The Oxford Companion to Beer definition of
International Bitterness Units (IBUs)
are the internationally agreed-upon standard for measuring bitterness in beer.
This has, not surprisingly, led to some confusion. The complexity notwithstanding, for the brewer, IBU values are an important quality control measurement for defining beer flavor and for determining whether a particular batch of beer is true to its style or brand specifications. In practical terms, 1 IBU equals 1 mg/l or 1 ppm of iso-alpha acids in solution. IBU values, therefore, give useful information about a brew’s bitterness intensity. There is an elaborate formula that incorporates such variables as hop utilization, which allows brewers to calculate the expected IBUs of their beers during recipe formulation.
IBU values measured in the wort in the brewhouse drop dramatically, and largely unpredictably, during fermentation. This is why wort IBUs and beer IBUs are always two distinctly separate values and a brewer’s initial IBU calculations are only estimates of the true bitterness of the finished beer. Measuring the true IBU value of beer requires complicated laboratory techniques such as ultraviolet light (UV) spectrophotometric assay or high-pressure liquid chromatography (HPLC).
Regardless of how IBU values are derived, however, they do not provide information about the quality of the bitterness. In wine, for instance, tannin content can be measured, but this does not tell anything about the smoothness, roughness, or astringency of the wine. Likewise, low-IBU brews, such as many malt liquors, for instance, can taste rough, whereas high-IBU beers, such as well-brewed rich Russian imperial stouts, can taste smooth and velvety. Also, measured IBUs in beer, like tannins in wine, decrease as the beverage ages. Some beers, therefore, may be very tough and bitter in their youth—barley wines tend to be a typical example—but may become supple and balanced after a few years of cellaring.
For all its recent use in the public sphere, where it sometimes even appears in craft beer advertising, the IBU is a laboratory construct that was never meant to leave the laboratory. Its purpose is to help brewers formulate beers and then keep them consistent from batch to batch. The usefulness of the IBU to the beer consumer is highly debatable. Once the beer leaves the laboratory context, many non-iso alpha acid factors, including other hop components, roast character, carbonation, water chemistry, and residual sugar, may exert such influence as to make the IBU an entirely unreliable indicator of actual perceived bitterness.