Cohumulone is one of five alpha acid analogs in hop resin, the others being adhumulone, humulone, prehumulone, and posthumulone. These analogs differ only slightly from each other in their molecular structures, and together they serve as precursors to iso-alpha acids, the predominant contributors of bitterness in beer. Cohumulone and humulone levels vary between roughly 20% and 50% of total alpha acids, depending on variety, whereas adhumulone levels tend to be fairly constant across varieties, at about 10% to 15% of alpha acids. Pre- and posthumulone, on the other hand, play only minor roles. Most hop analyses will specifically show a cohumulone percentage. This is because brewers believe iso-cohumulone contributes a rougher, harsher quality of bitterness than other iso-alpha acids. Traditional aroma hops tend to be low in cohumulone, whereas some bittering varieties have higher levels. Because brewers tend to associate low cohumulone with a finer quality of bitterness, hop breeders are favoring new cultivars with low cohumulone levels. When hops oxidize, alpha acids, including cohumulone, change their molecular makeup, producing isobutryic acid, which has a distinctive rancid, sour, cheesy odor.