Why Scotch Distilleries Love their Mills

If you’ve ever been on a distillery visit in Scotland, you’ve probably seen a mill. You’ve been told its brand (Boby, maybe, or the famously self-defeating Porteus brand). You’ve probably heard about that all-important ratio of flour to grits to hulls (usually in the ballpark of 10%/70%/20%). While it’s not the sexiest piece of equipment, the device that grinds whole malted barley grains into grist is certainly one of the most critical tools in the entire distillery.

The early stages of whisky making look a lot like brewing beer: malted barley is ground up and brewed in hot water (a process called mashing) to extract as much of the sugar from the malt as possible. Then resulting sweet liquid, called wort, is drained off and sent to the fermentation stage.

It sounds simple, but in practice, mashing is surprisingly complicated. Different water temperatures extract different sugars and other compounds from the barley, so many whisky makers use several different watering phases, each one applied at a different temperature. Many reserve the final wash of each mash, which contains the least amount of sugar, to re-heat and use for the first watering of the next mash, which conserves resources.

After this malting barley is kilned, it’ll need to be milled, and the right grind is essential.

Then, there’s the matter of separating the wort from the grain. In the simplest mash tuns, water is sprinkled from the top of the vessel and allowed to trickle through the grain bed. The wort then drains out from a valve in the bottom of the mash ton underneath a false bottom, which is essentially a screen placed a small distance above the bottom of the vessel. Lauter tuns are equipped with a rake that agitates the grain bed, helping the liquid drain more quickly. (They can also be run in reverse, helping clear the spent grain out of the tun.) And some large distilleries even have mash filters, expensive pieces of equipment that are even more efficient than lauter tuns. But for each of these systems to work, the grind of the grist has to be just right.

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“Milling malted barley is all about a trade-off between adequate rate of extract recovery and the percentage of sugars collected,” says Paul Hughes, assistant professor in the food science and technology department at Oregon State University. “So coarse milling gives a fast run-off but lower sugar yields.” Conversely, finely milled grist yields more sugar, but at a cost: it takes a longer time for water to filter through the grist.

Anybody who’s ever tried to make French press coffee with very finely ground beans knows how hard it can be to force water through a dense particulate bed, and whisky making is no different. In fact, it might be worse, because flour and water makes paste. Because of this, the grind of the mill is measured and evaluated every day to ensure the ratio of flour to grits to husks is just right. Too much grit, and yields are damaged. Too much flour? Well, let’s just say that imagining the nightmare of coping with a gummed-up mash tun filled with thousands of pounds of wet flour is enough to make anyone understand exactly why Scotch distillers are so proud of their hardworking mills.