Types of Malt Milling
The purpose of milling is to reduce the size of the malt particles and to expose the endosperm to attack by enzymes during the mashing process, so that the greatest conversion of starch to fermentable sugar is achieved in the shortest possible time. The acceptable range of particle size is determined by the wort separation system being used—mash tun, lauter tun, or mash filter. For example, mash tuns require comparatively coarse grists while lauter tuns can use finer grists and mash filters still finer grists. Fine grinding provides more surface area of contact between the water and the starch, leading to more complete reaction of the starch.
In breweries, dry milling is commonly performed by roller mills or hammer mills. If the wort separation involves using a mash tun or lauter tun, roller mills are employed that crush the grain between two or more cylindrical rollers. Hammer mills are largely used for mash filters and continuous brewing systems. screen. The resulting particles resemble flour.
As mentioned, the objective of milling is to break and expose the starchy endosperm of the seeds without excessive damage to the husk. Excessive damage to the husks should be avoided because they act as a processing aid during wort separation, helping to maintain grain bed porosity during lautering. In addition, if the husks are damaged phenolic compounds, i.e., tannins can be extracted into the wort giving an undesired astringency to the beer.
Two-, Four-, and Six-Roller Mills. Two-roller mills are single-pass mills commonly used by craft breweries and/or for well-modified malts. They are considerably less expensive than comparable four-roller, five-roller, and six-roller malt mills. However, the uniformity of milling fineness is not as good as with multi-roller mills. While two-roller mills are still in operation, they have largely been succeeded by multi-roller mills, which are of a four- or six-roller design. For the larger craft brewers or major brewers, the choice will be either a four roller or six roller mills.
Milling Safety. Grain handling systems should be well-ventilated to prevent the possibility of a grain-dust explosion. A tiny spark created by the metal rollers hitting a stone or staple in the malt can be enough to ignite an explosion. For this reason, grain cleaners are necessary.
A refinement to dry milling employed by numerous breweries is conditioning of malt with low-pressure steam. This practice minimizes the risk of fracturing the malt husks by the mill rollers, thus the husks become tougher and more flexible due to absorbed moisture, while the endosperm remains dry and friable. The resulting grist is said to be characterized by a husk fraction containing a very high proportion of split husks with very few endosperm particles adhering.
An alternative to dry malt milling is to partially rehydrate the malt or at least steep condition the husk. In wet milling the malt is fed into a conditioning chamber prior to milling where it is sprayed with warm water at 50 to 70 degrees C (122–158°F) for a controlled time such that the uptake of moisture by the husk is considerably enhanced about 20 to 30 percent; the endosperm stays dry enough to fracture under pressure. The duration and temperature of steeping depends on the modification and the moisture content of the malt.
Hammer mills can be used only in brewhouses that employ a mash filter instead of conventional wort separation methods using the mash bed itself to filter and run-off the clarified wort, either in the mash tun (typical British infusion system) or a lauter tun. A hammer mill consists of a rotor made of two or more plates with pins to carry the hammers. e-walled vessels that are subject to environmental temperature fluctuations.
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