In breweries, dry milling is commonly performed using roller mills or hammer mills. Roller mills are employed if the wort separation involves using a mash tun or lauter tun. Hammer mills are largely used for the later generation of mash filters and continuous brewing systems.
Roller mills are particularly suited for milling malt when the primary objective is to leave the malt husk intact. An intact husk helps with wort separation and may reduce extraction of tannins and other undesirable components.
Two-roll mills are single-pass mills commonly used by craft breweries and/or for well-modified malts.
Multi-roll mills provide greater control of the rate of feed of the unground malt, the spacing between rolls, and the rate of speed, either uniform or differential, at which the rolls are driven.
A hammer mill consists of a rotor made of two or more plates with pins to carry the hammers. Hammers are simply flat metal bars with a hole at one or both ends. They may have some type of edge preparation such as hard facing or carbide coating to provide better wear resistance.
A refinement to dry milling employed by numerous breweries is conditioning of malt with steam or warm water. This practice minimizes the risk of fracturing the malt husks, thus making the husks tougher and more flexible due to absorbed moisture, while keeping the endosperm dry and friable.
Dry milling and conditioned dry milling operations have the advantage that the crushed malt can be sampled by the brewer and visually assessed for uncrushed kernels, excessive tearing of the husks, and excessive flour.
Wet milling is very common in Africa and Asia, as it simplifies the grinding process, but it is not common in the United States. In a wet milling operation, the whole uncrushed malt is pre-steeped in hot water until the husks reach a water content of approximately 20% and the endosperm remains nearly dry, resulting in a semi-plastic, almost pasty consistency.