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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"/>.
Measuring BU's directly requires costly laboratory equipment, and so researchers and enthusiasts have made some progress in creating models that attempt to estimate bitterness units. These models are generally based off of boil time, alpha acid percent of the hops used, weight of the hops used, post-boil volume of wort, and gravity of the wort. These include the [http://www.realbeer.com/hops/research.html Tinseth] model, the Garetz model, and the [http://realbeer.com/hops/FAQ.html#units Rager] model. These models are often used in brewing software, such as [https://beersmith.com/blog/2021/09/23/hop-utilization-models-for-beer-brewing-compared/ BeerSmith™] and [https://www.brewersfriend.com/ibu-calculator/ Brewer's Friend]. [https://alchemyoverlord.wordpress.com/2021/11/10/ibus-and-the-smph-model/ Other models such as Another model called the so-called 'SMPH' model] have has been proposed by John Paul Hosom as potential updates to older models to address updated model that addresses their limitations in regard to newer hopping techniques such as whirlpool hopping and dry hopping , as well as accounting for IBU's from non-iso-alpha acid compounds (see also [http://thebrulab.libsyn.com/episode-076-a-modern-method-for-predicting-ibu-w-john-paul-hosom The Brü Lab Podcast Episode 076 | A Modern Method For Predicting IBU w/ John Paul Hosom]). Other limitations include differences in brewhouse size and efficiencies. For example, the Tinseth model was developed on a homebrew system using whole leaf hops (see the Experimental Brewing podcast interview with Glenn Tinseth link below).
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