3A Sparkling Wines and Yeast Autolysis 71
Fig. 3A.2LTSEM images of yeast cells that have been in wine for 12 months.aSuperficial
ultrastructure of yeast cells.bImages of fractured yeast cells. (Mart ́ınez-Rodr ́ıguez AJ, Polo MC,
Carrascosa AV (2001). Int J Food Microbiol 71:45–51. Copyright (2001). Elsevier)
have been considered to be the best markers of yeast proteolytic
activity (Fornairon-Bonnefond et al. 2002; Lurton et al. 1989; Mart ́ınez-Rodr ́ıguez
and Polo 2000a).
During the aging of wine in contact with yeast there is a steady drop in the protein
content, since proteins are hydrolyzed to lower molecular weight compounds. For
this reason, sparkling wines have lower values of proteins than those usually present
in the base wine. Peptides and amino acids are, generally, considered the major
compounds of those released into the wine during autolysis. For this reason, several
authors have studied the role of protease activity in yeast autolysis and its rela-
tionship with foaming abilityand other sensorial properties (Alexandre et al. 2001;
Lurton et al. 1989). Several different types of proteases have been related with the
proteolytic activity during autolysis in sparkling wines (Komano et al. 1990; Lurton
et al. 1989; Olsen et al. 1999) and protease A is the main enzyme involved in the
process. Protease A presents the greatest activity in a wine pH range of 3–3.5. Lurton
et al. (1989) use pepstatine to inhibit protease A and obtain around 80% inhibition
in the release of nitrogen compounds, demonstrating the key role of protease A
in autolysis in an acid medium. More recently, Alexandre et al. (2001) found that
protease A was responsible for 60% of the nitrogen released during autolysis in
wine, suggesting that other acidic proteases may also be involved in the proteolytic
process. This has been confirmed by other authors who found that yeast autolysis is
mainly carried out by protease A, but with the help of other proteases.
Yeasts can release amino acids to the extracellular medium before autolysis
starts. This release occurs as a cellular response to the absence of nutrients from
the wine and was first defined as exsorption by Morfaux and Dupuy (1966). In
contrast to autolysis, this has been described as a passive excretion of amino acids
that occurs prior to the former. If we carry out a quantitative analysis of the increase
in amino acid concentration during the process of autolysis, we observe that this