Milling is the physical crushing of malt kernels into smaller particles in preparation for mashing and lautering. The various milling processes need to be manipulated carefully to find a balance between a grind that is too fine and one that is too coarse. See lautering and mashing. The finer the grind, the greater will be the amount of sugary wort that can be extracted from a given amount of grist. See wort. In theory at least, finely ground malt flour yields by far the most economical extraction in the brewhouse. In practice, however, the opposite is true; the finer the grind, the more the mash will clump and become sticky, the malt husks having been pulverized along with the starchy endosperm. Pulverized husk cannot not serve to “fluff up” the grain bed for the proper rinsing of sugars out of themash by the sparge water. See sparging. At the other extreme, a very coarse grind, although it would make for a well-draining grain bed in the lauter tun, would reduce the surface area of the grist that is exposed to the grain enzymes. Thus, beta-glucan, protein, and starch conversion could be deficient. See beta-glucans, proteins, and starch. Because both excessively fine and excessively coarse grinds mean a reduction in brewhouse extract yield, the actual grind used by most brewers in the real world is a compromise between theory and practice. For optimum brewhouse efficiency and yield, therefore, the mill must be set to produce a mealy rather than chunky or floury grist, whereby the gap setting depends on many factors: the type, size, hardness, and friability of the kernels; the way the grist has been prepared for milling (wet or dry); and the type of equipment that does the milling. See dry milling, friability, hammer mill, and wet milling. As a general rule, a combined mash–lauter tun (as is used for single temperature infusion mashes in the UK and in some small breweries elsewhere) requires a fairly coarse grind so as to avoid a poor runoff or stuck mash. A separate lauter tun can usually take a finer grind, which gives better extract efficiency; a mash filter can take an almost floury grind. A mash filter is thus by far the most efficient wort-extraction device, but it is also the most expensive, which is why it is usually employed only in larger breweries. See lauter tun, mash filter, and mash tun.

The grist mill at the Hook Norton Brewery in Oxfordshire, England, dates from 1899 and is powered by a steam engine from the same year. cath harries

Mill settings, therefore, are always a balance between two incompatible requirements, the maximization of yield and the practical workflow in a brewhouse. At the same time, they have an enormous influence on the biochemical transformations in the mash tun, on brewhouse yields, on the composition and the quality of the wort, and finally on the taste of the beer. A fine, flour-like powder—although the most desirable grind for rapid and optimal enzymatic conversions of all grist components—might also leach too many undesirable substances into the wort and beer, such as silicates, lipids, husk-derived tannins, and large-molecular proteins, which can cause deposits, hazes, and a shorter shelf life. Given the variability of the grist from one harvest year to the next, from one vendor to the next, and between the grain bills for different recipes, milling is as much of a fine art as are the other parts of the brewhouse process.