Biomedical Engineering Reference
In-Depth Information
Table 6.4
Selected metabolites produced by
L. sanfranciscensis
and
S. cerevisiae
inoculated in
dough with various dough y
ields (data expressed as peak chromatographic area) [
51
]
L. sanfranciscensis
S. cerevisiae
DY 220
DY 146
DY 220
DY 146
Compound
Ethanol
7.72
7.80
8.10
8.34
Isobutanol
6.19
7.46
7.15
7.31
Isoamyl alcohol
7.60
7.47
7.77
7.97
Acetoin
6.42
6.69
5.63
6.19
2,3-Butanediol
n.d.
n.d.
5.55
5.35
Butyric acid
6.42
6.48
6.59
6.57
Ethyl decanoate
n.d.
n.d.
6.12
5.44
Ethyl-9-decenoate
5.78
6.29
n.d.
n.d.
Hexanoic acid
7.64
7.34
6.79
6.95
Phenylethanol
6.23
6.28
7.87
7.93
Ethyl hexadecanoate
n.d.
n.d.
6.17
6.41
Standard deviation values of the three repetitions in this experiment were less than 10%
n.d. Below detection limit
and yeast forms of the parasitic fungus
Histoplasma capsulatum
provided the first
example of an apparent quorum-sensing mechanism in eukaryotes [
73
] .
Saccharomyces cerevisiae
may show the morphological transition from the yeast
to the filamentous forms in response to environmental cues. Growth under nitrogen-
limiting conditions is an example, which activates several signal transduction
pathways. One of these routes is the Ras-cAMP-dependent protein kinase (PKA)
pathway. Chen and Fink [
74
] provided some intriguing molecular clues to PKA sig-
nalling. The conditioned medium of
S. cerevisiae
stationary-phase cultures markedly
induces the filamentous growth. Induction was partially related to the fivefold stimu-
lation of the transcription of
FLO11,
a gene essential for filamentous growth. The
active molecules were purified and were found to be phenylethanol and tryptophol, two
aromatic alcohols derived from phenylalanine and trytophan, respectively, which are
always present in yeast-fermented foods. The addition of both alcohols causes the
more vigorous filamentous growth and the induction of
FLO11
. On the basis of the
bacterial paradigm, these compounds have the features of quorum-sensing molecules.
The synthesis of these molecules is the highest during the stationary phase of growth,
the addition of tryptophol induces the expression of genes (
ARO9
and
ARO10
), which
are required to convert tryptophan to tryptophol, and mutants unable to synthesise these
alcohols show a markedly decreased filamentous growth and
FLO11
expression.
Preliminary results on inter-species communication during sourdough fermenta-
tion have been reported [
51
]. As shown in Fig.
6.6
, the exposure of
C. milleri
to the
cell-free wheat flour hydrolyzed (WFH) medium, previously inoculated with
L. sanfranciscensis
, induced morphological changes and early autolysis of the yeast.
Figure
6.7
shows another possible effect of the inter-species signalling mecha-
nism, which is mediated by lactic acid bacteria metabolites. After 2 h of exposure to
