Alkali Pretreatment of Rice Straw to Enhance Enzymatic Hydrolysis for Bioethanol Production

Abstract

The study was conducted to evaluate the effect of alkali pretreatment of rice straw on enzymatic hydrolysis for bioethanol production. The experiments conducted were divided into two studies. For Study 1, rice straw was treated with different concentrations (0.5%, 1.0%, and 1.5%) of sodium hydroxide (NaOH) and steeping durations (15, 30, and 45 hours). Alkali pretreated rice straws were subjected to enzymatic hydrolysis using cellulase complex (NS22074). In the second study, untreated rice straw as well as rice straw treated with NaOH (conditions as determined in the first study), were subjected to enzymatic hydrolysis using three different enzyme combinations of cellulase complex (NS22074), xylanase (NS22036), and hemicellulase (NS22002), and yeast fermentation using Saccharomyces cerevisae. Results of Study 1 revealed that NaOH concentrations and steeping durations significantly affect the enzymatic hydrolysis of rice straw to fermentable sugars. Rice straws treated with 1.5% NaOH solution steeped for 45 hours had the highest reducing sugar of 0.5402 g and conversion efficiency at 56.85%. Thus, these conditions were used in Study 2. In this study, it was found that pretreatment and different enzyme combinations have a highly significant effect on enzymatic conversion of rice straw to fermentable sugars. Alkali pretreated rice straw hydrolyzed using three enzyme combinations—cellulose complex, xylanase, and hemicellulase—obtained the highest reducing sugar of 0.7396 g and conversion efficiency at 77.88%. Alkali pretreatment, specifically sodium hydroxide (NaOH) at 1.5% concentration, and steeping for 45 hours of rice straw was an effective pretreatment to enhance enzymatic hydrolysis in terms of increase in reducing sugar and conversion efficiency. Similarly, alkali pretreatment of rice straw had significant advantage over untreated rice straw in enhancing the effect of enzymes cellulase complex, xylanase, and hemicellulase in enzymatic hydrolysis. After yeast fermentation using Saccharomyces cerevisiae, ethanol yield of the study ranged from 0.0354 g to 0.0781 g/g glucose, or 41.18 to 51.51% of the maximum theoretical yield.