Grain mills are used to crush grains in preparation for brewing. Crushed grains are susceptible to oxidization, so brewers prefer to crush grains within a few days of brewing.
Crushing grains is a very important step in the brewing process, as the crush of the grains determines the efficiency of the mashing process. Efficiency is the percentage of fermentables in the grain (by weight) that are absorbed into the wort during the mashing process.
Finely crushed grains result in higher mashing efficiency, but form a poor grain bed that can easily become "stuck", halting the water flowing through the mash tun during the sparging process. This condition is called a "stuck mash". Coarsely crushed grains allow for free flowing of sparge water, but produce lower efficiency.
An ideal grain mill finely crushes the interior of the grains while leaving the grain husks largely intact. These husks then form a grain bed that allows sparging with high efficiency. Dual roller grain mills are considered the best option for home brewers to achieve this type of milling.
Type of Mills
- Corona Mill - A Corona mill (or the similar Victoria mill) looks very much like a sausage grinder. It mills the grain by forcing it between two rotating plates. Corona mills are the cheapest to purchase, but can be difficult to adjust as they tend to powder the grain and husk rather than leaving the husk intact. Unlike most of the other mills mentioned below, it is primarily designed for milling flour.
- Single Roller Mill - A single roller malt mill presses the grains between a moving roller and immobile roller or plate. This results in a better crush than the corona mill, but still produces some powdering of the husk.
- Dual Roller Mill - A dual roller malt mill provides the best overall price/performance for a homebrewer. If properly adjusted, it will crush the interior of the grain while leaving the husks largely intact.
- Triple Roller Mill - A triple roller mill provides the best crush available to a homebrewer, but at a significance price increase from the dual roller mills. Triple roller mills have only become widely available since 2010, and are still fairly uncommon due to their price.