alphaacid

Alpha acids are a class of bitter-tasting compounds found in the resins of hops (Humulus lupulus). The primary alpha acids are humulone, cohumulone, and adhumulone, along with several minor members. They are responsible for much of the bitterness in beer and define, in large part, the bittering potential of a hop variety. The alpha acid content of hops is typically expressed as a percentage by weight, and it is used to estimate how much bitterness a given amount of hops can contribute during boiling.

When wort is boiled, alpha acids undergo an isomerization reaction to iso-alpha acids, which are more soluble and impart a characteristic bitterness. The extent of isomerization—and thus the resulting bitterness—depends on factors such as boil time, wort gravity, and temperature, as well as the form of hops used (whole hops, pellets, or extracts). Bitterness is commonly expressed in bitterness units (IBU), a measure influenced by the alpha acid content, hop quantity, boil duration, and post-boil processing.

Alpha acids vary widely among hop varieties. High-alpha hops yield more bitterness per unit weight and are often used in bittering regimes, while low-alpha hops are favored for aroma and flavor additions later in the boil or during dry hopping. Storage and handling influence alpha acid stability: they degrade with time, light exposure, and heat. For best shelf life, hops are stored in cool, dark conditions, with pellets generally offering longer storage than whole hops and freezing providing extended stability for long-term storage.

In brewing literature, the term "alpha acids" is standard, while "alphaacid" may appear as a shorthand or variant spelling.