Production of a cellulolytic enzyme system in mixed-culture solid-state fermentation of soybean hulls supplemented with wheat bran

Abstract

Solid-state fermentation of soybean hulls supplemented with wheat bran using a co-culture of Trichoderma reesei and Aspergillus oryzae was performed. Three parameters — initial moisture content, incubation temperature, and initial pH — were optimized in culture flasks using response surface methodology. Parameter optimization was carried out with respect to filter paper activity and β-glucosidase activity in the culture. Temperature of 30 °C, pH of 5, and moisture content of 70% were found to be optimum. Optimized parameters were used for laboratory scale-up in static tray fermenters. The maximum filter paper activity of 10.7 FPU/g-ds and β-glucosidase of 10.7 IU/g-ds were obtained after 96-h incubation period in static tray fermenters in agreement with optimized activities at shake flask level. The results of static tray fermentation also highlighted the importance of mixed-culture fermentation. Both enzyme activities and volumetric productivities of enzyme produced were significantly higher in mixed-culture fermentation as compared to mono-culture static tray fermentation. Expression profile of cellulase system was characterized using SDS-PAGE and it indicated the presence of all the five major activities corresponding to β-glucosidase, CBH I, CBH II, EG I and xylanase. Enzyme broth was centrifuged and concentrated in an ultrafiltration cell. The concentrate was used for enzymatic saccharification of pretreated wheat straw and the potential of an indigenously developed enzyme concoction was reported in terms of saccharification efficiency. Pretreatment using both acid and alkali was carried out, and differences in sugar yield due to differences in composition as a result of pretreatment were reported. Results showed that alkali treatment generated higher sugars as compared to acid pretreatment. This was due to lignin removal and concentration of the cellulosic fraction. Present work showed that solid-state fermentation in a static tray bioreactor is a valuable technique for producing a system of enzymes with balanced activities that can efficiently saccharify lignocellulosic biomass like wheat straw.