You’ve undoubtedly seen it printed on packaging or read it on a web page. And as a craft beer connoisseur, you probably already know that an IBU is an acronym for International Bitterness Unit. And you’re likely aware that it’s a measurement of the bitterness in a beer. But there’s certainly more to know than that. And this article will explore the origins of the IBU, how it’s measured, and explain the differences between perceived and actual bitterness.

Methods for Measuring Bitterness

The measurement of a beer’s bitterness dates back to the mid-20th century and its process has evolved over time. Before the introduction of the IBU, there were two methods for measuring the bitterness in beer. And here’s where it gets technical:

In 1955, P. Kolbach and H. Schilfarth extracted the bitter substance from beer by using chloroform and then weighed the dried extract. But the solvent extracted other substances as well and it was necessary to measure the unhopped wort to produce a correction.

The process was refined by F.L. Rigby and J.L. Bethune by taking iso-octane extracts of the acidified beer and diluted them methyl. This produced an alkaline that could be measured by ultraviolet light and those results were agreed upon by a tasting panel.

Seeking improved accuracy, Rigby and Bethune developed Method II in which the iso-octane extract was washed with acidified methanol to remove impurities.

Both methods calculated results as iso-humulones and this process became the industry standard.

Laboratories adopted this method and producing results for the “isohumulone content” in beer became routine. But achieving those results was a lengthy process until A.B. Moltke and M. Meilgaard came along. The pair produced a faster approach by using a higher ultraviolet wavelength that allowed skipping the process of making the extract alkaline or washing it.

Happy ending, right? Not exactly.

Here Comes the IBU

It was accepted by all that the Rigby & Bethune method produced accurate results. But because it was such a lengthy process, faster alternatives were preferred.

Therefore in 1964, the Analysis Committee of the European Brewery Convention (EBC) and the Isohumulone Sub-committee of the American Society of Brewing Chemists (ASBC) conducted studies to determine the accuracy of both methods.

It was initially believed that the Moltke & Meilgaard calculation produced the same results but it was later learned that the sampled beers were all in the same range. Apparently, the Rigby & Bethune and Moltke & Meilgaard methods produce the same results at 28 parts per million (or what would be 28 IBUs).

But the variation between the two widens as the calculated results move higher or lower along the scale.

Both the EBC and the ASBC concluded that the Rigby & Bethune method was more accurate and they both agreed that it was undesirable to have two methods that produced such varying results.

The two Societies decided to adopt the Rigby & Bethune method to measure the bitterness of beer and that those results would be named “International Bitterness Units”. Note: at that time, the ASBC named the unit to be used in the United States as the “Isohumulone Bitterness Unit” but its values were identical.

For more details regarding the measurement process, have a look at this PDF.

Perceived Versus Actual Bitterness

Bittering hops are added early in the boil and the longer those hops boil, the more bittering acids are released. The IBU is essentially a measurement of how much hop resin (or iso-alpha acids) is in the final product. The IBU scale begins at 0 and doesn’t have a ceiling. The higher the IBU count, the more hop resin is in the beer, and therefore it tastes more bitter, right? Well, not exactly. Those numbers can be deceiving.

The bitterness can be masked by sweetness, such as what’s found in barley malt sugars. Most of the sugars are consumed by the yeast during fermentation but those that remain are expressed as ending or finishing gravity. For example, let’s say an American Pale Ale and an American Stout have the same alcohol content and the same IBU count. But the finishing gravity for the Pale Ale is 2 degrees Plato and the Stout is 4 degrees Plato. The Pale Ale will taste much more bitter because the Stout has more sugars left following the fermentation.

In Summary

The most important takeaways are that IBUs measure a beer’s bitterness but that count may not be indicative of just how bitter it might taste. Other factors need to be considered as well. Also, measuring IBUs is a complicated science that involves spectrometers, iso-octanes, acids, solvents, and complex machinery like centrifuges.