How do you measure a beer’s bitterness? By its hoppy taste? Its pungent smell? Try again: A beer’s bitterness is gauged by its International Bittering Units (IBU). But what is that? We asked a few knowledgable brewers in the area—including Avery Brewing Company’s head brewer, Matt Thrall, and Oskar Blues Brewery‘s head brewer, Tim Matthews—to explain IBU to us.

IBU is a scale that goes from 1­ to 100, and measures the amount of isomerized alpha acids in a beer. Hops contain alpha acids, and when they are boiled, the alpha acids become isomerized—or their physical structure starts to change—and they become bitter in flavor. Your standard Budweiser has 7 IBU, while Avery’s IPA has 70 and the Maharaja, an imperial IPA, has right around 100.

Bittering hops get added early on in the boil. The longer the hops boil, the bitterer the beer becomes—but the flavor and aroma fade away. It’s just like parsley or herbs, says Thrall. “If you add them early, it will bitter out the dish but you’ll lose the flavor.” IBU primarily come from that early addition of hops.

And, unsurprisingly, ambitious brewers have already outgrown the IBU scale. “The scale was developed with an understanding that most humans can’t detect bitterness after 100. 110 tastes just as bitter as 100,” says Thrall. “But it was thought up well before beers were even approaching that level.”

The actual bitterness experienced by the drinker is affected by other factors, though. Hops’ bitterness serves to offset the malty sweetness of beer, so if a brew—such as Avery’s Mephistopheles Stout, which has 80 IBU—has a lot of malt, it won’t taste very bitter, regardless of a high IBU level.

Still, IBU has nothing to do with the hoppiness we actually taste. That wonderful flavor comes from a totally different acid. Near the end of the boil, brewers usually add another load or two of hops to create the desired flavor and aroma, which come from the hops’ beta acids. For some concoctions, brewers will do a dry hop and throw some hop pellets into the fermentation vessel right before it is packaged to add an extra aromatic punch.

If you want to conduct your own little taste test, crack a can of Oskar Blues’ Dale’s Pale Ale and a can of their G’Knight. Dale’s has slightly more IBU, but G’Knight is considered hoppier due to its prominent dry hop. Either way, you get what Thrall calls that “mouth feeling sticky thing.”

—Image courtesy of Shutterstock