Experimental optimization of thermochemical pretreatment of sal (Shorea robusta) sawdust by Central Composite Design study for bioethanol production by co-fermentation using Saccharomyces cerevisiae (MTCC-36) and Pichia stipitis (NCIM-3498)

摘要

Production of bioethanol was attempted using sawdust of sal (Shorea robusta), a hardwood tree species known for its excellent quality timber. Sawdust, a lignocellulosic biomass waste is generated in plenty from the furniture industry. To the best of our knowledge, this is the first study that uses design of experiment (DoE) approach for enhancing total reducing sugar (TRS) from thermochemically pretreated sal sawdust (SS) biomass. Statistical optimization of thermochemical pretreatment was performed using Central Composite Design (CCD) tool of Response Surface Methodology (RSM) using three process parameters (biomass loading, chemical concentration and incubation time). It resulted in about 2.05-fold more reducing sugars (7.0 g L-1) at 121 °C, 1 bar pressure and 30 min incubation time as compared to OVAT approach (3.4 g L-1). Enzymatic hydrolysis carried out by commercial cellulase and pectinase (enzyme loading 5U/g biomass) enzymes facilitated liberation of 19.09 g L-1 TRS. The optimum delignification percentage obtained was 27.27 ± 0.20% under optimized thermochemical conditions. Delignification estimation studies were done using Folin-Ciocalteu (F-C) and phloroglucinol-HCl test. Further analysis involved Powder X-Ray Diffraction (PXRD), Scanning Electron Microscopy (SEM) analysis and Fourier-transform infrared (FTIR) data which clearly confirmed the breakdown of cellulose and hemicellulose, removal of lignin and morphological changes in the pretreated SS biomass. Fermentation using co-culture of Saccharomyces cerevisiae (MTCC-36) and Pichia stipitis (NCIM-3498) at 30 °C, pH 5.0 resulted in 9.43 g L-1 bioethanol concentration after 24 h with a conversion efficiency of 97%. These results indicate that sal sawdust can serve as a promising feedstock for bioethanol production.