Structural characterization and antioxidant property of released exopolysaccharides from Lactobacillus delbrueckii ssp. bulgaricus SRFM-1

Three released exopolysaccharide fractions (r-EPS1, r-EPS2 and r-EPS3) were isolated from the fermented milk of Lactobacillus delbrueckii ssp. bulgaricus SRFM-1 and purified by anion exchange chromatography, and characterizations of the structures were conducted. The r-EPS1 and r-EPS2 were homogenous with the average molecular weights of 3.97 × 105 Da and 3.86 × 105 Da, respectively. Three r-EPS fractions were composed of galactose and glucose with a molar ratio of 1.23: 1.00, 1.33: 1.00 and 1.00: 1.34, respectively. Structural characterization indicated that the r-EPS1 contained a backbone of →6-β-d-Galp-(1 → 4)-β-d-Glcp-(1 → 4)-α-d-Galp-(1 → 4)-β-d-Galp-(1 → 6)-β-d-Galp-(1 → 4)-β-d-Glcp-(1 → 4)-α-d-Galp-(1 → 4)-β-d-Galp-(1 → 4)-α-d-Glcp-(1→, and had three branching points which existed in terminal with D-Glcp residues with α/β-d-(1 → 6) linkages. The r-EPS2 was composed of →6-β-d-Galp-(1 → 4)-β-d-Glcp-(1 → 6)-α-d-Galp-(1→ as the backbone chain with a branching point which also existed in terminal D-Glcp residue with β-(1 → 6) linkage. In addition, three r-EPS fractions exhibited strong scavenging activities on superoxide radical, hydroxyl radical, DPPH radical and chelating activity on ferrous ion, and their antioxidant activities decreased in the order of r-EPS1 > r-EPS2 > r-EPS3.