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 one of the oldest and the most widely consumed type of alcoholic drink in the world, and the third most popular drink overall after water and tea. It is produced by the brewing and fermentation of starches, mainly derived from ce ...

and are in the class of compounds known as iso-alpha acids. They are found in hops.

Beer

The bitterness of beer is measured according to the

International Bitterness Units scale When drinking beer, there are many factors to be considered. Principal among them are bitterness, the variety of flavours present in the beverage and their intensity, alcohol content, and colour. Standards for those characteristics allow a more o ...

, 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 process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

d by riboflavin to generate free-radical 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 gro ...

which causes beer to develop a "skunky" flavor.

Formation

Isohumulones are generated by the isomerization of humulone.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