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Hops
Hop Varieties
Although there is only one hop species (Humulus lupulus) that is useful for brewing, 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. As their name implies, bittering hop varieties are those that impart bitter flavor to beer and have high alpha acid levels. Aroma hops, with low- to medium alpha levels, mainly impart characteristic hop aromas to beer. Generally, bittering hops are added at the beginning of the boil, a process referred to as "kettle hopping" or "bitter hopping," while aroma hops are added in the final stages of the boil for their aromatic and flavoring properties, a process referred to as "late hopping." Some hops that are used for their bitterness also have desirable aroma properties, just as there are some aroma varieties known for their high levels of alpha acids. Challenger and Northdown are two such varieties that have both bittering and aroma properties.
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