Iso-Alpha Acids are the thermally induced isomers of alpha acids and the principal source of bitterness in beer. They are bitter, have surface activity, and are reactive with oxygen and other beer constituents. As a class of compounds, they are composed of individual acid homologs, which are adhumulone, cohumulone, humulone, prehumulone, and posthumulone. The isomerization from an alpha acid into an iso-alpha acid is a chemical process that maintains the alpha acid’s original material composition but essentially rearranges the compound’s molecular structure, altering its chemical properties and reactions.

Compared with other constituents of beer, such as carbohydrates from malt, ethanol, and carbon dioxide, iso-alpha acids occur at relatively low levels—typically at 8 to 25 ppm and in rare cases as high as 100 ppm. Although only present in minute amounts, even small changes in the concentration of iso-alpha acids can have a disproportionate impact on the beer’s flavor, its bitterness, and its overall drinkability. Iso-alpha acids are intensely bitter and have a human detection threshold in beer of approximately 6 to 7 ppm. Because bitterness perception is somewhat subjective, some studies allow for a wider range of 4 to 11 ppm. See bitterness. To put this into perspective, many American lager brands have iso-alpha acid levels at or below 10 ppm, which actually means that a significant portion of beer-drinking consumers may not even detect any bitterness in some American mass market lagers. Different beer styles typically have very different bitterness levels as part of their specifications. European lagers, for instance, may have an average iso-alpha acid content of 20 to 30 ppm, British ales of 25 to 40 ppm, and the wide range of modern craft brewed beers of 10 ppm to 100 ppm.

Iso-alpha acids are formed typically during wort boiling, when hops are added, and alpha acids are extracted from the hops’ lupulin glands (or from the hop resin in hop extracts). Isomerization is the result of the effect of heat on the alpha acids. The process is both time and temperature dependent. The longer the alpha acids are exposed to the rolling wort boil, the more alpha acids are converted into iso-alpha acids and the more bitterness is created in the wort and beer. See boiling. Hops that are added late in the boil, on the other hand, still release their aroma oils—and these will not all evaporate into the kettle stack—but a much smaller proportion of their alpha acids will become iso-alpha acids.

Hop acids can also be isomerized before they are added to the wort kettle. For this there are several approaches. There are preisomerized hop pellets on the market. These are produced by mixing finely milled hops with a small amount of about 1.5% by weight of magnesium oxide. This mixture is then pelletized, vacuum packed, and stored hot at about 45°C (113°F) to 55°C (131°F) for 10 to 14 days. During this time, the small amount of magnesium serves as a catalyst to accelerate the formation of iso-alpha acids. The addition of isomerized kettle extracts is another method. These are prepared similarly to preisomerized hop pellets by mixing magnesium oxide or a potassium carbonate/hydroxide solution with a hop resin extract and then heating the mixture to produce iso-alpha acids. Then there are preisomerized iso-alpha acid extracts, which are produced by separating alpha acids from beta acids after supercritical CO₂ or ethanol extraction and then processing the result further downstream to convert alpha acids to iso-alpha acids. These extracts are used either during fermentation or, more typically, postfermentation to give beer the desired level of bitterness. The unit of measurement of bitterness in beer and wort is the international bitterness unit. See hop extracts and international bitterness units (ibus).