The role of carboxyl group number in hydroxy acid pretreatment for xylooligosaccharide production, enzymatic saccharification, and antioxidant lignin recovery from Gleditsia sinensis pods.

This study proposes a sustainable biorefinery strategy for converting waste Gleditsia sinensis pods into high-value fine chemicals using hydroxy acids, with the carboxyl group number (CGN) serving as a critical molecular design parameter. The results showed that increasing CGN changes the composition of XOS, increases XOS yield, promotes glucose conversion, and protects lignin structure. The highest xylooligosaccharide (XOS) yield of 62.88% was achieved with citric acid pretreatment (CGN = 3), which reduced furfural formation by 43.8% compared to lactic acid (CGN = 1), resulting in 98.05% glucose conversion. The lignin structure remained largely intact, with a retention rate of 53.04% per 100 Ar of β-O-4' bonds and phenolic hydroxyl content of 4.36 mmol/g. This preserved lignin structure enhanced antioxidant activity (75.38%) in DPPH radical scavenging compared to BHT. Besides, XOS produced via acid pretreatment at 150 °C for 30 min showed enhanced prebiotic effects, promoting the growth of Bifidobacterium adolescentis by 53% and increasing propionic acid production by 300% relative to commercial fructooligosaccharides. Structural analyses indicated that CGN protects lignin through intramolecular hydrogen bonding and chelation, thereby suppressing condensation. Overall, this environmentally sustainable biorefinery approach efficiently converts agricultural waste into high-value products, supporting advances in the food, feed, and materials sectors.