Characterization of physical properties of flour and starch obtained from gamma-irradiated white rice

Physical and structural characteristics of rice flour and starch obtained from gamma-irradiated white rice were determined. Pasting viscosities of the rice flour and starch, analyzed by using a Rapid Visco Analyser, decreased continuously with the increase in irradiation dosage. Differential scanning calorimetry showed that gelatinization onset, peak and conclusion temperatures of rice flour and starch changed slightly but the enthalpy change decreased significantly with increase of irradiation dosage. All irradiated starch displayed an A-type X-ray diffraction pattern like the native starch. Gel permeation chromatography showed that the blue value ratio of the first peak (amylopectin) to the second one (amylose) decreased with the increase of the irradiation dosage. The weight-average molecular weight (M-w) and gyration radius (R-z) of amylopectin analyzed by using HPSEC-MALLS-RI (high-performance size-exclusion chromatography equipped with multiangle laser-light scattering and refractive index detector) decreased gradually from 1.48 x 10(9) (M-w) and 384.1 nm (R-z) of native rice starch to 2.36 x 10(8) (M,,) and 236.8 nm of 9 kGy-irradiated starch. The branch chain-length distribution of amylopectins determined by HFAEC-ENZ-PAD (high-performance anion-exchange chromatography with amyloglucosidase post-column on-line reactor and pulsed amperometric detector) showed that gamma irradiation had no significant effect on the amylopectin branch chains with 13 <= DP <= 24 and 37 <= DP, but produced more branch chains with 6 <= DP <= 12 when the irradiation dosage was less than 9 kGy. It might be deduced that gamma irradiation caused the breakage of the amylopectin chains at the amorphous regions, but had little effects on the crystalline regions of starch granules, especially at low dosage irradiation.