Hops are the cone-like flowers of the female hop vine (Humulus Lupulus). At the base of the petal, within the hop flower, there is a soft resin called Lupulin Oil that gives beer its bitter flavor and "hoppy" aroma. Hops were originally introduced by brewers to help inhibit the growth of beer spoiling bacteria, to maintain flavor stability, and to help retain the head (foam) of the beer. There are over 50 different varieties of hops which are grown commercially in Germany, southern England, southern Australia, Tasmania and Washington State. They are selected for their unique bitterness in flavor or aroma. Hops contribute to the flavor of the beer through its hop oils and to the bitterness with its resins.
Hop oils are employed in the brew to give it a characteristic aroma and flavor. These oils are soluble in water and are highly volatile. Because of this, hops used for flavoring is added to the mixture (see Wort) at the end of it's boil. Alpha acids and Beta acids are the resins contributing to bitterness. They are conventionally measured on a mass percent basis. Of the two, alpha acids are the most dominant in bitterness contribution. Both alpha and beta acids are not soluble in water, in order to allow them to contribute to the bitterness of the beer, they must be modified. This is accomplished with alpha acids by a sustained boil. Beta acids on the other hand, can only be made soluble by oxidation, a process which destroys many of the other flavors and may impart "off" tastes to the beerandis thus not employed.
Hops are broadly classified into two groups. Noble hops are low in bitterness and have a pleasant aroma and flavor. High-alpha hops have high bitterness but a less pleasant aroma. The high-alpha hops can be used more economically due to the less amount required. However, the flavor of the beer is slightly sacrificed.
Years ago, the recipes for beer would call for hops measured in ounces. Since the bitterness for each type of hop was not accounted, the beer's flavor was inconsistent from batch to batch. Today, the bitterness for each type of hop is rated by alpha acid units (AAU). It is calculated by the alpha acid percentage in the hops multiplied by the weight of the hops in ounces. Consistent control of the bitterness of the beer is achievable.
Since hops are a plant, they can be dried and compressed called whole hops. Pelletized hops are hop flowers that are ground into a powder and pressed into pellets for easier use and storage. The negative aspect of hops is that they are easily perishable. Heat and oxygen can deteriorate hops in a matter of weeks if not stored and handled properly. Pelletized hops can be stored at room temperature only if they are vacuumed packed or packed with nitrogen (a non-oxidizing atmosphere). Hops that are not packaged in this manner must be stored refrigerated or frozen.