106 M. Ugliano
in order to improve colour stability of white wines (Muller-Sp ̈ath 1990; Artajona
et al.1990; Dubourdieu and Lavigne1990; Cheynier et al. 1991). Accordingly, must
phenolics are extensively oxidised by controlled supplementation of must with oxy-
gen, and the oxidized forms are then removed during pre-fermentative clarification.
Although hyperoxygenation has been shown to provide wines with increased colour
stability (Schneider 1998), tentative evidenceis available that it can be detrimental
to aroma compounds that contribute to the varietal character of wine (Dubourdieu
and Lavigne 1990; Ribereau-Gayon et al. 2006).
4.3 Lipoxygenase and Hydroxyperoxide Cleaving Enzymes
Six carbon atom (C 6 ) compounds are the major groupof volatile compounds formed
during the pre-fermentative stage of winemaking. This group includes volatiles such
as hexanal, hexanol,cis-3-hexenol,trans-2-hexenol, andcis-andtrans-isomersof
2- and 3-hexenal, which, depending on concentration, can have a detrimental effect
on wine quality due to their grassy, herbaceous odors (Crouzet 1986).
This group of compounds is formed from the lipid fraction of the juice, through
the sequential action of several enzymes. In Fig. 4.2. the mechanism described
by Crouzet (1986) for the formation ofcis-3- andtrans-2-hexanal and their cor-
responding alcohol is shown. Linolenic acid is a common constituent of grape
juice. Through the action of a lipoxygenase enzyme, linolenic acid is transformed
Linolenic acid13-Hydroperoxylinolenic acid
9-Hydroperoxylinolenic acidcis-3-Hexenal trans-2-Hexenalcis-3-HexenolLipoxygenaseHydroxyperoxyde
cleaving enzymeAlcohol
dehydrogenasetrans-2-HexenolFig. 4.2Mechanism of formation of six carbon atom volatile compounds from longchain fatty
acids during the pre-fermentative stages of vinification