Analysis of processing properties of rice flour by micro/nano-scale grinding

The objectives of this study are to grind rice grains into micro/nano-scale size, and to make clear the processing properties of the flour. In micro-scale rice flours using dry jet grinding and dry hammer mill, the flour suspension with the finest mean size (3 µm) showed a smaller amount of change in absorbance within time than those of coarse flours (15-120 µm). The finest flour showed better dispersibility and also highest starch damage (35%). For the mid-sized flours, the mean size of particle in the suspension was 15 μm at 0 min but decreased to 7 μm at 25 min, along with absorbance. This result shows that particles of 7 μm mean size remained in suspension at 25 min when the coarser flours had settled down. In comparison between commercial rice starch granule (CS) and the ground starch granule (GS), the mean size and absorbance of CS decreased a little at 25 min but those of GS remained unaltered. The value of starch damage was higher in GS (17%) than in CS (6%), and that in the finest rice flour was higher. From this discussion, it was considered that a small mean size (<5 μm) and a high degree of starch damage contributed greatly to maintain the dispersibility in the suspension. In comparison for viscosity at 30% concentration batter, the finest flour indicated highest viscosity. In nano-scale grinding, it was found that the rice flour of nano-scale mean size could be produced by combination of the jet mill and the wet media mill. The mean size of the ground rice flour slurry was approximately 650 nm. But it was also found in a laser diffraction particle size analyzer that the measuring condition with or without ultrasonic irradiation greatly affects the result of size distribution in the rice flours with nano-scale mean size.