342 M. Ugliano and P.A. Henschke
(Timberlake and Bridle 1976). These pigments are chemically more stable than
anthocyanins and contribute to the colour of aged red wines.
Formation of lactones can occur though chemical cyclisation, although enzy-
matic formation is generally more frequent in nature. Glutamic acid is the presumed
precursor for-lactones formation during fermentation (Wurz et al. 1988). The
mechanisms regulating the formation of lactones in wine are not known.
8D.4.4.3 Modulating Factors
Yeast species and strain, juice clarification, fermentation temperature, medium
composition, extreme aeration of fermentation and SO 2 addition can affect the
formation of aldehydes during fermentation (Romano et al. 1994). Aldehydes
are unstable compounds, which are readily reduced to corresponding alcohols by
yeast dehydrogenases and reductases. Therefore, wine aldehyde content is rela-
tively low immediately after fermentation unless fermentation arrested either due
to yeast physiological factors (stuck fermentation) or intervention by the wine-
maker. SO 2 addition to must, before fermentation, has a strong influence on total
acetaldehyde content of wine by the formation of the stable adduct acetaldehyde-
hydroxysulfonate. Acetaldehyde production is highly variable amongst strains of
Saccharomyces cerevisiae, which can range from 6 mg/L to 190 mg/L, with sulfite-
resistant and high sulfite producing strains typically accumulating more acetalde-
hyde as a result of adduct formation.
8D.4.5 Volatile Sulfur Compounds
8D.4.5.1 Significance
Volatile sulfur compounds are potent aroma compounds that have very low sensory
threshold values, in the low ng/L to g/L range (Mestres et al. 2000; Rauhut 1993).
A large number of sulfur compounds appear in association with fermentation,
those that have received more attention are summarised in Table 8D.3. The types
of compounds and their concentrations in wine are highly variable. The majority
of sulfur compounds are associated with off-flavours and considered by many as
undesirable or a defect in wine by contributing aromas of rotten egg, cooked cab-
bage, onion and rubber (Bell and Henschke 2005; Fedrizzi et al. 2007; Mestres
et al. 2000; Park et al. 1994; Rauhut 1993; Rauhut et al. 1996, 1998; Rib ́ereau-
Gayon et al. 2000a, 2000b; Swiegers and Pretorius 2007). One exception is
represented by the long-chain polyfunctional sulfur compounds, which at low
concentrations are characterised by pleasant, fruity aromas, although high concen-
trations of these compounds can also be objectionable (Dubourdieu et al. 2006;
Mestres et al. 2000; Swiegers et al. 2005; Swiegers and Pretorius 2007). Dimethyl
sulfide (DMS) also appears to be an exception in that the asparagus, corn, molasses
aromas can be considered pleasant in some wines when present at low suprathresh-
old concentrations. Odours of quince, truffles and metallic have also been reported
to depend on DMS concentration, wine type and age. DMS appears to be especially