HIGH SOLIDS SIMULTANEOUS SACCHARIFICATION AND FERMENTATION OF PRETREATED WHEAT STRAW TO ETHANOL

Wheat straw was pretreated with dilute (0.5%) sulfuric acid at 140-degrees-C for 1 h. Pretreated straw solids were washed with deionized water to neutrality and then stored frozen at -20-degrees-C. The approximate composition of the pretreated straw solids was 64% cellulose, 33% lignin, and 2% xylan. The cellulose in the pretreated wheat straw solids was converted to ethanol in batch simultaneous saccharification and fermentation experiments at 37-degrees-C using cellulase enzyme from Trichoderma reesei (Genencor 150 L) with or without supplementation with beta-glucosidase from Aspergillus niger (Novozyme 188) to produce glucose sugar and the yeast Saccharomyces cerevisiae to ferment the glucose into ethanol. The initial cellulose concentrations were adjusted to 7.5, 10, 12.5, 15, 17.5, and 20% (w/w). Since wheat straw particles do not form slurries at these concentrations and cannot be mixed with conventional impeller mixers used in laboratory fermenters, a simple rotary fermenter was designed and fabricated for these experiments. The results of the simultaneous saccharification and fermentation (SSF) experiments indicate that the cellulose in pretreated wheat straw can be efficiently fermented into ethanol for up to a 15% cellulose concentration (24.4% straw concentration). Above this concentration, the cells lose their viability apparently because of ethanol inhibition. The maximum ethanol concentration achieved of 57 g/L with 20.2% straw concentration (12.5% cellulose) yielded the highest sugar productivity of 1.27 g/Kg.h ethanol productivity of 0.62 g/Kg.h along with a combined ethanol and sugar yield of 70%.