Wine Chemistry and Biochemistry

286 D. Dubourdieu and T. Tominaga

8B.5 Precursors of Volatile Thiols Derived from Cysteine

Sauvignon blanc musts, like those of many grape varieties except Muscats, are not

highly odoriferous. The characteristic aroma of the grape variety appears during

alcoholic fermentation. Peynaud (1980) had aremarkable intuition of the existence

of aroma precursors in Sauvignon blanc must, which he described as follows: ‘When

you taste a thick-skinned, golden Sauvignon blanc grape, you can detect its char-

acteristic flavor, although it is not very intense. In the same way, freshly pressed

juice is not highly odoriferous, and the initial flavor is quite discreet. Between 20

and 30 s after you have swallowed it, an intense aromatic Sauvignon blanc aftertaste

suddenly appears in the rear nasal cavity. Fermentation brings out the primary aroma

hidden in the fruit. Wine has more fruit aroma than grapes, etc. Fermentation reveals

the aroma and releases the odoriferous substances from the grapes. This “aroma

recurrence” or “aftertaste” could be explained by the action of the mouth enzymes

on the aroma precursors.

The existence of a non-glycosilated precursor of 4MSP in Sauvignon blanc

grapes and must was first demonstrated by Darriet (1993). The structure of this

compound was elucidated by Tominaga et al. (1995, 1998c). A -lyase specific to

S-cysteine conjugates was used to release 3-SH, 4-MSP and 4-MSPOH from a non-

volatile extract of Sauvignon blanc aroma precursors, suggesting that these three

thiols were present in grapes in cysteinylated form:S-3-(hexan-1-ol)-L-cysteine,

S-4-(4-methylpentan-2-one)-L-cysteine, andS-4-(4-methylpentan-2-ol)-L-cysteine

(Fig. 8B.5). This was formally established by gas-phase chromatography/mass spec-
trometry of the precursors in trimethylsilylated form.S-3-(Hexan-1-ol)-L-cysteine

has also been identified in passion fruit juice (Tominaga and Dubourdieu 2000)

(Fig. 8B.6).

Methods to measure cysteinylated aroma precursors in must were developed by

Peyrot des Gachons et al. (2000) and Murat et al. (2001b). They allowed the location

of thiol precursors to be determined in Sauvignon blanc grapes (Fig. 8B.7). Around

80% of P-4MSP is in the juice and 50% of P-3SH in the skin. The skin contact

improves the must aromatic potential, but its main effect is on the P-3SH content

(Fig. 8B.8). Similarly, in Cabernet Sauvignon and Merlot grapes, 60% of the P-3SH

is located in the skins This method has been also applied to study the influence of

S CH

βα

2 CH COOH

NH 2

SH H 3 CC COOH

ß-lyase

+

O

+

S-cysteine conjugate

mercaptan pyruvic acid ammonium

R

R NH 3

Fig. 8B.5 , -Elimination reaction ofS-cysteine conjugate -lyase (EC 4.4.1.13)