Bioproduction of Ethanol in SHF and SSF from Cassava Stalks

Abstract

Cellulose biomass is being investigated as a potential substrate for bioethanol production. Cassava stalks were successfully converted to ethanol by fermentation using Saccharomyces cerevisiae TISTR5048, S. cerevisiae KM1195, S. cerevisiae KM7253 and co-culture of S. cerevisiae TISTR5048 and Candida tropicalis TISTR5045. The objective of this study was to assess the fermentable sugars production from cassava stalks by dilute-acid, dilute-base and distilled water pretreatment and enzymatic hydrolysis. Another objective was to study the conversion of the fermentable sugars into ethanol by mono-culture and co-culture of yeast strain of S. cerevisiae and C. tropicalis. Cassava stalks were milled to flour and dried overnight in hot-air oven. Cassava stalks at 1.5% (w/v) in 0.1 M sulfuric acid was pretreated for 30 min at 135°C under the pressure of 15 lb/in2. The pretreated cassava stalk suspensions were neutralized to pH 5.5 for saccharification process. The enzyme solution (cellulose, xylanase and pectinase solubilized in buffer pH 5.0) was used for hydrolysis of the pretreated cassava stalk at 50°C for 24 h. The hydrolysate was supplemented with additional nutrients. The culture was incubated at 30°C. The pretreatment of the stalk with dilute-acid hydrolysis resulted in 0.57 g/g sugar yield enzymatic hydrolysis, which was higher than dilute-alkaline-pretreated and distilled water-pretreated stalk. The sugar hydrolysate was bioconverted to ethanol with separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF). The highest ethanol yields of 98.43% and 95.29% were obtained in SHF and SSF, respectively by S. cerevisiae KM1195. The fermentation time of SSF process was 24–32 h shorter than that of the SHF (≈56 h), but the ethanol productions (5.42-6.22 g/L for SSF; 5.9-6.23 g/L for SHF) were not significantly different.