Biomedical Engineering Reference
In-Depth Information
Reutericyclin is a tetramic acid derivative with a broad spectrum of activity
against Gram-positive bacteria, including rope-forming bacilli [
142,
143
] . To date,
all reutericyclin-producing strains were isolated from an industrial rye sourdough.
Reutericyclin is produced to active concentrations in sourdough and the persistence
of
L. reuteri
strains during long-term propagation of a rye sourdough was attributed
to the synthesis of reutericyclin (for a review, see [
144
] ). The use of reutericyclin-
producing
L. reuteri
as a sourdough starter also caused a delay in the growth of
Bacillus
sp. during bread storage [
144
] .
7.7
Metabolism of Phenolic Compounds and Lipids
The major phenolic compound in wheat and rye is ferulic acid bound to cell wall
polysaccharides [
145
]. Changes in the ferulic acid content during sourdough
fermentation are predominantly the result of oxidation reactions and cereal enzymes
[
146,
147
]. In contrast, cereal grains used in African cereal fermentations, particularly
sorghum and millet, are rich in phenolic compounds, including phenolic acids, phe-
nolic acid esters, desoxyanthocyanidins, and tannins [
148,
149
] . In these cereal
grains, metabolism of phenolic compounds influences product properties, and the
content of antimicrobial phenolic compounds influences the microbial ecology of
sorghum sourdough fermentations [
150
]. A review on the metabolism of food phe-
nolics by lactic acid bacteria, based predominantly on isolates from wine and veg-
etable fermentations, is provided by Rodriguez et al. [
151
] .
Conversion of phenolic compounds is based on glycosyl hydrolases, for example
b-glucosidase and a-rhamnosidase, which convert flavonoid glycosides to the cor-
responding aglycones [
152,
153
], esterase degrading methyl gallate, tannins, or
phenolic acid esters [
154
], and decarboxylases and reductases with activity on phe-
nolic acids [
151
] . The strain-speci fi c metabolism of phenolic compounds is particu-
larly well described for
L. plantarum
[
151,
155
] . Decarboxylation of hydroxyl
cinnamic acids generates the corresponding vinyl derivatives [
156
] . Reductases
hydrogenate the double bond of hydroxyl cinnamic acids or their decarboxylated
vinyl derivatives [
151,
156
]. The spectrum of activities is strain specific; organisms
capable of conversion of hydroxy cinnamic acids harbor reductase activity, decar-
boxylase activity, or both [
150,
151,
156
]. Analysis of phenolic compounds during
sorghum sourdough fermentation revealed that strain-specific glycosyl hydrolase,
decarboxylase, and phenolic acid reductase contributed to the conversion of pheno-
lic compounds [
151
] .
Phenolic compounds, including phenolic compounds from sorghum and mil-
let, exhibit strong antimicrobial activity [
148,
150
]. Particularly the activity of
hydroxy benzoic and hydroxy cinnamic acids is well characterized [
157
] . The
resistance of lactic acid bacteria towards phenolic acids is highly strain specific
and the strain-specific capability for phenolic acid conversion corresponds to
resistance [
157,
158
]. Because the conversion of hydroxy cinnamic acids by
reductase- or decarboxylase activities reduced the antimicrobial activity of caffeic
