Caution: technical stuff and math follow!
What do you think when you see a beer recipe that calls for hops measured not in ounces but in HBU’s? or when you read a label and it tells you the beer has so many IBU’s? Ever notice on that packet of hops you bought at the Home Brew Store that the hops are rated based on aa%? What gives? How do you figure out how much to use? It’s not as complicated as it looks at first blush.
AAU’s and aa%: The relative bitterness of a given crop of hops is measured based on the acidity of the flower. The acid that brewers are primarily concerned with is the “alpha acid”, or aa – this is measured in a percentage, hence aa%. The higher the number, the more acidic and thus the more bitter the hop. The Hallertau I used this morning measured 3%, the Perle 6%, so the Perle were twice as bitter as the Hallertau. An AAU (alpha acid unit) is the product of the percentage of bitterness times the amount used – 1 ounce of those 3% Hallertau provides 3 AAU’s, one ounce of the Perle provides 6 AAU’s.
More common and relevant to the homebrewer is the HBU – Homebrewer’s Bitterness Unit – but it’s exactly the same thing as an AAU. If your recipe calls for 6 HBU’s of a given hop, work backwards. If you have a Chinook hop at 12% aa, you need to use 1/2 oz. to get 6 HBU’s (or AAU’s) – right?
IBU’s – International Bitterness Units – are the result of using the hop. There is actually an esoteric table which gives an indication of how bitter a beer will be based on how much of which hops are boiled, and for how long. So IBU’s are the sum total of all the hops added in a given beer, multiplied by a factor depending on length of boil and volume and strength of the wort. For more information on this, I recommend the excellent discussion in Greg Noonan’s “New Brewing Lager Beer”.
In a nutshell, the average homebrewer can use the following simplified tables, which I used in “North American Clonebrews”, although it is based on a table Greg devised for the “Seven Barrel Brewery Brewers’ Handbook”.
Assuming moderate/average strength wort, in a 5-gallon boil:
- 0 minutes, or dry-hopping – .7
- 5 minutes – .7
- 15 minutes – 1.25
- 20 minutes – 1.5
- 30 minutes – 2.4
- 45 minutes – 4.0
- 60 minutes – 4.25
- 90 minutes – 4.5
So one would multiply the HBU’s (aa% times ounces) times the above factors, to get the IBU’s.
If one boiled only one ounce of a 4.5% aa hop for 60 minutes, one would have 19.1 IBU’s (4.5 x 4.25)
But if one also boiled one ounce of a 2% aa hop for 5 minutes one would end up with 20.5 IBU’s (add another 1.4, or 2 x .7).
So work backwards – if you know the approximate IBU rating you are shooting for, it is relatively easy to calculate the amount of hops, the desired aa%, and the length of boil. At least I think it is…
Looking at the recipe I posted earlier for the Dortmund Export, we see a total of 22.5 IBU’s. 3.2 of that came from the first addition of Hallertau (.25 x 3 x 4.25); 12 from the 45-minute addition of Perle (.5 x 6 x 4); 5.4 from the second batch of Hallertau (.75 x 3 x 2.4) and the last 1.9 from the final addition of Hallertau (.5 x 3 x 1.25).
So a quick quiz: If you boil 1/2 oz. of Galena at 8% aa for 90 minutes, 1 oz. Tettnang at 4% for 45 minutes, and dry hop with 1 oz. Saaz at 5%, how many IBU’s will the beer have? (Answer follows)
.5 x 8 x 4.5 = 18; 4 x 4 = 16; 5 x .7 = 3.5; total 18 + 16 + 3.5 =37.5 IBU’s.