All About Hops - Urban Brewers
Hops are the green, cone-shaped flowers of the female hop plant (Humulus lupulus), and are used by brewers to provide bitterness, flavor and aroma to their beers. Additionally, hops have anti-bacterial/preservative qualities that aid with keeping beer from spoiling. Hops contain alpha acids (AA%), which are the primary bittering agent used to balance the sweetness in the beer imparted by malts during the brewing process. Hops also contain several essential oils that lend beers their unique flavors and aromas. 

Hops can be categorized by the role they provide in the brewing process: bittering, aroma, and dual purpose. Bittering hops are normally high in alpha acids and low in essential oil content. In contrast, aroma hops contain high levels of various essential oils while having a lower percentage of alpha acids. Finally, dual purpose hops are found to have a more balanced chemical distribution between alpha acids and essential oils.

Hops can provide a vast spectrum of flavors and aromas, but can be generalized as follows: piney & citrusy - normally associated with big and bold American style IPAs, think of “C” hops, such as Cascade, Centennial and Citra; floral & spicy - normally associated with “noble” hops such as, Saaz and Tettnanger, which lend traditional German and Czech pilsners and lagers their characteristic flavor profiles; earthy & delicate - normally associated with English hops, such as EKG and Fuggles, these hops are mild and subtle with hints of earth, wood and spice; fruity & juicy - normally associated with New Zealand hop varieties, such as, Nelson Sauvin and Motueka, with a flavors ranging from crushed grapes to tropical fruit and citrus. There are no limits to what can be done with this very versitle ingredient.

Hops are packaged and sold in several forms: leaf hops, pellet hops (most common) and cryo hops. Leaf hops are dried and kept whole and are normally used when dry hopping. Pellet hops are leaf hops that are ground and pressed into pellets; these hops tend to be more stable, thus providing better extraction efficiency. Cryo hops consist of lupulin powder that has been separated from the whole cone, and are said to pack a more pungent punch of aroma and flavor without all of the vegetative cone material; these hops are normally used in IPAs that require several rounds of dry hopping.