Bioconversion of Beet Molasses to Exopolysaccharides by High-Sucrose-Tolerant Lactic Acid Bacteria: Strain Screening, Fermentation Optimization and In Vitro Bioactivity Evaluation

Production of functional exopolysaccharides (EPS) by lactic acid bacteria (LAB) is a promising strategy for the value-added utilization of beet molasses. This study aimed to isolate high-sucrose-tolerant LAB strains from beet molasses and evaluate their potential for crude EPS production. A total of 13 LAB strains were initially isolated from beet molasses, among which seven isolates were selected as high-sucrose-tolerant candidates based on their growth performance under high-sucrose conditions. Further crude EPS-production screening showed that two isolates, Leuconostoc mesenteroides C4 and Pediococcus pentosaceus T1, produced relatively high crude EPS yields. These two isolates were then identified by 16S rRNA gene sequence analysis. Further, L. mesenteroides C4 was selected as a candidate, and preliminary OFAT screening followed by Box–Behnken response surface methodology (RSM) was employed to optimize the conditions for crude EPS production. The antioxidant activity and antibiofilm activity of the crude EPS-C4 fraction were also evaluated. The RSM-predicted optimal conditions for crude EPS production by L. mesenteroides C4 were 350 g/L of beet molasses, an initial pH of 6.0, a fermentation time of 30 h, a fermentation temperature of 37 °C, an inoculum size of 2.3%, a loading volume of 85%, and a shaking speed of 108 r/min. Under these optimal conditions, a maximum crude EPS yield of 16.99 ± 0.22 g/L was obtained, which was approximately 5.0-fold higher than the yield obtained under the initial fermentation condition (3.40 g/L). Furthermore, crude EPS-C4 exhibited in vitro free-radical-scavenging activity against DPPH and ABTS+ radicals, with scavenging rates of 97.00% and 67.35% at 6.0 mg/mL, respectively. The crude EPS-C4 fraction also inhibited biofilm formation by Escherichia coli and Staphylococcus aureus, with inhibition rates of 60.68% and 68.69% at 8.0 mg/mL, respectively. These findings provide preliminary evidence that beet molasses can be used as a substrate for LAB-EPS production and suggest that crude EPS-C4 is a promising candidate for further purification, structural characterization, safety assessment, and application-oriented evaluation.