From shake flasks to a bench-scale bioreactor: scaling up exopolysaccharide production in fed-batch cultivation of Ensifer meliloti SEMIA 135

Abstract

BACKGROUND

This study investigates the production scale-up of exopolysaccharide (EPS) by Ensifer meliloti SEMIA 135 from shake flasks to a 5 L bench-scale fed-batch bioreactor. Soybean molasses was the cost-effective nutrient source. The primary aim was to improve conditions for maximizing EPS yield and minimizing production costs.

RESULTS

Various molasses concentrations (30 to 50 g L⁻¹), with and without yeast extract and mineral salts, were tested in shake flasks. Agitation and operational modes (batch and fed-batch) were then evaluated in the bioreactor. Results showed that yeast extract is unnecessary (a cost-reduction factor) since soybean molasses supplemented with mineral salts achieved similar EPS production. The highest molasses concentrations inhibited the EPS synthesis. Best-performing bioreactor conditions (200 rpm, 1.25 vvm) in the fed-batch mode yielded 14.4 g L⁻¹ EPS. It meant increase of 67% by comparison with the batch mode. EPS exhibited high molecular weight (9.266 × 10⁷ Da), polyanionic character (zeta potential −36.03 mV at pH 7) and functional groups which are typical of rhizobial EPS. Its rheological behavior followed the Herschel-Bulkley model, i.e., viscosity decreased at high pH, and it showed remarkable thermal stability (melting point was 203 °C).

CONCLUSION

This study highlights the potential of soybean molasses in cost-effective EPS production. Fed-batch conditions improved EPS yields significantly. EPS properties underline its potential for industrial applications and, thus, generation of knowledge about scalable biopolymer production. © 2025 The Author(s). Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).