8D Yeasts and Wine Flavour 349
The most important aliphatic compounds comprise the C 6 compounds, including
hexanol, which give a green (cut grass) character in wine. These C 6 compounds are
found in wines of many varieties.
8D.5.1.2 Metabolism and Modulating Factors
With the exception of monoterpenes in aromatic grape varieties, few of the aroma-
active secondary metabolites in mature grapes are present in sufficient concentra-
tion to provide a distinctive aroma or taste. Most of these compounds exist as
non-volatile and odourless glycoconjugates.These glycoconjugates are either glu-
cosides, disaccharides or trisaccharides, with the disaccharide glycosides represent-
ing the major source of aroma compounds. They all contain a glucosyl moiety, but
for the disaccharide glycosides, the glucose moiety is further substituted with -
L-arabinofuranosyl, - L-rhamnopyranosyl, - D-xylopyranosyl or -apiofuranosyl
sugars (Williamset al. 1982).
Hydrolytic release of the sugar moiety(ies) is necessary to liberate the agly-
con, which then becomes volatile and aroma-active. Several mechanism have been
proposed, including acid-induced hydrolysis and grape endogenous and microbial
hydrolytic enzymes (Fig 8D.9). Acid-catalysed hydrolysis, as the result of the mild
acidic conditions of must, has generally been observed to be too slow to explain
major release during fermentation (Williams et al. 1982; Ugliano et al. 2006). Nev-
ertheless, acidic hydrolysis is considered the important mechanism operating during
wine aging. Yeast associated hydrolaseactivities, on the other hand, induce a sig-
nificant release of aglycons, which vary according to the structures of the sugars
and aglycon moieties (Ugliano et al. 2006). Various extracellular enzymes with
hydrolytic activities, such as -glucosidase, -arabinosidase, -rhamnosidase, -
xylosidase or -apiosidase, have been described inSaccharomyces cerevisiaeand
non-Saccharomycesspecies (Charoenchai et al. 1997; Darriet et al. 1988; Ugliano
et al. 2006). Recent evidence suggests that exo- -glucanase could be responsible
for yeast-associated hydrolysis of glycoconjugates (Gil et al. 2005).
In addition to glycoside hydrolysis, someS. cerevisiaeand non-Saccharomyces
species can, under certain conditions, synthesize detectable amounts of monoter-
penes in the absence of grape precursors (Chambon et al. 1990; Carrau et al. 2005).
They are believed to derive from the sterol biosynthetic pathway in which geranyl-
PP is synthesised from the intermediate isopentenyl-PP by geranyl-PP synthase. The
various monoterpenes (C 10 ) geraniol, nerol, citronellol, linalool and -terpineol, are
then produced by various chemical, or possibly enzymatic transformation reactions,
involving isomerisations, reductions and cyclisations. The sesquiterpene (C 15 )far-
nesol is derived from the sterol pathway intermediate farnesyl-PP and is partially
isomerised to nerolidol. Carrau et al. (2005) recently hypothesized that biosynthesis
of the C 10 terpenes is derived from the leucine-mevalonic acid pathway and, thus, is
independent of sterol metabolism. The same authors also showed that factors such
as assimilable nitrogen and oxygen differentially regulated production of monoter-
penes and sesquiterpenes bySaccharomyces cerevisiae. Combined high nitrogen