Along with alpha acids, hops contain beta acids, principally lupulone, colupulone and adlupulone. These are rarely considered separately, but the beta acids as a whole are important to a beer's flavor.
The beta acids do not produce as much bitterness during the boil as the alpha acids, but during fermentation and storage, as alpha acid bitterness breaks down, beta acids slowly create bitterness through oxidation. This affects the long-term character of aged and lagered beers.
Beta acids are given by hop producers as either a total percentage of beta acid in the hops by weight, or as a ratio of alpha to beta acids. Some people consider beta acid bitterness to be "harsher" than alpha acid (or at least humulone) bitterness, and look for hops with low total beta acids. However, the traditional noble hops generally have an alpha to beta acid ratio of close to 1:1, which is therefore considered desirable by some. Other brewers prefer a 2:1 ratio, which is thought to yield the most constant bitterness in aged beers.
