Abstract: The present study was undertaken with two aims: 1) to determine the optimal preparation process for selenium-modified soluble dietary fiber from millet and 2) to explore the influence of selenium modification on the structure and antioxidant activity of millet water-soluble dietary fiber (SDF) and on its effect on the ability of intestinal flora to produce tryptophan. SDF was modified by a nitric acid-sodium selenite method, and the selenization conditions were optimized by the Taguchi method. The molecular mass, particle morphology, functional groups and crystallinity of the native and modified SDF were determined by gel permeation chromatography (GPC), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy and X-ray diffraction. In addition, their antioxidant activity was compared with each other, and the change of the tryptophan content in the culture broth of mouse intestinal microbes utilizing Se-SDF as a carbon source was analyzed. The results showed that the optimal conditions of modification were as follows: reaction temperature of 40 ℃, reaction time of 6 h, 3 mL of 5 mg/mL Na2SeO3, and 0.65 g of BaCl2. Under these conditions, the yield of Se-SDF was 10.56% with a selenium content of 2.69 mg/g. The relative molecular mass of the Se-SDF was increased compared with the native SDF, and the surface showed a porous honeycomb-like structure with large pore size. The degree of polymerization was reduced relative to that of the native SDF, and functional groups such as Se=O, Se–OH and Se–O–C were appeared in the Se-SDF. Despite no obvious change in the crystal structure, the crystallinity of SDF was decreased after the modification. The hydroxyl and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity and total antioxidant capacity of SDF as well as its promoting effect on the tryptophan-producing capability of the mouse intestinal flora were enhanced after the modification. In summary, the Se-SDF prepared using the optimized process can be used to produce dietary selenium and tryptophan supplements.

Key words: millet; soluble dietary fiber; modification; structure; tryptophan