What's the deal with AAU's?

I've only made 3 brews to date (so my experience isn't much), but every recipe I follow makes no mention of the AAU on the hops I need. They only list them by name. Are AAU's an un-needed known? I understand what they stand for, and it seems like it would be important to note on the recipe, so I'm keeping record of it in my brewday notes. The lady in the LBS keeps asking me what AAU I need, and I never know what to tell her. Also, does anybody know of a website where I can download a chart or something for hops percentage of bittering usage based on boil time?

I think the confusion may stem from the terminology. Hops have an alpha acid content, such as 10% let's say. Recipes usually call for an amount of hops given by AAU's (alpha acid units), which are related to but different from the alpha acid content of the hops. So, the formula is:

AAU = (alpha acid content) x (weight in ounces)

or to calculate how much hops you need given you know the AAU and AA:
(weight in ounces) = AAU / (alpha acid content)

If you have a recipe calling for 5 AAU's of a 10% AA hop, you would need:

5 AAU / 10% AA = 0.5 ounces of hops.

Usually you just buy the hops, whatever the AA is (it varies from year to year and crop to crop), and then given the AAU's in your recipe you calculate the amount of hops to use.

Groovy. So can AAU's be used before the boil to help determine IBU's in the final brew? And I guess a chart that helps with that is ultimately what I'm looking for. Basically I want to be able to know the IBU I'm looking for, and adjust my hopping as necessary. Unless I'm way off base with terminology here.

No, you're pretty close. I don't actually know the IBU formulas as I just use Promash. You could use the beer reciprocator if you don't have any software that calculates it. Actually, I doubt the formulas are that difficult, I just know them offhand.

It's a function of AAU's and time to get to IBU's in the brew.

And just to make it more complicated, AAU's are also knows as HBU's (Homebrew Units) in some circles.

AAUs are especially helpful if you are using a different bittering hop than the type the recipe calls for. i.e. if the recipe calls for 2 ozs Northern Brewer at 8% AA, but you want to use Columbus at 14%, you know that you need 16 AAUs, which 1.14 ozs of the Columbus.

Groovy, thanks for the help guys. And thanks to anybody else that has anything to add.

Here's a great page that explains the calculations: http://www.howtobrew.com/section1/chapter5-5.html

There's also a page describing this in Papazian's book. If you don't have it, you need to make that your next purchase!

If you go to google, and search on "convert aau to ibu" you won't get a direct answer, but you will get a lot of good links

-a.

JeepGuy said:

Are AAU's an un-needed known?

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There are known knowns, known unknowns, unknown knowns and unknown unknowns but ive never heard of an un-needed known?

horaay for double-speak!

heres a simple formula for ibu's:

amount hops (oz.) x a.a.u x percent utilization/ 6.7

your utilization will range from 20 (2-3 gallon boils) to 35 (5-6 gallon boils) for 60 min.

so if you have 1 ounce of chinnoook @ 8 a.a.u and you boil 3 gallons for 60 min
youll get 1x8x20/6.7=24 ibu's

for a 1/2 hour do 10 percent etc...(1x8x10/6.7=12)
or for a 4 gallon boil do like 30 percent utilization.

the 6.7 is actually 5 (gallons) x 1.34 or 6.7

Bjorn Borg said:

There are known knowns, known unknowns, unknown knowns and unknown unknowns but ive never heard of an un-needed known?

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I have! An "unneeded known" is me replying to this....my opinions are totally unnecessary......but you knew I couldn't keep my mouth shut!!