258 R. Baumes
but monoterpenoids are also abundant as free compounds in Muscat and some other
aromatic cultivars only. As a lot of these aglycons are odorants or precursors of
odorants, their odorless glycosidic progenitors make up a reserve of grape flavor,
which is generally more abundant than the free one (G ̈unata et al. 1985a, b, 1986;
Strauss et al. 1986; G ̈unata et al. 1989a; Voirin et al. 1992). Indeed, these glycosides
are susceptible to the release of their volatile aglycons in wine through hydrolysis.
These aglycons are considered to be part of the varietal aroma only, except for the
volatile vinylphenols, which are mostly generated from phenolic acids during wine-
making. With regard to the other aglycons, C6 compounds, and fusel alcohols, they
occur in grape in much lower amount than the corresponding volatiles formed during
the prefermentary and fermentary steps of winemaking, so that their influence on
wine aroma is not significant.
The total amount of glycoconjugates is not much different from a few mg/kg in
grapes of neutral varieties (Kotseridis 1999; Schneider 2001; Schneider et al. 2002;
S ́egurel 2005; Dagan 2006), but it can be much higher in aromatic varieties, such
as Muscats, in which it is up to ten times as much (G ̈unata et al. 1985b; Voirin
1990; Voirin et al. 1992). The distribution of free and glycoconjugates among the
solid and liquid fractions of different white and red grape varieties is quite vari-
able and depends mostly on the aglycon structure, but generally the skin contains
≥50% of their total content, which is not much different from free volatiles (Gomez
et al. 1994; G ̈unata et al. 1985a; Park et al. 1991; Wilson et al. 1986). That is inter-
esting on a technological basis, as this potential of aroma precursors, as well as the
typical free aroma, can be enhanced by processes, such as skin contact or carbonic
anaerobiosis (Baumes et al. 1989; Bitteur et al. 1992; Mc Mahon et al. 1999).
During berry maturation, the biosynthesis of glycoconjugates extends from
the first stage of fruit formation (berry set), to the end of berry maturation, but
the changes in glycoconjugate levels before and after v ́eraison are dependent on the
aglycon structure and on grape variety (Baumes et al. 2002; G ̈unata 1984; Marais
1987; Park et al. 1991;Williams et al. 1984; Wilson et al. 1984). However, at the
end of berry maturation, there is no simple relationship of glycoconjugate contents
to the enological parameters used by winemakers to define maturity of grape (sugar
and acid contents, pH) (Dagan 2006; Schneider 2001; S ́egurel 2005). Therefore,
more simple and less time-consuming methods than the reference analysis meth-
ods used in research laboratories to quantitate glycoconjugates should be valuable
tools, providing winemakers with a useful index to evaluate the aroma glycosidic
potential of grapes. Two methods were developed to reach that goal – the enzymatic
red-free glycosyl-glucose (GG) method, which allows the total determination of gly-
coconjugates (Iland et al. 1996; Williamset al. 1995), and an FT-IR method, which
allows glycoconjugate quantitation according to the different groups of aglycons
(Schneider et al. 2005).
The formation of odorants from grape glycosides is generally a very slow chem-
ical process, occurring mainly during wine aging, the kinetics of which depends on
the structure of the aglycon and its site of glycosylation, on temperature and pH of
the wine, but which seems independent on the structure of the sugar moiety (De
La Presa-Owens and Noble 1997; Marais 1983; Puglisi et al. 2007; Skouroumounis