is the most common device for crushing malt and grain into grist in preparation for the mash. Roller mills differ by the number of tandem operating rollers and by the treatment of the malt or grain prior to and during the milling operation. There is dry milling, conditioned dry milling, and wet milling. The actual milling takes place between a pair of cylindrical rollers that spin downward in opposite directions. The rollers are flat or, in newer mills, serrated, and they rotate at differing or identical velocities. They are specially cast to have the hard surface necessary for milling. Rollers turning at different velocities cause the grain to press together and shear into smaller pieces, whereas serrated edges also cut the grain. Care must be taken to prevent the serrated edges from dulling on hard objects, such as stone or metal. To prevent this, a stone separator and a magnet are often installed in the malt or grain path right before the mill. The magnet is also an essential precaution against dust explosions that may be sparked by metal objects being scraped by the rollers.

Varying the distance between the rollers determines the fineness of the grind as well as the speed with which the milling can be accomplished. This, in turn, has an effect on overall brewhouse efficiency and yield. Milling malt for a normal brew should not take more than 1 h to 2 h. The time depends on the length of the rollers, the moisture content of the malt, the roller speed, and the type of roller surface. In some roller mills, the grist may be separated by size and some grist fractions may be sent through the mill a second or even third time.

The simplest, most common, but least efficient mill is a two-roller mill, which is usually found in small breweries or brewpubs. Both rollers spin at the same speed. The malt passes through the rollers only once. The use of such mills, therefore, requires well-modified and homogenous malts.

Four-roller mills are more common in mid-size breweries. They consist of two pairs of rollers stacked one on top of the other. Malt passes through the first set of rollers and is directed to a sieve that lets finer grist fall through directly. The remaining, coarser grist is directed to the second pair of rollers.

Six-roller mills are the most flexible. They can be adapted to different dry malts to produce grist for any mash. After the first pair of rollers, the grist is separated into three fractions by a set of two sieves, whereby the finest fraction, which is basically flour, is directed away for collection. A coarser fraction bypasses a second pair of rollers and is sent directly to a third pair to be milled again, whereas the coarsest fraction, which consists of husks with endosperm remnants, is directed to the second pair, also to be milled again. This fraction is then sifted to separate it into husks, coarse grist, and flour. The coarse fraction from this milling step is then directed to the third pair of rollers to be milled a third time.

The fractioning that is possible with four- and six-roller mills allows for the optional separation of husks from the grist. The husks can then be added to the mash later, reducing the leaching of astringent polyphenols. When adding husks to the mash, however, it is essential that the husks be free of endosperm remnants, because these contain unconverted starches that could end up in the wort and beer as hazes.

Some modern roller mills condition the malt with water before the milling process. This process, called “conditioned dry milling,” involves moistening the grist to increase the level of water in the husks but not significantly in the endosperm. The moistened grist is than milled normally. The moisture makes the husks less brittle, which results in larger husk fractions. This results in numerous advantages, including more efficient and faster lautering, higher brewhouse yields, and milder beers. One disadvantage is an increased risk of microbiological contamination from moist deposits inside the mill itself. These mills, therefore, must be cleaned thoroughly after use.

In the process called wet milling, the grist is effectively steeped in water until it contains as much as 30% moisture. Done properly, the husks become elastic and the endosperm is squeezed out during milling using a simple two-roller mill.

See also grist, milling, and wet milling.