Enhancing the valorization efficiency of Camellia oil extraction wastes through sequential green acid pretreatment and solid-state fermentation-based enzymatic hydrolysis

This study focuses on the underutilized Camellia oleifera oil extraction wastes, namely C. oleifera shell (COS) and cake (COC), which are often overlooked due to their high recalcitrance and antimicrobial properties. COS was found to be an ideal substrate for producing on-site enzymes through solid-state fermentation (SSF) that can be used for lignocellulosic biomass degradation. Additionally, pretreating COS with 2 % (w/v) oxalic acid greatly enhanced sugar production, achieving 9.22 and 5.24 times more sugar compared to room temperature and thermal pretreatment methods, respectively. The highest sugar of 28 g/L was achieved by utilizing a mixture of COS and COC. Enzymatic hydrolysis performance was evaluated through three routes. In the first two routes, the whole pretreated slurry was employed, except the pH was adjusted in route 2. In route 3, the pretreated hydrolysate was recovered, while the solid residue underwent further enzymatic hydrolysis. Ultimately, route 3 resulted in the highest sugar of 20.58 g/L, using SSF enzymes and oxalic acid pretreated solid residues of COS and COC mixture. This value is twice the performance achieved using a single substrate of COS. The fine-tuning strategy employed in route 3 led to (1) achieving high sugar recovery from pretreatment and enzymatic hydrolysis, (2) minimizing water and alkaline regulator consumption, and (3) optimizing resource utilization efficiency. The effectiveness of this approach was also attributed to the synergistic effect of oxalic acid pretreatment and the use of mixed substrate. The possible mechanisms underlying these processes were discussed, offering potential directions for future studies aimed at developing even more efficient approaches.