One of the most common questions we get here at MoreBeer! is “what is the ideal gap setting for my grain mill?” While that seems like a pretty straight-forward question, the answer couldn't be more nuanced. The “ideal” gap setting for producing a fine crush on my grain mill, might not be the right crush at all for your brewing system. Your brewing system might require a coarser crush to prevent a stuck mash and your mill might have different diameter rollers. Couple that with the fact that grain varies in kernel size and crushability from different harvests and maltsters, this opens up a real discussion.
If you’ve ever bought pre-milled malt
from a homebrew shop or online retailer, you’ve probably noticed that the crush is very coarse. This is done because this is the “safest” crush that should work on the most brewing systems, however, it is not great for brewing efficiency. A lot of published homebrewing recipes
take this into consideration, and will have incorporated an extra pound or two of malt into the recipe’s grain bill. An extra pound or two per batch isn’t going to cost much on a homebrewing scale, but in a commercial setting, it will add up greatly over time. While professional brewers need high brewing efficiency to keep the cost per batch down, for homebrewers, securing consistent brewing efficiency is more of a right of passage, or proof of an understanding of the craft, more than a way to save money on ingredients.
This makes it easier to understand how there can’t be just one magic, universal gap setting for homebrewing. When it comes to gap settings, you’ll always want to “mind the gap,” which is just another way of saying that your crushed grain batch after batch requires some attention and modification to be consistent. Understanding the difference between coarse and fine crushes helps with efficiency, and will help to prevent a stuck mash. Consistency is key, both in replicating recipes, and for understanding results in your finished beer.
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Understanding a COA and a Sieve Test
When I started working full time in the industry with MoreBeer!, one of the first projects I began working on helped me to learn a great deal about malt in general. We were working with Viking Malt
to develop a new base malt geared for the North American market, which led me to begin familiarizing myself with all of the aspects of a malt analysis sheet, otherwise known as a Certificate of Analysis, or COA. On a COA, you’ll see a lot of detailed information, and two of the most important specs with regards to gap settings and crush are “friability” and “lot assortment,” which we’ll focus on in this article. Friability is the tendency for a kernel to break into smaller pieces when pressure is applied. Lot assortment is the indicator of the kernel sizes of that batch of malt. It is calculated by counting the weight of kernels in a sample assortment caught on different sized screens. Ideally you want a majority of your kernels to be plump and caught in the top screen.
To help demonstrate how friability and lot assortment affect your crush at different gap settings I performed a sieve test on several popular base malts. The base malts I used were -- Viking Xtra Pale
, Rahr 2-Row
, and Admiral Maltings Gallagher’s Best
. Using a standard 2-roller homebrew mill
, I crushed each malt using four different gap settings. The gap settings were 1.25 mm, 1.0 mm, 0.75 mm and 0.50 mm, and I used a feeler gauge to confirm these gap setting between each milling. After milling, I performed a grain sieve test on the different malts and crushes, and recorded all of the results. While performing a sieve test is pretty straightforward, doing sixteen of them takes quite some time.
To mimic this test, you’ll want to place three rubber balls on each of the 3 standard US test sieve sizes, with the bottom pan under the finest screen -- #14, #30, and #60. The sieves must be stacked in a specific order, with the widest screen at the top (#14) and the finest (#60) on the bottom. The grain that’s being measured is then put into the top sieve, and the lid is put on before beginning the testing process.
For my example, I used 100 grams to make it easier, but up to 130 grams can be used at a time with the US standard test sieves. For the most accurate representation of the crush, your test should include husks, kernels, and powder. You’ll need a smooth and flat surface to test in order to slide the test sieves back and forth a few times. The sieves have to move 18 inches in one direction and then the other in about 0.5 seconds each way, for a total of 1 second in each cycle. Every 15 seconds, you’ll need to pat the sieves downward on the work surface. This cycle must be maintained for three minutes total, and once completed, empty each sieve, brush them out completely, and weigh and record the results. You can calculate the percentage of each sieve by dividing each fraction by the sum of all fractions weights, then multiplied by 100.
Example: #14 + #30 + #60 + Pan = Sum
#14 / Sum x 100 = percentage
Next, repeat for screens 30, 60, and the pan. Because we started with 100 grams of sample material, the percentages will be really close to the original fractions.
Knowing the friability and lot assortment of these malts prior to conducting this test (Figure A), I was anticipating a greater variance. To my surprise, the biggest gap (pun intended) in percentages was 7%. This percentage is still almost enough of a difference to go from a “standard crush” to a “fine crush” (Figure F) with the exact same gap setting, by doing nothing other than switching the base malts. This is where that old phrase “mind the gap,” becomes valuable. In this context, “mind the gap” isn’t just an old adage, it’s meant to act as a reminder of the importance of checking your gap setting with each batch of beer that you brew.
If we use Admiral Malts milling spec (Figure F) as our reference point, we can see that with these specific lots of grain the following gap settings landed us roughly in these spec ranges.
So...What is the Ideal Gap Setting for my Grain Mill?
Now, knowing what we know, let’s see if we can attempt to answer the original question: “what is the ideal gap setting for my grain mill?” We should assume we’re talking about most 2-roller homebrew mills
, which have a 1.25” diameter roller, and you’re brewing with a mash tun that has a good false bottom. With these things in mind, you could start with a gap of a little over 1.0 mm, and adjust from there. If you don’t have a feeler gauge to measure the gap, you can use a credit card to help measure, which is about 0.76 mm. When we talk about an “ideal” gap setting, you want to adjust the gap to find a highly efficient setting to work for your brewing system, but which is not fine enough to cause a stuck mash.
Milling your grains at home allows you to work with the freshest possible ingredients, but it will also allow you to dial in the best crush on your grains for maximum efficiency. Because barley is an agricultural product, its friability and kernel size assortment is always going to vary year to year with the harvest, and from maltster to maltster. The most important takeaway from this article should be that gap settings are not universal, and the crush should be monitored during every milling to ensure that the malt you’re working with is being properly crushed. While sieve tests are a great way to get the most ideal crush for your brewing system dialed in, paying close attention to the crush quality during each milling you do with help you to get the most consistent crush. When you master achieving consistent crushes, consistent batches are what are sure to follow, and brewing consistency is what makes a good brewer.
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