Killer response from Nicole:
oxidized hop compounds get pretty confusing!
the paper/cardboard is from trans-2-nonenal, which comes from a precursor - an unsaturated fatty acid in malt.
hop compounds are a little trickier. The classic "old hop" aroma is isovaleric acid, which is an oxidized fatty acid from hops, and is the same compound found in foot sweat and in some "stinky" cheeses. So "cheesy" is a hop off flavor... the thing is this usually comes from beer MADE with old hops...
SO a really hop forward beer that is old/oxidized/exposed to heat will first of all, loose the intense and fresh complexity you might expect. The first aromas to go are those derived from dry hopping and very very late aroma additions - and happen to be the least water-solluble (makes sense, right?) monoterpnes such as myrcene, as well as compounds like linalool, limonene, and others don't want to stay in solution and will leave. They would rather stick to the cap, can liner, or glom onto something that isn't water and is precipitating out of solution.
After those other terpenes and terpenoids go... goodbye fresh pine needle, citrus, floral, and other aromas...
The bitterness derrived from isomerized alpha acids also start to subside.
Other flavor compounds start to oxidize and change. Beta acids and others have compounds that can become more bitter as they oxidize.
Oxidized flavor and aroma compounds in hops could be compared to those of any vegetable or fruit... think canned v. fresh. I often get notes of canned peach and stale tea... fruit and tea likely have some of the exact same compounds as hops that are causing the particular flavor (most "flavor active" compounds occur in multiple places, but it is the magical combination of compounds that make up for the "essence" of a particular fruit, veg, etc).
To sum it up, let's say you have an oxidized american IPA... it is just starting to show signs... You might have enhanced carmel flavors (malt derrived) notes of stone fruit like canned peaches (hops) and a less pleasant, flat bitterness (hops). There might be a popsickle stick aroma or no aroma at all where you're expecting a fresh burst of straight-up dryhop action. This is, unfortunately what most bottled IPAs taste like due to poor handling!
-N
Pretty cool of her to take the time to share. Thanks Nicole!
Mills
