Optimization of bacterial biomass production using soybean molasses-based medium as a potential component for bean crop inoculant development

This study aimed to optimize bacterial biomass production using a soybean molasses-based medium in shaken flask cultures as a potential component for bean crop inoculant development. Bradyrhizobium yuanmingense SEMIA 6462, Bradyrhizobium pachyrhizi SEMIA 6464, Rhizobium tropici SEMIA 4077, and Rhizobium freirei SEMIA 4080 were evaluated. B. yuanmingense SEMIA 6462 presented the highest biomass production, which was 5.48 g L⁻¹ after 72 h of cultivation. For this microorganism, a Plackett-Burman (PB) design with ten variables followed by a 2⁴ Central Composite Rotatable Design (CCRD) were proposed to optimize biomass production. The variables with the greatest influence were soybean molasses, sodium chloride, calcium chloride dihydrate, and yeast extract concentrations, with the pH adjusted to 7, while dibasic and monobasic potassium phosphates, magnesium sulfate heptahydrate, manganese chloride heptahydrate, and Tween 80 were removed from the medium. The best conditions established by an empirical model and validated by experimental assays were 100 g L⁻¹ soybean molasses, 0.8 g L⁻¹ yeast extract, 0.45 g L⁻¹ sodium chloride, and 0.3 g L⁻¹ dihydrate calcium chloride. Under these conditions, biomass production reached 38.6 g L⁻¹, representing a 604.93 % increase from the initial value (5.48 g L⁻¹) while also reducing the medium cost per kilogram of biomass by 70 %. The results demonstrate the efficacy of cultivation conditions in increasing rhizobia biomass production, highlighting the potential of soybean molasses as an alternative and sustainable substrate to produce microbial inoculants.