IBU (International Bitterness Units) – In beer, the bitterness is caused by the tannins and iso-humulones of hops. Iso-humulones are chemical compounds that contribute to the bitter taste of beer and are in the class of compounds known as iso-alpha acids which are found in hops. Bitterness of hops is perceived in the taste, and can vary person to person. The amount of bitterness in a beer is one of the defining characteristics of a beer style.
IBUs are a simple measurement of the levels of a class of bitter compounds found in the finished beer. It is not the “hoppiness” and not even how bitter you might perceive the beer to be. You can have two beers, both with an IBU of 60, and one of those beers you may perceive to be way more bitter than the one next to it. IBU ratings are fairly broad, but use it as a general guide when shopping for beer. Think of it this way: your average American lager, such as Budweiser, has 10 IBUs, Pale Ales have between 30-40 IBUs (Sierra Nevada’s Pale Ale has 38 IBUs), your average IPA has between 40 and 60 IBUs (Dogfish Head’s 60min. IPA has 60 IBUs), and double IPAs clock in somewhere between 60 and 110 IBUs (Captain Lawrence’s Palate Shifter clocks in at 90 IBUs).
I wont bore you too much with how IBUs are actually measured, but basically some finished beer is put in a vial with some hydrochloric acid and an organic solvent, its then shaken for 15 minutes to create an emulsion, and then a bunch of other science-y stuff goes on. In the end the scientist is looking for how much of these certain wavelength’s are absorbed by specific molecules.
When hops age, one thing that drops is their humulone, which is the precursor to iso-humulone. If the humulone content goes down, the available amount of iso-humulones, or bitterness in the beer will also goes down. In other words, brewers don’t set out to make a beer with a specific IBU rating, they use IBUs as a gauge to ensure that they’re consistently hopping their beers properly, given that the hops are constantly changing while in storage. And as a customer you should also be using IBUs as just a gauge when shopping for your beer. Two different beers with the same IBU will be perceived totally different from each other based on malt content and other variables, but at least you’ll have a round about idea.