If you’ve been around the craft beer or homebrewing scenes for a long time then you’ve probably heard the acronym “IBU.” You may be aware that IPA’s have lots of IBUs or even that it has something to do with hoppiness. That may still leave you wondering, what, exactly, is IBU?
IBU is short for International Bitterness Unit. It measures the amount of iso-alpha acids in the beer. Iso-alpha acids are chemical compounds found in hops that contribute to a beers bitterness. As a curious aside, iso-humolones are one group of iso-alpha acids, which is why Allagash Brewing calls there Belgian IPA Hugh Malone: it’s a pun, you see.
Different hop varieties have different levels of alpha acids. Multiplying the amount of hops used by the percentage of alpha acids gives a measure called Alpha Acid Units. With a little more work, IBU can be estimated from AAU using a relatively simple calculation. Large professional breweries use ultraviolet spectrometry to get a more precise IBU measure, but these hand calculations can be good for smaller brewers. The calculation can be found here, or if you’re not mathematically inclined, this calculator should help you work it out. Basically, the more hops that are used in making a beer, the higher their alpha acid percentage, and the longer the time they spend in the boil of the brew, the higher the beer’s final IBU will be.
Is IBU a measure of the bitterness of the beer? Not quite. Certainly, styles of beer that have very low IBUs, like American Lagers such as Budweiser or Coors (around 10 IBUs) will not be that bitter, where styles that have high IBUs, like IPAs (from around 60 IBUs upwards) will be more bitter. Two beers with the same IBU could, however, have different perceived bitterness levels. Stouts and Pilsners can seem very different in bitterness even if they both come in around 30 or 40 IBU. That’s because, in the stout, the stronger malt presence balances out the hops. This chart compares typical IBU levels of different beer styles.
On the whole, fans of hoppy beers will prefer beers with high IBUs. What are some of the highest IBU beers, then? Danish brewing group Mikkeller has produced beers that claim 1,000 and 2,007 IBUs. A Barrie, Ontario Brewery called Flying Monkeys makes one called Alpha Fornication that they claim has 2,500 IBUs. Unfortunately for these brewers, there might be a limit to the possible IBU level of a beer. First, there is a limit to how much of the iso-alpha acids can be dissolved in water (or water-based beer). A Popular Science column suggested that this may result in an upper limit of 110 IBU, but noted that there was no verification for that particular number – or any other. Many claims exist that the upper limit is between 100 and 120 IBU, about the level of the Wormtown Hopulence IPA reviewed last week in this column. There may also be an upper limit to the amount of bitterness that can perceived by the human sense of taste. Although, again, there are no substantiated numbers, the limit for perceived bitterness might be as low as 70 IBU.
IBU is an interesting measure and a good rule of thumb, but ultimately what really matters is the taste of the beer. Pennsylvania’s Victory Brewing uses spectrometry to measure the IBU of its beers, but uses those measurements more as a test of consistency between batches. Human taste testing is still used, because, ultimately beer is made for human tastes.