Enhanced sweet sorghum stalk to ethanol by fungus Mucor indicus using solid state fermentation followed by simultaneous saccharification and fermentation

Abstract

Sweet sorghum is one of the most promising energy crops for ethanol production. Fungal solid state fermentation (FSSF) of dry sweet sorghum stalk particles (DSSSPs) for ethanol production was conducted using fungus Mucor indicus and followed by simultaneous saccharification and fermentation of the residual solid without any pretreatment and addition of fresh microorganism cells. The effects of important variables including temperature (28, 32, and 36°C), moisture level (65, 75, 80, and 85%), initial fungal biomass concentration (0.001, 1, and 5g/L), and particle size (<80, 20–80, and >20mesh) on the yield of ethanol production by FSSF were investigated. The results showed that M. indicus was able to utilize almost all the glucose and fructose within 48h, whereas the maximum ethanol yield (0.48g produced ethanol/g consumed sugars) was achieved by FSSF at 32°C, 80% moisture, and particle size of 20–80mesh with 5g/L fungal biomass concentration. Moreover, simultaneous saccharification and fermentation of the stalk glucan (10, 25, and 50g/L) was performed at 32, 35, and 37°C with different cellulase and β-glucosidase enzymes loading for 48h. In the best case, 85.6% of ethanol yield was achieved when 50g glucan/L was saccharified using 15FPU cellulase and 30IU β-glucosidase per gram glucan and simultaneously fermented to ethanol at 37°C for 48h. The results indicated that the FSSF acted as a pretreatment stage and assisted the subsequent simultaneous saccharification and fermentation process of the residual solid, resulted in up to 4.3 times improvement in the ethanol production yield.