Bitterness Units, known widely as the international bitterness unit (IBU) or sometimes the shortened form bitterness unit (BU is the internationally accepted unit for describing bitterness in beer). For brewers, the IBU is an important metric for defining beer styles, beer flavor, and a particular beer’s “trueness to brand.” The IBU measurement is an important quality control measurement because it gives information regarding bitterness intensity. More specifically, it is a measure of iso-alpha acids and other bitterness compounds present in wort or beer where 1 IBU is equal to 1 mg/l or 1 ppm iso-alpha acid in solution. Brewers calculate the expected IBUs of their beers when formulating a recipe or incorporating a new hop into a beer. Beers can range from 1 to about 100 IBUs. The saturation point of iso-alpha acid in beer is approximately 110 IBUs. Brewer’s initial calculations are only an estimation of bitterness and the true IBUs of a beer must be measured. IBUs measured in the original wort will drop dramatically during fermentation, and therefore wort IBUs and beer IBUs are two very different things. IBUs are measured in a brewing laboratory by either ultraviolet (UV) light spectrophotometric assay or high-pressure liquid chromatography (HPLC) methods. The UV method is common and is often done even in small brewery laboratories. It tends to be less accurate in measuring specifically iso- alpha acids, whereas the HPLC methods performed in larger, more sophisticated brewery labs are very accurate. HPLC can detect, separate, and measure specific analogues and non-iso-alpha acid bittering compounds that can originate in the hard resins, beta fraction, or other hop fractions. Trained flavor panelists are able to taste and approximate IBU values quite accurately. However, the sweetness and malty component of a beer can counterbalance and cover the bitterness in a beer, making it harder to determine the IBUs by taste, especially in higher-gravity, more assertive beer styles. IBUs do not give any information about the quality of bitterness. To use wine as an example, it is possible to measure a wine’s tannin content, but this measurement does not convey whether tannins are smooth and well integrated or rough and astringent. It is the same with the IBU, which should be of far more interest to the commercial brewer and brewing scientist than to the consumer, who will learn relatively little about beer flavor from a number. Measured IBUs in beer, like tannins in wine, decrease as a beer ages. A barley wine that may be very tough and bitter in its youth may therefore become supple over a number of years if aged properly. Examples of typical IBU ranges in various beer styles include the following: American light lagers (5–10 IBUs), Bavarian hefeweizens (8–12 IBUs), amber lager (20–25 IBUs), American pale ale (35–40 IBUs), American India pale ale (IPA; 55–70 IBUs), and “double IPAs” and American barley wines (65–100 IBUs).