
8D.5.4 Oak Derived Volatile Compounds
8D.5.4.1 Significance
Oak barrels have long been used in winemaking for their convenience as containers
to ferment, store, and transport wine. Nevertheless, during fermentation and mat-
uration in contact with oak, wine undergoes a series of complex transformations,
which can result in significant changes to its aroma composition and quality. In
particular, toasted oak staves, commonly employed in cask manufacturing, con-
tain significant amounts of potent aroma compounds, such as vanillin and vanillin
derivatives, volatile phenols, and lactones. The accumulation of these compounds in
wine during barrel storage results in increased intensity of sensory descriptors, such
as vanilla, spice, and coconut, which are characteristic of these compounds. Optimal
balance between oak-derived sensory attributes and grape- and fermentation-derived
aromas is particularly important for the achievement of the desired level of wine
aroma complexity.
8D.5.4.2 Metabolism and Modulating Factors
When fermentation is carried out in oak barrels, some of the compounds migrat-
ing from the wood to fermenting grape juice can be transformed by yeast through
enzymatic reactions. The main reactions are reductive, such as conversion of car-
bonyl compounds to the corresponding alcohols. Amongst these, the powerful
odorant vanillin can be reduced to vanillic alcohol during fermentation (Chatonnet
et al. 1992a). As the odour threshold for vanillic is much lower then vanillin, it has
been suggested that this transformation contributes to reducing the ‘woody’ charac-
ter of wines that have been fermented in barrel (Chatonnet et al. 1992a). A similar
reaction results in the formation of furfuryl alcohol from furfural during barrel fer-
mentation (Marsal et al. 1988). 2-Furanmethanethiol (furfuryl mercaptan) can also
be formed from furfural, possibly catalysed by yeast cysteine desulfhydrase, which
releases H 2 S from cysteine under conditions of low assimilable nitrogen (Tominaga
et al. 2000). This compound has a roasted coffee-like aroma.