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===International Bitterness Unit===
Beer bitterness is often described in terms of International Bitterness Units (IBU), or more accurately, Bitterness Units (BU). The European Brewery Convention has adopted the [https://europeanbreweryconvention.eu/new-international-method-on-bitter-compounds-in-dry-hopped-beers/ “E.B.C. Bitterness Units,”] determined in a similar way and recently updated for dry hopped beers to account for humulinones, as a uniform method that best expresses the true bitter flavor value of beer <ref name="asbc_ibu">[https://www.asbcnet.org/Methods/BeerMethods/Pages/default.aspx ASBC Methods of Analysis website. Retrieved 02/11/2022.]</ref>. These measurements seek to measure the amount of iso-alpha acids, which contribute the majority of bitterness to beer. There are, however, other compounds that contribute to bitterness, such as oxidized alpha and beta acids (see [[Hops#Chemistry_and_Characteristics|Aged Hops]] below). These methods include using [https://en.wikipedia.org/wiki/Spectrophotometry spectrophotometry], [https://en.wikipedia.org/wiki/High-performance_liquid_chromatography High-Performance Liquid Chromatography-Ultraviolet (HPLC-UV)], and [https://en.wikipedia.org/wiki/Liquid_chromatography%E2%80%93mass_spectrometry liquid chromatography–mass spectroscopy (LC–MS)]. The ASBC describes these methods in depth on [https://www.asbcnet.org/Methods/BeerMethods/Pages/default.aspx their website] (see method 23; requires membership to read). While many brewers argue that the IBU measurement is not that helpful for communicating bitterness to consumers, it is generally agreed upon that IBU measurements are very useful to brewers who are seeking consistency in their products <ref>[https://beerandbrewing.com/dictionary/eej03p6ZUI/ The Oxford Companion to Beer definition of International Bitterness Units (IBUs). Retrieved 02/12/2022.]</ref>.
Compounds other than iso-alpha acids present several challenges to traditional methods of measuring BU's. Many hop compounds other than iso-alpha acids that have varying levels of perceived bitterness are detected at the same wavelength as iso-alpha acids using spectrophotometry. In addition, oxidized alpha acids are known to contribute to bitterness. As a result, updated methods of using High-Performance Liquid Chromatography-Ultraviolet (HPLC-UV) and liquid chromatography–mass spectroscopy (LC–MS) are generally recommended for measuring bitterness units in dry hopped beers (although models that account for bitterness contribution from compounds other than iso-alpha acids have not yet been completed). In addition to these challenges, iso-alpha acids and other hop compounds that contribute to bitterness degrade over time, thus the perceived bitterness of beer tends to become weaker as beer ages <ref>[https://www.chromatographyonline.com/view/liquid-chromatography-mass-spectrometry-analysis-of-hop-derived-humulone-and-isohumulone-constituents-in-beer-the-bitter-truth-of-hops-utilization-during-brewing Liquid Chromatography–Mass Spectrometry Analysis of Hop-Derived Humulone and Isohumulone Constituents in Beer: The Bitter Truth of Hops Utilization During Brewing. Bruce C. Hamper, Nicholas Viriyasiri, Aaron Boland, Lorna Espinosa, Hunter J. Campbell, Kurt Driesner, Michael McKeever. January 1, 2022. LCGC Europe, January 2022, Volume 35, Issue 01. Pages: 32–37.]</ref><ref name="asbc_ibu"/>.