CHARACTERIZATION OF EXOCELLULAR SLIME PRODUCED BY BACTERIAL STARTER CULTURES USED IN THE MANUFACTURE OF FERMENTED DAIRY PRODUCTS (YOGURT, SKIM MILK, ROPY)

Exopolysaccharide producing (ropy) strains of lactic acid bacteria were isolated from commercial starter culture and yogurts. The exopolymers were investigated by scanning electron microscopic analysis and found to be affiliated with the cell surface and the hydrated casein micelles in fermented skim milk. The monosaccharide composition of the exopolymers was determined by a gas chromatographic method and the monomers were found to be galactose and glucose in a 2:1 ratio. ('13)C-NMR spectra obtained for each of the polymers were similar. The spectra were comprised of 18 major peaks. Three of the signals were obtained in the region of the spectrum in which the anomeric carbon of the glycosyl residues is located. Coupled ('13)C-NMR spectra gave coupling constants for the anomeric carbon of the glycosyl residues which suggest that the first two glycosyl residues are attached via a (beta)-linkage and the third via an (alpha)-linkage. The accumulated data suggest that the polymers isolated from the individual species have similar chemical structures. Rheological evaluation of skim milk fermented with the ropy strains showed a decreased susceptibility to syneresis and an increased viscosity as compared to skim milk fermented with non-ropy strains. However, frozen yogurt manufactured using a ropy starter culture was not found to be significantly different in perceived texture as compared to one manufactured using a non-ropy starter culture.