Effect of Soluble and Insoluble Dietary Fibers from Extrusion Cooked Rice Bran on the Properties of Rice Starch and Their Interactions

The effects of soluble and insoluble dietary fibers from unextruded and extruded rice bran on the pasting properties, thermal properties, retrogradation properties, crystallinity properties and microstructure of rice starch were studied. The interaction between dietary fibers from extruded rice bran and rice starch was investigated by means of texture analysis, nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The results showed that compared with un-extruded rice bran dietary fibers, soluble and insoluble dietary fibers from extruded rice bran significantly increased the breakdown value by 74.09% and 128.36%, respectively, but significantly decreased the peak viscosity, trough viscosity, final viscosity, peak time, and pasting temperature of rice starch. Soluble dietary fibers from extruded rice bran facilitated the transformation of free water into strongly bound water in rice starch gels, while insoluble dietary fibers from extruded rice bran promoted the transformation of free water into weakly bound water. Compared with un-extruded rice bran dietary fibers, soluble and insoluble dietary fibers from extruded rice bran decreased the setback value of rice starch by 62.59% and 44.81%, respectively, and also decreased the retrogradation rate, relative crystallinity, hardness, cohesion, resilience, gumminess, chewiness and ratio between peak heights at 1047 cm^-1 and 1022 cm^-1 of rice starch gels. The surface of starch gels added with extruded rice bran soluble and insoluble dietary fibers was smoother with larger crack, showing that extrusion improved the inhibitory effect of rice bran dietary fibers on the retrogradation of rice starch, and extruded soluble dietary fiber had a more prominent effect than extruded insoluble dietary fibers.