Enhanced Microwave-Assisted Xylooligosaccharide Production from Corn Stover via Ball Milling and Citric Acid-Induced Dielectric Regulation

Abstract

This work targets xylooligosaccharide (XOS) production from corn stover, yet conventional hydrothermal methods yield low DP2–6 XOS selectivity. Short-chain DP2–6 XOS exert favorable prebiotic effects, while existing processes fail to enrich this fraction efficiently. In this study, an integrated strategy combining ball milling (BM) pretreatment, citric acid (CA) catalysis, and microwave (M)-assisted hydrothermal treatment was developed to enhance XOS production from corn stover, followed by enzymatic hydrolysis of the solid residue to recover monosaccharides. Among five acids evaluated (sulfuric acid, oxalic acid, citric acid, benzoic acid, and acetic acid), citric acid (0.09 g/g corn stover) was identified as the optimal catalyst. Under microwave heating at 130 °C for 60 min, total XOS reached 12.43 ± 0.34 g/L with a 66.88 ± 1.82% yield, representing a 3.4-fold increase relative to single pretreatment, of which only 6.83 g/L was DP2-6 prebiotic XOS. Mechanistically, the synergistic effect of ball milling and citric acid significantly disrupts the compact structure of the raw material, increases the exposure of polar groups, and improves dielectric properties, thereby enhancing the conversion efficiency of microwave energy and ultimately achieving the selective hydrolysis of hemicellulose. Structural characterization, including TGA and SEM, verified the gradual breakdown of the biomass matrix. Enzymatic hydrolysis of the BMCA-M residue at a solid loading of 15% (w/w) using 20 FPU/g cellulase yielded approximately 86.19 g/L of total sugars, with a feedstock-based target saccharide recovery rate of 50.33%. This work provides an integrated biorefinery pathway for corn stover valorization, while we also discuss the limitation of abundant long-chain XOS restricting its direct application as high-purity prebiotic raw materials.