Characterization and Rheological Properties of a New Exopolysaccharide Overproduced by Rhizobium sp. L01

The exopolysaccharides produced by rhizobia play an important role in their biotechnological and bioremediation properties. The characteristics and properties of an exopolysaccharide produced by Rhizobium sp. L01 were investigated. Strain Rhizobium sp. L01 was identified as Rhizobium tropici and produced a high yield of exopolysaccharides (REPS-L01), reaching 22.8 g/L after 63 h of fermentation in a 5 L bioreactor with glucose as the carbon source. REPS-L01 was composed of glucose and galactose in a ratio of 2.95:1, carrying pyruvate, acetate, and succinate groups. REPS-L01 had good shear-thinning properties in aqueous solutions at various concentrations and revealed typical non-crosslinked polymer properties. REPS-L01 revealed thermal stability up to 275 °C. REPS-L01 had the potential to be thicker, being suitable for use under conditions ranging from 4 to 60 °C, pH between 2 and 12, and salt concentrations up to 20,000 mg/L. REPS-L01 showed strong emulsifying activity, particularly with n-hexane; even at concentrations as low as 0.25 wt%, the emulsification index could reach more than 50%. Even more impressively, stable n-hexane emulsion gel was formed with 2 wt% REPS-L01 solution. Rheological studies showed that the solid-like emulsion gel had a high storage modulus, and the SEM studies of the emulsion gel indicated that n-hexane could fill the pores of REPS-L01.