Milling — Theory and Practical Consideration
Milling is an important step in the brewing process. The purpose of milling is to crush the grain kernels in order to expose the starches to liquid during the mashing process. Ideally, the kernels should be crushed in a way such that the inner starches are ground into relatively fine particles, but not so much as to turn the starch component into flour. A finer crush produces a higher surface area of the starch in the mash, and this allows the starch to gelatinize more easily and be more readily available for conversion into sugars. This should, theoretically, produce a higher extract per pound of malted barley. But there is a practical trade-off regarding fineness of crush. If the grain starch components are so finely crushed that there is a significant amount of flour in the mash, there is an increased likelihood of a “stuck” mash and the corresponding reduced extract efficiency. Additionally, if the grain is crushed to the point of flour, it is likely that the grain husks have also been ground up to a point such that there will be an increase in the amount of tannins and other less desirable husk components extracted into the wort.
It is important to obtain a good crush such that the inner starches are ground into relatively fine pieces, but the kernel husk is left mostly intact. Allowing the husks to remain relatively intact is crucial, as the husks form a filter bed in the lauter tun and prevent the run off from becoming stuck. The filter bed also helps to clarify the wort during the run off. If the husks are ground too finely, they will not form a good filter bed structure. Without a good filter bed, sparging and run off will be problematic.
Roller Mill Configuration
The most common type of milling equipment used by homebrewers is a hopper-fed roller mill. The roller mills used by homebrewers typically consists of two metal rollers separated by a gap. Usually only one roller is powered and the second roller is allowed to freely rotate. Because of this, the powered roller rotates faster. Grain is fed by gravity into the mill from a hopper, as shown in Figure 1 below.
Roller Mill Crush Quality
There are many variables that affect the quality of crush when using a roller mill. Physical characteristics of the grain and the configuration and operational parameters of the mill directly influence the quality of the crushed grain.
Crushed Grain Quality
The physical characteristics of the grain will impact the quality of the crush. Important variables related to the grain include moisture content, grain size distribution and friability. Grain with higher moisture content is more pliable and less brittle, and so will be broken apart less during milling than grain with lower moisture content. Grain size distribution is important because it is related directly to the variability of the size distribution of the crushed grain. Grain with less variability in size distribution will produce crushed grain with a more uniform size distribution. Grain friability is related to moisture content and refers to the overall tendency of the grain to shatter under stress. Grains that are characterized as very friable will be very easily broken into smaller pieces during milling.
Configuration and Operational Parameters
Roller mills crush grains through a combination of forces. If the mill rollers rotate at the same speed, compression is the primary force used. If the mill rollers rotate at different speeds, both shearing and compression forces are important. If the mill roller surfaces are grooved or textured, a tearing or grinding force is also present.
Mill roller gap distance is important because a smaller gap will produce a finer crush. A typical gap setting for a homebrewery roller mill is between about 0.035–0.055 inches (0.9–1.4 mm). (You can measure the gap size with a tool designed to measure spark plug gaps.)
A more uniform roller gap will produce crushed grain with less variability in the particle size. Faster absolute speed will produce a finer crush, while a slower absolute speed will produce a coarser crush. Relative differential speed of the rollers is directly related to the amount of shear that the grain experiences during milling. Rollers with a higher amount of differential speed (one roller moving much faster than the other) will impart a much greater amount of shear stress to the grain and will produce a finer crush. The higher shear stress is also more likely to tear apart the grain husks.
Roughness and disposition of the roller surfaces is important because the way the grain contacts the rollers during the milling process affects the magnitude and types of forces that are transferred to the grain. Some rollers are textured and configured in such a way as to purposefully try to impact the way that force is transmitted, and therefore impact the characteristics of the crushed grain. The impact of roughness and roller disposition is illustrated by research that was conducted by Fang and Campbell. These researchers investigated the effect of roller disposition on the breakage of grain kernels during roller milling and found that different dispositions produce different particle size distributions. Figure 2 (above) shows the roller dispositions that were investigated. It illustrates four different scenarios in which the surface characteristics and the working angles of the texture of the roller surfaces are different. Because of these differences, the forces acting on the grain are different in terms of direction, magnitude and shear-force intensity, and so the grain is crushed differently. In the S-S (sharp to sharp) disposition scenario, grain husk and endosperm break together resulting in a broad and relatively even distribution of particle sizes. The grain husk is not left intact. In the D-D (dull to dull) disposition scenario, the force transmitted by the mill rollers to the grain is primarily compressive, which causes disintegration of the fragile endosperm while leaving the grain husk relatively intact. This results in a U-shaped particle size distribution, with many small (endosperm) and large (husk) particles, but few in the midsize range. The particle size distributions resulting from the mixed dispositions (S-D and D-S) lie between these extremes. The differences in particle size distribution clearly show the importance of mill roller roughness and disposition on the quality of the crushed grain.
Practical Considerations
You can determine the quality of your crushed grain by visually examining the grain discharged from your mill. Ideally you should see few or no uncrushed kernels, and have the majority of the kernels broken into two to four pieces with only a very minimal amount of flour. As a rule of thumb, having approximately 70% (by weight) of crushed grain retained on a #14 sieve is a good crush.
If your grain looks like it is crushed too finely, or if you get high extract efficiencies, but experience stuck mashes and astringent beers, you have several options. You can widen your mill gap setting or slow down the speed of the mill rollers, either by cranking more slowly (in the case of a hand-cranked mill) or by manually “throttling” the speed of drill. Ideally, you can motorize your grain mill with a motor and a system of sheaves to run at an optimal speed (for homerew-sized roller mills) of 150–200 rpm, as opposed to the over 1,500 rpm that most drills run at. Construction details are explained in the online article at: http://www.byo.com/component/resource/article/1171-motorize-your-grain-mill-projects
If your grain looks like it is not crushed enough, or if you get lower than desired extract efficiencies, you can crush more finely by doing the following — adjusting your mill to a smaller gap setting or increasing the speed of the mill rollers. You can also mill the grain a second or third time.
In general, ensure that the mill roller surfaces are clean and dry before milling. Use a spark plug gap gauge to ensure that the gap between the mill rollers is uniform along the full length of the rollers. Be sure that the grain being fed into the mill is entering the mill roller gap constantly, and that the grain is spread across the whole length of the rollers as much as possible. Some commercial breweries condition (wet) their grains slightly immediately before milling to yield smaller starch particles but larger husk particles. See the March-April 2010 issue of BYO for how to do this at home. Properly milled grains are very important to the quality of your finished beer. Take the time to run some trials in which you vary your mill gap and roller speed. “Sacrificing” a little grain in this way will allow you get a better crush, allowing for better extract efficiency, easier run off and no astringency.
Figure 1
Figure 2
