Alpha acid

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Alpha acid is the natural acid in the cone of the hop plant that gives bitterness to beer. Some international brewers use only 'high alpha' or super alpha' hops that give a high level of bitterness but little aroma.

Hop resins are composed of two main acids: alpha and beta acids.

Alpha acids have a mild antibiotic/bacteriostatic effect against Gram-positive bacteria, and favour the exclusive activity of brewing yeast in the fermentation of beer.

Beta acids do not isomerise during the boil of wort, and have a negligible effect on beer flavour. 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 oxidize and oxidized beta acids form sulfur compounds such as DMS (dimethyl sulfide) that can give beer off-flavours of rotten vegetables or cooked corn.

The flavour imparted by hops varies by type and use: hops boiled with the beer (known as "bittering hops") produce bitterness, while hops added to beer later impart some degree of "hop flavour" (if during the final 10 minutes of boil) or "hop aroma" (if during the final 3 minutes, or less, of boil) and a lesser degree of bitterness. Adding hops after the wort has cooled and the beer has fermented is known asdry hopping, and adds hop aroma, but no bitterness. 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. Unboiled hops are only mildly bitter.