72 A.J. Mart ́ınez-Rodr ́ıguez and E. Pueyo
is only a few milligrams per litre. Two possible explanations have been suggested
for this observation: first, that protease A is an endoprotease that produce peptides
rather than amino acids, and, second, that amino acids released are later transformed
by decarboxylation and desamination reactions, which cause a reduction in the final
amino acid concentration.
Peptides are the majority product of autolysis and although they have been used
as markers of the process, they are less used than amino acids, mainly because of the
complexity of the analytical techniques available for their analysis (Moreno-Arribas
et al. 2002). It has been observed that high molecular weight peptides mainly of a
hydrophobic nature are released in the first steps of the process (Moreno-Arribas
et al. 1998a). These large peptides are hydrolyzed in the steps following autolysis,
giving rise to less hydrophobic peptides of lower molecular weight and to free amino
acids. The concentration of total wine amino acids increases before the concen-
tration of free amino acids, demonstrating that first peptides are released and that
later these are hydrolysed into amino acids (Moreno-Arribas et al. 1996). The final
concentration of peptides in sparkling wines can be influenced by different variables
such as temperature, wine-aging time, the yeast strain used in the second fermenta-
tion etc (Mart ́ınez-Rodr ́ıguez et al. 2002). By studyingthe amino acid composition
of the peptides present in sparkling wines, it has been shown that both threonine and
serine are present at the highest levels (Moreno-Arribas et al. 1998b), demonstrating
that the peptides present in sparkling winesare mainly derived from yeast autolysis,
since these two amino acids are involved in glycosidic bonds between the proteins
and mannanes of the cell wall (Klis et al. 2002).
The polysaccharides present in sparkling wines can come from either the grape
or from the yeasts and present constitutional differences. Arabinose is the sugar
with the greatest presence in base winepolysaccharides, whilein sparkling wines
mannose is the majority sugar in the polysaccharides (43%), followed by glucose
(31%) (Mart ́ınez-Rodr ́ıguez and Polo 2000b). During aging and yeast autolysis of
the yeasts in sparkling wines, enzymatic activity (proteases and glucanases) causes
the breakdown of glucanes and the release of cell wall mannoproteins (Feuillat
et al 2003). Among the wine glycoproteins, the yeast mannoproteins, released dur-
ing fermentation and autolysis, have been particularly studied in recent years for
their ability, among others, to improve the tartaric stability (Moine-Ledoux and
Dubourdieu 2000) and to increase wine stability against the protein haze (Dupin
et al. 2000). Also, mannoproteins have been associated with improved foaming
properties in sparkling wines (Nunez et al. 2006). Different authors have observed
that the concentration ofpolysaccharides containingglucose and mannose in their
structure increases three- or fourfold during the aging of sparkling wines in con-
tact with yeast. The differences observed in the amount of mannoprotein and other
polysaccharides released during autolysisinto the wine by yeast depend on several
variables such as yeast strain, temperature, and time of aging (Caridi 2006).
Other compounds released during autolysis are present in lower amounts, such as
lipids and nucleic acids, but could play an important role in the sensorial character
of the final wine. Lipids may affect wine flavour in that the fatty acids released
could give rise to volatile components withlow sensory thresholds, either directly
or through derivatives such as esters, ketones and aldehydes (Charpentier and