8D Yeasts and Wine Flavour 347
et al. 1973). Deficiencies in pyridoxine, a cofactor in the methionine biosynthetic
pathway, can also result in large production of H 2 S, particularly in the case of yeast
strains that are unable to synthesize this vitamin (Monk 1986). Lack of biotin can
also cause an increase in the formation of H 2 S (Thomas and Surdin-Kerjan 1997),
possibly due to the role of this vitamin as cofactor in the biosynthesis ofO-acetyl-
L-homoserine (Bohlscheid et al. 2007). Addition of commercially available fermen-
tation nutrients containing the above-mentioned vitamins is a common measure to
reduce the risk of H 2 S production in the wine industry.
Nevertheless, under winemaking conditions, nutrient supplementation has not
eliminated the risk of H 2 S production (Henschke and Jiranek 1991; Jiranek et al.
1995a; Park et al. 2000; Spiropoulos et al.2000). Recent genetic studies highlight
the complexity of regulation of the sulfate reductive assimilation pathway (Lin-
derholm et al. 2006, 2008; Spiropoulos et al. 2000). Controlling sulfite reductase
activity is an obvious target for lowering excessive H 2 S production to acceptable
levels but so far no commercial strains have been developed (Sutherland et al. 2003;
Zambonelli et al. 1975). An inability to control H 2 S liberation by over-expression
of genes associate with H 2 S sequestration (MET17)andS-amino acid biosynthesis
(CYS4andMET6) suggests that metabolite flux might be a limiting factor rather
than inadequate enzyme activities. Analysis of the S-amino acids biosynthetic path-
way, by studying various mutants carrying gene defects, suggests that some of
these genes, or substrates or products of their corresponding protein products, might
play key roles in regulating sulfate reduction (Linderholm et al. 2008). Interactions
between the sulfate assimilation pathway and amino acid pathways and various
metabolites (acetaldehyde) provide an insight into the complexity of pathway regu-
lation (Aranda and del Olmo 2004; Backhus et al. 2001; Marks et al. 2003).
Organic Volatile Sulfur Compounds
Conditions that favour H 2 S production also favour production of other volatile sulfur
compounds, methanethiol and methanethioacetate (Rauhut et al. 1996), suggesting
a metabolic link to methionine catabolism. Catabolism of this amino acid essen-
tially follows the Ehrlich pathway, as shown in Fig 8D.5. The first step involves
transamination to yield the keto acid -keto-gamma-(methylthio)butyric acid, which
is decarboxylated to 3-(methylthio)-1-proprionaldehyde specifically by Ydr380wp,
and reduced to 3-(methylthio)-1-propanol (methionol) by alcohol dehydrogenases
(Fig 8D.8). The production of methionol is likely to be regulated in a similar man-
ner to that of other higher alcohols, that is, higher concentrations are formed in
low to moderate nitrogen musts (Hern ́andez-Orte et al. 2005). Both methionine and
-keto--(methylthio)butyric acid can act as a source of methanethiol through a
demethiolase step, which also produces alpha-ketobutyric acid (Perp`ete et al. 2006).
Yeast strains with intense H 2 S production also produce higher amounts of thioacetic
acid esters of methanethiol and ethanethiol (Rauhut et al. 1996); it is likely that these
thiols are esterified by alcohol acetyltransferase in the same way as ethyl acetate is
formed from and acetic acid. The formation of acetic acid esters is a major problem
in winemaking since these compounds are not removed from wine during fining