Biomedical Engineering Reference
In-Depth Information
dough yield and presence of sugars, salts and polysaccharides (osmotic stress),
oxygen (oxidative stress), temperature fluctuations (heat shock and cold stress) and
interactions between lactic acid bacteria and yeasts (e.g.
S. cerevisiae, C. milleri
and
L. sanfranciscensis
), and between yeasts (e.g.
S. cerevisiae
and
C. milleri
).
6.2.2.1
Low Temperature
In bakery practice, the temperature of the sourdough is an important parameter to
control the growth of lactobacilli and yeasts [
21
] . Häggman and Salovaara [
40
]
studied the effect of process parameters, including low temperature, on the leaven-
ing of rye dough. Under the experimental conditions, the endogenous
C. milleri
was
responsible for leavening, especially when the temperature was set at 22°C. The low
temperature of fermentation also slowed the acidification rate, thus favouring an
extended production of CO
2
by
C. milleri
. An appropriate modulation of the tem-
perature and the use of temperatures ranging from 4°C to 8°C is a practical approach
to control the fermentation rate and to program the working times.
The effects of the low temperature and other environmental parameters on yeast
physiology are conditioned by the exposure dynamics [
41
] . Transcriptional
responses during adaptation to suboptimal temperatures permitting growth (10-
20°C) [
42,
43
] differed from those found after exposure to temperatures below
10°C, where growth ceases [
44,
45
]. Two different mechanisms of response are
distinguished: (1) the early cold response (ECR), occurring within 12 h; and (2) the
late cold response (LCR), occurring later than 12 h [
43
] .
Tai et al. [
41
] used chemostat cultivation to compare different culture conditions
and/or microbial strains at fixed specific growth rate. They observed 15% of the
genes that showed a consistent transcriptional response in previous batch-culture
studies on cold adaptation [
42,
43,
45
] were also identified under chemostat condi-
tions at 30°C [
46
]. In batch cultures, the exposure of
S. cerevisiae
to low tempera-
tures invariably induces an increased synthesis of storage carbohydrates (especially
trehalose) and transcriptional up-regulation of genes involved in storage carbohy-
drate metabolism [
47
]. Transcriptional induction of the trehalose-biosynthesis genes
TPS1 and TPS2 is consistently observed in cold-shock studies, and after exposure
to near-freezing conditions. Several other genes such as HSP12, HSP26, HSP42,
HSP104, YRO2 and SSE2, and those encoding the three cell-wall mannoproteins
(Tip1p, Tir1p, and Tir2p), fatty-acid desaturase (Ole1p), which influences the mem-
brane fluidity, and Nsr1p, a nucleolar protein required for pre-rRNA processing and
ribosome biogenesis, were consistently associated with cold shock [
41,
43,
45
] . The
stress response element (STRE) binding factors Msn/Msn4 are implicated in the
coordinate regulation of low-temperature-responsive genes [
43,
47
] . Consistently,
many genes induced upon a temperature downshift were also induced under a vari-
ety of other stress conditions [
43
]. Contrarily to batch cultures, where the low tem-
perature adaptation is accompanied by marked ESR (Environmental Stress
Response) (29% of the differentially expressed genes in batch cultures at low tem-
perature respond to ESR as well), only three ESR-induced genes (YCP1, VPS73,
