92 R. Peinado and J. Mauricio
3B.3.5 Organic Acids
Malolactic fermentation usually occurs insobretablaswine; as a result, the wine
incorporated into the aging system contains no appreciable concentrations of malic
acid. The decrease in tartaric acid contentsduring biological aging of the wine is a
result of crystal precipitations.
Gluconic acid is widely used as a measure of rotting. Wines containing con-
centrations of this acid below 1 g/L are suitable for biological aging. Flor yeasts
metabolize such acid during biological aging without altering the quality of the final
wine (Peinado et al. 2003, 2006a).
Acetic acid is produced at concentrations rarely exceeding 0.7 g/L by yeasts dur-
ing fermentation. The acid is metabolized by flor yeasts during the biological aging
process, its concentration being reducedthrough consumption via acetyl-CoA for
incorporation into the Krebs cycle or fatty acid synthesis.
Other fatty acids result from the metabolism of flor yeasts during the bio-
logical aging of wine. Butanoic, isobutanoic and 2- and 3-methylbutanoic acids
increase its content with the aging time. Isobutanoic acid reaches the highest
concentrations (8 mg/L), and those of the other acids range from 2 to 5 mg/L.
Medium-chain fatty acids (hexanoic, octanoic and decanoic) exhibit the opposite
behaviour and can even completely disappear from the medium after prolonged
aging (Cort ́es 2002).
3B.3.6 Higher Alcohols and Ester
Higher alcohols are very important contributors to the aroma of fino wines. Although
their overall content changes little during the biological aging, some individual
alcohols exhibit marked changes. Thus, the contents in isobutanol, 2-phenylethanol
and isoamyl alcohols (2-methyl-1-butanol and 3-methyl-butanol) increase only
slightly with time; by contrast, that in propanol can be doubled during the process
(Moreno 2005).
The production of higher alcohols from their corresponding amino acids is cou-
pled to the oxidation of NADH; therefore, yeasts use them as alternative electron
acceptors in the absence of oxygen. These alcohols are largely produced in the
fourth, third and secondcriadera, coinciding with the periods of greatest activity
of flor yeasts.
Yeast autolysis has a strong effect on the contents in higher alcohols, as shown
by the presence of propanol, isobutanol and isoamyl alcohols in yeast extracts from
wines under biological aging (Mu ̃noz et al. 2002).
Ester concentrations change by effect ofsynthetic and hydrolysis reactions, and
also as a result of enzymatic activity of the flor yeasts. The effect of the latter
depends on the specific yeast race prevailing in the medium and its physiological
status (Mauricio et al. 1993; Plata et al. 1998).
Broadly speaking, the contents in acetates of higher alcohols decreases through
hydrolysis over the first few months of aging; by contrast, those in ethyl esters