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Hops
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====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>.
Aging hops while exposed to oxygen develops a cheesy aroma due to [[Isovaleric Acid|isovaleric acid]], isobutyric acid, and 2-methylbutyric acid. These acids are produced by the oxidative cleavage of acyl side chains of the hop resins <ref name="Briggs_2004" />. These cheesy oxidation compounds can be esterified to form wine-like and fruity tasting compounds (see [[Hops#Esters|Esters]] below and [[Aging_and_Storage#Acids_and_Esters|Aging and Storage]]) <ref name="Shellhammer, Vollmer and Sharp, CBC 2015"/>.
Storage conditions and variety play a large role in how acid content in hops changes over time. Beta acids are generally more resistant to oxidation than alpha acids. A study by Mikyška and Krofta (2012) found that after 12 months of storage at 20°C in open air, pellet hops lost 64-88% of their alpha acid content and 51-83% of the beta acid content, with the beta acids dropping off more significantly after 6 months (alpha acid content declined steadily throughout the aging period). These amounts varied with different Czech hop varieties (Saaz, Sládek, Premiant, and Agnus), and beta acids degraded slower than alpha acids as seen below <ref name="Mikyška_2012" /> (percentages listed below are how much percent was lost):
Hop oils also generally degrade over time, however, their degradation rates are more complex. [http://pubs.acs.org/doi/abs/10.1021/jf00070a043 Lam et al. (1986)] found that aging both cascade and North American grown Hallertauer Mittelfrueh resulted in an increase in grapefruit-like character, although the compound that caused this was not identified. In the case of Cascade the intensity of this flavor correlated with the age of the hops <ref name="Lam et al., 1986"> [http://pubs.acs.org/doi/abs/10.1021/jf00070a043 Aging of Hops and Their Contribution to Beer Flavor. Lam et al. 1986.] </ref>. In the Hallertauer hops, aging resulted in an increase in a spicy/herbal character <ref name="Lam et al., 1986"/>, which is in agreement with reports of oxidized sesquiterpenes (specifically humulenol II, humulene diepoxides, caryophyllene, and to a lesser extent humulene monoepoxides and alpha-humulene) contributing a spicy/herbal flavor to beer <ref name="Goiris et al., 2002">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2002.tb00129.x/abstract Goiris et al., 2002]</ref><ref name="Mikyška_2012" />. Many of the oils followed in the Lam et al. (1986) study which increased during a short accelerated aging period (2 weeks at 90°F) then decreased during extended aging (60 additional days at 90°F). The cascade hops lost more of the fruity/citrusy hop oils (myrecene, linalool, and geranial) than Hallertauer, suggesting that different strains of hops can withstand aging better than others. The concentration of hop oils are affected by the brewing process and fermentation (see the table) <ref name="Lam et al., 1986"/>. Another study found that beta-ionone (classified as a ketone, and characterized as "floral" and "woody" <ref>[http://www.thegoodscentscompany.com/data/rw1006632.html Beta-ionone. Good Scents Company. Retrieved 11/22/2016.]</ref>) increased in beers brewed with hops that were aged for 30 days at 40°C versus beers brewed with aged hops <ref name="kishimoto_2007" />.
A recent study at the Shellhammer lab looked at how trained panelists and consumers perceived a lager beer dry hopped with slightly oxidized Hallertau Mittelfrüh hops (exposed to oxygen once, then stored at 38°C for two weeks) versus highly oxidized (daily exposure to oxygen and stored at 38°C for two weeks). They found that the trained panelists detected more characteristics that are associated with noble hops; e.g. more woody, earthy, and herbal characteristics in the lager beers dry hopped with oxidized hops. They also found the oxidized hopped beers to be more bitter (probably due to oxidized alpha and beta acids). Consumers were not statistically able to tell the difference. The study determined that oxidized hops might serve to provide nuanced increases in noble hop character <ref>[http://www.asbcnet.org/publications/journal/vol/2017/Pages/ASBCJ-2017-1287-01.aspx Aroma Properties of Lager Beer Dry-Hopped with Oxidized Hops. Daniel M. Vollmer, Victor Algazzali, and Thomas H. Shellhammer. 2017.]</ref>. Aging hops while exposed to oxygen develops a cheesy aroma due to [[Isovaleric Acid|isovaleric acid]], isobutyric acid, and 2-methylbutyric acid. These acids are produced by the oxidative cleavage of acyl side chains of the hop resins <ref name="Briggs_2004" />. These cheesy oxidation compounds can be esterified to form fruity tasting compounds<ref name="Shellhammer, Vollmer and Sharp, CBC 2015"/>.
* [http://scottjanish.com/increasing-bitterness-dry-hopping/ "Increasing Bitterness By Dry Hopping", article by Scott Janish on oxidized alpha acids.]