Analysis of Milling Quality of Reasonably Well Milled Rice with Three Different Grain Shapes

In order to explore changes in the milling quality of reasonably well milled rice with different grain shapes, the bran degree, milling degree and broken rice rate of short-grained (‘Jihong 6’), medium-grained (‘Daohuaxiang’) and long-grained (‘Yexiangyoulisi’, ‘Yexiang’ for short) brown rice were measured after milling at different temperatures and for different periods. The results showed that the bran degree decreased exponentially with an increase in milling pressure for ‘Jihong 6’ and ‘Daohuaxiang’ and an increase in milling time for ‘Yexiang’. For reasonably well milled rice with bran degree from 7.0% to 2.0%, as specified by China’s national standard GB/T 1354–2018, the percentage change in milling degree of ‘Jihong 6’, ‘Daohuaxiang’ and ‘Yexiang’ were approximately 5.03%, 2.98% and 0.18%, respectively, indicating that the milling degree of reasonably well milled rice did not change more significantly with increasing grain length. The broken rice rates of ‘Daohuaxiang’, ‘Jihong 6’ and ‘Yexiang’ increased by 5.32, 0.25 and 0.21 percentage points, respectively, and the white rice yields decreased by 1.38, 2.74 and 0.21 percentage points, respectively. The change in broken rice rate of ‘Yexiang’ for reasonably well milled rice was the smallest. The broken rice rates of ‘Daohuaxiang’, ‘Jihong 6’ and ‘Yexiang’ at bran degree of 0.10% increased by 10.36–4.77, 1.85–1.60, and 10.05–9.84 percentage points, respectively compared with those at bran degree from 7.0% to 2.0%, and the white rice yields decreased by 4.19–2.81, 7.02–4.28 and 10.65–10.44 percentage points, respectively. So, the effect of changes in broken rice rate at bran degree of 0.10% on white rice yield was more pronounced. ‘Daohuaxiang’ with bran degree of 2.10%–2.45% met germ retention rate of less than 20% for high-quality japonica rice specified in the national standard, while ‘Jihong 6’ and ‘Yexiang’ with bran degree from 7.0% to 2.0% did not meet this requirement. At bran degree from 7.0% to 2.0%, the whiteness of ‘Daohuaxiang’, ‘Jihong 6’ and ‘Yexiang’ increased by 6.65, 2.80 and 2.69 percentage points, respectively. There was no significant regular change in the breaking strength of the three rice varieties. The bran degree had a significant correlation with the degree of milling, whiteness and germ retention rate in reasonably well milled rice. As observed by scanning electron microscopy (SEM), the difference in thickness between the dorsal and ventral regions of ‘Yexiang’ was the smallest. When the bran degree was about 2.00%, the ventral aleurone layer of the three types of rice was almost entirely removed, while much of the aleurone layer was remained in the dorsal part. When the bran degree was about 0.10%, a small amount of the dorsal aleurone layer was retained for ‘Daohuaxiang’ and ‘Jihong 6’, while the dorsal aleurone layer of ‘Yexiang’ was completely removed. Therefore, the milling degree required for uniform whitening of brown rice and the processing quality vary with cultivar and grain shape.