Sequential pretreatment of sugarcane bagasse by alkali and organosolv for improved delignification and cellulose saccharification by chimera and cellobiohydrolase for bioethanol production

Sequential pretreatments for sugarcane bagasse (scb) by NaOH followed by organosolv under mild conditions were evaluated for cellulose recovery and dilignification. The best-optimized sequential pretreatment of scb was obtained at 10% (w/v) of raw scb loading at 1% (w/v) NaOH (50 °C, 2 h) followed by treatment with organosolv (85%, v/v phosphoric acid, 50 °C, 1 h) with chilled acetone. This sequentially pretreated scb showed cellulose recovery, 66.1% (w/w) and delignification, 83.2% (w/w). NaOH or organosolv pretreated scb showed lower cellulose recovery 47.4% (w/w) or 54.5% (w/w) with lower delignification, 61% (w/w) or 56% (w/w), respectively. Pretreated solid residue of sequentially pretreated scb was enzymatically saccharified by chimera (β-glucosidase and endoglucanase, Ct GH1-L1- Ct GH5-F194A) and cellobiohydrolase ( Ct CBH5A) cloned from Clostridium thermocellum . Enzymatic hydrolysate of best sequentially pretreated scb gave total reducing sugar (TRS) yield, 230 mg/g and glucose yield, 137 mg/g pretreated scb. Only organosolv pretreated scb gave TRS yield, 112.5 mg/g and glucose yield, 72 mg/g of pretreated scb. Thus, sequentially pretreated scb resulted in 37% higher enzymatic digestibility than only orgnaosolv pretreated scb. Higher enzymatic digestibility was supported by higher crystallinity index CrI (45%) than those obtained with only organosolv pretreated (38%) or raw scb (25%). Field Emission Scanning Electron Microscope (FESEM) and Fourier-transform infrared (FT-IR) analyses showed enhanced cellulose exposure in sequentially pretreated scb. Preliminary investigation of bioethanol production at small scale by separate hydrolysis and fermentation (SHF) of enzymatic hydrolysate from best sequentially pretreated scb by Saccharomyces cerevisiae gave maximum ethanol yield of 0.42 g/g of glucose.