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The Ins and Outs of Alpha Acid Units

Alpha acid units offer a convenient way to adjust recipes for differences in alpha acids—the substances that make beer bitter.

Dave Carpenter Aug 23, 2015 - 5 min read

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The Northern Hemisphere hops harvest is almost upon us. Before you know it, a whole new flock (herd? pack? litter?) of Cascades, Goldings, Hallertaus, and Mosaics will make their way to homebrew stores far and wide. Each year’s crop is a little different from the last, especially in terms of alpha acids, the substances that, after having been boiled, make beer bitter.

Homebrew recipes usually express hops additions by weight. That’s fine for flavor and aroma, but the earlier in the boil you add the hops, the more important it is to know the alpha acid percentage. After all, a 5 percent alpha-acid hops addition boiled for an hour will deliver half the bitterness of the same weight of a 10 percent alpha-acid hops boiled for the same length of time. Alpha acid units offer a convenient way to adjust recipes for these kinds of differences.

An alpha acid unit (AAU) is simply the weight of a hops addition multiplied by its alpha acid percentage:

AAU = Weight x Alpha acid percentage

What it amounts to is the actual amount of alpha acid, by weight, that you add to the kettle in a given hops addition. If you know how much alpha acid you need to add, then you’re free to swap hops left and right, as long as you keep that number the same.

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Let’s illustrate it with an example. You’re brewing a basic American pale ale, and the recipe calls for a 60-minute addition of one ounce (28 grams) of Chinook hops with 12 percent alpha acids. Let’s also say that you were unable to get Chinook at the homebrew store, so you bought Nugget instead. But your Nugget hops are 14 percent alpha acids, not 12 percent. In order to do the substitution, you’d first calculate the number of AAUs that the original recipe called for with Chinook:

AAU = 1 ounce (28 gram) x 0.12 = 0.12 ounce

Now you want to deliver that same amount of alpha acid using your 14 percent Nugget hops, so just use the equation the other way around:

Weight (Nugget) = AAU ÷ Alpha acid percentage (Nugget)

Weight (Nugget) = 0.12 ounce (3.4 gram) ÷ 0.14
Weight (Nugget) = 0.85 ounce (24 gram)

So, in order to achieve the same level of bitterness using our 14 percent Nugget, we only need 0.85 ounces (24 grams). Now, this isn’t a big change, and in all honesty, you could probably get away with just swapping them ounce for ounce, and your friends would be none the wiser.

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But sometimes alpha acids can be very different, even among the same hops variety. A few years ago, I bought a large quantity of East Kent Goldings (EKGs) that had alpha acid content of just over 4 percent, and I formulated a number of single-hops recipes around that crop (I was experimenting with cask ales).

The next year, the EKGs I purchased came in at a whopping 6.7 percent, which makes a big difference:

AAU = 2 ounces (57 gram) x 0.04 = 0.08 ounce

Weight (High alpha) = AAU ÷ Alpha acid percentage (High alpha)

Weight (High alpha) = 0.08 ounce ÷ 0.067
Weight (High alpha) = 1.2 ounce (34 gram)

In this case, I needed to reduce my bittering addition by almost half to account for the difference in alpha acid.

Remember, the later you add hops in the boil, the less bitterness they contribute. So AAUs are strictly just relevant for early additions. You can substitute hops late in the boil, and all you’ll change are the aromatic and flavor qualities of your beer. Practically speaking, it’s probably worth thinking about AAUs only for hops that will be boiled for half an hour or longer. The stuff at the end doesn’t matter as much.

To really dial in the process, you need to work with International Bitterness Units (IBUs), which is a topic unto itself. But the AAU works just fine for 60-minute bittering additions, and it’s a convenient way to swap the hops you have for the hops in a recipe.

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