Biomedical Engineering Reference
In-Depth Information
Fig. 6.4
Yeast recovery ability in terms of volume increase of a dough inoculated with the selected
strain (P6). The dough was initially stored for 7 days at 4°C (
yellow square
), and then incubated at
35°C
and EMI1) showed higher transcription at 12°C under chemostat cultures.
Conversely, 88 ESR-induced genes showed a consistently lower transcript level at
12°C [
41
]. Studies on cold adaptation in batch cultures of
S. cerevisiae
revealed a
clear transcriptional up-regulation at low temperature of chaperone-encoding genes
such as HSP26 and HSP42 [
43,
44
]. The proteins encoded by these genes prevent
the aggregation of cytosolic proteins during heat shock [
41
] .
In bakery practice, refrigeration of dough or sourdough is used to control
fermentation. Under the refrigerated conditions, yeasts have to maintain and then to
recover their fermentative capacity in a very short period of time (15-30 min). At
4-8°C, many yeast strains continue to ferment at a slow rate and induce a slight
increase of the dough volume. The fermentation stops at 4°C. Only selected strains
recover their leavening capacity when the dough temperature is raised to 28-35°C
(Gottardi et al. unpublished data) (Fig.
6.4
). As shown in Fig.
6.4
, these strains
increased the dough volume within 15 min also when the dough had been kept for
7 days at 4°C.
6.2.2.2
Acidity
The acidity of the sourdough depends on lactic and acetic acid production by lactic
acid bacteria [
40
]. Usually, low temperatures of sourdough fermentation delay the
lactic acidification and decrease the time of yeast exposure to high acidity. A low
fermentation temperature was suggested as a means to improving the synthesis of
CO
2
by yeasts [
40
] .
Because only the non-dissociated acids diffuse into the cell, the type of acid
more than the pH determines yeast inhibition [
49
]. Mainly the level of dissociation
of acetic acids affects the leavening capacity [
40
]. The non-dissociated form of the
