Beer Fact Friday: International Bitterness Units (IBU)

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We’ve mentioned International Bitterness Units, or IBUs, briefly in previous Beer Fact Fridays, but it’s time to dive in for a more in-depth look. IBUs are the agreed-upon measurement of bitterness in the beer industry, and give a general indication of a beer’s bitterness roughly on a scale from 0-100. Kilt Lifter, which has pretty low bitterness, has about 21 IBUs; Hop Knot, which you see here, has close to 50.

Here’s how that number is actually determined: A specific volume of beer is acidified with hydrochloric acid, then shaken for 15 minutes with a substance called iso-octane. This encourages the extraction of iso-alpha acids—the hop compounds that give beer a majority of its bitterness—out of the beer and into the iso-octane. The iso-octane, now full of iso-alpha acids, is then placed inside a device called a spectrophotometer, which measures the absorbance of light as it passes through a liquid. The solution is measured at a light wavelength of 275 nanometers—the level determined to be the best approximation of the maximum absorbance of the various bitter molecules—and the calculated absorption rate is multiplied by 50. The number that results is the beer’s IBU. Simple, right?

The main idea of this process is to measure the amount of iso-alpha acids that are dissolved in the beer. But measuring bitterness this way isn’t without its problems. Hop alpha acids aren’t the only compound that absorbs light at the wavelength used to measure IBUs; ingredients such as spices or citrus peel can increase the absorption reading and lead to IBU levels much higher than a beer’s actual perceived bitterness. IBUs also only tell part of the story of a beer’s bitterness. A malt-focused beer like an imperial stout may have sky-high IBU levels, but because the amount of residual sugar in these beers is also usually pretty high, the actual amount of bitterness you perceive when drinking it might be minimal. An IBU level of 60 tastes quite different in a strong, malty beer than it does in something light like a Pilsner.

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