Alpha acids have a mild antibiotic/bacteriostatic effect against Gram-positive bacteria, and favor the exclusive activity of brewing yeast in the fermentation of beer. Alpha acids are responsible for the bitter flavor in the beer.
Beta acids do not isomerize during the boil of wort, and have a negligible effect on beer flavor. Instead they contribute to beer’s bitter aroma, and high beta acid hop varieties are often added at the end of the wort boil for aroma. Beta acids may oxidize into compounds that can give beer off-flavors of rotten vegetables or cooked corn.
The effect of hops on the finished beer varies by type and use, though there are two main hop types: bittering and aroma. Bittering hops have higher concentrations of alpha acids, and are responsible for the large majority of the bitter flavor of a beer. European (so called “noble”) hops typically average 5–9% alpha acids by weight, and the newer American species typically ranging from 8–19% aabw. Aroma hops usually have a lower concentration of alpha acids (~5%) and are the primary contributors of hop aroma and (non-bitter) flavor. Bittering hops are boiled for a longer period of time, typically 60–90 minutes, in order to maximize the isomerization of the alpha acids. They often have inferior aromatic properties, as the aromatic compounds evaporate off during the boil.
The degree of bitterness imparted by hops depends on the degree to which otherwise insoluble alpha acids (AAs) are isomerized during the boil, and the impact of a given amount of hops is specified in International Bitterness Units (IBUs). Unboiled hops are only mildly bitter. On the other hand, the (non-bitter) flavor and aroma of hops come from the essential oils, which evaporate during the boil.
Aroma hops are typically added to the wort later to prevent the evaporation of the essential oils, to impart “hop flavor” (if during the final 10 minutes of boil) or “hop aroma” (if during the final 3 minutes, or less, of boil). Aroma hops are often added after the wort has cooled and the beer has fermented, a technique known as “dry hopping” which contributes to the hop aroma. The four major essential oils in hops are Myrcene, Humulene, Caryophyllene, and Farnesene which comprise about 60–80% of the essential oils for most hop varieties.
Today there is a substantial amount of “dual-use” hops as well, which have high concentrations of alpha acids and good aromatic properties. These can be added to the boil at any time, depending on the desired effect.
Flavors and aromas are described appreciatively using terms which include “grassy”, “floral”, “citrus”, “spicy”, “piney,” “lemony,” and “earthy”. Most of the common commercial lagers have fairly low hop influence, while true pilseners should have noticeable noble hop aroma and certain ales (particularly the highly-hopped style known as India Pale Ale, or IPA) can have high levels of bitterness.