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Beta acid is a, bitter hop resin. It is one of the two main compounds found in the soft resins in hop lupulin glands.
It is harsher in flavor than alpha acids but is almost insoluble at normal wort pH values. Therefore, it does not contribute significantly to the beer's bitterness. However, if oxidized during wort boiling or beer storage, beta acids may form bitter compounds. Beta acids do contribute to the preservation of the beer.
The amount present in a given hop is expressed as a percent of the total beta acids in the hop cone.