Differences in physicochemical, morphological, and structural properties between rice starch and rice flour modified by dry heat treatment

The differential influence of dry heat treatment on pasting, thermal, morphological, and crystalline structural properties of glutinous rice starch (GRS) and glutinous rice flour (GRF) in a dry state for 0, 2, or 4h at 130 degrees C were studied. The results revealed that the viscosity and thermal stability of GRS and GRF increased after dry heat treatment. Peak viscosity of GRF rose nearly threefold from 1156 to 3145cP. The number of disulphide bonds in GRF increased from 1.93 to 3.19mol/g, indicating the covalent SS linkages after dry heat treatment may result in the changes of protein structure, which may greatly influence properties of dry heated flour. Morphology of gelatinized GRS was denser, and a strong laminar structure was observed for gelatinized GRF after dry heat treatment. Fourier transformation infrared spectroscopy indicated that both GRS and GRF had relatively higher transmittance intensities in the region of 1068 and 1022cm(-1). X-ray diffraction showed that the crystallinity of GRS increased from 32.82 to 35.02% and GRF increased from 29.32 to 34.14% after dry heat treatment. Dry heat treatment had greater effects on viscosity and structural properties of GRF than those of GRS.