3B Biologically Aged Wines 91
aroma. This compound typically reaches concentrations from 350 to 450 mg/L and
occasionally as a high as 1000 mg/L (Mart ́ınez et al. 1998). Acetaldehyde results
mainly from the oxidation of ethanol by alcohol dehydrogenase II (ADH II), which
produces NADH. This isoenzyme is repressed by glucose, so it plays an active role
in the biological aging of wine. Flor yeasts races with a high specific activity of
ADH II in pure cultures have been found to release large amounts of acetalde-
hyde into wine (Mauricio et al. 1997). Theacetaldehyde content increases as the
wine ages; however, the most marked changes occur in the scales containing the
youngest wine, where yeast metabolism is more intense (Berlanga et al. 2004b;
Garc ́ıa-Maiquez 1988; Mart ́ınez de la Ossa et al. 1987).
Acetaldehyde takes part in a number of reactions during the biological aging of
wine one of the most important of which is the formation of 1,1-diethoxyethane.
This acetal results from the combination of acetaldehyde and ethanol. Its concentra-
tion, which is closely related to those of the parent compounds, can easily exceed
100 mg/L (Mu ̃noz et al. 2005). Also, 1,1-diethoxyethane is the only acetal contribut-
ing to the aroma of wine (Eti ́evant 1991).
Acetaldehyde is also involved in the formation of acetoin and 2,3-butanediol.
Although some authors have stressed the significance of a chemical pathway,
these compounds are more likely to originate from yeast metabolism (Romano and
Suzzi 1996).
Acetaldehyde also takes part in the synthesis of sotolon (Guichard et al. 1997;
Pham et al. 1995).
Other reactions involving this aldehyde include the following: (1) combination
with sulphite ion, which substantially increases the proportion of bound sulphite in
wine; (2) formation of addition compoundswith some polyphenols such as tannins
and procyanidins, where it acts as a “bridging molecule” (Haslam and Lilley 1998);
and (3) chemical oxidation to acetic acid,which only occurs to a small extent and
has little influence on wine composition and quality.
3B.3.4 Nitrogen Compounds
Biological aging reduces the contents in amino acids, ammonium ion and urea of
wine (Mauricio and Ortega 1997). Amino acids constitute the main source of nitro-
gen for yeasts; especially important in this respect is L-proline (Botella et al. 1990;
Mauricio and Ortega 1997; Valero et al. 2003), which is not degraded by the yeasts
during fermentation since this requires the presence of molecular oxygen (Ingledew
et al. 1987). Its transformation into glutamic acid allows flor yeasts to synthesize all
other amino acids they require to grow (Cooper 1982; Botella et al. 1990).
Flor yeast may be able to use amino acids not only as nitrogen source but also
as redox agent to balance the oxidation-reduction potential under conditions of
restricted oxygen. Thus, amino acids as threonine, methionine, cysteine, tryptophan,
and proline can be released to the wine to restore the intracellular redox balance
by means of the oxidation of NADH in excess (Berlanga et al. 2006; Mauricio
et al. 2001; Moreno-Arribas and Polo 2005; Valero et al. 2003).