A greener, mild, and efficient bioprocess for the pretreatment and saccharification of rice straw

Abstract

Open rice straw burning has created hazardous effects on the environment and human health. Biological pretreatment of rice straw has been proven as an environmental-benign and economical feasible process for the production of biofuels and value-added products under milder and greener conditions by avoiding harmful chemicals and toxic reagents. A consolidated bioprocess for the production of cellulolytic enzymes and biological pretreatment was developed to achieve a cost-effective saccharification of rice straw. Biodegradation of biomass was slightly enhanced with increase in cultivation time of microbial pretreatment. Furthermore, the modification in biomass due to biological pretreatment weakened the interaction between hemicellulose and lignin that resulted in significant reduction in lignin using Bacillus subtilis subsp. subtilis JJBS300, Myceliophthora thermophila BJTLRMDU3, and Aspergillus oryzae SBS50 for biological pretreatment of rice straw. Bacterial culture produced maximum FPase, CMCase, and β-glucosidase of 28, 17, and 20.36 U/g DMR after 2 days during pretreatment process, whereas M. thermophila produced maximum FPase (85.10 U/g DMR), CMCase (96.89 U/g DMR), and β-glucosidase (91.92 U/g DMR) after 9 days. The mold A.oryzae SBS50 also produced maximum FPase (57.41 U/g DMR), CMCase (55.36 U/g DMR), and β-glucosidase (48.04 U/g DMR) after 9 days of pretreatment. Maximum amount of reducing sugars of 52.41, 86.74, and 49.59 mg/g substrate were liberated from 6-, 5-, and 3-day-old biological pretreated rice straw by B. subtilis, M. thermophila, and A. oryzae, respectively, after enzymatic hydrolysis for 6 h at 60 oC and pH 5.0. Laccase-pretreated rice straw followed by enzymastic saccharification resulted in liberation of 162.82 mg/g reducing sugars at pH 5.0 and 60 °C after 6h using 20 U/g cellulase. Simultaneous laccase pretreatment and saccharification (SLPS) process further enhanced the liberated reducing sugars, i.e., 179.47 mg/g substrate. FTIR, XRD, and SEM analyses showed marked morphological changes as a result of delignification after biological pretreatment of rice straw. Biological pretreatment being an environmental-benign process causing no harm to the environment in comparison to physical and chemical pretreatments of lignocellulosic biomass could be a better pretreatment strategy for bioremediation of lignocellulosic substrates.