Changes

'''Acids''': oxidized During aging, both alpha and beta acids oxidize and degrade. This corresponds with an increase in the [http://methods.asbcnet.org/summaries/hops-12.aspx Hop Storage Index (HSI)] <ref name="Lam et al., 1986"/>. Oxidized alpha acids (humulinones) are similar in taste perception to iso-α-acids<ref name="Shellhammer, Vollmer and Sharp, CBC 2015"/>. Oxidized beta acids (hulupones) also contribute to perception of bitterness.

[[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.]]]  '''Oils''': 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, 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>. 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, 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"/>.