Physiology and Biochemistry of Sourdough Yeasts - Sourdough Biotechnology - page 164

accordingly. The pathway for the synthesis of glycerol is shown in Fig. 6.3 . The
NAD-dependent glycerol-3-phosphate dehydrogenase (Gpd) and the glycerol-3-
phosphatase (Gpp) catalyse the two step reactions. Saccharomyces cerevisiae also
possesses genes that might encode the enzyme glycerol dehydrogenase ( GCY1 and
YPR1 ) and the dihydroxyacetone kinase ( DAK1 and DAK2 ). These two enzymes are
involved in the pathway for glycerol degradation [ 50 ]. The pathway for glycerol via
Gpd and Gpp converts NADH to NAD, while the conversion of glycerol to dihy-
droxyacetone phosphate via Gcy1p and Dak1p reduces NADP to NADPH. Hence, the
glycerol-dihydroxyacetone phosphate cycle mainly acts as transhydrogenase for the
interconversion of NADH to NADPH. Since stress conditions require high levels of
NADPH to manage reactive oxygen species, this pathway assumes an important role.
The capacity for other NADPH-generating reactions, such as those of the pentose
phosphate pathway, is also increased under stress conditions. When salt-stressed yeast
is inoculated in dough with high sugar content, the fermentation time was significantly
reduced and bread-specific volume increased due to glycerol accumulation and a less
dense gluten network. Moreover, two-step industrial fermentation, including a pre-
adaptation to osmotic stress, in order to enhance flavour, has been proposed [ 51 ] .

6.2.2.4 Membrane Lipids as Modulators of Stress Tolerance in Yeasts

Biological membranes are the barrier that separates cells from their environment
and are the primary target for damage during environmental stress. Sudden changes
of environmental conditions cause alteration in the organisation and structure of
membrane lipids, and alter the function of many cellular activities. Homeoviscous
adaptation to low temperature maintains the molecular order of membrane lipids as
well as the activity of membrane-associated enzymes and transporters. To date,
most of the research in this field has focused on the connection between the physical
state of the membrane and cold and ethanol tolerance. In organisms producing etha-
nol, including yeasts, intra-cellular ethanol freely diffuses into an external medium.
At high concentration, the ethanol present in the external medium acts as a chemical
stress. A long-standing conundrum is the mechanism by which cells growing in the
presence of a high concentration of ethanol modify their membrane composition.
The plasma membrane lipid is the main site of impediment and interaction with
ethanol. The altered lipid composition, following the exposure to ethanol, combats
the deleterious effect. The ethanol-dependent modification of phospholipids' fatty
acid composition was also shown in S. cerevisiae [ 52 ] . The addition of ethanol
(0.5-1.5 M) to S. cerevisiae leads to a progressive decrease of the proportion of
saturated fatty acids (SFAs) (mainly 16:0) and to a corresponding increase of the
mono-unsaturated fatty acid residues, especially 18:1. This increased unsaturation
favours an increase of the fluidity of the membrane. On the basis of similar observa-
tions in Escherichia coli , it was suggested that increased unsaturation was not due
to the additional synthesis of unsaturated fatty acids (UFAs), but was the result of
the decrease of levels of SFAs. An exception to this hypothesis was the increase of
18:0 in the presence of ethanol [ 52- 54 ]. It is assumed that the presence of ethanol