Hop AAUs, or “Homebrewing, why do you make me do math?”

I recently picked up a recipe from Brew Your Own for a SMaSH pale ale. In the ingredient list, it called for 12 AAU of Citra (1 oz @ 12% alpha acid), not a weight. This was new to me and going to BeerSmith, I couldn’t find a way to enter an amount of hops and have it give me an AAU value. Help!

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So I googled “AAU hops” and found some interesting information. Of course, John Palmer’s web site came up first in the results, under the title “Hop Bittering Calculations”. I knew I was on the right track. So according to his page, AAU is calculated by multiplying a certain weight (in ounces) of hops by its % Alpha Acids. As an example, if I have 1.5 oz of 6.4% AA Perle hops, that gives me:

AAU = Weight (oz) x % Alpha Acids

AAU = 1.5 oz x 6.4 = 9.6 AAUs of Perle

So the next important question that you might be asking is “Who cares?” It’s important though, especially for hops that have large variances in their alpha acid percentages. Have you ever looked at the same hops variety over time? The alpha acid % can vary widely based on many factors, such as time of year, soil composition, and what location they were grown in. If you have such large variances, it’s meaningless to put in a recipe that you need 1 ounce of that hop. One ounce in January may contribute something very differrent than an ounce in July. That’s where AAUs come to the rescue.

AAUs put all hops on a level playing field. Let’s go back to my recipe. I need 12 AAU of Citra. If I have a 1 oz pouch of Citra at 14.5% alpha acids, here’s what I’d do:

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To get 12 AAUs of Citra hops at 14.5% alpha acid, you would need:
12 AAU = ? oz x 14.5% AA
? oz = 12 AAU / 14.5.5% AA
You would need 0.827 oz of Citra hops (12/14.5)

The more alpha acids your hops have, the less of that hop you need to get the same contribution. It’s actually a pretty elegant solution to variances in alpha acids. Now you know what to do if you run across a recipe that doesn’t tell you how many ounces of a hop to put in, but instead tells you to add this many AAUs of that hop.

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