A Study on the Valorization of Rice Straw into Different Value-Added Products and Biofuels

This work depicts that rice straw (RS), which is one of the major lignocellulosic wastes all over the world and causing many environmental problems, has considerable amounts of protein, ash, macronutrients, and micronutrients of approximately 11.38%, 16.77%, 2.27 mg/kg, and 771.9 mg/kg, respectively; besides, a C/N ratio of 15.18, a total N, P₂O5, and K₂O content of 1.85%, and a considerably low concentration of undesirable heavy metals and silica of approximately 77.69 mg/kg and 109 mg/kg are also present, which recommends its applicability as a precursor feedstock for the production of organic fertilizer and animal fodder. The batch solid-state fermentation (SSF) of RS by Trichoderma longibrachiatum DSMZ 16517 produced considerable amount of total reducing sugars (TRS) of approximately 339.2 mg TRS/g RS under the optimum operatic conditions of 20% (w:v) substrate concentration, pH 7, 1% inoculum size, a 9-day incubation period, and 30°C incubation temperature. The readily available and cost-effective agroindustrial waste, sugarcane molasses, proved to enhance the fungal biomass growth and (hemi) cellulolytic enzymes activities. The inoculated RS-SSF batch process with T. longibrachiatum precultured on 10% molasses enhanced the (hemi) cellulolytic enzymatic activities and TRS production rate by approximately 5.82 and 3.8 folds, respectively, relative to that inoculated by T. longibrachiatum precultured in the conventional potato dextrose broth medium. The separate hydrolysis and fermentation processes by different yeast strains Candida tropicalis DSM 70156, C. shehatae ATCC 58779, and Saccharomyces cerevisiae ATCC 64712 revealed an efficient bioethanol yield and productivity that ranged between 0.36 and 0.38 g/g sugars and 0.22 and 0.23 g/L/h, respectively, with concomitant competent fermentation efficiencies that ranged between 48.35% and 51.25%. The proximate analysis of rice straw before and after fungal hydrolysis proved calorific values of approximately 15.8 MJ/kg and 16.05 MJ/kg, respectively, recommending their applicability as primary and secondary solid biofuels. Thus, this study proved the waste prosperity of RS for environmental opulence and sustainability.