Wine Chemistry and Biochemistry

604 A. Bertrand and A.A. Beloqui

CH 3 C

O

H

3H 2 S + CH

OH

SH

3 CH 3

SS

H 3 C S CH 3

CH 3

+CHH 2 O 3CH 3 2 SH

6 H+

Fig. 10.7Reaction of H 2 S with ethanal to produce ethanethiol

Mercaptanes can be oxidised to disulphides and the latter can be reduced to the

respective mercaptanes with the addition of SO 2 (Tanner 1969; Bobet et al. 1990).
The reaction between H 2 S and ethanal to produce 2,4,6-trimethyl-S-trithiane and

ethanethiol has been proposed by many authors (Neuberg and Grauer 1952; Wilson

and Katz 1972; Rankine 1963) (Fig. 10.7).

In the red wines we analysed (more than 200), COS was not detected in any of

the healthy wines (without defects) nor in the wines with reduction defects.

Ethanethiol has a very weak perception threshold (1.1 g/L) according to Goniak

and Noble (1987), and has never been detected in wines without reduction defects.

In some of the wines with a very high H 2 S concentration (estimated at around

20
g/L), the ethanethiol concentrations are very low (lower than 2 g/L). Also,

concentrations in some white wines are higher than those found in red wines (Lav-

igne 1996). The very different chemicalcomposition of a whitewine compared to

a red wine (especially in relation to the presence of phenolic compounds) could

explain the difference in results. On the other hand, no author has demonstrated the

formation of ethanethiol from H 2 S, ethanol or ethanal in a red wine.

10.5.2 Trithiolanes

Other reaction mechanisms have also been proposed between ethanal and H 2 S

(Werkhoff et al. 1991). H 2 S reacts with ethanal through 1-hydroxy-ethanethiol

(1-sulphanylethanol) forming ethane-1,1-dithiol, a basic compound in many pro-

duction cycles of other sulphur compounds (Zhang et al. 1988). This compound

has been isolated and identified in yeast extracts by Werkhoff et al. (1989). These

authors claim that the aroma it produces depends on its concentration, and is similar

to that of “onion”, “leek”, “cabbage” or “peanut”. In the presence of oxygen, ethane-

1,1-dithiol can be oxidised to producecis-/trans-3,6-dimethyl-1,2,4,5-tetrathiane

that has rubbery aromas (Werkhoff et al. 1989). After oxidation/cyclization,cis-

/trans-3,5-dimethyl-1,2,4-trithiolane is produced that can give the wine “meaty”,

“spicy” and “garlic” aromas. These compounds were first detected in roast and

boiled chicken, pork and beef (Brinkmann et al. 1972). The reaction between 1,1-

ethanedithiol and H 2 Sformscis-/trans-4,7-dimethyl-1,2,3,5,6-pentathiepane that

produces meaty aromas (Chen and Ho 1986). The reactions involved and cited by

the above authors are described (Fig. 10.8).