Biomedical Engineering Reference
In-Depth Information
number of species produce dextran, the best-documented exopolysaccharide formed
by heterofermentative LAB. In sourdoughs, the dextran-producing species
Weissella
cibaria
and
Weissella confusa
are most frequently found. Both taxa are positioned
together on one of the four phylogenetic branches in this genus based on 16S rRNA
gene analyses [
90
], but can be differentiated using restriction analysis of the
ampli fi ed 16S rDNA [
91
] , randomly ampli fi ed polymorphic DNA-PCR (RAPD-
PCR) [
92
], and PCR targeting the ribosomal ITS [
26
] . Within the facultatively het-
erofermentative pediococci, the species
Pediococcus acidilactici
and
Pediococcus
pentosaceus
are most commonly found in sourdoughs. Differentiation between
these two biochemically and phylogenetically related species can be achieved by
fingerprinting methods such as ribotyping [
93,
94
], restriction analysis of the
amplified 16S rDNA (16S-ARDRA) [
95
] , RAPD-PCR [
96,
97
] , and by sequence
analyses of the 16S rRNA gene, the ribosomal ITS regions and the heat-shock pro-
tein 60 gene [
98
]. In the obligately heterofermentative genus
Leuconostoc
, the
majority of sourdough isolates so far identified belong to
Leuconostoc mesenteroi-
des
and
Leuconostoc citreum.
In the former species, further taxonomic distinction is
made at subspecies level between
Le. mesenteroides
subsp
. mesenteroides
,
Le. mes-
enteroides
subsp
. dextranicum,
and
Le. mesenteroides
subsp.
cremoris
(
www.bacte-
rio.cict.fr/l/leuconostoc.html
). The three subspecies can be separated by RAPD-PCR
fi ngerprinting [
99
] .
5.2
Microbial Species Diversity of Sourdoughs
5.2.1
In fl uence of Geography
5.2.1.1
The Origin of Sourdough
Historically, sourdough production started as a
conditio sine qua non
to process
cereals for the production of baked goods [
100
]. Indeed, thousands of years ago the
first bread production must have been based on spontaneous wild lactic acid fer-
mentation whether or not associated with yeasts and with little or no leavening.
Leavening could not have been very pronounced because of the use of barley
(
Hordeum vulgare
) and ancient grains [such as spelt (
Triticum aestivum
subsp.
spelta
), emmer (
Triticum turgidum
subsp.
dicoccum
), and kamut (
T. turgidum
subsp.
turanicum
)] in early times and no addition of yeast for leavening. This form of flat
(sour) bread production is still daily practice in many countries of the world, in
particular in African countries and the Middle East. Leavening must have been an
accidental discovery when yeasts from the air or the flour were allowed to ferment
the cereal dough mixture extensively. However, it was only from the late nineteenth
century onwards that yeast starter cultures were introduced for bread production
from wheat (
T. aestivum
and
Triticum durum
) flour, first by using brewing yeasts as
remnants of beer production followed by intentionally produced commercial bak-
ers' yeast,
S. cerevisiae
. Consequently, sourdough bread must have been consumed
