Rheological characteristics and supramolecular structure of the exopolysaccharide produced by Lactobacillus fermentum MTCC 25067.

Rheological properties and supramolecular structure of the exopolysaccharide (EPS) secreted by Lactobacillus fermentum MTCC 25067 were investigated. The critical concentration representing the lower-limit of the semi-dilute regime was estimated to be 0.71 g/L from the concentration dependence of zero-shear specific viscosity. The storage modulus (G') of a 20 g/L EPS solution was greater than the loss modulus (G″) at 0.1-25 Hz. Approximately linear increases in G' and G″ determined at a frequency of 1 Hz and a strain of 0.01 during cooling from 80 to 25 °C were an indication that the EPS did not undergo thermally-induced cooperative conformational transitions typical of gelling polysaccharides. Atomic force microscopy images revealed that EPS molecules were not completely dissociated into individual molecules in an aqueous solution but remained to form three-dimensional networks. The gel-like dynamic viscoelasticity of the 20 g/L EPS solution was thus attributed to the existence of supramolecular assemblies resulting from significant degrees of intermolecular association of the EPS in the solution.