Although there is only one hop species (Humulus lupulus) that is used for brewing beer, there are a number of varieties (technically known as "cultivars") in that species, each with its own spectrum of characteristics. Varieties of hops are chosen for the properties of bitterness, flavor, or bouquet that they will lend to the beer. Hop varieties can be roughly divided into two classes, bittering hops and aroma hops, although there are hops that can be considered dual-purpose.
As their name implies, bittering hop varieties are those that impart bitter flavor to beer and have high alpha acid levels. Bittering hops usually have a high alpha acid content.
Aroma hops, with low- to medium alpha levels, mainly impart characteristic hop aromas to beer. In Europe, aroma hops are mostly grown, with a smaller but growing emphasis on bittering hop varieties.
Dual-purpose hops tend to have intermediate levels of alpha acids together with desirable aroma properties.
Traditionally, hops have been named after their place of origin. For example, Hallertau hops are named after the Hallertau district north of Munich, Germany.
Different cultivars are important for the production of specific styles of beers. For instance, Bullion (a strong odor, high myrcene, and high alpha acid hop) is suitable for strongly bittered ales, but would not be suitable for final hopping of light lager.
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