Production of fatty-acyl-glutamate biosurfactant by Bacillus subtilis on soybean co-products

•Fermentation of soybean co-products produced less FA-Glu than glucose-based medium.•Enzymatic hydrolysis converted 65% of soybean hull (SBH) solids into soluble sugars.•Fermenting SBH hydrolysate (solid-free) produced 60% more FA-Glu titers than control.•SBH hydrolysate is a potential feedstock for biosurfactant production. Fatty-acyl-glutamate (FA-Glu), a surfactin variant has been successfully produced using a genetically modified strain of Bacillus subtilis grown on glucose. However, yields with soybean hulls (SBH) replacing glucose were lower. This work was undertaken to reduce the yield loss when using SBH as the carbon source and to evaluate two other soy by-products, namely fiber and skim from aqueous oil extraction as alternative carbon and nitrogen sources. Fermentation of soybean hulls, fibers and skim at various concentrations produced lower FA-Glu titers compared to S-7 medium. Neither increasing their amount nor supplementing with glucose increased the FA-Glu titer, suggesting the presence of an inhibitor in these feedstocks. By using a mixture of polysaccharide-degrading enzymes, over 65% of SBH solids were converted to soluble carbohydrates. FA-Glu titers obtained from SBH hydrolysates containing residual hull solids were still low; however, with the removal of the solids, cell growth improved and FA-Glu yield was 60% higher than with glucose. Thus, this low-cost material can be converted to a substrate for production of FA-Glu biosurfactant. Unmodified fiber and skim components of aqueous oil extraction were not beneficial.