Process development for sodium carbonate pretreatment and enzymatic saccharification of rice straw for bioethanol production

Abstract

Optimization of pretreatment of rice straw was carried out with sodium carbonate. Initially, sodium carbonate pretreatment of rice straw followed by saccharification using partially purified cellulolytic enzymes of thermophilic mould Myceliophthora thermophila BJTLRMDU3 (20 U/g) at 60 °C and pH 5.0 after 6 h, released about 3-fold more reducing sugars with autoclaving (154.93 mg/g substrate) as compared to room temperature (51.01 mg/g substrate). Further optimization was carried out using statistical tools by response surface methodology (RSM) selecting three variables i.e., sodium carbonate concentration, substrate concentration and autoclaving time. Statistical optimization has resulted in enhanced liberation of reducing sugars i.e., 531.2 mg/g substrate using 5% rice straw, 1.25% sodium carbonate and autoclave time of 30 min after saccharification for 6 h. Furthermore, maximum liberation of reducing sugars (728.45 mg/g substrate) from pretreated rice straw was recorded after 48 h. Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) data clearly confirmed the breakdown of cellulose and hemicellulose, removal of lignin and morphological changes in pretreated rice straw. Fermentation of enzymatic hydrolysate (20%) was carried out by Saccharomyces cerevisiae at 30 °C, pH 6.0, 150 rpm that resulted in 18.07 g/L ethanol after 72 h.