Alpha Acids are the principal components in lupulin, the resin of the hop cone. They are of great interest to brewers because they are the main bittering agent in hops. Chemically, alpha acids reside in the soft-resin fraction of the lupulin, which is soluble in hexane. They are expressed as a percentage of the total weight of the hop and exist as complex hexagonal molecules. Alpha acid analogues include humulone, cohumulone, and adhumulone, which, when isomerized to isohumulones (iso-alpha acids) through the boiling process, bring bitterness to beer. Alpha acids in their non- isomerized form are stubbornly insoluble in aqueous solutions such as beer. They are considered only the precursor to the isomerized compounds that are measurable in finished beer. See bitterness. There is some debate among experts as to which of the humulones gives the cleanest bitterness. There seems to be agreement, however, that high levels of cohumulone are an indicator for potentially harsh bitterness. Cohumulone levels are listed on a hop analysis, usually right next to the alpha acid percentage. Many of the classic brewing varieties such as Saaz and Hallertauer Mittelfrueh have very low cohumulone levels.

The alpha acid level of hops is measured in a laboratory. When dissolved in beer as iso-alpha acids, the unit of measurement for bitterness is International Bitterness Units. See international bitterness units (ibus). The hops’ alpha acid value is then used by the brewer to formulate a recipe for the beer’s final bitterness. Brewers adjust hopping rates based primarily on the selected hop’s alpha acid content expressed as a percentage of the hop’s weight (commonly in a range of 2%–18%) and on the expected “utilization rate” of that hop in a given brewing system during a particular brewing process. See hop utilization. Because the kettle process is inherently less efficient at extracting isomerized alpha acids from hops than a laboratory process can be, the brewhouse utilization rate rarely exceeds 30%. This means that for hops with 9% alpha acids by weight, only a maximum of 3% of the hops’ weight will ultimately end up in the beer as bittering. When hops are added to the kettle at different times during the boil, their utilization rates, too, differ. This is because isomerization does not happen all at once, but is a function of the length of time the hop is exposed to high heat in an aqueous solution. Depending on the timing of a hop addition, therefore, utilization rates may vary from as low as 5% for late additions to about 30% for early additions. Hops added after the boil in the whirlpool or hop back do not isomerize efficiently, and those added during dry hopping do not isomerize at all. See dry hopping. When more than one hop variety is used, the contribution of each variety to the beer’s final bitterness is calculated separately and then added up.

Hop varieties are often grouped into four categories based on their alpha acid content. There are the low-alpha aroma varieties with alpha acid levels of perhaps 2.5%–6%; there are dual-purpose varieties with alpha acid levels of perhaps 6%–10%; there are high-alpha bittering varieties with alpha acid levels of perhaps 10%–15%; and then there are super alpha bittering varieties with alpha-acid levels of perhaps 14%–18%. Some recent experimental hop plantings have even been assayed at 22%. Many high- and super alpha varieties find their way into hop extract production and are ultimately sold solely for their alpha acid content. Experimental craft brewers, however, have adopted a few super alpha varieties for their unique flavor characteristics.

Hop alpha acid levels are highest at the point of harvest and diminish gradually and continuously during storage because of oxidation. This is especially true of baled whole hops. Refrigerating or even freezing hops after they have been harvested and kiln dried helps to delay oxidation and preserve their alpha acids longer. Hops that have been processed into pellets or into concentrated hop extracts, on the other hand, tend to maintain their alpha acid levels better. World hop production is often measured by the total amount of alpha acids produced in a given harvest year. The annual world alpha acid demand is currently between 7,000 and 7,500 metric tons.

See also bitterness and hops.