Platform Study on the Development of a Nondetoxified Rice Straw Hydrolysate to Its Application in Lipid Production from Mortierella alpina

In the present work, a mild acid saccharification approach for rice (Oryza sativa) straw was developed by response surface optimization. A 2(3) central composite experimental design with variable factors, such as duration (minutes), mild acid concentration percentage (v/v) and minimum solid loading percentage (w/v) was chosen to optimize the hydrolysis process, fixing the operating temperature to a moderate minimum of 121 degrees C. The study determined a solid loading of 5%, an acid concentration of 0.75%, and a residence duration of 150 min to be optimum, for enhanced reducing sugar release. To compensate for the reduced saccharification efficiency under mild operating conditions, a novel approach of multitier saccharification was implemented under optimal conditions of saccharification. High-performance liquid chromatography (HPLC) analysis revealed the final reducing sugar yield of 0.76 g reducing sugar per gram of straw (47.9 g/L and 35 g/L concentration in two consecutive saccharification cycles) and glucose, galactose, and xylose as major reducing sugars in hydrolysate. On assessing the hydrolysate as a fermentable substrate with and without detoxification for oleaginous strain Mortierella alpina, nondetoxified hydrolysate surpassed detoxified hydrolysate for growth support, sugar consumption, and lipid accumulation. HPLC analysis indicated the complete absence of furfurals and hydroxymethylfurfurals in the nondetoxified hydrolysate. Therefore, the rice straw hydrolysate produced by multitier mild acid saccharification approach was suitable for microbial propagation without requiring detoxification, suggesting it to be a potential fermentable substrate for microbial lipid production, as well as for other prospective bioprocesses.