Biomedical Engineering Reference
In-Depth Information
numbers above those of the adventitious microbiota upon fermentation of a
fl our-water mixture [
57
]. However, whether the type of flour mainly directs the
growth of sourdough LAB species remains controversial [
25,
26,
47,
89,
105,
107,
121,
123,
135
]. For instance, whereas it was assumed that rice sourdough fermenta-
tion selects for
Lb. spicheri
[
80
], this LAB species cannot always be found in rice
sourdoughs [
123
]. This has been ascribed to competitiveness of the microorganisms
that are present in the sourdough ecosystem. Indeed, microbial interactions between
the spontaneous microbiota and an added sourdough starter culture may lead to the
dominance of autochthonous LAB species and/or strains. Among other mecha-
nisms, competitiveness may explain the apparent prevalence of LAB species in
specific sourdough preparations, such as evidenced by single reports on
Lb. amylo-
vorus
in rye sourdoughs [
120
] ,
Lactobacillus sakei
in amaranth sourdoughs [
122
] ,
and
Lb. pontis
in teff sourdoughs [
47
] . Yet, spontaneous sourdough fermentations
carried out in the laboratory with flour as the sole nonsterile ingredient indicate that
the type and quality (microbiological and nutritional) of the cereal flour used is
indeed an important source of autochthonous LAB and yeasts occurring in the ripe
sourdoughs [
40,
107
]. Hence, the flour plays a key role in establishing stable micro-
bial consortia within a short time. In this context, it has been shown that laboratory
sourdoughs based on wheat, rye, or spelt, backslopped daily for 10 days at 30 °C,
whether or not initiated with a
Lb. sanfranciscensis
starter culture [as tested in the
case of wheat sourdough fermentations in the study of Siragusa
et al.
[
135
] ], reach
an equilibrium of LAB species through a three-step fermentation process: (1) preva-
lence of sourdough-atypical LAB species (e.g.,
Enterococcus
spp. and
Lc. lactis
subsp.
lactis
); (2) prevalence of sourdough-typical LAB species (e.g., species of
Lactobacillus
,
Leuconostoc
,
Pediococcus
, and
Weissella
); and (3) prevalence of
highly adapted sourdough-typical LAB species (e.g.,
Lb. fermentum
and
Lb. plan-
tarum
) [
107-
109,
135
]. Indeed, it has been shown that the LAB species
Lb. fermen-
tum
(strictly heterofermentative) and
Lb. plantarum
(facultatively heterofermentative)
dominate several sourdough fermentation processes, irrespective of the type of flour
or the addition of starter cultures that are not robust enough [
47,
108,
123,
136-
138,
142,
172,
174,
175
] . Concerning yeasts,
C. glabrata
and
W. anomalus
prevail dur-
ing laboratory sourdough fermentations [
40
]. Further, it has been shown that previ-
ous introduction of flour into the bakery environment helps to build up a so-called
house microbiota that serves as an important inoculum for subsequent bakery sour-
dough fermentations [
155
]. Indeed, LAB strains adapted to the sourdough and bak-
ery environment (apparatus, air, etc.), which have been shown to be genetically
indistinguishable, may be repetitively introduced in consecutive sourdough batches
during backslopping. The widespread use of bakers' yeast may be responsible for
the prevalence of
S. cerevisiae
in bakery sourdoughs [
26,
29,
31,
34-
36,
39,
40
] .
However, there are also indications through reliable molecular data of a large strain
diversity of
S. cerevisiae
in single sourdoughs, that suggest an autochthonous wheat
flour origin of this yeast species in sourdough too [
27,
30,
34
] . Supportive of an
autochthonous origin of
S. cerevisiae
is also the presence of this species in rye flour [
29
] .
Yet, during laboratory fermentations with flour as the sole nonsterile ingredient and
without added bakers' yeast, other species such as
C. glabrata
and
W. anomalus
