In this study, crude polysaccharides from soybean and natto were prepared by water extraction followed by ethanol precipitation. The chemical compositions, structural characteristics, water solubility, water-holding capacity (WHC) and fat-binding capacity (FBC) of soybean and natto polysaccharides were analyzed. Their in vitro antioxidant, hypoglycemic, and lipid-lowering activities were compared and analyzed. The results showed that the content of uronic acid was significantly higher in natto polysaccharide than in soybean polysaccharide (P < 0.05). The molecular masses of soybean and natto polysaccharides were 5.256 and 33.532 ku, respectively, and the monosaccharide compositions of soybean and natto polysaccharides were similar in the types but different in the proportions of monosaccharide. The surface of soybean polysaccharide was rough, whereas the surface of natto polysaccharide was smooth and dense. The water solubility of natto polysaccharide was 2.04 times as high as that of soybean polysaccharide, and the FBC was 2.99 times as high as that of natto polysaccharide. Natto polysaccharide exhibited better antioxidant activity, with half maximal inhibitory concentration (IC₅₀) of (0.049 ± 0.015) and (2.640 ± 0.072) mg/mL for scavenging of 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radical, respectively. The IC₅₀ for the inhibition of α-amylase activity by natto polysaccharide was (3.297 ± 0.395) mg/mL. Natto polysaccharide had significantly higher in vitro hypoglycemic activity (P < 0.05), stronger cholate binding capacity and in vitro hypolipidemic activity than soybean polysaccharide. This study has provided an important theoretical basis for the structural analysis and biological activity evaluation of natto polysaccharide.

In order to comprehensively evaluate the nutritional value of common bean polysaccharides, shiitake mushroom polysaccharides were used as the reference standard in this study. Fourier transform infrared spectroscopy (FTIR), high performance liquid chromatography (HPLC), high performance gel permeation chromatography (HPGPC), scanning electron microscopy (SEM) and Congo red assay were used to characterize the structures, monosaccharide compositions, molecular masses, and microstructures of the two kinds of polysaccharides. Their water solubility and water-holding capacity were compared, and their antioxidant, hypoglycemic and hypolipidemic activities in vitro were evaluated comprehensively. The results showed that the yield of common bean polysaccharides was significantly higher than that of shiitake mushroom polysaccharides, (7.01 ± 0.57)% versus (5.04 ± 0.38)% (P < 0.05), and the content of uronic acid in common bean polysaccharides was 2.21 times higher than that of shiitake mushroom polysaccharides. The major polysaccharide components of common bean and shiitake mushroom had molecular masses of 77.157 and 559.245 kDa, respectively. Common bean polysaccharides contained a small amount of esterified pectic polysaccharides with a loose and rough microstructure, while shiitake mushroom polysaccharides were heteropolysaccharides with a triple helical structure containing α- and β-pyran rings, which exhibited a compact and smooth microstructure. The water solubility and water-holding capacity of shiitake mushroom polysaccharides were 1.61 and 6.07 times higher than those of common bean polysaccharides, respectively. The in vitro antioxidant capacity, hypoglycemic activity and hypolipidemia activity of common bean polysaccharides were higher than those of shiitake mushroom polysaccharides, 2.43 versus 1.82. In conclusion, common bean is an ideal food resource for developing active polysaccharide products.