adhumulone,
From The Oxford Companion to Beer
one of five identified alpha acid analogues in hop resin, the others being cohumulone, humulone, prehumulone, and posthumulone. Collectively, these alpha acid analogues serve as the precursors to iso-alpha acids, the predominant contributors of bitterness in beer. Adhumulone levels in hops are typically constant at 15% of the total alpha acid content across all varieties, whereas cohumulone and humulone levels vary (20%–50%) depending on variety. Pre- and posthumulone are minor constituents. Only slight differences in molecular structures differentiate all analogues of the alpha acids. In the case of adhumulone, the side group of the molecule is 2-methylbutyryl. Oxidation of hops leads to cleavage of this side group and the production of 2-methylbutryic acid, which has a distinctive, pungent odor similar to Roquefort cheese. This is why oxidized hops are often described as “cheesy.”
This definition is from The Oxford Companion to Beer, edited by Garrett Oliver. © Oxford University Press 2012.