Effect of the degree of milling on the microstructure and composition of japonica rice

The effect of the degree of milling (DM) on the distribution of the residual bran layer on the surface of rice grains and subsequent rice quality is essential for rice milling technology. This study used scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to examine the microstructure of japonica rice grains, the microstructural changes in different bran portions after milling, and the content and distribution changes of the elements and nutrients in rice after milling to identify the components that can quantify bran residue according to DM. The SEM results revealed that the thickness of the bran layer and the depth of the grooves in different parts of a rice grain varied, while the depth of the grooves differed between varieties. When DM was 12%, aleurone layer (Al) cells embedded in endosperm (En) remained in the dorsal area. It was speculated that this was the reason why there was no significant difference in bran degree when the DM estimated via grain weight loss increased from 10% to 14% (P > 0.05). The EDS elemental mapping image showed the qualitative distribution of the elements, which was condensed in the bran and uniform in the En. The compositional changes revealed that milling caused the loss of rice nutrients, and the loss rate of DM at 0–10% was more serious than that of DM at 10%–14%. Higher DM (>10%) could remove marginal bran from the dorsal groove. Therefore, the crude protein, ash, crude fat, dietary fiber, vitamin B1 (VB1), and vitamin B2 (VB2) content did not change significantly (P > 0.05), while the magnesium (Mg), kalium (K), and phosphorus (P) levels decreased substantially (P < 0.05). The microstructural and compositional changes indicated that the DM estimated via the grain weight loss increased from 2% to 10%, with the bran decreasing progressively. The DM estimated via the grain weight loss increased from 10% to 14%, with minimal changes in the residual bran in the dorsal grooves of the rice grains, while the Mg, P, and K levels presented precise DM indices during the rice milling process. This result will provide a theoretical reference for the accurate evaluations of DM.