The mechanism of bran layer removal in friction rice mills

Moderate removal of the bran layer plays an important role in reducing broken rice and nutrient loss during rice milling. However, the moderate removal of bran layer remains a challenge due to limited understanding of the mechanisms of rice bran removal. The mechanism of bran layer removal was studied by analysing the motion characteristics of rice grains on a rice sieve and the tissue structure and mechanical properties of the bran layer from different parts of rice grain. The results showed that the removal order of bran layer is firstly, lateral, followed by ventral and finally dorsal. In the milling chamber the grains on the sieve are almost always in contact with the rice sieve by the lateral, resulting in the lateral bran being removed the first during milling. The dorsal bran layer has a thicker tissue structure and stronger mechanical properties, which leads to the dorsal bran layer being removed after the ventral bran layer. The mechanism of bran layer removal reduces the efficiency and quality of milling. These findings should helpful for the process optimisation of moderate milling and for the design of rice mills.