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: