are the internationally agreed-upon standard for measuring bitterness in beer. See bitterness. Sometimes referred to by the shortened acronym BU, for Bitterness Units, IBUs are calculated values composed of the quantity of material in wort or beer derived from hop resin (alpha acids), multiplied by the fraction 5/7. See alpha acids. This IBU method was developed in the 1950s and 1960s, when most brewers used unrefrigerated baled hops, which, by the time the hops were actually used in the brew kettle, had often lost between 40% and 80% of their alpha acid–derived bittering potential. Instead they had obtained some 20% to 60% of their bittering power from oxidation products of the hop resins. As a result, the true bitterness in beer did not correlate very well with a simple measurement of its iso-alpha acids, expressed as milligrams of iso-alpha acids per liter of beer. See hulupones and iso- alpha acids. The IBU analysis was developed precisely to overcome this discrepancy. The correction factor of 5/7 in the IBU calculation was selected because it was assumed that this was the fraction of hop resin–derived material, which, in the average beer of the day, was actually iso-alpha acids. In beers for which this assumption did not hold, of course, the values for IBUs and milligrams per liter of iso-alpha acids were still not the same.

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. See hop utilization. Beers can range from 1 to about 100 IBUs, whereby the taste threshold for most humans is roughly between 4 and 9 IBUs—different studies suggest slightly different sensitivity intervals, but all within this range. The theoretical saturation point of iso-alpha acids in beer is approximately 110 IBUs, which corresponds to 78.6 IBUs (5/7 × 110). In practice, however, this value is rarely achieved because it assumes that there are no other hop-derived resins in the beer, which is rarely the case. American mass market lagers have typical IBU ranges of 5 to 10 IBUs, Bavarian hefeweizens 8 to 12 IBUs, amber lagers 20 to 25 IBUs, American pale ales 35 to 40 IBUs, American India pale ales (IPAs) 55 to 70 IBUs, and “double IPAs” and American barley wines 65 to 100 IBUs.

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). See chromatography. The UV method is more common and can usually be performed even by small brewery laboratories, but it tends to be less accurate than the more sophisticated HPLC method, for which only large laboratories tend to be equipped. Trained flavor panelists, too, are often able to taste and approximate IBU values in beer with reasonable accuracy. However, any strong sweetness and too many malty notes, especially in higher-gravity, more assertive beers, can counterbalance and cover up much of the bitterness and thus make bitterness assessments based purely on tasting more difficult.

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.