Biomedical Engineering Reference
In-Depth Information
acid two to fivefold, metabolism was recently identified as a mechanism of
detoxi fi cation [
157
]. To date, lactic acid bacteria capable of conversion of pheno-
lic compounds were predominantly isolated from fermented beverages (wine,
whisky, beer), or African sorghum fermentations. These organisms are likely a
suitable source for starter cultures of phenolic-rich cereals and pseudocereals
used in gluten-free baking [
159
] .
Lipid metabolism in sourdoughs is poorly described and particularly lipase activ-
ity of sourdough lactic acid bacteria appears not to be relevant. However, sourdough
fermentation influences lipid oxidation and the influence of lipid oxidation products
on bread flavor. During flour storage and during dough mixing, chemical oxidation
and cereal-derived lipoxygenase, respectively, convert free linoleic acid to lipid per-
oxides. Peroxides are chemically converted to lipid aldehydes, i.e., (E)-2-nonenal
and (E,E)-2,4-decadienal, with a strong influence on the flavor of the bread crumb
[
21
]. Heterofermentative lactobacilli reduce these aldehydes to the corresponding
alcohols with a much lower flavor threshold through alcohol reductase activity.
Homofermentative lactobacilli may increase lipid oxidation through formation of
hydrogen peroxide [
21
]. Baker's yeast also reduced flavor-active aldehydes result-
ing from lipid oxidation, but slower and through different metabolic pathways [
21
] .
Thiol accumulation by sourdough lactic acid bacteria [
56
] may additionally in fl uence
lipid oxidation. Cysteine and related thiol compounds reduce linoleic acid perox-
ides to the corresponding hydroxy fatty acids [
160
], and thus interrupts the reaction
cascade leading to flavor-active aldehydes.
7.8
Cell-to-Cell Communication
Bacteria synthesize, release, sense, and respond to small signaling molecules that
are defined as auto-inducers. The signaling molecules accumulate in the environ-
ment and lead to a series of physiological and biochemical responses when the
quorum (threshold concentration) is reached. The term quorum sensing is derived
from this consideration, and is mostly used to describe cell-to-cell communication
[
161
]. Overall, the mechanisms of intraspecies communication include the use of
acyl-homoserine lactones (AHL) and auto-inducing peptides (AIP) for Gram-
negative and -positive bacteria, respectively [
162,
163
] . The interspecies communi-
cation is mainly based on signaling molecules such as furanone derivatives. The
mechanisms involve the LuxS protein or AIP molecules and the three-component
regulatory system (3CRS) [
161
]. Several studies considered the microbial dynamics
during sourdough fermentation [
164
]. Within this complex food ecosystem, an
understanding of the mechanisms of interspecies communication should give new
insights into the physiological response of sourdough lactic acid bacteria and the
interactions in an heterogeneous microbial community that govern growth and
metabolism. The mechanism of cell communication was studied in
L. sanfrancis-
censis
CB1 co-cultivated with other sourdough lactic acid bacteria [
165
] . The high-
est number of dead and/or damaged cells of
L. sanfranciscensis
CB1 was found
