Biomedical Engineering Reference
In-Depth Information
early fermentation process but die off when a significant pH decrease occurs upon
fermentation [
89,
108
] .
Although sourdough fermentation proceeds anaerobically, the presence of oxy-
gen in the beginning of the fermentation and when small amounts of dough (high
ratio of surface to volume) are used may favor certain LAB and yeast species [
179,
190,
192
]. For instance, mild aeration has a positive influence on the competitive-
ness of
Lb. amylovorus
DCE 471 [
179
] . Similarly,
P. kudriavzevii
can only grow
when enough oxygen is available during fermentation [
190
] . Further, the ionic
strength and salt concentration of the dough affects microbial growth [
160,
179,
193,
194
]. Similarly, the presence of organic acids in and the buffering capacity of
the flour influence growth of both yeasts and LAB [
160,
179
] . In general, sourdough
LAB are acid-tolerant and their growth is favored in the presence of salt, as is the
case for, for instance,
Lb. amylovorus
DCE 471 [
188,
193
] . Alternatively, the growth
of
C. humilis
and
S. exiguus
is completely inhibited by 4% NaCl; also, the growth
of these yeasts is strongly inhibited in the presence of acetic acid and to a much
lesser extent by lactic acid [
160,
179
] .
Whereas backslopping practices select for mainly heterofermentative LAB, the
amounts of dough used for backslopping and the frequency of the refreshments
determine the community dynamics and stability of the sourdough microbiota as
well. The amount of backslopping dough defines the initial pH and in this way
influences the growth and acidification rates of the LAB species involved [
107,
189,
190
]. Also, the amount of backslopping dough determines the dough yield and
hence the availability of water (water activity of the dough). Short refreshment
times may select for rapidly growing LAB species, which in turn depends on the
fermentation temperature and influences the acidification rate. In this regard,
Lb.
fermentum
is most competitive at 30 °C and 37 °C with backsloppings every 24 h,
while a mixture of
Lb. fermentum
and
Lb. plantarum
prevails at 30 °C with back-
sloppings every 48 h [
175
]. This may explain why
Lb. sanfranciscensis
is some-
times missed during laboratory-scale fermentation processes [
136,
175
] . Also, a
short refreshment time seems to favor
C. humilis
during sourdough fermentation
compared to
S. cerevisiae
[
190
] .
Finally, interactions between LAB and yeasts are an important aspect for the
community dynamics and stability of the sourdough microbiota [
50,
57,
165,
189,
195,
196
]. Interactions encompass both cooperative and antagonistic ones. During
some sourdough fermentation processes yeasts cannot develop at all, perhaps
because of inhibition of yeast growth by nutritional competition or the presence of
inhibitory compounds [
47,
123,
175
]. In other processes, mutualistic interactions
lead to stable associations, not only between LAB species and yeasts (besides
Lb.
sanfranciscensis
/
C. humilis
, also
Lb. sanfranciscensis
/
K. barnettii
,
Lb. plantarum
/
S.
cerevisiae
, and
Lb. brevis
/
Candida
spp.) but also among LAB species (e.g., between
Lb. sanfranciscensis
and
Lb. plantarum
or
Lb. paralimentarius
) [
40,
41,
58,
69,
142
]. Nevertheless, the competitiveness of LAB and yeasts in sourdough seems to
be strain-specific and not species-specific, as has been shown for
Lb. sanfranciscen-
sis
[
135
] and
Lb. plantarum
strains [
138
] in wheat sourdoughs recently.
