Biomedical Engineering Reference
In-Depth Information
when the activity of the enzyme hexokinase is not induced [
37
] . The accumulation of
glucose was not found for some strains of
L. sanfranciscensis
that were cultivated in
the presence of maltose and fructose. Therefore, it was hypothesized that once liber-
ated the glucose is used through the activity of the enzyme hexokinase, which is in
turn induced by the presence of fructose [
38
] .
Some strains of
L. sanfranciscensis
have the capacity to express b -glucosidase
activity, but this activity is repressed by glucose [
41
] . Lactobacilli from sourdough
also show simultaneous rather than consecutive fermentation of pentoses and hexo-
ses. Compared to growth on maltose as the only energy source,
L. alimentarius
15 F,
L. brevis
10A,
L. fermentum
1 F e
L. plantarum
20B showed the highest
growth and acidification rates, and the highest cell yield when cultivated in the pres-
ence of xylose, ribose, and arabinose [
42
]. Other sourdough lactic acid bacteria
showed the highest performance in terms of growth and synthesis of acetic acid
when cultivated in the presence of a mixture of pentose carbohydrates [
43
] . The
presence of pentoses induces the synthesis of the phosphoketolase enzyme in facul-
tatively heterofermentative lactic acid bacteria (e.g.,
L. plantarum
) and allows the
second half of the 6-PG/PK pathway to proceed (Fig.
7.2
). The selection of lactoba-
cilli with the capacity to ferment pentose carbohydrates is thus a suitable alternative
to sucrose addition to increase acetate formation.
7.4
Proteolysis and Catabolism of Free Amino Acids
7.4.1
Proteolysis
Lactic acid bacteria are characterized by multiple amino acid auxotrophies. Lactic
acid bacteria depend on substrate-derived proteases or on the activity of their
proteolytic system to satisfy the nitrogen metabolism [
44
] . The proteolytic sys-
tems of lactic acid bacteria includes serine cell-envelope-associated proteinase
(CEP - PrtP) which is associated to the cell wall, oligopeptide and amino acid
transporters, and a large number of intracellular peptidases [
45
] (Fig.
7.5
).
Contrary to lactic acid bacteria in dairy fermentations,
L. sanfranciscensis
ATCC
27651
T
and most other sourdough lactobacilli do not possess a cell-envelope-
associated proteinase and depend on cereal-associated proteases [
46,
47
] . The
comparison between sourdoughs and chemically acidified doughs showed that the
degradation of the native proteins is mainly due to the activity of cereal endoge-
nous proteinases [
46,
48,
49
]. The acidification by sourdough lactic acid bacteria
favors the activation of aspartate-proteinases from cereals, which have an optimal
pH of activity that ranged between 3.0 and 4.5 [
50
] . Nevertheless, selected strains
of sourdough lactobacilli showed the capacity to hydrolyze albumins, globulins,
and gliadins during fermentation [
51-
53
]. Oligopeptides (4-40 amino acids) are
transported inside the bacterial cell and hydrolyzed through a complex system of
peptidases. An overview of the intracellular peptidases of
L. sanfranciscensis
is
shown in Fig.
7.6
. Several intracellular peptidases of
L. sanfranciscensis
CB1
were biochemically characterized: a 65-kDa metal-dipeptidase (PepV), with
