Bioprocess engineering as a tool to modulate the rheological performance of Ensifer meliloti SEMIA 135 exopolysaccharide

The controlled production of microbial polysaccharides offers a powerful route to design renewable materials with tailored functionalities. In this work, the influence of bioreactor hydrodynamics on the structure and macroscopic properties of the exopolysaccharide produced by Ensifer meliloti SEMIA 135 was systematically investigated. Aeration (0.8-2.2 vvm) and agitation (59-341 rpm) were varied according to a Central Composite Rotatable Design, revealing a strong interaction effect on polymer composition and viscosity. Variations in cultivation conditions modulated the apparent viscosity from 68.7 to 228.3 Pa·s, correlated with a variation in total uronic acid content from 2.91% to 11.96%, confirming the key role of charge density in defining molecular organization. The polymer formed weak, pseudoplastic, and thixotropic gels, with the storage modulus (G') consistently higher than the loss modulus (G″) across all frequencies. Its viscoelastic profile was highly dependent on concentration (0.01-3.0 wt%), pH (3-9), and temperature, showing a sol-gel transition near 70 °C and enhanced elasticity at acidic and alkaline pH. Deviations from the Cox-Merz rule indicated the presence of a structured associative network rather than a simple entangled solution. Overall, this study demonstrates that fine-tuning of cultivation conditions can be used as a predictive tool to control the molecular architecture and rheological performance of bacterial polysaccharides, establishing a rational bioprocessing framework for the development of functional biomaterials.