Integrated hydrothermal and microbial valorization of almond shells into xylooligosaccharides and cellobionic acid

Abstract Almond shells are abundant lignocellulosic by-products with great potential for sustainable valorization. This study developed an integrated biorefinery process to produce xylooligosaccharides (XOS) and cellobionic acid (CBA) through hydrothermal pretreatment and fungal simultaneous saccharification and fermentation. Pretreatment under subcritical water conditions (165.9–194.1 ℃, 5.9–34.1 min) was optimized using response surface methodology and single-variable analysis. The optimal condition, selected to maximize XOS while minimizing xylose formation, was 180 ℃ for 8 min with 10% (w/v) solids, producing 18.1 ± 0.1 g/L XOS with a yield of 68.9 ± 0.3%. Low xylose (0.4 ± 0.0 g/L) and negligible glucose formation indicated selective oligomer production while preserving cellulose. The hydrolysate contained mainly short-chain XOS, with degrees of polymerization ranging from DP2 to DP10, and was particularly enriched in xylotriose and xylotetraose. Pretreated solids containing cellulose (15 g/L, ~ 46.3 mM) were converted to CBA using the engineered thermophilic fungus Thermothelomyces heterothallica TH-11 at 40 ℃ without enzyme addition. CBA production reached 31.8 ± 0.6 mM with a yield of 89.0 ± 0.5% based on cellulose consumed. These results demonstrate an environmentally friendly strategy for converting almond shells into value-added oligosaccharides and sugar acids, supporting circular bioeconomy development in the agri-food sector.