Mechanism of rice bran removal at individual grain and population levels in abrasive rice mill

Abstract

In the process of rice bran layer removal using abrasive rice mills, over-milling will result in nutritional loss, while under-milling will result in poor palatability. However, achieving moderate milling with an abrasive rice mill can be challenging due to the rice bran layer removal mechanism. This study investigates the mechanism of bran layer removal in abrasive rice mills by analysing the wear and structural characteristics on the rice surface, as well as the motion of rice grains in the milling chamber. The results showed that surface wear due to the contact of the rice grains with the grit was the main reason for debranning. At the individual grain level, the process of removing the bran layer in the abrasive rice mill is phased, synchronised, and orderly. The removal process can be divided into three stages depending on the morphology of the residual bran layer and the wear mechanism. The rotation motion leads to the synchronous removal of the bran layer in different regions of the rice grains. The bran layer in different regions is removed sequentially due to the varying number of depressions. At the rice population level, the axial and radial positions exchange of the rice grains in the milling chamber ensures overall uniformity in removing the rice bran layer. These findings are valuable for optimising the design of the abrasive mills and guiding the mill uniformity in similar types of mills.