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.
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.
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.