How is semolina separated from bran during wheat flour milling?

In modern wheat flour milling, the separation of semolina from bran is achieved through a gradual reduction process involving a series of mechanical steps.1

After the wheat is partially torn open by grooved break rolls, the resulting mixture is processed using the following methods:

• Sieving: The product leaving the rolls is passed through machines called plan-sifters to separate the coarse endosperm nodules (semolina) from the larger pieces of grain that still have bran attached.1
• Air Purification: Devices known as air purifiers are used to remove branny impurities from the semolina stocks.1
• Reduction Rolls: Once cleaned, the semolina is fed into smooth steel reduction rolls. These rolls further reduce the semolina into fine flour while simultaneously flattening the remaining bran particles, which facilitates their removal during subsequent sieving stages.1
• Scratch Rolls: These are specifically employed to scrape any remaining flour particles from the branny stocks.1

Through this series of repeated grinding, sifting, and air purification, the millers progressively isolate the pure endosperm from the bran.1