Rice straw saccharification using cellulolytic cocktail from Aspergillus tubingensis and structure alterations studies of the wall polymer

Abstract

Optimization of saccharification of rice straw plays a crucial role in harnessing maximum fermentable sugars through enzymatic hydrolysis of this complex polymer. The present study was aimed to evaluate the applicability of A. tubingensis NKBP-55 cellulolytic cocktail for generation of fermentable sugars and to carry out parametric optimization of the saccharification process followed by batch ethanolic fermentation of the rice straw hydrolysate. Also, evaluation of the alterations (composition, crystallinity and structural morphology) occurring in the wall polymer structure after sequential treatments was studied. The cellulolytic cocktail, under the statistically optimized conditions (rice straw 4% w/v and pH 5.0) liberated 578 mg fermentable sugars (glucose 463 mg, xylose 114.35 and cellobiose 0.71 mg) per gram of pretreated rice straw showing 2.17-fold enhancement. Candida shehatae NCIM 3501 utilized both glucose and xylose and produced 16.15 g/L bioethanol along with yield (YP/S) 0.50 after 24 h of fermentation. Its bioethanol production was observed to be better than Saccharomyces cerevisiae (13.39 g/L) and their co-culture (15.57 g/L). Sequential changes brought about by the enzymatic treatment of the solid substrate revealed by X-ray diffraction and FTIR analyses showed decrease in the absolute crystallinity (CrI) (54.80 to 48.31%) and total crystallinity index (TCI) (1.002 to 0.991) and confirmed the degradation of the crystalline domain of cellulose by enzyme. The study demonstrated the potential of cellulolytic cocktail produced by A. tubingensis NKBP-55 in rice straw saccharification and its efficient fermentation by conventional and xylose-fermenting yeast in batch ethanolic fermentation.