Wine Chemistry and Biochemistry

350 M. Ugliano and P.A. Henschke

and microaerobiotic conditions stimulated monoterpene production whereas low

nitrogen and anaerobic conditions favoured sesquiterpenes.

The monoterpene, citronellol, has been shown to form by the yeast-catalysed

reduction of free geraniol present in the fermenting medium (Gramatica et al. 1982).

Ugliano et al. (2006) suggested that, in low monoterpene grape varieties, in which

low concentrations of free geraniol are present, citronellol can be formed by the

yeast-driven reduction of free geraniol derived from glycoside hydrolysis (Fig 8D.9).

The presence of ergosterol under aerobic conditions may favour the retention of

geraniol and produce a higher ratio of citronellol to geraniol during fermentation,

enhancing the aromatic quality of wine (Vaudano et al. 2004).

It is generally accepted that C 13 -norisoprenoids, including the powerful odorants

-damascenone, -ionone, and 1,1,6- trimethyl-1,2-dihydronaphtalene (TDN) are

formed through acid-catalyzed transformation of grape-derived precursors (Sefton

1998). However, recent studies have shown that yeast strain can affect the con-

centration of -damascenone and -ionone at the end of fermentation (Loscos

et al. 2007; Hern ́andez-Orte et al. 2008). Enzymatic hydrolysis of different pre-

cursors followed by acid-catalyzed transformations of the aglycon has been indi-

cated as a possible pathway for the formation of these compounds during fermen-

tation (Hern ́andez-Orte et al. 2008), although the actual mechanisms involved in

the yeast-mediated formation of norisoprenoids have still to be clarified. Though

Fermentation

Biosynthesis

Sugar

Ethanol

+

CO 2

Geranyl-

glycosides

Geraniol

Geraniol

Citronellol

Geraniol-PP

H+

glycosidases

glucanases

Free monoterpene

alcohols

Non-volatile monoterpene

glycosides

Acid

hydrolysis

Biotrans-

formation

Enzymatic

hydrolysis

Geraniol Citronellol

Fig. 8D.9Acid-catalysed (H+) and enzyme-catalysed hydrolytic release of grape glycosidically-
bound geraniol and its biosynthesis and biotransformation bySaccharomyces cerevisiae