Hops are the flowers of the humulus lupulis plant. Hops bitterness balances the sweetness of the malt, while aromatics provide an extra dimension of flavor for the beer. Hundreds of different varieties of hops exist, each giving unique bittering and aromatic properties.
Many different varieties exist, each giving different properties to the beer. There are three categories of hops, and all hops will fall into one of these categories. Bittering hops are those particularly well-suited to providing balance for the beer. Aroma hops are those with more favorable flavor and aromatics. Dual-use hops are those with both excellent bittering and aromatic properties.
Within the aroma hops categories, there are a few groupings of hops that share similar properties.
First, noble hops are the classic European hops. Namely Hallertau, Saaz, and Tettnang, these hops feature light, delicate floral and spicy aromatics, and are best used in German and Czech lagers, as well as Belgian ales.
English hops are best suited for use in malt-focused beers, as their clean, floral, and woodsy aromatics blend very well with malt aromas and English yeast esters. Most common among these are East Kent Goldings and Fuggles, though other varieties exist.
Third are the American "C" hops; Cascade, Centennial, Chinook, and Columbus. These hops share bold, strong bittering and citrus-forward aromatics, and are commonly used in hop-forward American ales, such as Pale Ales, IPAs, and Double IPAs.
The reason different categories, and varieties, of hops exist is due to the oils within the hops. Hops contain dozens of different essential oils and other flavor-giving chemicals, each in different proportions between varieties. Most important to know are alpha-acids.
Alpha acids, when boiled in wort, are isomerized, causing bitterness compounds to form. Hops package labels will always have an alpha-acid percentage; the higher the percentage, the more bitterness the brewer can expect from the hop.
Alpha acids are used to calculate the IBUs (international bitterness units) of a beer, which are a direct measure of how bitter a beer is. Alpha acids for bittering hops can range between 10-16%, aroma hops typically will be 2-8%, and dual-use hops typically 8-14%. This may not always be the case, however.
Newer hops varieties, such as Citra or Mosaic, are grown for their total oil content. As a result, these hops tend to have higher-than-average alpha acids. When purchasing hops, it is useful to know both the alpha-acids, as well as the category (and, for aroma and dual-use hops, the types of flavors and aromas that can be expected) of the hops.
Boil timings and usage
The amount of time the hop is boiled for will have a direct effect on the final beer. The longer a hop is boiled, the more isomerization occurs, and the more bitter the beer will be. Additionally, the longer a hop is boiled, the more aromatics are boiled off, and the less aromatic the beer will be. For this reason, beers will have different hops added at different, specific timings throughout the boil.
Bittering hops are usually added at the beginning of the boil (the 60-minute mark). Sometimes a secondary bittering addition may be added at the 45- or 30-minute mark. Flavoring additions (where the addition will mostly give flavor, with some aroma and bitterness contribution) are usually added between the 30- to 15-minute mark. Aromatic additions are added in the last 15 minutes of the boil.
Aromatic additions can become more complex in the case of hop-focused beers (IPAs and double IPAs). These beers will typically call for either a flame-out or whirlpool addition, as well as a dry-hop addition. Hoppy beers use these more complex additions to added greater depth of hops aromatics and flavor.
Flame-out additions are simply adding hops at the end of the boil, just after the heat source has been turned off. Whirlpool additions are similar; the heat is turned off, the wort is cooled down to ~160-180°F, the chilling is halted, and hops are added. The hops are allowed a contact time of ~15-30 minutes. After this time, the chilling continues.
Dry-hopping, while similar in purpose to flame-out and whirlpool additions, is done much later. After the beer has mostly fermented, the fermenter is opened and additional hops are added. The hops will steep in the fermenter for ~5 days. The beer is then racked and packaged. Flame-out, whirlpool, and dry-hop additions are all done to maximize aromatics, and minimize bitterness in the beer.
The amount of hops used at each different stage is important. Typically, bittering additions will account for most the IBUs of the beer, with flavor and aroma additions adding up to a much smaller proportion. Using too much hops at the wrong stage will throw off the balance of the beer. Most beers will steadily increase the amount of hops used throughout the brew. Malt or yeast focused beers will have simple hop schedules, typically a small bittering addition and a slightly larger flavor or aroma addition. IPA additions will use a small bittering charge, and increase the rate of hopping drastically over the brew. An example hops bill for an IPA could look like:
1 oz. Warrior @ 60
½ oz. Citra, ½ oz. Mosaic @15
1 oz. Citra, 1 oz. Mosaic @ whirlpool
2 oz. Citra, 2 oz. Mosaic @ Dry-hop 5 days