6A Amino Acids and Biogenic Amines 165
content increases during grape maturation, and can reach 90% of the total nitrogen
in musts from table wine grapes. At wine harvest, 70% of the organic nitrogen is
constituted by amino acids, 3% by proteins and 2% by peptides.
Free amino acids in musts are of paramount importance, since they constitute a
source of nitrogen for yeasts in alcoholic fermentation, for lactic acid bacteria in
malolactic fermentation and can also be a source of aromatic compounds. In certain
cases, some amino acids can produce undesirable compounds in wines, such as ethyl
carbamate, biogenic amines, ochratoxin A (from 2-phenylalanine) and -carbolines
(from tryptophane) (Herraiz and Ough 1993; Herraiz et al. 1993).
6A.2.1 Evolution of Free Amino Acids During Alcoholic
Fermentation
The free amino acids present in wine may be of different origins. Those from the
grape may be partially or totally metabolised by the yeasts during fermentation.
Other free amino acids are released by yeasts at the end of fermentation or by
proteolysis processes during yeast autolysis. The two most abundant free amino
acids are proline and arginine and both have glutamic acid as a precursor, which
is also among the most abundant (Moreno-Arribas et al. 1998). Changes in free
amino acids during alcoholic fermentation have been the focus of numerous and
important studies. At the start of fermentation, yeasts use the nitrogen from ammo-
nium salts for their development, followed by the nitrogen from free amino acids.
Some of these, namely, arginine, glutamic acid, glutamine, aspartic acid, asparagine,
threonine and serine, are preferentially assimilated. Simultaneously, by enzymatic
processes, the yeasts degrade the proteins to peptides and to amino acids until fer-
mentation has finished. The presence of exogenous proteases in yeasts has been
demonstrated by Feuillat et al. (1980). Autolysis and the release of amino acids
by yeasts also contribute to the increasedconcentration of free amino acids after
fermentation (this subject is discussed in more detail in Chapter 3A. – Sparkling
Wines and Yeast Autolysis)
Numerous factors are involved in nitrogen uptake by yeasts during alcohol
fermentation. These include temperature (L ́opez et al. 1996), concentration of
sugars – at higher sugar concentrations more nitrogen is required (Agenbach 1977) –
and the concentration of oxygen (Ingledew and Kunkee 1985). Addition of weak
concentrations of ammonia nitrogen (for example 0.23 mg/L of diammonium phos-
phate, a value close to the maximum permitted for nitrogen enrichment) has little
effect an amino acid utilization (Monteiroand Bisson 1992). In contrast, the addi-
tion of high doses of this salt (2.0 g/L) greatly reduces arginine consumption and
degradation, and the formation of polyamines, such as putrescine, spermine and
spermidine.
In a study by Dizy and Polo (1996), the majority amino acids in grape must of the
Malvar variety were glutamine, arginine, proline and-aminobutyric acid (GABA).
The authors found that the concentration of proline rose during fermentation, while