Valorization of Oat Husk for the Production of Fermentable Sugars, Xylooligosaccharides, and Inulinase via Deep Eutectic Solvent and Microwave-Assisted Pretreatment

This study presents an integrated valorization strategy for oat husks through microwave-assisted pretreatment using a deep eutectic solvent (DES) composed of choline chloride and glycerol (1:2). The process was designed to enhance the release of fermentable sugars, enable xylooligosaccharide (XOS) production, and support inulinase production by Aspergillus niger A42 via submerged fermentation of the hydrolysate and solid-state fermentation of the residual biomass. Response surface methodology (RSM) was applied to evaluate the effects of microwave power, treatment time, and liquid-to-solid ratio (LSR) on fermentable sugar content (FSC) and total phenolic compounds (TPCs). Following pretreatment, the biomass was hydrolyzed using 1.99% sulfuric acid for 1 min. Optimal pretreatment conditions (350 W, 30 s, LSR 4 w/w) yielded an FSC of 51.14 g/L. Additionally, 230.78 mg/L xylohexaose and 6.47 mg/L xylotetraose were detected. Submerged fermentation of the liquid fraction with A. niger A42 resulted in inulinase and invertase activities of 60.45 U/mL and 21.83 U/mL, respectively. Solid-state fermentation of the pretreated solids produced 37.03 U/mL inulinase and 17.64 U/mL invertase. The integration of microwave-assisted DES pretreatment, dilute acid hydrolysis, and fungal fermentation established a robust strategy for the sequential production of XOS, fermentable sugars, and inulinase from oat husks, supporting their comprehensive utilization within a sustainable biorefinery framework.