Malt milling for microbreweries
The purpose of milling is to reduce the malt to a particle size, which will yield the most economical extract (wort) that will run satisfactorily under brewhouse conditions and throughout the brewing process. The more extensive the malt milling, the greater the yield of extract. However, fine grinding can lead to subsequent wort separation problems and loss of extract from spent grains during the wort separation process. Therefore, the brewer needs to consider the equipment used in the brewhouse when determining the grain size for grinding the malt. For example, a wort tank requires a relatively coarse grind, while a fermenter can use a finer grind and a wort filter still requires a finer grind.
In breweries, dry grinding is usually performed by roller mills or hammer mills. If wort separation involves the use of a wort tank or percolation tank, a roller mill is used. Hammer mills are mainly used for later wort filters and continuous brewing systems.
Roller mills are particularly suitable for milling malt when the main objective is to keep the malt husk intact. The intact husk helps wort separation and reduces the extraction of tannins and other undesirable components.
Two-Roll Mills – Two-roll mills are single-pass mills that are commonly used by craft breweries and/or for improving malt.
Multi-Roll Mills – Multi-roll mills allow better control of the feed rate of unground malt, the spacing between rolls, and the speed of the driven rolls, either uniformly or differently.
A hammer mill consists of a rotor consisting of two or more plates with pins to carry the hammers. The hammers are simple flat metal bars with a hole at one or both ends. They may have some type of edge finish, such as hardfacing or carbide coating, to provide better wear resistance.
An improvement on the dry milling method used by many breweries is the conditioning of the malt with steam or warm water. This practice minimizes the risk of malt shell breakage, so the malt shell becomes tougher and more flexible as it absorbs water, while the endosperm remains dry and friable.
The advantage of dry milling and conditional dry milling operations is that the brewer can sample the crushed malt and visually inspect it for uncrushed grains, excessively torn hulls and excess flour.
Wet milling is very common in Africa and Asia because it simplifies the milling process, but it is not common in the United States. In wet milling operations, the entire uncrushed malt is pre-soaked in hot water to bring the moisture content of the hulls to about 20%, while the endosperm remains almost dry, which results in a semi-soft, almost paste-like consistency.