Glossary
Alpha acid
The group of hop resins — principally humulone, cohumulone and adhumulone — that isomerise during the boil to become beer’s main source of bitterness.
Also called alpha acids, aa, alpha.
Alpha acid content is quoted as a percentage of hop weight and is the single most important number on a hop package, because it determines how much hop is needed for a given bitterness. It varies by variety from around 2% for a traditional English aroma hop to over 18% for a modern high-alpha bittering variety.
It also varies by harvest year, growing conditions and storage, which is why a responsible supplier states the actual figure for the lot rather than the variety average. Alpha acids degrade in storage, faster when warm and in contact with oxygen, so an old hop bitters less than its label claims.
Related
- Isomerisation — The reaction that makes alpha acids bitter and soluble.
- Beta acid — The related resin group that does not isomerise.
- Cohumulone — The alpha acid fraction argued to bitter differently.
- IBU — The measure of the isomerised product in finished beer.
More vocabulary
ABV · ABW · Alcohol unit · Alkalinity · Apparent attenuation · Attenuation · Beta acid · Cohumulone · Degrees Lovibond · Degrees Plato
See every term on the glossary index.
Last reviewed 2026-08-16.
Evidence
What BeerHQ knows about this, and how. Every statement carries the state of the evidence behind it rather than being presented flatly as fact.
A hop variety’s alpha acid content varies materially by harvest year, growing region and storage conditions, so a variety-level figure is a typical range rather than a specification and suppliers state the analysis for each lot.
Backed by a source
Sources: Hop Growers of America variety data; Brewing: Science and Practice — print
Alpha acids oxidise and lose bittering potential in storage, faster at higher temperature and with oxygen exposure, which is why hops are packaged under vacuum or inert gas and kept cold.
Backed by a source
Sources: For the Love of Hops — print; Brewing: Science and Practice — print
The iso-alpha-acids that hops contribute do not merely taste bitter — they act as proton ionophores, collapsing the proton gradient across a bacterial membrane, which is the mechanism behind hops preserving beer.
Checked against the source
Source: Bokulich & Bamforth, "The Microbiology of Malting and Brewing" (2013), Gram-positive bacteria (Microbiol Mol Biol Rev 77(2):157–172)