Structural Characterization of Mixed Rice Straw and Deoiled Algal Cake-Based Substrate as a Potential Bioenergy Feedstock for Microbial Lipids and Carotenoid Production

Open-field burning of rice straw contributes significantly to deterioration of air quality. Similarly, defatted algal residue is a surplus by-product of the algal biofuel industry. This study emphasizes on the two-step conversion of agro-industrial residues (rice straw and Scenedesmus obliquus deoiled biomass) into biodiesel and carotenoids using oleaginous red yeast Rhodotorula mucilaginosa R2. The effect of biological pretreatment on degradation of lignocellulosic substrates followed by catalytic saccharification was evaluated using suitable analytical techniques. Mixed biomass favoured the growth and secretion of extracellular cellulose degrading enzymes by Penicillium citrinum PKB20 resulting in the highest specific activity of endoglucanase (83.02 IU/mg), xylanase (103 IU/mg) and β-glucosidase (13.8 IU/mg) under solid-state fermentation. The hydrolysis of pre-treated substrate was achieved with a saccharification yield of 41.41 %. Batch fermentation of detoxified hydrolysate amended with synthetic yeast propagation or lipid production media (YEPD or C/N 75) resulted in maximum lipid content and lipid yield of 30.43 % and 1.19 g/L respectively. GC analysis revealed that oleic acid (29.4-50.39 %) was the predominant fatty acid. The crude carotenoid extract was composed of torularhodin, torulene and β-carotene and exhibited strong antioxidant activity.