Lesson 4 - Milling Principles
This lesson explains the objectives of milling and how mills are designed to achieve them. It details how malt is conditioned for dry milling and how we analyse grist particle size profiles. Finally, it explains how milling affects downstream brewing processes.
Roller Speeds and Mill Gap
As we saw in the last section, roller speeds affect the throughput of the mill. If this is too slow, the mill won’t operate quickly enough and if it is too fast, the grains will bounce around and ride on top of the rolls rather than being pulled into the milling gap.
Roller speeds range from 250 rpm to 440 rpm (peripheral speeds of 2-2.4 m/s). Where there are more than two rollers, the husk cutting rollers tend to run slower than those processing the grits. It goes without saying that unless the rollers are different lengths and sizes, the rollers at the top of the mill must run slower than those at the bottom, or material will build up on the lower rollers.
Differential roller speeds assist with shearing the grain.
In many mills, one roller (the lead roller) will run faster than its partner (lag roller) will. The ratio of speeds is usually 1.25:1, and this along with the gripping action of the fluting helps to shear the grain apart. In some mills a motor drives the lead roller and the lag motor is driven from the lead roller via a gearing. In some modern mills, each roller is driven by a motor controlled by a variable frequency drive. Modern mills feature speed detectors that measure the RPM of each roller and can vary the speed differentials between each pair to control the degree of shear. Simple mills used in most small breweries have pairs of rollers operating at the same speed with no adjustment possible.
Mill gap
The mill gap determines the extent to which the grain is crushed. Setting and maintaining the gap is essential for determining the particle size profile of the grist. We will cover typical gaps for each of the mill types in the Milling Technology lesson. Gap sizes are optimised at the brewery to obtain the target particle size distribution and wort separation performance.
The mill gap and condition of the rollers needs to be checked regularly. Some modern mills have condition monitoring, but all benefit from a regular visual inspection. The gap can be verified by inserting feeler gauges between the rollers or by rolling a length of soft metal between the rollers and checking that it has been squashed to the necessary extent. The evenness of the rollers can be checked by rolling a square card between the rollers and confirming that it stays straight as it moves. A visual inspection would check for the following:
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Damage to rollers (nicks, scuffs, and chips)
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Damage to flutes (dulling of angles, flattening of grooves)
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Deformation of rollers (curves in surfaces, eccentric rotation)
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Wear on bearings (uneven and stiff rotation)
Any significant problems identified will lead to a replacement or refurbishment of the rollers. Mills should also be inspected for the build up of dust.