8D Yeasts and Wine Flavour 351
not yet demonstrated, it is likely that aliphatic-, shikimate-, benzene-derivatives-
and phenol-glycoconjugates are similarly hydrolysed predominantly by yeast hydro-
lases during fermentation. In addition to this mechanism, it is well documented that
the volatile phenols, 4-vinyl phenol and 4-vinyl guaiacol can be formed bySac-
charomyces cerevisiaethrough non-oxidative decarboxylation of hydroxycinnamic
acid precursors,p-coumaric and ferulic acids, respectively (Chatonnet et al. 1993).
This enzymatic activity is naturally inhibited by catechins, which explains why, in
comparison to white wines, red and rose wines typically exhibit much lower concen-
trations of vinyl phenols, despite having more precursors (Chatonnet et al. 1993).
Reaction with anthocyanins to form pyranoanthocyanins might also contribute to
lower vinyl phenols in red wines.
8D.5.2 Long-Chain Polyfunctional Thiols
8D.5.2.1 Significance
The long-chain polyfunctional volatile sulfur compounds, represented by 4-mercapto-
4-methylpentan-2-one (4-MMP), 4-mercapto-4-methylpentan-2-ol (4-MMPOH), 3-
mercaptohexan-1-ol (3-MH) and 3-mercaptohexyl acetate (3-MHA), are amongst
the most potent aroma compounds identified in wine with sensory thresholds
of 0.8–60 ng/L (Dubourdieu etal. 2006). These potent volatile thiols are major
contributors to the varietal aroma of Sauvignon Blanc wines, and are associated
with aroma characteristics such as box tree, blackcurrant, grapefruit, passionfruit
and citrus zest (Darriet et al. 1995; Tominaga et al. 1995, 1998; Dubourdieu
et al. 2006). The compound 4-MMP is also present in wines made from Scheurebe,
Gew ̈urztraminer, Riesling, Colombard, Petit Manseng, Semillon, Cabernet Sauvi-
gnon and Merlot.
8D.5.2.2 Metabolism and Modulating Factors
Free forms of the long-chain polyfunctional thiols 4-MMP, 3-MH and 3-MHA
are virtually absent in grapes. However, a large number ofVitis viniferavarieties
contain variable amounts of 4-MMP and 3-MH in the form of non-volatile
S-cysteinyl- and related conjugates.Saccharomyces cerevisiaecan hydrolyse these
cysteine conjugates, liberating free 4-MMP and 3-MH (Fig 8D.10) (Dubourdieu
et al. 2006; Howell et al. 2004b; Murat et al. 2001; Swiegers et al. 2008c). The
mechanism of uptake of the non-volatile thiol precursors and the factors that control
this are still to be determined. However, ammonium appears to suppress thiol pro-
duction suggesting that precursors are excluded from the cell by nitrogen catabolite
repression control, which regulates the uptake of nitrogen sources (Subileau et al.
2008; Thibon et al. 2008).
Release of the free thiol from the cysteinyl-conjugate appears to involve carbon-
sulfur lyase activity (Howell et al. 2005; Tominaga et al. 1995). Overexpression
and deletion experiments inSaccharomyces cerevisiaehave shown that tryptophane