Biomedical Engineering Reference
In-Depth Information
7.6.1
Antifungal Compounds from Sourdough
Lactic Acid Bacteria
Antifungal compounds synthesized by sourdough lactic acid bacteria include
diacetyl, hydrogen peroxide, acetate, propionate, caproate, 3-hydroxy fatty acids,
phenyllactate, cyclic dipeptides, reuterin, and fungicidal peptides [
120
] . Numerous
reports describe a substantial increase of the mould-free shelf life of bread owing to
antifungal activities of sourdough lactic acid bacteria [
121,
122
] . However, metab-
olites of lactic acid bacteria responsible for the antifungal effect remain in most
cases unknown. Acetate inhibits fungal growth only at concentrations that
significantly impair the sensory and textural quality of bread [
30,
107,
123
] .
Phenyllactate and 4-hydroxyphenyllactate were initially characterized as antifun-
gal metabolites from
L. plantarum
ITM21B [
124
]. Lactobacilli capable of produc-
ing phenyllactate and hydroxyphenyllactate delayed the growth of
Aspergillus
niger
and
Penicillium roqueforti
for up to seven days of bread storage [
125
] . The
concentration of phenyllactate accumulated by lactobacilli in sourdough fermenta-
tion, however, is about 100 fold lower than its minimal inhibitory concentration
[
74,
126
] and the antifungal effect of the sourdough is thus not attributable to phe-
nyllactate accumulation. Likewise, the levels of antifungal cyclic dipeptides in
bread are 1,000 fold lower than their MIC, but above the threshold imparting a
metallic and bitter taste [
127
]. Accordingly, the antifungal effect of sourdough lac-
tic acid bacteria is attributed to a synergistic activity of several compounds. The
antifungal effect of
L. reuteri
,
L. plantarum
, and
L. brevis
in the presence of Ca
propionate (0.2%, w/w) [
128
] delayed fungal growth by 8 days compared to the
bread started with baker's yeast alone. It was hypothesized that a synergistic activ-
ity between acetic acid and phenyllactate with chemical preservatives was respon-
sible for this effect. The antifungal effect of
L. buchneri
and
L. diolivorans
against
growth of four moulds on bread was attributed to a combination of acetate and
propionate [
30
]. The preservative effect of
L. amylovorus
[
121,
122
] was attributed
to the synergistic activity of more than ten antifungal compounds, including phe-
nyllactate, phenolic acids, fatty acids, and cyclic dipeptides [
129
] .
In addition to the antifungal metabolites of lactic acid bacteria, inhibitory pep-
tides derived from the substrate may also contribute to the preservative effect.
A water-extract from beans in combination with sourdough fermented with
L. brevis
AM7 contained three natural inhibitory compounds, two phaseolins (NCBI n. gi
130169, gi 403594) and one lectin (NCBI n. gi 130007) [
130
] . Antifungal peptides
were also identified from the water-extract of sourdough. The combined activity of
the above compounds determined a delay in fungal growth of up to 21 days, lead-
ing to a shelf life for the bread that was comparable to that found when using Ca
propionate (0.3% w/w). A very extensive bread shelf life was also achieved by
using sourdough lactic acid bacteria and amaranth flour or wheat germ [
131,
132
] .
Sourdough fermented with the nonconventional yeast
Wickeramomices anomalus
LCF1695 and
L. plantarum
was shown to exhibit strong antifungal activity based
on synergistic activities between the inhibitory peptides liberated by
L. plantarum
