Changes

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 oxidize 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" />. This 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 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>.

{| class="wikitable sortable"

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| Open air at 20°C for 12 months

| || '''Alpha acids''' || -80% || -88.3% || -64.3% || -78.2%

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| Vacuum sealed at 20°C for 12 months

Oxidized alpha acids (humulinones) are similar in taste perception to iso-α-acids, but have been described as less bitter (about 66% as bitter) <ref name="Shellhammer, Vollmer and Sharp, CBC 2015"/><ref name="Maye_2016" />. Humulinone content increases in hops after being pelletized (whole leaf hops have less humulinones). In fresh pellet hops that have a relatively low humulinone content, the humulinones contribute little to the bitterness of the beer when boiled, however when dry hopped they readily dissolve into the beer and have a significant impact on the beer's bitterness. With heavy dry hopping, the humulinones also decrease iso-alpha acid content of beer with more than 20 IBU's, but not in beer with less than 25 IBU. The decrease in iso-alpha acids and perceived bitterness/IBU is partially made up for the bitterness of the humulinones themselves (humulinones are picked up in IBU measurements with a [http://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Kinetics/Reaction_Rates/Experimental_Determination_of_Kinetcs/Spectrophotometry spectrophotometer]). In beers with less than 25 IBU, high dry hopping rates greatly increase the bitterness/IBU due to the bitter humulinones. The rate of humulinone formation is limiting, meaning that humulinone formation occurs rapidly during hop pelletization, and the concentration peaks during this time (researchers found that further exposure to air did not increase humulinone content). Scientists believe that this is because when whole leaf hops are baled, only 20% of lupulin glands are broken, whereas when they are pelletized 100% of the lupuline glands are broken. The exact mechanism by which alpha acids are converted to humulinones is not known <ref name="Maye_2016" />. Humulinone content in long-aged hops (1+ years) has not been studied.

[[File:Lam Hop Oils.jpg|thumb|upright=2.5|[http://pubs.acs.org/doi/pdf/10.1021/jf00070a043 Data from "Aging of hops and their contribution to beer flavor" by Kai C. Lam, Robert T. Foster, and Max L. Deinzer.] '''Aged I''': 2 weeks at 90°F; '''Aged II''': 60 additional days at 90°F.]]