Isohumulones are compounds

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 that contribute to the bitter taste of beer. They are in the class of compounds known as iso-alpha acids. The bitterness of beer is measured according to the International Bitterness Units scale, with one IBU corresponding to 1 part per million of isohumulone. When beer is exposed to light, these compounds can be decomposed in a reaction catalyzed by riboflavin

Riboflavin

Riboflavin, also known as vitamin B2 or additive E101, is an easily absorbed micronutrient with a key role in maintaining health in humans and animals. It is the central component of the cofactors FAD and FMN, and is therefore required by all flavoproteins. As such, vitamin B2 is required for a...

, creating 3-methylbut-2-ene-1-thiol, a thiol

Thiol

In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...

 which causes beer to develop a "skunky" flavor.

Isohumulones are generated by the isomerization of humulone

Humulone

Humulone is a bitter-tasting chemical compound found in the resin of mature hops . Humulone is a prevalent member of the class of compounds known as alpha acids, which collectively give beer its characteristic bitter flavor.-Chemistry:In terms of structure, humulone is a phloroglucinol derivative...

.