8A Wine Aroma Precursors 263
although its hydrate is a wine constituent (Schneider 2001; Escudero et al. 2002).
Similar pathways could also explain theformation of other odorous volatile thi-
ols identified in wine and also in beer, such as 3-methyl-3-sulfanylbutan-1-ol, 2-
methyl-3-sulfanylbutan-1-ol, or 3-sulfanylpentan-1-ol, which have noS-cysteine or
S-glutathione conjugates identified asyet (Bailly et al. 2006; Huyng-Ba et al. 2003;
Sarrazin et al. 2007; Tominaga et al. 2000b, 2003a,b; Vermeulen et al. 2006).
During wine aging, the levels of all these thiols decrease, but this decrease is
very dependent on the oxidative conditions of the aging. Thus, lees, sulfur diox-
ide, glutathione, and anthocyanins have protective effects on these volatile thiols,
whereas increased contact with oxygen, particularly in presence of derivatives of
catechin, favors their degradation (Murat et al. 2003; Blanchard et al. 2004). As
for the evolution of these odorous thiols during the conservation of the wine, their
contents generally decrease, but this phenomenon appears very dependent on the
oxydative reactions related to this conservation. Thus, the factors which prevent the
decrease of the reducing potential of the wine (contact limited with oxygen, dioxide
of sulphur, lees, glutathion, anthocyanes) limit the degradation of these odorous thiol
(Murat et al. 2003; Blanchard et al. 2004).
8A.7 Precursors of Dimethylsulfide
Dimethylsulfide (DMS) has been known as a wine constituent for a long time,
as well as its formation during yeast fermentation and wine aging (Anness and
Bamforth 1982; Eti ́evant 1991), but the ability of DMS precursors, differing from
DMSO, to release this compound through a chemical process during wine aging,
and their occurrence in grape, were reported recently (Loscos Deodad et al. 2007;
S ́egurel et al. 2004, 2005). DMS is a powerful odorant, which has an olfactive per-
ception threshold of 27 g/L in red wine (Anocibar Beloqui et al. 1996) and 25
g/L in white wine (Spedding and Raut 1982). It is the key-aroma compound in
truffle (Talou et al. 1987), an aroma descriptor often attributed to bottled bouquet
in aged premium red wines and late harvest wines (Anocibar-Beloqui et al. 1996;
Anocibar-Beloqui et al. 1998; Dagan 2006; De Mora et al. 1987; Du Plessis and
Loubser 1974; Spedding and Raut 1982). However, its influence on wine aroma was
perceived sometimes negatively (Spedding and Raut 1982; Goniak and Noble 1987).
During the alcoholic fermentation, DMS is biogenerated by yeast from DMSO
and some sulfur amino acids, such as cysteine, cystine or glutathione (Anocibar
Beloqui 1998; Bamforth and Anness 1981; De Mora et al. 1986; Schreier et al. 1976),
but DMSO levels in must are generally very low (S ́egurel 2005). In addition, lactic
acid bacteria and yeast cannot generate DMS from methionine (Schreier et al. 1976;
Pripis-Nicolau et al. 2004). Anyway, the amount produced during yeast fermentation
is low, and as DMS is quite volatile (vapour pressure 53 KPa at 20◦C) (Lestremau
et al. 2003), it is mainly stripped off by CO 2. Therefore, its levels in young wine
after fermentation are generally lower than its olfactive preception threshold, but
they can be higher in wines thought to have sulfide-related off-odors, in which DMS