Enzymatic hydrolysis of soybean hulls to produce a potential carbon source for cultivation of exopolysaccharide-producing diazotrophic bacteria

Abstract

Lignocellulosic biomass, such as soybean hulls, constitutes a promising alternative substrate for application in biotechnological processes, as it is abundant, inexpensive, and renewable. However, its use is limited by the inability of bacteria and yeasts to assimilate it. To solve this problem, pretreatment and enzymatic hydrolysis conditions of soybean hulls were studied to produce a substrate capable of assimilating by exopolysaccharide-producing diazotrophic bacteria. Pretreatments by microwave (power of 1.4 kW during 30 s, 60 s, or 120 s) or autoclave (1.1 atm and 121 °C during 15 min, 30 min, or 60 min) were tested to favor the enzymatic action. Celluclast 1.5L® was used as a biocatalyst. The best results for pretreatment were obtained in an autoclave (121 °C for 15 min), with an increase of 6.7 times in the concentration of reducing sugars (40.4 g/L) in relation to hydrolysis without any previous pretreatment (6.0 g/L). The best hydrolysis conditions were established as a temperature of 50 °C, pH 5.3, and enzyme/substrate ratio of 20 FPU/g, reaching 40.4 g/L of reducing sugars. The hydrolysate was composed mainly of cellobiose, followed of glucose, xylose, and arabinose. It was used for the cultivation of Mesorhizobium sp. Semia 816, producing 1.26 g/L biomass and 3.93 g/L exopolysaccharides (EPS). An increase in viscosity with a pH decrease was observed, as well as a high degradation temperature. These results proved that soybean hull hydrolysate can be assimilated by Mesorhizobium sp. Semia 816 to produce EPS with potential industrial applications.