Why add hops?? Bitterness, Flavor, Aroma   Hops add bitterness to balance malt sweetness, flavor and bouquet to beer. Each variety of hop contributes unique flavors and aromas. Bitterness: The amount of bitterness hops impart to beer is directly related to the alpha acids (%AA) content. A hop variety of 8% AA has twice the bitterness of a variety of 4% AA. Hops must be boiled to extract the bitterness. A 45 minute boil will extract nearly all the bitterness possible while a boil of 15 minutes will extract about half. Hops high in AA are often referred to as bittering or boiling hops although any hop boiled for an extended period is in fact a bittering hop. A popular unit in homebrewing is the HBU (Home Bittering Units). The number of HBU in a recipe is calculated by multiplying the ounces of hops used by its %AA. For example, 2 oz of Bullion (%AA=9.0) would give 18 HBU (assuming a lengthy, >45 minute, boil). A 5 gallon batch of beer will typically have 6-20 HBU depending on the style. Conversion factors for HBU's and International Bittering Units (IBU) are given here. Flavor: Hops used for flavoring are called finishing or aromatic hops. The flavor components of hops are aromatic oils. These oils are easily boiled away so finishing hops should not be boiled for more than 15 minutes. They are usually added in the last 5-10 minutes of the boil. However, all hops add flavor even if boiled for long periods (as discussed later). Finishing hops are generally low in alpha acid content. Hops with medium %AA are often used for both bittering and flavoring and are called combination hops. Bouquet: The components of hops that contribute to the 'hop nose', essential oils, are volatile and difficult to keep in beer. One method is to add the hops very late in the boil or even after turning off the heat. Additionally some beers are dry-hopped, the process of adding hops to the beer either late in the fermentation or in the keg. Commercial beers which are dry-hopped are Samuel Adams Boston Lager and Anchor Liberty Ale. Smell the bouquet immediately after opening the bottle. You can also get hop bouquet by making a hop tea at bottling. Bittering hops have flavor too Every hop imparts a unique flavor even if boiled extensively. This is most evident for Cascade (American pale ales), Saaz (Pilsner Urquell), and Northern Brewer (Anchor Steam). Fuggle and Goldings are clearly discernible when used as bittering hops. Other hops such as Tettnang and Hallertau have less distinctive flavors, but flavor nonetheless. Thus, feel free to try different hops for bittering and follow the subtle and sometimes not so subtle flavor changes. Dry Hopping Another method of obtaining wonderful hop flavor and aroma is to dry hop. Add 1-2 ounces of hops late in the fermentation or keg.\ Wait ~4-7 days and remove. If left too long, the beer picks up a grassy tone. It's easier to place the hops in a nylon bag with marbles to sink the bag and a long string to ease removal. Commercial examples of dry hopping are Anchor Liberty Ale which uses whole hops and Samuel Adams which uses pellets. Dry-hopping and hop tea yield great hop aroma and flavor, although somewhat different. Hop plugs are good for dry-hopping and are used primarily for that in England. Multiple Hop Additions Hops are often added at various intervals of the boil. For example, some brewers add hops for 60, 30, 15, and 10 minutes before wort cooling. All of these additions contribute differing character to the beer. As a rule of thumb, the closer the addition to the end of the boil, the more distinguishable the effect of the addition.