Abstract: The conditions for removing cadmium from rice flour by water soaking were optimized using response surface methodology. The effects of the optimized treatment on nutrient retention were determined. We also investigated the effects on the morphology, thermodynamic characteristics, starch crystal structure and functional groups of rice flour by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FTIR), respectively. The results showed that the optimal conditions that provided the maximum remove rate of cadmium of (0.459 ± 0.006) mg/kg (cadmium content (0.176 ± 0.003) mg/kg) were determined as follows: temperature 42 ℃, solid to water ratio 1:3, and soaking time 47 h. Under these conditions, protein content of rice flour decreased significantly (P < 0.05), and the contents of fat, starch and ash decreased slightly. The onset temperature of gelatinization of rice flour starch decreased after water soaking, while the peak temperature changed little, indicating damaged crystal layer, reduced stability and possibly increased amylose content of rice starch. On the other hand, the soaking treatment did not destroy the crystal components of starch granules in rice flour. The d-spacing of the soaked rice flour decreased slightly, indicating that soaking may lead to deterioration in hardness, chewiness and resilience and cohesiveness. No new chemical bonds or groups were formed during the process of water soaking.

Key words: soaking, cadmium, rice flour, removal, quality