There have been
a lot of articles published in peer reviewed journals on this. The excerpt below is from an old
Brewing Techniques article from 2004 that does a decent job of scratching the surface, and references for further reading of published articles are cited at the bottom.
Hops start to lose their a-acids and oils as soon as they are harvested. The rate of loss depends on the storage temperature, the amount of air present, and the hop variety. The lower the temperature, the less the hops deteriorate. It has been shown that the rate of loss halves for every 15 degrees C (27 degrees F) drop in temperature (2).
That said, I rarely use IBUs for hoppy beers, because the majority of my hop additions are at or post flame out (all calculate at zero). For hop forward beers I pay more attention to the weight, oil content and "freshness" (as calculated by a highly scientific "whiff" test) of the hops that I'm using than the calculated bitterness units. Ironically, I do pay attention to IBUs when making beers that are yeast or malt driven, and in those cases it is more to make sure that I have some bitterness structure to hold up the beer. I normally do not use an
alpha acid loss calculator, but it does make sense to do so.