Abstract: Millet bran dietary fiber (MBDF) was modified by combined ultrasonic-microwave treatment in this study. The structures of the original and modified MBDF were investigated by means of scanning electron microscope (SEM), gel permeation chromatography-refractive index-multi-angle light scattering (GPC-RI-MALS), X-ray diffraction, infrared spectroscopy and ion chromatography. The inhibitory effect of the original and modified soluble dietary fiber (SDF) from millet bran on α-glucosidase activity was explored using in vitro α-glucosidase inhibitor model. SEM images showed that the surface of SDF became rough, loose and porous after modification and was composed of aggregated small particles. The results of GPC-RI-MALS indicated that the molecular mass of modified millet bran SDF was smaller than that of the original one. The crystallinity of the modified insoluble dietary fiber from millet bran was increased as indicated by X-ray diffraction analysis. Fourier transform infrared spectroscopic analysis illustrated that the chemical functional groups of the original millet bran SDF did not change after modification. Both the original and modified millet bran SDF had the characteristic absorption peaks of carbohydrates. Ion chromatography showed that both the original and modified millet bran SDF were composed of arabinose, galactose, glucose, xylose, mannose and fructose, with differences being observed in the monosaccharide contents. Both the original and modified millet bran SDF could inhibit the activity of α-glucosidase, while the inhibitory effect of modified millet bran SDF was stronger than that of the original one, with a half-maximal inhibitory concentration of 0.415 mg/mL.

Key words: soluble dietary fiber, modification, structural analysis, α-glucosidase, inhibitory effect