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It has been established that many factors influence the initial content of alpha and beta acids, such as cultivar (variety), cultivation area, climate and environmental conditions, harvest year, and harvest date. For example, early harvest dates result in hop cones that are not fully developed and contain less alpha/beta acids, and late harvest dates can produce hop cones with lower alpha/beta acids as these acids begin to degrade. Thus, optimal harvest dates occur when the hop cones reach their maximum size but before they become brittle and hop acid degradation begins. Skomra and Koziara-Ciupa (2020) found that harvest date also affects how alpha and beta acids degrade over time during storage. They looked at alpha and beta acid degradation differences for 4 varieties of hops grown at the same farm, but harvest at 3-4 different dates (7-10 days between harvest dates) <ref name="Skomra_2020">[https://www.pja.iung.pulawy.pl/stability-of-the-hop-bitter-acids-during-long-term-storage-of-cones/ Stability of the hop bitter acids during long-term storage of cones with different maturity degree. Urszula Skomra, Marta Koziara-Ciupa. Polish Journal of Agronomy 2020, 40, 16–24. DOI: 10.26114/pja.iung.406.2020.40.03.]</ref>. The hops were stored at 5°C and 20°C in aerobic conditions for 12 months <ref>Private correspondence with Urszula Skomra by Dan Pixley. 07/21/2020.]</ref>. As expected, the lower storage temperature of 5°C preserved the alpha and beta acids better than the warmer 20°C storage temperature. In general, an earlier harvest date also resulted in greater degradation of alpha and beta acids, but there were some exceptions to this. For example, Sybilla Harvest Date II produced the most stable alpha acid content (harvesting too early or too late resulted in less stable alpha acid content), while for Magnet Harvest Date II resulted in the most degradation of alpha and beta acids and Harvest Date III was preferable for alpha acid stability. In general, the stability of beta acids depended more on the harvest date than the stability of alpha acids <ref name="Skomra_2020" />. See the table below.
Skomra and Koziara-Ciupa (2020) also reported that for the first 3 to 6 months (3 months for , the colder alpha and beta acids were relatively stable. The cooler storage and 6 months for had a longer period of initial stability than the warmer storage conditions). After the initial period of stability, the alpha and beta acids were relatively stable, and then began a rapid drop tended to occur in most varieties until around 9 monthsof total storage. From 9-12 months, the rate of decline in alpha and beta acids was much slower and seemed to somewhat stabilize <ref name="Skomra_2020" />.
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