Biomedical Engineering Reference
In-Depth Information
Glucansucrases and fructansucrases use one of the two constituting monosaccharides
of sucrose for poly- or oligosaccharide synthesis, the other monosaccharide, fructose
and glucose, respectively, accumulates in the fermentation substrate. Levansucrases also
release the a-galactosides melibiose, manninotriose, and manninotetraose from
raf fi nose, stachyose, and verbascose, respectively [
40
] . In
L. sanfranciscensis
LTH2590,
levansucrase is the only enzyme capable of sucrose and raffinose hydrolysis [
40,
98
] . In
L. reuteri
and
Lc. mesenteroides
, levansucrase, glucansucrase and sucrose phosphory-
lase constitute alternative pathways for sucrose (and raffinose) metabolism. Metabolism
by extracellular levansucrases is the preferred metabolic route for raffinose, stachyose,
and verbascose, presumably because the resulting a-galactosides have a smaller degree
of polymerization, which facilitates transport across the cytoplasmic membrane [
40
] .
Glucansucrase activity invariably accumulates fructose and thus supports acetate forma-
tion by heterofermentative lactic acid bacteria [
98
]. Because high quantities of acetate
have detrimental effects on the structure of wheat bread [
107
] , dextran-producing
Weissella
spp. that do not convert fructose to mannitol with concomitant acetate forma-
tion exhibited superior performance in baking applications when compared to
L. reuteri
or
L. sanfranciscensis
[
91,
92,
108
] .
A contribution of HoPS formation to stress resistance of lactic acid bacteria has
particularly been demonstrated for
L. reuteri
. Here, HoPS or fructo-oligosaccharides
increase survival at acid conditions during the stationary phase [
103
] , and improves
survival of freeze-dried
L. reuteri
during storage [
109
] . The protective effects of
levan and fructo-oligosaccharides are partially attributable to their specific interac-
tion with phospholipids of biological membranes [
109,
110
] . However, protective
effects are not limited to levan and fructo-oligosaccharides; dextran and b -glucan
also improved acid resistance and stationary phase survival [
86,
111
] .
Lactic acid bacteria producing HoPS from sucrose in laboratory medium
generally also produce HoPS during growth in sourdough [
39
] . HoPS concentrations
typically range from 1 to 8 g/kg dough [
39,
91,
92,
98,
112
]; more than 10 g/kg
were reported in optimized, pH-controlled fermentations [
113
] . Glucansucrases
and fructansucrases are extracellular enzymes, thus, their activity is governed by
ambient conditions rather than intracellular pH and substrate concentrations
(Fig.
7.12
). The optimum pH of levansucrases and glucansucrases of sourdough
lactobacilli ranges from 4.5 to 5.5, matching the pH profile of sourdough fermenta-
tions [
98,
99
]. HoPS yields in sourdough were maximized by pH-controlled fer-
mentations at a pH of 4.7 [
113
]. Because sucrose acts both as glycosyl-donor and
glycosyl-acceptor in HoPS formation, the sucrose concentration has a decisive
influence on the ratio of sucrose hydrolysis, oligosaccharide formation, and poly-
saccharide formation. Below 10 g/kg sucrose, sucrose hydrolysis is the predominant
reaction [
98,
114
]. Increasing sucrose concentrations initially increase the forma-
tion of levan over hydrolysis. After a further increase of the sucrose concentration,
sucrose increasingly acts as a glycosylacceptor and oligosaccharide formation is the
predominant reaction catalyzed by glucansucrases or fructansucrases [
98,
114,
115
] .
In sourdough fermentation with 100 g/kg sucrose,
L. sanfranciscensis
LTH2590
produced 11 g/kg fructo-oligosaccharides but only 5 g/kg levan [
98
] . High reuteran
yields from
L. reuteri
TMW1.106 were achieved in pH static fed-batch fermentations
