318 M. Ugliano and P.A. Henschke
phosphate affects flavour metabolism. Similarly, metal ions primarily function as
cofactors in enzymatic reactions but again the link between availability and flavour
metabolism is essentially unknown. Conversely, certain vitamins, particularly biotin
and pantothenic acid, are known to affect the formation of aroma compounds due to
their variable content in must.
Yeast characteristics are largely determined by its genetic constitution, never-
theless the balance of nutrients in each grape must can strongly affect growth and
metabolite production, which is a determinant of wine composition, flavour and
style. Few wine strains are well characterised and little information exists to guide
winemakers on using nutrients to modulate flavour. Apart from the routine measure-
ment of sugar and, to a growing extent, -amino nitrogen concentration, few rapid
methods for measuring other important nutrients are available to winemakers. For
this reason proprietary nutrient mixtures are widely used, according to wine regu-
lations. Because the balance of nutrients, such as sugar, amino acids and vitamins,
appear to depend on the viticultural conditions, regions which favour a particular
balance of nutrients might owe some of the regional characteristics of wine, to
nutrient composition, as well as the winery microflora, at least when winemaker
intervention is minimal.
8D.2.2 Biotransformation of Flavour Precursors
Apart from sweetness, acidity and astringency, grapes and musts exhibit relatively
low flavour when compared to wines. Furthermore, wines made from most grape
varieties posses a characteristic aroma andflavour, which is attributed to, at least
in part, grape-derived flavour-active precursor compounds (Dubourdieu et al. 2006;
Francis et al. 1999). During fermentation and subsequent wine aging these non-
volatile, odourless compounds undergo transformation into volatile, aromatic com-
pounds. A variety of yeast biochemical mechanisms are involved, including hydrol-
ysis, transformation reactions such as reduction, esterification, and decarboxylation,
oxidation and metabolite-induced condensation reactions. Examples of these reac-
tions are given in Table 8D.2, and will be discussed in more detail later in this
chapter.
The best studied enzyme-catalysed reactions relate to two important classes
of odourless flavour-precursor compounds, namely glycosides and cysteinylated
conjugates. Grape varieties related toVitis viniferacv. Muscat contain terpenoid
glycosides, which give rise to the intense floral Muscat aroma after fermentation
(Cordonnier and Bayonove 1974; Strauss et al. 1986). Yeasts possess hydrolytic
enzymes with - D-glucopyranosidase ( -glucosidase) and -glucanase activities,
which are capable of enhancing the hydrolysis of glycosides during fermentation
(Darriet et al. 1988; Gil et al. 2005; Ugliano et al. 2006). In addition to containing
glycosides,Vitis viniferaL. cv. Sauvignon Blanc contains non-volatileS-cysteinyl-
conjugates, which are responsible for the characteristic passion fruit/grape fruit
and box-hedge aromas that develop during fermentation (Dubourdieu et al. 2006).