I have written about hop storage and degradation before https://mashematics.net/wp/hop-storage-age-and-degradation/
There is no doubt that a major factor when it comes to storing hops is the loss of Alpha Acids, thus the Hop Storage Index but with the continuing trend to very late and dry hopping the Alpha Acids become increasingly less significant. Hop oils (myrcene, geraniaol, linalool, humelene, caryophyllene,farnesene and others) are critical aroma components of modern so called craft beer.
For commercial brewers using 5kg or more bags, unopened since they packed at the pellet maufacturing facility and kept below 5C at least three years is a reasonable shelf life, but for homebrewers who buy repackaged hops it is a real buyer beware situation.
Some work was carried out on hop oil reduction during storage under various conditions can be found here https://www.mdpi.com/2304-8158/11/19/3089
“Hop essential oils are composed of numerous compounds, of which the majority are volatile, so it would be expected that for proper storage, maintaining low temperatures to prevent evaporation would be enough. However, the results from our study show that another important factor is access to oxygen. In all varieties, the amount of oil lost was higher if oxygen was accessible.
Based on HCA, the samples stored at low temperatures and anaerobic conditions had the most similar composition to the original one for the longest period of time. The statistical differences between samples also show that in the initial months, the amount of changes in hop oil content and changes in oil composition are more variety dependent (especially in cones), but after, the storage conditions take the main role. Besides that, regardless of the storage conditions, some varieties lost a higher amount of oil, which proves that some varieties are inherently more stable than others. The majority of hop is crushed into pellets, since the pellets take up much less space. Another reason is the stability of pellets, since they are known to be more stable than hop cones. The higher stability is known to be the consequence of the thin film formed around the pellets, to prevent oxidation and evaporation. However, from cluster diagrams and other results, it is seen that this is true only if no access to oxygen is allowed. In the process of pelleting, the hop cones are crushed into pellets, causing cracks in lupulin glands, which contain essential oil. These cracks leave the oil more accessible to oxygen and allow a higher evaporation rate. As a consequence, the hop cones are more stable in aerobic conditions than pellets. On the contrary, the pellets performed better when anaerobic conditions were investigated. Therefore, pellets are a better option for storage, but only if anaerobic conditions are enabled. However, after 8 months, the pellets stored under anaerobic conditions and room temperature are more similar to the 2-year-old pellets stored under anaerobic conditions and in a cold room than to the original one. Myrcene, linalool, beta-caryophyllene, alpha-humulene, and beta-farnesene are the most frequent describers of the profile of hop oil. The differences in the dynamics of changes within the same variety and form indicate that during long-time storage, not only does evaporation of compounds occur, but some other biotransformation also takes place”
Good news is though that as a homebrewer you can still keep your hops in good condition by following a few simple rules:
First and most important buy properly packaged hops, that is vacuum sealed bags in oxygen and light barrier bags that have been kept cold. Clear single layer PE ziplock peanut bags whilst affording some oxygen barrier are a very poor choice.
If buying larger amounts than you might use in one batch simply vac-pac yourself in barrier bags.
Keep them cold.
Buy from Mashematics.