Biomedical Engineering Reference
In-Depth Information
of the total number of colonies of that particular morphotype. Each morphoptype's
percentage provides important species abundance data in the sourdough if more
than one yeast species is isolated. The generation of pure cultures is crucial before
characterization and identification of the isolates. The purity should be tested micro-
scopically and on antibiotic-free medium to exclude any carry-over of the accompa-
nying bacterial microbiota.
5.3.2
Isolation of Sourdough LAB
Isolation of LAB from sourdough environments is challenging for three main rea-
sons. First, sourdoughs are complex ecosystems not only in terms of their microbial
composition but also in terms of the interactive effects among types of breadmaking
processes and ingredients. The utilization of soluble carbohydrates by LAB and,
thus, their energy yield are greatly influenced by the associated yeasts and vary
according to the type of carbohydrates [
195
]. However, as many media for selective
isolation of LAB incorporate yeast-inhibiting agents such as cycloheximide, pima-
ricin, and amphotericin B
,
the trophic interaction between LAB and yeasts is in
these cases disturbed, which may affect the recovery potential of LAB strains that
strongly rely on this association. Secondly, sourdough fermentation is a dynamic
process in which fast-acidifying LAB initially dominate the ecosystem and are then
gradually replaced by typical sourdough LAB that largely contribute to the organo-
leptic and textural properties of the end product. Depending on whether the early
subdominant LAB and/or the final dominant LAB are the target of the isolation
approach, it may thus be necessary to include multiple samples taken at different
time points. Finally, the LAB communities in sourdoughs may consist of metaboli-
cally very diverse groups, including obligately homofermentative and facultatively
or obligately heterofermentative species. As some of these species have specific
growth requirements in terms of the incubation medium and conditions (e.g., tem-
perature, pH, atmosphere, etc.), it seems inevitable that different medium formula-
tions and/or sets of incubation parameters are required to cover the entire metabolic
LAB spectrum present in a sourdough sample.
Initially, sourdough LAB were mostly isolated on de Man-Rogosa-Sharpe (MRS)
medium [
202
], which is the general medium used for the isolation and enumeration
of lactobacilli from fermented food products. The MRS medium contains glucose
as the main carbohydrate source. Triggered by growing insights in the species diver-
sity of sourdough-associated LAB, a number of more specialized media have been
developed for the selective isolation of typical sourdough species. For the specific
detection of
Lb. sanfranciscensis
, Kline and Sugihara [
75
] proposed the SourDough
Bacteria medium which contains maltose as the carbohydrate source in addition to
freshly prepared yeast extract (FYE) to further enhance growth. The Sanfrancisco
medium was developed for the isolation and description of
Lb. pontis
and
Lb. min-
densis
[
79,
203
]. This medium contains three carbohydrates (maltose, fructose, and
glucose), FYE, cysteine and rye or wheat bran. In parallel to the design of new
