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Hops
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===Chemistry and Characteristics===
During aging and if exposed to oxygen, compounds in hops oxidize into different compounds known collectively as "oxidative polar compounds" or OPC's. Hao et al. (2020) found 39 different OPC's in aged hops, with 15 of them derived from alpha acids, 15 derived from isomerized alpha acids, and 9 from beta-acids.
====Acids====
During aging, both alpha and beta acids oxidize and degrade with warmer temperatures and more oxygen exposure having a greater impact. The generally accepted theory is that oxygen interacts directly with hop acids. This event is called "autooxidation". An alternative theory to this is that oxygen indirectly oxidizes acids by first oxidizing the hop oils and turning them into pro-oxidants, which then oxidize the hop acids which are mixed in with the oils within the lupulin glands <ref name="Algazzali_2014" />. The oxidation of hop acids corresponds with an increase in the [http://methods.asbcnet.org/summaries/hops-12.aspx Hop Storage Index (HSI)], which is a practical way of measuring the overall freshness of hops. As the oxidation of hop oils rises, the measured HSI number on a lot of hops increases <ref name="Lam et al., 1986"/><ref name="Maye_2016">[http://www.hopsteiner.com/wp-content/uploads/2016/07/TQ-53-1-0227-01.pdf Humulinone Formation in Hops and Hop Pellets and Its Implications for Dry Hopped Beers. John Paul Maye, Robert Smith, and Jeremy Leker. 2016.]</ref>. These oxidized compounds lead to a higher amount of non-alpha-acid bitterness compounds in aged hops and have a remarkable effect on the bitterness of the beer. The bitterness from oxidized hop compounds has been described as more earthy, harsh, and astringent than the sharper, cleaner bitterness from iso-alpha acids <ref name="Mikyška_2012">[http://onlinelibrary.wiley.com/doi/10.1002/jib.40/abstract Assessment of changes in hop resins and polyphenols during long-term storage. Alexandr Mikyška and Karel Krofta. 2012.]</ref>.