Response surface methodology optimization of alkaline extraction of polysaccharides from rapeseed meal: Structural characterization and antioxidant activities

Rapeseed meal, a major by-product of oilseed processing, constitutes a rich and underutilized source of natural polysaccharides with promising functional potential. In this study, alkaline extraction was optimized using Box–Behnken design to isolate meal husk polysaccharide (HMP) and meal kernel polysaccharide (KMP) from rapeseed meal husk and kernel, respectively. Comprehensive physicochemical characterization revealed that both HMP and KMP were low-heterogeneity polysaccharides composed primarily of arabinose, galactose, glucose, xylose, and galacturonic acid. Despite originating from distinct anatomical parts, HMP and KMP exhibited comparable molecular weight distributions, Fourier-transform infrared (FTIR) spectra, and X-ray diffraction (XRD) patterns, indicative of amorphous or semi-crystalline structural features. Methylation and Nuclear magnetic resonance (NMR) analysis indicated that the arabinan featured (1 → 3)/(1 → 5) backbone organized around 2,3,5-trisubstituted residues serving as branching hubs; arabinan segments were grafted onto GalA at O-4, and the RG-I repeating unit [→2)-Rha-(1 → 4)-GalA-(1→] was present. Additionally, continuous (1 → 4)-xylan together with β-(1 → 4)-galactan and β-(1 → 4)-glucan bearing O-6 branches was identified, indicating that HMP and KMP were arabinan-dominated composite polysaccharides. Thermogravimetric analysis (TGA) demonstrated good thermal stability for both HMP and KMP. The slightly lower overall mass loss observed for KMP suggests greater thermal stability. Antioxidant activity assays, including 1,1-diphenyl-2-picrylhydrazyl (DPPH), and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and hydroxyl radical scavenging, confirmed the notable radical-scavenging capacities of both HMP and KMP, with KMP demonstrating relatively higher activity in the DPPH and ABTS systems. Scanning electron microscopy further revealed morphological differences, with HMP displaying a smoother and denser surface, whereas KMP exhibited a more porous and irregular microstructure. Additionally, both HMP and KMP effectively inhibited browning in fresh-cut apple models, underscoring their potential as natural antioxidant agents and edible coating materials. These findings provide theoretical and practical foundation for the valorization of rapeseed meal polysaccharides in the development of functional food and preservation technologies.