Physiology and Biochemistry of Sourdough Yeasts - Sourdough Biotechnology - page 158

Generally, the level of glycerol produced is ca. 0.03 mmol/g of sourdough.
Paramithios et al. [ 18 ] reported that the synthesis of glycerol is positively affected
by the co-cultivation of S. cerevisiae with L. sanfranciscenis and L. brevis . Exposure
of baker's yeast to salt stress also increases glycerol production [ 22 ] .
The glycolytic pathway and related enzymes were conserved during evolution,
even though the mechanisms of controlling the carbon and energy metabolism have
been adapted to the needs of each species. Saccharomyces cerevisiae switches to a
mixed respiro-fermentative metabolism, resulting in ethanol production, as soon as the
external glucose concentration exceeds 0.8 mmol/kg [ 23 ] . Hence, S. cerevisiae controls
fermentation versus respiration primarily in response to the concentration of glucose.
The aerobic synthesis of ethanol by S. cerevisiae depends on the relative capacities of
the fermentative and respiratory pathways. High glucose levels result in the glycolytic
rate exceeding that of the pyruvate dehydrogenase (Pdh) reaction, thereby generating
an overflow towards pyruvate decarboxylase (Pdc) and, hence, ethanol production. At
low external glucose levels and in the presence of oxygen, S. cerevisiae does not
synthesise ethanol [ 24 ]. The uptake of glucose into S. cerevisiae is controlled by multiple
hexose transporters (Hxts) [ 25 ], which have different substrate specificity and affinity,
and are expressed under different, overlapping conditions [ 26 ] . When S. cerevisiae is
exponentially growing under aerobic conditions with glucose or fructose as carbon
sources, glucose degradation proceeds mainly via aerobic fermentation. When yeast is
growing under aerobic conditions on mannose or galactose, degradation proceeds
simultaneously via respiration and fermentation.
Control of carbohydrate metabolism in yeasts is of both fundamental and practical
significance. It is regulated by different mechanisms depending on the genus and
species as well as on environmental conditions. These mechanisms have been called
Pasteur, Kluyver, Custers and Crabtree effects, glucose or catabolite repression, and
genera or catabolite inactivation [ 27 ] . Table 6.1 describes these regulatory phenomena
in S. cerevisiae and other genera, including also the species occurring in sourdough.
The term Crabtree effect defines the inhibition of the synthesis of respiratory
enzymes in the presence of oxygen and high concentration of glucose. Candida and
Pichia , which are frequently associated with sourdough, do not show the Crabtree
effect. Depending on environmental conditions, especially, the type of carbon
sources, yeasts adjust the energy metabolism according to a process referred to as
carbon catabolite repression. Two extreme cases are exponential growth on glucose
or ethanol, which lead to the almost exclusive fermentation of the former with
extensive secretion of ethanol or to exclusive respiration to carbon dioxide, respec-
tively. Both, fully respiratory and fermentative metabolism are mediated by differ-
entially active transcription factors. During fermentation, the transcription factor
complex of Tup1p, Ssn6p and Mig1p mainly represses the expression of respiratory,
gluconeogenic and alternative carbon source utilisation genes [ 28 ] . The minimal
activity of the citric acid cycle for biosynthetic purposes is ensured mainly by the
Rtg transcriptional activators [ 29 ]. During respiratory growth on non-fermentable
carbon sources, the respiratory genes of the citric acid cycle and the respiratory
chain are highly induced. This is triggered by the Hap transcription factors, a global
activator complex of respiratory genes [ 29 ]. Activation of genes for gluconeogenesis
during growth on non-fermentable carbon sources is achieved by the transcriptional