Physiology and Biochemistry of Lactic Acid Bacteria - Sourdough Biotechnology - page 194

2

1

NH 3 + α -ketobutyrate + cysteine homocysteine + pyruvate + NH 3

cystathionine

3

Fig. 7.11 Cystathionine metabolism (Adapted from [ 169 ] ). 1 Cystathionine- g -lyase, 2 cystathio-
nine-b -lyase, 3 cystathionine- b -synthetases

7.5 Synthesis of Exopolysaccharides

The formation of exopolysaccharide by lactic acid bacteria during sourdough
fermentation improves culture survival and stress resistance, impacts dough rheology
and bread texture, and allows production of bread with specific nutritional functionality.
EPS from lactic acid bacteria can be categorized on the basis of their composition or
their biosynthesis. EPS composed of only one type of constituting monosaccharide are
classified as homopolysaccharides (HoPS). Examples include levan, which is composed
of fructose, or dextran, which is composed of glucose. Heteropolysaccharides (HePS)
are composed of two or more constituting monosaccharides. Lactic acid bacteria employ
two alternative routes for EPS synthesis, intracellular synthesis from sugar nucleotides
by glycosyltransferases, and extracellular synthesis from sucrose by glucansucrases or
fructansucrases. EPS produced by glucansucrase or fructansucrase activity are invari-
ably HoPS composed of glucose or fructose, respectively [ 80, 81 ] . EPS produced by
intracellular glycosyltransferases predominantly are HePS with glucose, galactose,
N-acetylglucosamine, N-acetylgalactosamine, rhamnose, or fucose as constituting
monosaccharides [ 82- 84 ]. Noticeable exceptions of HoPS produced by intracellular
glycosyltransferases include a galactan from Lc. lactis [ 85 ] and b-glucan produced by
Pediococcus parvulus and Oenococcus oeni [ 86 ] .
Large-scale screening of strain collections of sourdough lactic acid bacteria
revealed that very few isolates are capable of HePS formation [ 39, 87- 89 ] ; only one
report documents formation of HePS in sourdough [ 90 ]. In contrast, HoPS forma-
tion is a frequent trait of sourdough lactic acid bacteria and any sourdough is likely
to contain at least one HoPS-forming strain [ 39, 88 ]. Moreover, the in situ formation
of HoPS during sourdough fermentation as well as the effect of HoPS on bread
quality is well documented ([ 91, 92 ]), for a review see [ 93 ] and Chap. 8 ) . Subsequent
paragraphs will hence only briefly discuss HePS formation and focus on HoPS
formation and applications.

7.5.1 EPS Biosynthesis and HoPS Structure

HePS formation by lactic acid bacteria is mediated by large gene clusters that are
encoded on plasmids or the chromosome (for reviews, see [ 82- 84, 94 ] ). EPS gene
clusters generally code for proteins regulating EPS biosynthesis (EpsA in
Streptococcus thermophilus Sfi6), polymerization, export, and chain length
determination (EpsB, EpsC, and EpsD in S. thermophilus Sfi6), and one or several