Partial characterization and dynamics of synthesis of high molecular mass exopolysaccharides from Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus

Exopolysaccharide (EPS) preparations from Lactobacillus delbrueckii ssp. bulgaricus (L. bulgaricus) strains LBB.B26 and LBB.B332 and Streptococcus thermophilus strains LBB.T54 and LBB.T6V were characterized using ion-exchange chromatography and gel filtration. All four preparations contained a neutral EPS with molecular mass in the range of 1.3−1.6 × 10⁶ Da (HMM-EPS). The EPS preparations from the two L. bulgaricus strains also contained an acidic low molecular mass EPS fraction (LMM-EPS) comprising from 10% to 34% of the total EPS yield. HMM-EPS preparations were subjected to High Pressure Liquid Chromatography (HPLC) analysis of monomer sugars after complete hydrolysis. Glucose, galactose and/or rhamnose in different ratios proved to be the principal sugars building the HMM-EPS from all four strains. The chemical composition of HMM-EPS was strictly strain-specific. The LMM-EPS contained galactose. The viscosifying properties of the four different HMM-EPS varied greatly with intrinsic viscosity in the range from 0.26 (strain B26) to 2.38 (strain T6V). For 24 h the two L. bulgaricus strains accumulated more HMM-EPS in milk (>70 mg l⁻¹) than S. thermophilus strains T54 and T6V (<30 mg l⁻¹), but maximal yields were reached earlier with cocci (8 h) than with rods (16–24 h). The contribution of HMM-EPS production to increased viscosity of fermented milk was demonstrated for all of the tested strains grown as monocultures or as mixed yogurt starters compared to non-EPS producing S. thermophilus LBB.A and poor EPS-producer L. bulgaricus LBB.B5. The extent of increased viscosity was strongly dependent on the nature of the produced HMM-EPS, rather than simply on polymer yield.