Confused about the link between Aplha Acids and Cohumulone

Basically, alpha-acids are a class of compounds known as humulones. They consist of a complex hexagonal molecule with several side chains, with ketone and alcohol groups. Examples include humulone, cohumulone, adhumulone, posthumulone, and prehumulone. Each different humulone differs in the make up of the side chain, for instance, humulone has a side chain of isovalerate attached, while cohumulone has isobutyrate as its side chain. These side chains can become detached during extended storage under poor conditions and result in cheesy flavors associated sometimes with old hops. It has become accepted dogma among brewers to think of each of these humulones to have different bitter characteristics. There are some that swear that the bitterness associated with cohumulone is "harsher" than that from humulone. Other studies have shown no difference in sensory impact when each of the different humulones are compared. Nevertheless, the humulone:cohumulone ratio is now quoted in hop analyses and new varieties are being bred with low cohumulone levels in mind. Historically, the most highly prized hop varieties - including noble hops such as Hallertau, Tettnang and Saaz - also happen to be those that have low cohumulone levels.

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