The bittering, flavor, and aroma characteristics of hops are created by a two major types of chemical compound contained in the cone-shaped hop flower: acids and oils. The acids in question are alpha acids (humulone, adhumulone, and cohumulone), which for iso-alpha acids in the boil, and beta acids; the oils are the hop essential oils (primarily humulene, myrcene, caryophyllene and farnesene).
The alpha acids are the hop components most familiar to home brewers. Every package of hops sold to homebrewers indicates the alpha acid percentage to allow the brewer to calculate the bitterness he or she wants in the finished beer. The alpha acid percentage represents the amount of the hop, by weight, that is composed of alpha acids. For more information on these calculations, see the article on bitterness.
The "alpha acid" percentage actually encompasses multiple separate chemicals, each of which adds a unique flavor and style of bitterness. The primary alpha acids are:
Humulone is the primary alpha acid occurring in most hops. It is thought to give a desirable "soft" bittering to the finished beer.
Traditionally, cohumulone has been considered to add a harsh, unpleasant bitterness to beer, and so low-cohumulone varieties were considered more desirable for brewing purposes; most noble hops have relatively low cohumulone. For this reason, cohumulone is often the only alpha acid identified specifically by hop producers. Cohumulone is indicated as a percentage (by weight) of the total alpha acid content of a hop.
However, recently the role of cohumulone has been called into question, as new high-alpha hop varieties that are also high in cohumulone have come onto the market which are considered to impart a good bitterness.
The third identified alpha acid is adhumulone, which usually occurs in relatively small amounts. Its effect on bitterness and flavor is not well understood.
When added to boiling wort, alpha acids go into solution almost immediately. However, the bitterness they impart to beer appears only gradually, as the alpha acids are slowly isomerized in the boil to form isomerized alpha acids or iso-alpha acids. This is why bittering hop additions are normally done at the beginning of a boil of 60 minutes or longer.
Each alpha acid has a corresponding iso-alpha acid. The iso-alpha acid isohumulone, the isomerized form of humulone, is important to brewers because it reacts with riboflavin in the presence of light to form MBT to form a skunk-like flavor and odor.
Along with alpha acids, hops contain beta acids, principally lupulone, colupulone and adlupulone. These are rarely considered separately, but the beta acids as a whole are important to a beer's flavor.
The beta acids do not produce as much bitterness during the boil as the alpha acids, but during fermentation and storage, as alpha acid bitterness breaks down, beta acids slowly create bitterness through oxidation. This affects the long-term character of aged and lagered beers.
Beta acids are given by hop producers as either a total percentage of beta acid in the hops by weight, or as a ratio of alpha to beta acids. Some people consider beta acid bitterness to be "harsher" than alpha acid (or at least humulone) bitterness, and look for hops with low total beta acids. However, the traditional noble hops generally have an alpha to beta acid ratio of close to 1:1, which is therefore considered desirable by some. Other brewers prefer a 2:1 ratio, which is thought to yield the most constant bitterness in aged beers.
While alpha acids contribute most of the bitterness to beer, most of the hop flavor and aroma is contributed by volatile essential oils. Hop producers generally indicate the total percentage of essential oils by weight in a given hop, and sometimes identify specific oils by percentage of total oil.
Because hop oils are highly volatile, traditionally the aroma and flavor characteristics of hops were obtained by late hop additions or dry hopping, allowing the oils to be absorbed into the wort but not leaving them in the boil long enough to boil off. Traditionally flavor additions were made in the last fifteen minutes or less of the boil, while aroma additions were made with five minutes or less of boil remaining. However, recently home and craft brewers have been experimenting with the recently rediscovered technique of first wort hopping, which allows hops added early in the brewing process to survive the boil and lend flavor and aroma to the finished beer.
The principal hop essential oils are:
Humulene is thought to lend the distinctive "noble" character to noble hops; most varieties traditionally considered noble are high in humulene, while many bittering hop varieties have very low levels. The noble character is strongest when the hops are used in dry hopping or late hop additions; if boiled for longer periods, humulene lends the finished beer an herbal or spicy character.
Myrcene yields flavors that were not traditionally considered desirable by European brewers, and noble hops are very low in myrcene. However, many American hop varieties are very high in myrcene; it makes up up to 60% of total oil in Cascade and up to 70% in Amarillo. Also found in some citrus fruits, myrcene lends American hops many of their distinctive flavors.
When added late in, or after, the boil, myrcene adds the intense, pungent aroma associated with American dry-hopped beers. When boiled for longer periods, it yields the characteristic citrus and pine aromas of American craft beer.
Caryophyllene adds a spicy, herbal character similar to humulene when boiled. Its effect on flavor when fresh is not well understood.
Farnesene makes up a very low percentage of total oil in most hop varieties. However, it is considered significant because it makes up a substantial proportion of some noble hops. Its effect on flavor and aroma is unknown.