Impact of rice friction milling on the bran layer and on kernel structure

Moderate processing can improve the edible rice grain quality and nutrition. The grain structural-removal behavior was researched based on the macro and micro-structure analysis. Grain bran layer 3D profiling combined with iodine solution staining is applied for geometrical-structural analysis. Results show that rice grain can be divided into five structural region based on bran layer removal capacity, lateral > ventral > ventral groove > dorsal > dorsal groove. The relationship between grain geometric parameters and collision possibility is in-depth analyzed, results indicated large grain surface curvature leads to grain surface layer hard removal capacity. The region's bran layer thickness distribution affects its removal behavior and obey its removal capacity. The regional bran layer thickness order: dorsal(59.56 μm) > dorsal groove(50.62 μm) > ventral(43.91 μm) > ventral groove(42.83 μm) > lateral(37.08 μm). The bran layer micro-morphology impacts removal behavior, dorsal region with the groove structure, the bottom surface layer in grain groove structure can be removed when groove structure is damaged. The ventral groove depth is less than dorsal groove, showing better removal ability. The bran layer color in L*A*B*space shows strong correlation with remaining layer thickness, reflecting grain milling degree. Combined effects of grain geometry-parameters, bran layer thickness and micro-structure lead the final bran layer removal behavior. This study provides theoretical and practical basis for grain moderate processing.