Isohumulones are

chemical compounds A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

that contribute to the bitter taste of

beer Beer is an alcoholic beverage produced by the brewing and fermentation of starches from cereal grain—most commonly malted barley, although wheat, maize (corn), rice, and oats are also used. The grain is mashed to convert starch in the ...

and are in the class of compounds known as

iso-alpha acid Alpha acids (α acids) are a class of chemical compounds primarily of importance to the production of beer. They are found in the resin glands of the flowers of the hops, hop plant and are the source of hop Taste#Bitterness, bitterness. Alpha a ...

s. They are found in

hops Hops are the flowers (also called seed cones or strobiles) of the hop plant ''Humulus lupulus'', a member of the Cannabaceae family of flowering plants. They are used primarily as a bittering, flavouring, and stability agent in beer, to whic ...

Beer

The bitterness of beer is measured according to the International Bitterness Units scale, with one IBU corresponding to one part-per-million of isohumulone. When beer is exposed to light, these compounds can decompose in a reaction

catalyze Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

d by

riboflavin Riboflavin, also known as vitamin B2, is a vitamin found in food and sold as a dietary supplement. It is essential to the formation of two major coenzymes, flavin mononucleotide and flavin adenine dinucleotide. These coenzymes are involved in ...

to generate

free-radical In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spont ...

species by the homolytic cleavage of the exocyclic carbon-carbon bond. The cleaved acyl side-chain radical then decomposes further, expelling carbon monoxide and generating 1,1-dimethylallyl radical. This radical can finally react with sulfur-containing amino acids, such as cysteine, to create 3-methylbut-2-ene-1-thiol, a

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

which causes beer to develop a "skunky" flavor.

Formation

Isohumulones are generated by the isomerization of

humulone Humulone (α-lupulic acid), a Vinylogy, vinylogous type of organic acid, is a Bitterness (taste), bitter-tasting chemical compound found in the resin of mature hops (''Humulus lupulus''). Humulone is a prevalent member of the class of compounds ...

.Esslinger, H. M. and Narziss, L. 2003. “Beer.” in ''Ullmann's Encyclopedia of Industrial Chemistry''. Wiley-VCH Verlag GmbH & Co. KGaA, 2009 S-Humulone Isomerization

References

{{reflist Bitter compounds Cyclopentenes 3-Hydroxypropenals Humulus

Beer

Formation

See also

References