314 M. Ugliano and P.A. Henschke
8D.1 Introduction
Vinification is a complex process involving interactions between microbes and
grape must, resulting in wine. The key microorganisms are fermentative yeasts,
which transform an acidic, sweet, low flavoured grape must into a distinctive,
highly flavoured alcoholic beverage. In essence, this process is an alcoholic fer-
mentation involving biochemical conversion of hexose sugars to ethanol and carbon
dioxide. The central glycolytic pathway, which is coupled to the reductive forma-
tion of ethanol, provides energy, precursor molecules and reducing power for cell
growth, maintenance and reproduction. Inaddition, the glycolytic and associated
pathways form volatile and non-volatile metabolites that contribute to wine flavour.
The species of yeast(s), fermentation conditions and nutrient content of the must
modulate the production of these compounds, and consequently, a wine’s flavour
profile (Lambrechts and Pretorius 2000; Fleet 2003; Romano et al. 2003b; Swiegers
et al. 2000).
Over recent decades, research into the role of yeast in the development of wine
flavour has revealed complex interactions between this microbe and grape com-
pounds; many of these interactions contribute to the appearance, aroma, flavour and
texture of wine. When wood is used in fermentation, some wood-derived flavour
compounds can also be modified by yeasts. Together, all these compounds that are
present in must, and produced and modified during fermentation, and by other pro-
cesses, contribute to the distinctive ‘varietal’ character of wine.
Although many wines are still made with little intervention and rely on the
indigenous yeasts present in must, modern winemaking is founded on the use of
selected strains ofSaccharomyces cerevisiae,that have reliable fermentation prop-
erties coupled to good flavour characteristics. As our understanding of the impacts
that yeasts have on wine flavour has improved, strains have been selected that are
less susceptible to off-flavour formation and better augment different components
of wine flavour. Chemical identification of key flavour compounds and definition of
the genetic and biochemical basis of the associated metabolic pathways, and their
regulation, has often lagged behind the use of selected strains, but remains an essen-
tial step in the development of yeasts with improved flavour properties that are better
adapted to grape must (Henschke 1997; Pretorius 2000; Swiegers et al. 2005). Char-
acterisation ofSaccharomycesspecies has further expanded winemakers’ ability to
modulate wine flavour due to greater genetic and metabolic variability. Driven by the
search for even greater diversity of wine flavour, non-Saccharomycesspecies, which
originate from grapes, are also being evaluated, and novel inoculation strategies
developed, to exploit their flavour complexing properties (Jolly et al. 2006). Wild
yeasts, although representing an undefined mixture of species and strains, produce
flavour profiles, which cannot be easily achieved by other techniques.
This chapter will summarise the microbiological, physiological and biochemical
interactions between grape compounds and yeast metabolism that lead to the devel-
opment of wine flavour. The role of different yeasts and inoculation strategies in
the modulation of wine flavour will be highlighted, and the potential for improving
wine flavour by advanced screening and genetic techniques will be briefly discussed.