166 M.V. Moreno-Arribas and M.C. Polo
that of arginine diminished, because proline was derived from arginine metabolism.
During the first step of fermentation, almost all the free amino acids were consumed.
In the time between the end of fermentation and racking, there was an increase in
free amino acids, which these authors associate with yeast autolysis.
According to Jiranek et al. (1995), all the amino acids, except glycine, can be
removed from fermentation media to different degrees. Several authors (Cooper
1982; Henschke and Jiranek 1993, among others) have reported that the amino acids
that constitute the best yeast nitrogen source are glutamic acid, glutamine, aspartic
acid, asparagine, threonine, histidine, alanine, tyrosine and arginine. According to
Fraile et al. (2000), the formation of different alcohols takes place at the end of
the fermentation. Carrying out studies of partial least-squares regression models,
Hern ́andez-Orte et al. (2002) found that amino acid composition accounts for a high
proportion of the variance in the volatilecomposition. Moreover, the composition
of the amino acids remaining in the wine influences aromas during the maturing
process (Escudero et al. 2000).
The importance of adding amino acids to the must and their effect on the biosyn-
thesis of aromatic compounds by yeasts during alcoholic fermentation was evaluated
by Hern ́andez-Orte et al. (2005). These authors studied the effect of supplementing
a must of Airen variety with ammonium (100, 300 mg/L) and amino acids (dou-
bling the level of amino acids in that must) that was fermented with three different
yeast strains. Statistical treatments showed that the yeast strain is the major factor
affecting wine volatile composition, but must nitrogen supplementation also has an
influence. From the sensory point of view, must supplementation brings about a
decrease in sulphur notes and an increase in citric flavour. Also, the effect of adding
selected amino acids (phenylalanine, alanine, aspartic acid and threonine) to grape
juice on the generation of aroma compounds and on amino acid uptake were studied
(Hern ́andez-Orte et al. 2006a). The addition of amino acids produced differences in
fermentation kinetics. Higher alcohols were generated at the same time as ethanol.
From the sensorial point of view, the wines with more amino acids were better
valued by the panel of tasters because the sulphured notes decreased significantly
while the floral notes increased.
6A.2.2 Evolution of Free Amino Acids During Malolactic
Fermentation
In the study of the changes in free amino acids during malolactic fermentation,
the concentration of some of these (arginine, glycine, tyrosine, phenylalanine, histi-
dine, -alanine and serine) was found to decrease sharply, while the concentrations
of the other amino acids tended to increase significantly (aspartic acid, glutamic
acid, leucine, methionine, isoleucine and tryptophan) (Davis et al. 1986). Although
there is only limited information about the presence of proteases in wine lactic
acid bacteria, a higher concentration of amino acids was observed at the end of
bacterial growth in the wine compared to the start. However, to date little attention