The mill crushes the grain between steel rollers. Mills have between one to three pairs of rollers. More complex mills give a greater uniformity of and control over particle size. Milling breaks apart the grains, allowing them to make contact with the water. More finely crushed grain give better contact with the water during mashing, yielding more efficient conversion of starch to sugar, but the fine particles can impede flow during wort separation.

In most breweries, pulverizing the grain hull is avoided because intact hulls help the flow characteristics and powdered hulls can give off-flavors.

This topic is covered in The Chemistry of Beer: The Science in the Suds, Chapter 7.