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).
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Alpha acids
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.
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.
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
Humulone is the primary alpha acid occurring in most hops. It is thought to give a desirable "soft" bittering to the finished beer.
Cohumulone
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.
Adhumulone
The third identified alpha acid is adhumulone, which usually occurs in relatively small amounts. Its effect on bitterness and flavor is not well understood.
Beta acids
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.
Essential oils
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.
The principal hop essential oils are:
Humulene
Humulene is
