Physiology and Biochemistry of Sourdough Yeasts - Sourdough Biotechnology - page 155

A global signal indicates nitrogen derepression, and a second, pathway-specific
signal indicates the presence of a substrate or of an intermediate of the pathway.
This control permits the selective expression of enzymes for a specific catabolic
pathway from many potential candidates within the nitrogen regulatory circuit.
However, some systems are controlled only by nitrogen metabolite repression and
not by induction [ 16 ] .
The cell yield of yeasts seemed to be unaffected by the presence of lactic acid
bacteria in sourdough. Saccharomyces cerevisiae grown as a monoculture in dough
attained 8 log CFU/g [ 18 ]. Almost the same value of cell count was found under
co-culture with lactic acid bacteria. Conversely, the maximum specific growth rate
was negatively affected. According to Meroth et al. [ 6 ], cell counts of endogenous
yeasts of ca. 7.5 log CFU/g were reached during the first fermentation period at
30°C, with a dough yield of 200, and remained stable until the end of fermentation.
During fermentation of traditional sourdough products (e.g. Panettone or Colomba),
the total yeast counts reached the level of 8-8.5 log CFU/g, after addition of baker's
yeast [ 19 ]. Yeast cells may encounter different environmental states. Maintaining
optimal functionality in the presence of such external variability is a central evolu-
tionary constraint. Feedback mechanisms directly link gene expression with internal
need. Physiological variables such as the rate of biomass production, the cellular
pools of nutrients or the energy feedbacks, to properly tune gene expression with the
corresponding functional needs, are part of these strategies [ 17 ] .
Yeast cells starved for particular amino acids or treated with an inhibitor of protein
synthesis (cycloheximide) may exhibit a phenomenon referred to as the stringent
response. In S. cerevisiae , the stringent response results in a rapid inhibition of the
synthesis of rRNA (but not of tRNA or mRNA) and it is thought to be signalled
when cells sense a rapid decrease of the overall rate of protein biosynthesis. Nitrogen
compounds and, especially, amino acids are not the limiting factors for yeasts.
Saccharomyces cerevisiae and the other species, which naturally occur in sour-
doughs, are prototrophic for amino acids. Figure 6.2 shows the concentration of
various free amino acids during fermentation of kamut flour with baker's yeast and
sourdough. Compared to the dough leavened with baker's yeast, the fermentation by
sourdough lactic acid bacteria caused an increase of some amino acids. Fermentation
with baker's yeast resulted in the decrease of free amino acids due to yeast metabo-
lism (see below). Because of the high value of pH during fermentation with yeasts
alone, the activation of flour endogenous proteinases is poor, and the concentration
of free amino acid increases only after the yeasts have reached the stationary phase
of growth [ 20 ] .
Co-fermentation with lactic acid bacteria and yeasts determines environmen-
tal fluctuations in terms of availability of nutrients, synthesis of organic acids,
pH decrease and changes of the rheological properties of sourdough. Brandt
et al. [ 21 ] evaluated the effect of process parameters on the growth and metabo-
lism of L. sanfranciscensis and C. milleri during rye sourdough fermentation.
pH did not affect the growth of the yeast in the range 3.5-5.5, whereas the growth
of L. sanfranciscensis was inhibited at pH 4.0. The concentration of NaCl of 4%
(wt/wt of flour) inhibited the growth of L. sanfranciscensis but not that of C. milleri .