Biomedical Engineering Reference
In-Depth Information
substrates with a high buffering capacity, for example whole wheat flour or bran,
allows the production of sourdough or sourdough products with a high concentration
of organic acids and a corresponding high total titrable acidity.
The response of sourdough lactic acid bacteria to high salt concentrations is also
species specific. Generally, obligate heterofermentative lactobacilli are more sensi-
tive to NaCl in comparison to other lactobacilli. For example, the heterofermentative
L. sanfranciscensis
and
L. pontis
are inhibited by 4% NaCl whereas
L. plantarum
and
L. amylovorus
tolerate up to 6% NaCl [
9,
10,
12
] .
7.3
Metabolism of Carbohydrates
Lactic acid bacteria belong to three metabolic categories: (1) obligately homofer-
mentative organisms, which ferment hexoses through the EMP (Embden-Meyerhof-
Parnas) pathway to lactate as the major end product of carbohydrate metabolism
(Fig.
7.1
). Pentoses are not fermented. Examples of obligately homofermentative
lactobacilli occurring in sourdough include
L. delbrueckii
,
L. acidophilus
,
L. farciminis
,
L. amylovorus
, and
L. mindensis
. (2) Obligately heterofermentative
organisms, which ferment hexoses and pentoses through the 6-PG/PK
(6-phosphogluconate/phosphoketolase) pathway and synthesize equimolecular
amounts of lactate and ethanol or acetate. CO
2
is additionally produced from hexo-
ses (Fig.
7.2
). Key examples of obligately heterofermentative lactobacilli in sour-
dough are
L. sanfranciscensis
,
L. rossiae
,
L. brevis
,
L. pontis
, and
L. fermentum
.
(3) Facultatively heterofermentative organisms, which ferment hexoses through the
EMP pathway, and pentoses and gluconate through the 6-PG/PK pathway. Examples
of facultatively homofermentative lactobacilli occurring in sourdough include
L. plan-
tarum
,
L. alimentarius
,
L. paralimentarius
, and
L. curvatus
[
13
] .
Homofermentative metabolism of hexoses under anaerobic conditions yields
2 mol ATP per mol hexose. In contrast, heterofermentative metabolism of hexoses
under anaerobic conditions yields only 1 mol ATP per mol hexose unless co-substrates
are present (Fig.
7.1
). Contrarily to other fermented foods where obligately homo-
fermentative species have the major role, obligately heterofermentative lactic acid
bacteria are dominant in sourdough fermentations [
6
]. Several factors determine the
dominance of heterofermentative strains: (1) the metabolism of maltose via maltose
phosphorylase activity, simultaneous fermentation of hexoses and pentoses through
the 6-PG/PK pathway, and the use of fructose and other substrates as an external
acceptor of electrons; (2) the optimal pH and temperature which often coincides
with the values of sourdough fermentation; (3) the capacity of showing alternative
phenotype responses and to markedly adapt under various environmental stresses;
and (4) the synthesis of a large spectrum of antimicrobial compounds [
6
] . The vari-
able sources of fermentable carbohydrates in sourdough determine phenotypic
responses that include the use of external acceptors of electrons, the preferential
and/or simultaneous use of nonconventional energy sources, and the interaction with
exogenous or endogenous enzymes from the flour.
