Biomedical Engineering Reference
In-Depth Information
health- promoting effects [
69,
166
]. In this context, strain robustness and fitness
towards microbial competitors and environmental conditions should be the driving
force in the selection of useful starters for sourdough fermentation processes, as it
has been shown that autochthonous strains often emerge [
69,
135,
138,
142
] .
However, studies on the industrial exploitation of sourdough starter cultures are
scarce [
183,
184
]. Recently, the use of starter cultures in type I propagated sour-
doughs has been investigated [
89,
123,
135,
185
]. It is of course well known that the
fermentation temperature affects the ratio of lactic acid to acetic acid [
185,
187
] . In
general, homofermentative LAB starter cultures are used at high temperature and
for short fermentation times (e.g., 37 °C for 36 h) and heterofermentative LAB
starter cultures are used at low temperature and for long fermentation times (e.g.,
25 °C for 48 h), resulting in sourdoughs with mainly lactic acid and acetic acid,
respectively. However, it would be of great value to know the circumstances for the
expression of other functional properties that are of added value to sourdoughs
[
182
] .
The fermentation temperature, one of the criteria to distinguish type I and II sour-
doughs, is essential for the community dynamics and stability of a sourdough micro-
biota [
29,
160,
175,
179,
185,
188,
189
]. For instance, spontaneous wheat sourdough
backslopping fermentations (type I) carried out at 23 °C for 10 days select for
Le.
citreum
instead of
Lb. fermentum
that prevails at 30 °C and 37 °C [
175
] . Similarly,
rye fermentations initiated with commercial sourdough starter cultures maintain the
presence of
Lb. mindensis
and
Lb. sanfranciscensis
at 25 °C (type I), but select for
Lactobacillus crispatus
and
Lb. pontis
at 30 °C and for
Lb. crispatus
,
Lb. frumenti
,
and
Lb. panis
at 40 °C (both type II) [
184
] . Whereas
Lb. sanfranciscensis
prefers
long fermentation times at relatively low temperature, conditions that often prevail
during type I sourdough preparations, this species grows optimally at 32 °C [
159,
189
] . However, whereas
C. humilis
grows optimally at 27-28 °C but does not grow
above 35 °C [
160,
189
], the association of
Lb. sanfranciscensis
-
C. humilis
grows
optimally at 25 °C and 30 °C and may explain its stability between 20 °C and 30 °C
[
160,
190
] . The abundance of
Lb. sanfranciscensis
in wheat sourdoughs made at
ambient temperature indicates a low competitiveness of other LAB species such as
Lb. fermentum
that prefers higher temperatures for optimal growth. Similarly, tem-
perature may be responsible for a selection toward
Lb. helveticus
during Sudanese
sorghum sourdough fermentations, which are carried out at 37 °C [
148
] .
For the growth of sourdough LAB, also the pH plays an important role [
160,
179,
188,
189
] . For instance,
Lb. sanfranciscensis
cannot grow below pH 3.8-4.0 [
160,
189
] , whereas
C. humilis
is not influenced by the pH [
190
] . An optimal pH for
growth of around 5.0 has been found for
Lb. sanfranciscensis
. This pH value cor-
responds approximately to that observed during the first stage of dough fermenta-
tion. However, the growth of lactobacilli is favored over yeast growth at pH values
above 4.5 [
160
]. Hence, the rate of acidification of the dough may determine the
level of
Lb. sanfranciscensis
in the dough. Natural sourdough fermentations dis-
playing higher pH values are often dominated by a different microbiota, encom-
passing
Enterococcus
,
Lactococcus
,
Leuconostoc
,
Pediococcus
,
Streptococcus
, and
Weissella
, which are commonly present in the cereal flour [
57,
133
] or during the
