Is There Any Harm Milling Twice?

To mill once, or to mill twice? That is the question — but why shall a brewer mill at all? Brewers mill malted barley for two purposes, extract yield and husk preservation, and these purposes are opposed in terms of process optimization. Extract yield, measured by comparing wort density and volume to malt weight, increases as grist particle sizes decrease. In a lab setting, malt extract is measured on coarsely milled and finely milled malt samples, and the difference between the two values is one of many indices of malt modification. The lab method uses funnels and filter papers to separate wort from mash solids; time is not so critical because malt labs have lots of these filter set-ups and the time required for the filtration is just part of the method.

When it comes to practical brewing operations, the grist is rarely milled as finely as the finely milled malt used in lab analyses and brewers look to the coarse-ground, as-is (meaning the value is not on a dry weight basis) extract value (CG as-is) as a good indicator of what to expect in the brewhouse. Mill adjustment is one of the first things to look to when troubleshooting low extract yields. Brewhouse efficiency is calculated by comparing the extract yield in the brewery to the CG as-is value.

But extract yield is not the only value to focus on when it comes to efficiently producing a kettle full of wort. Time is money, and the gains in extract efficiency that come with adjusting the crush can easily be erased if wort collection times become too long. This is especially true in a commercial brewing operation where the most expensive piece of brewhouse equipment is the wort separation device (usually a mash tun, lauter tun, or a mash filter). The bottom line is that coarse grist makes for easier wort recovery.

So, you ordered unmilled malt, received some pre-milled and some unmilled, and decided to just mill it all to make things easiest. The good news is that everything turned out OK. If you had ended up with a stuck mash, I would be jumping into a discussion about how milling twice can bash up the malt husk and potentially cause you headaches. To neatly answer your first question about milling twice, try to avoid this in the future unless you are doing something to emulate multi-roll milling or are employing brew-in-a bag mashing.

Your question about milling flaked adjuncts is a really good subject with a bit of controversy surrounding it. For whatever reason, there are some brewers who are dead-set against milling flaked grains. One of the most common justifications given for this view is that milling flakes can end up with a gummy mash that does not make for easy wort separation. The primary point to this argument is that the stuff contained in flaked grains causes issues with run-off, so it’s best to keep most of this stuff in the flaked grains by minimizing extract efficiency. OK, that’s an interesting way to do things, but the other way is to mill the flakes and use less since the efficiency will increase with milling. Another totally radical approach is to mill the flakes and enzymatically address the gummy materials, primarily beta glucans and arabinoxylans, by adding a low-temperature mash rest and/or adding exogenous, cellulase enzymes to the mash.

I am in the mill the flakes camp. Flaked grains do not have any husk to be preserved and the majority of the solids contained in flakes are solubilized during mashing. There is no argument that cell wall constituents from certain flaked and unflaked grains, especially barley, oats, and rye, are quite effective at gumming up the works. The solution to this conundrum is found by varying the grist bill and/or mashing process, versus faking things by reducing yield.

As an aside, starchy adjuncts like rice and corn grits are typically milled finely because these ingredients are almost entirely comprised of starch and fine milling optimizes yield. Similarly, wheat malts and other huskless grains are often milled more finely than barley malts because there is no husk to preserve. Finally, when you do buy pre-milled grains, be mindful about storage conditions because they pick up moisture from the environment more readily than whole grains.

Response by Ashton Lewis.