Wine Chemistry and Biochemistry

8A Wine Aroma Precursors 261

Fig. 8A.6S-(L-Cysteine)
conjugates occurring in
grape, precursors of varietal
thiols

4-Mercapto-4-méthylpentan-2-one

4-Mercapto-4-méthylpentan-2-ol

3-Mercaptohexanol

L-Cysteine S-Substituent

HO 2 C

CH 2

H 2 N

CH

SR

Thiols Thiols released fromreleased from

SS--(L(L--cysteinecysteine) ) conjugatesconjugates

S-glutathione conjugate was identified in grape, particularly no precursor corre-

sponding to P4MMP and P4MMPOH.

As the analytical methods for assessing levels in grapeS-cysteine conjugates

were only recently available and are not easily implemented (Peyrot des Gachons

et al. 2000; Dagan 2006), few quantitative data on these compounds were pub-

lished. Their levels in must are low and do not exceed a 100 g/L for the most

abundant P3MH, in Sauvignon Blanc and Petit and Gros Manseng, and a few g/L

for P4MMP and P4MMPOH in Sauvignon Blanc, the latter being slightly more

abundant (Peyrot des Gachons et al. 2000, 2002a, 2005; Dagan 2006). The changes

in their levels during ripening were shown to be dependent on environmental condi-

tions, soil and climate parameters and vineyard management techniques (Peyrot des
Gachons et al. 2000, 2005; Chone et al. 2006; Dagan 2006). In Sauvignon Blanc

grape berry, the levels of P4MMP and P4MMPOH are equivalent in the juice and

skin, but the levels of P3MH are much higher in the skin (Peyrot des Gachons

et al. 2002a). A lower enrichment in P3MH in Merlot and Cabernet Sauvignon grape

skins was also reported (Murat et al. 2001a). Thus, skin contact in winemaking has

a higher impact on the contents of P3MH in the must than on those of the other two

precursors (Murat et al. 2001a; Peyrot des Gachons et al. 2002a).

Although they are few in number and not abundant, these odorless compounds

are very important aroma precursors in grape. Indeed, they generate in wine,

four very odorous volatile thiols, which make an important contribution to the

aroma of certain wine varieties : 3-sulfanylhexan-1-ol (3MH), 3-sulfanylhexyl

acetate (ac3MH), 4-methyl-4-sulfanylpentan-2-one (4MMP), and 4-methyl-4-

sulfanylpentan-2-ol (4MMPOH), with very low olfactive detection thresholds, 0.8

ng/L, 60 ng/L, 4.2 ng/L and 55 ng/L, in hydroalcoholic model solution, respec-

tively (Tominaga et al. 2000a). 3MH is the most abundant, and always present in

wine, whatever be the grape cultivar, in levels generally higher than its olfactive

perception threshold, up to a few g/L. In contrast, 4MMPOH levels in wine are

generally lower than its olfactive perception threshold. With regard to the other two

thiols, their levels in wine are very dependent on the grape cultivar, but they are

also very variable between different samples of the same cultivar. Thus, 4MMP and