I was just reading through Chapter 6 of Freshness by Bamforth, and I noticed the following passage:
"I think the answer lies with yeast. Yeast loves to reduce carbonyl substances. If you take a beer with pronounced cardboard/wet paper character and treat it with a good virile yeast, the stale notes will be removed."
I assume he's talking about beer in process or he means krausening and doesn't specifically say it. Otherwise, if you did it post fermentation, what would the yeast have to work on?
I really don't know (beer in process wouldn't yet be at the point where it has a cardboard/wet paper character would it?).
I'm going to read the rest of the book this week. It's short but pretty dense. And I believe he covers E-2-nonenal in greater detail in an earlier chapter. I'll post back if I see anything on point.
One other tidbit I noticed in Chapter 3:
"If you take a stale beer and add enough sodium metabisulfite to it, then you can clean up the flavor. This speaks to the fact that sodium metabisulfite binds carbonyl substances to produce so-called adducts, which no longer display the aged character."
The carbonyl group includes diacetyl, acetaldehyde, and E-2-nonenal.
I'm not saying the OP should do this or that to his beer; I just wanted to share a few things I came across that I think are interesting.