68 A.J. Mart ́ınez-Rodr ́ıguez and E. Pueyo
of the stationary growth phase and is associated with cell death (Babayan and
Bezrukov 1985; Charpentier and Feuillat 1993). When sugars and other nutrients
are consumed, the yeast cells turn to their own internal energy reserves, composed
of glycogen and other elements. Once these reserves become insufficient for the
continued energy demands of the cell, cell degeneration begins and autolysis is
triggered (Connew 1998).
Several authors have attempted to describe the mechanism to explain the process
of yeast autolysis (Arnold 1980; Charpentier and Feuillat 1993; Charpentier and
Freyssinet 1989). The general features of the mechanism proposed are as follows:
hydrolytic enzymes are released into the intracellular space due to the degradation
of cell endostructures. Initially, these enzymes are inhibited by specific cytoplasmic
inhibitors which are later degraded provoking the proteolytic activation of these
enzymes. Then, an accumulation of hydrolysis products is produced by the enzy-
matic degradation of intracellular macromolecules. When the cell wall pores are
large enough, the autolytic products are released into the extracellular environment.
Finally, a further autolytic degradation of more polymerized compounds into low
molecular weight compounds occurs inthe extracellular environment.
It is interesting to differentiate between the natural autolysis caused by cell aging
and death, and the induced autolysis. Induced autolysis is widely used in different
industrial applications, such as the production of yeast extracts or culture media.
Autolysis in industrial processes can be induced by several inductors such as tem-
perature, osmotic pressure, detergents, pH, etc. and the process is usually very fast,
around 48–72 h, depending on the inducer used. In the manufacture of sparkling
wines elaborated by the traditional method, yeast autolysis occurs under very spe-
cific conditions. The pH ranges from 3 to 3.5 and the temperature is generally low,
between 10◦C and 15◦C, the ethanol concentration is around 10% and CO 2 pressure
is high. These conditions are very different to the optimum conditions described for
autolysis to occur (Alexandre and Guilloux-Benatier 2006; Fornairon-Bonnefond
et al. 2002) and are responsible for the long duration of the process.
In the research carried out on autolysis in enology, two major approaches have
been followed to date: first, studying the breakdown of the yeast cell wall, by ana-
lyzing the change in its components during autolysis and carrying out structural
and ultrastructural studies, and second, analysing the different products released
into the medium during autolysis, following the changes in nitrogen compounds,
polysaccharides, glycoproteins, nucleicacids, lipids and other macromolecules.
3A.4.1 Breakdown of the Yeast Cell Wall During Autolysis
The yeast cell wall is a complex structure that accounts for 15–25% of the dry
weight of the cell. The mechanical strength of the wall is mainly due to the inner
layer, which consists of -1,3-glucans and chitin. The outer layer is mainly formed
by glycosylated mannoproteins emanating from the cell surface, which are involved,
among others, in cell-cell recognition events (Klis et al. 2002). The cell wall is a
multifunctional organelle, the primary function of which is to protect the protoplast.