344 M. Ugliano and P.A. Henschke
methanethiol and ethanethiol and 2-mercaptoethanol are more persistent in wine
to which they elicit objectionable odours of cooked cabbage, onion, rubber, and
poultry. Thioesters can also form during fermentation, and slowly hydrolyse under
the acidic conditions of wine to release the parent objectionable mercaptan (Rauhut
et al. 1998). Quantitatively, the most important low volatilitysulfur compound is
methionol, which possesses a potato, cauliflower odour. Unlike the lower boiling
point volatile sulfur compounds, methionol can be present in wine in the low mg/L
range, up to 5 mg/L.
8D.4.5.2 Metabolism and Modulating Factors
The appearance of volatile sulfur compounds in freshly fermented must is a com-
plex, multifaceted chemical and biochemical process. Most research has focused
on the induction of H 2 S metabolism in yeast, especially in response to changes in
nutrient composition (Eschenbruch1974; Henschke and Jiranek 1991; Rauhut 1993;
Jiranek et al. 1995a, 1996; Linderholm et al. 2006; Rankine 1963; Swiegers and Pre-
torius 2007; Spiropoulos et al. 2000; Vos and Gray 1979). Yeasts produce H 2 Sand
other volatile sulfur compounds by a number of pathways, which include chemical
reduction of elemental S, degradation of sulfur amino acids or reduction of sulfite
or sulfate. Apart from the reduction of inorganic sulfur to H 2 S, few of the chemical
and/or metabolic pathways for the formation of other volatile sulfur compounds
have been reported or verified in wine fermentation.
Hydrogen Sulfide
Elemental S (crystalline or colloidal form) can accumulate as a residue in must as
the result of its use as a vineyard agrochemical, where it is used to control grape
vine powdery mildewErysiphe necatorand various pests. Direct reduction of S to
H 2 S is induced by the highly reductive conditions that exist at the yeast cell surface
during fermentation. This mechanism is of little practical importance except when
the application of elemental S is not used according to manufacturers’ recommen-
dations, such as application within the recommended with-holding period before
grape harvest (Rankine 1963; Rauhut and K ̈urbel 1994; Thomas et al. 1993).
Along with elemental S, grape musts also contain other forms of inorganic sulfur.
Sulfate is indeed usually present in excess amounts (up to 700 mg/L) and sulfite
(up to 100 mg/L) is often added as an antioxidant and antimicrobial compound
(Henschke and Jiranek 1991). These two forms of inorganic S have been shown to
be the main sources of H 2 S formed during fermentation. Sulfate is accumulated
by specific transporters (Sul1p and Sul2p), activated with 2 moles of ATP and
reduced to H 2 S by the sulfate reductive assimilation pathway (Fig 8D.8). The last
step of this pathway reduces sulfite to H 2 S, catalysed by sulfite reductase, which
is encoded by theMET5andMET10genes. Sulfite, when present in the must,
enters the cell by diffusion across the plasma membrane (Stratford and Rose 1985a)
and can be directly reduced to sulfide (Hallinan et al. 1999; Jiranek et al. 1996;
Stratford and Rose 1985b). Sulfite is a favoured source of sulfur in many yeast and