Originally Posted by newbrewer
I'm new to brewing, I have read up about it for a while and am going to start. I am buying equipment tommorow and will start my first batch of ale soon but the recipe I found measures the amout of hops in aau. I have never heard of this measurement before and was wondering how to convert it to grams which is what the local ubrew sells by. thanks in advance
Hi newbrewer. Welcome to homebrewing!! Good luck and have fun.
Now on to your question ...
AAU stands for Alpha Acid Unit and its a measure of the bittering potential for a given hop. The same hop can have slightly different bittering levels from different sources and year-to-year. For instance, you can't be sure that Cascade hops will always be 6.3%. You might find some Cascade at 6.0% or 6.8%. By using AAUs you can better guess at the impact a given hop will have on your brew.
The calculation for AAUs is ...
AAU = (alpha acid %) * (weight)
So if your recipe calls for 10 AAU for bittering hops and you want to use Cascade hops with an Alpha Acid percentage of 6.8% you'd need 1.47 (or almost 1.5 oz) of hops to reach the 10 AAU
10 = 6.8 * 1.5 (I know it's actually 10.2 but I'm rounding )
Now let's say the Cascade crop comes in next year with a lower Alpha Acid percentage of 5%. If you used the same 1.5 oz you'd only get 7.5 AAU. To reach the same level of bitterness, you'd need to use 2 oz of hops.
It's all about consistency and repeatability.
You may also see these listed as HBU (Homebrew Bitterness Units). Charlie Papazian usually uses HBU. They are the same actually.
If you want to get real deep into this subject then try this page ... http://realbeer.com/hops/FAQ.html
Enjoy the new hobby.