Generating Fermentable Sugars from Rice Straw Using Functionally Active Cellulolytic Enzymes from Aspergillus niger HO

Among the three Aspergillus spp. niger, (A. oryzae, and A. fumigatus) screened for cellulolytic enzyme production potential, A. niger produced cellulolytic enzyme in relatively higher concentrations than the other two isolates. Enzyme produced by all three isolates was optimally active at pH 5.0. Celluloses from A niger and A fumigatus were optimally active at 55 degrees C, while the enzyme from A oryzae showed optimum activity at 50 degrees C. Cellulose from A niger and A fumigatus retained more than 80 and 70% activity, respectively, while cellulose from A oryzae could retain only 20% activity at 55 degrees C after 12 h. Cellulose from A niger exhibited better stability at higher temperatures than the enzyme from the other two Aspergillus spp., showing half-life (t(1/2)) of about 5 and 3 h at 70 and 80 degrees C, respectively. Zymogram revealed multiple forms of endoglucanase, cellobiohydrolase, and beta-glucosidase with molecular mass ranging between 28 and 154 kDa for cellulose from all three isolates. Hydrolysis of rice straw at 12.5% (w/v) with crude cellulose from A. niger HO resulted in fermentable sugar concentration and productivity of 66.2 g L-1 and 2.75 g L-1 h(-1) respectively, showing potential for the reported enzyme in biofuel industry.