Abstract: In this paper, the total sugar and protein contents, molecular mass, monosaccharide composition of polysaccharides extracted from soy hulls by ultrasonic-microwave synergistic extraction were determined, and they were further structurally characterized by Fourier transform infrared spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results showed that the total sugar content of the extract obtained by the synergistic method was up to (50.67 ± 3.36)%, which was significantly higher than that extracted by ultrasonic or microwave method alone. The polysaccharides extracted by the three methods were all mainly composed of galactose, mannose, galacturonic acid and rhamnose, while the synergistic extraction method significantly changed the proportions of the monosaccharides. The synergism reduced the polydispersity and unsaturation degree of soy hull polysaccharides and changed the position and quantity of branches rather than the type of sugar ring. Soy hull polysaccharides extracted by the synergistic method exhibited big spherical particles forming a network structure, indicating that this synergistic effect could promote the molecular aggregation of soy hull polysaccharides. The results from this study will provide theoretical support for the analysis of polysaccharide conformation.

Key words: ultrasonic-microwave synergistic extraction, soy hull polysaccharide, microstructure, atomic force microscopy