Sustainable production of exopolysaccharides from quinoa stalk hydrolysates using halotolerant Bacillus swezeyi: fermentation kinetics and product characterization

Microbial exopolysaccharides (EPSs) have attracted increasing attention due to their versatile applications across diverse areas. However, large-scale production of EPSs remains challenging due to the high production costs, primarily driven by the use of synthetic carbon sources. This study demonstrates the potential of quinoa stalk hydrolysates as a sustainable alternative for EPS production using a halotolerant bacterial strain that was isolated from a hypersaline environment and termed SU4M. The bacterial isolate was identified through 16S rRNA and gyrB sequencing as a Bacillus swezeyi strain, and was then cultivated in quinoa stalk hydrolysates. The hydrolysates were produced by acid-catalyzed hydrothermal pretreatment using either sulfuric acid or phosphoric acid, followed by enzymatic saccharification. Fermentation experiments conducted in both shake flasks and bioreactors demonstrated that B. swezeyi SU4M utilized glucose from the hydrolysates efficiently, resulting in significantly higher biomass (5.1 ± 0.1 g L⁻¹) and EPS production (1.2 ± <0.1 g L⁻¹) compared to synthetic media (4.3 ± 0.1 g L⁻¹ and 1.1 ± <0.1 g L⁻¹). The kinetic analysis revealed distinct substrate consumption rates and growth patterns, with hydrolysates enhancing EPS yields under single-pulse fed-batch conditions. Advanced characterization techniques, including compositional analysis, Fourier transform infrared (FTIR) spectroscopy, ¹H and ¹H-¹³C heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR), high-performance size-exclusion chromatography (HPSEC), and thermogravimetric analysis (TGA), confirmed that the EPSs derived from hydrolysates were heteropolysaccharides with close structural similarities to those obtained from synthetic media. These findings underscore the potential of quinoa stalk hydrolysates as a biobased alternative to synthetic media as a substrate for EPS production.