Abstract: Millet bran was pretreated by steam explosion and then subjected to combined ultrasonic-microwave treatment in order to modify millet bran dietary fiber (MBDF), namely to increase soluble dietary fiber (SDF) yield. The process conditions were optimized using response surface methodology. In addition, the structures of modified and unmodified MBDF were investigated by means of gel permeation chromatography-refractive index-multi-angle light scattering (GPCRI- MALS), infrared spectroscopy and X-ray diffraction. The steam explosion conditions were set as 1.0 MPa and 90 s for pressure and time, respectively. The optimal conditions for combined ultrasonic-microwave treatment were determined as microwave power of 535 W, solid-to-solvent ratio of 1:50 (g/mL) and time of 57 min, giving an SDF yield of as high as 10.841%. The results of GPC-RI-MALS showed that the molecular weight of modified millet bran SDF was smaller that of the unmodified one, due to the shorter molecular chain and the reduced degree of polymerization resulted from the modification. The infrared spectroscopic analysis illustrated that there were no significant changes in the chemical functional groups between the modified SDF and IDF, and both of them possessed the characteristic absorption peaks of carbohydrates. The X-ray diffraction analysis revealed that the crystallinity of the modified IDF increased, indicating that its non-crystalline region was partially degraded and converted to SDF. Scanning electron microscopic (SEM) observation showed that the surface of modified millet bran SDF became rough, loose and porous and was composed of aggregated small particles.

Key words: millet bran, combined ultrasonic-microwave treatment, steam explosion, soluble dietary fiber (SDF), response surface methodology, structure