Physiology and Biochemistry of Sourdough Yeasts - Sourdough Biotechnology - page 160

nucleic acid precursors and nucleotide coenzymes. The reduced carriers, NADH
and FADH 2 , are reoxidised in the respiratory chain located in the inner mitochon-
drial membrane. The energy released during the transfer of electrons is coupled to
the process of oxidative phosphorylation via ATP synthase, an enzyme complex
also located in the inner mitochondrial membrane and designed to synthesise ATP
from ADP and inorganic phosphate.
For the majority of industrial fermentations with bakers' yeast, the elevated capac-
ity to ferment available carbohydrates is an important characteristic, especially when
the biomass is exposed to high sugar concentrations and/or absence of oxygen.

6.2.2 Stress Response in Sourdough Yeasts

Yeasts are exposed to constant fluctuations of their growth conditions. Consequently,
they have to develop sophisticated responses to adapt to and survive under a variety
of conditions. Yeasts, as well as other organisms, employ a concerted response to
external stress [ 34 ]. One mechanism that yeast cells use to protect the cell from the
effects of environmental variation is to initiate a common gene expression program.
This program includes about 900 genes, whose expression is stereotypically altered
when yeast cells are shifted to stressful environments. The genes that participate in
this response amount to almost 14% of the currently predicted genes in the yeast
genome [ 35 ] .
The “general stress” transcription factor could be identified in the zinc-finger
transcription factor (Msn2) [ 14 ]. Normally, Msn2 is exported from the nucleus, and
a cyclin-dependent kinase (Srb10) is concomitantly repressed. Under stress, Msn2
re-localises to the nucleus and, with the relief of Srb10 repression, activates tran-
scription. The stress response is rapid, but quickly attenuated. Bose et al. [ 36 ]
showed that this attenuation is caused by a nuclear-dependent degradation of Msn2.
Msn2 rapidly disappeared from cells after heat or osmotic shock.
Process parameters, including temperature, dough yield, oxygen, pH, as well as
the composition of starter cultures, determine the quality and handling properties of
sourdough [ 37 ] and the metabolic response of microorganisms responsible for the
fermentation process [ 38 ]. The exposure of microbial cells to stressful and fluctuating
conditions during fermentation involves a broad transcriptional response with many
induced or repressed genes. The selective pressure exerted by environmental condi-
tions encountered by yeast cells during sourdough fermentation, accounts for the
consolidated dominance of selected yeast species. Nutrient availability likely modu-
lates the microbial ecology of sourdough. However, within the sourdough ecosys-
tem there are numerous mechanisms whereby one species may influence the growth
of another [ 38 ]. Although autochthonous bacteria and yeasts are adapted and com-
petitive in their respective environment, the dough environment can be described as
a stressful environment for microorganisms [ 39 ] .
The conditions of the sourdough microenvironment that principally affect yeast
responses and growth rate are: nutrient availability (starvation), pH (acid stress),