I totally agree oxidation can take on many forms, no arguments Amanda. What I was saying, not clearly enough evidently, is that I disagree that wet cardboard-y oxidation smells/aromas can only come from the hot side of the process. I say this because the only 3 times I ever experienced it were after unintentionally splashing my beer into the keg, to a greater extent than I ever had. I changed my racking process (and attention to detail) afterward and never experienced it again.
The key phrase in your argument is "attention to detail." Most brewers intentionally or unintentionally change multiple variables when they encounter a brewing problem. The only way to know if a process change results in an improved outcome is to hold everything else the experiment exactly the same. By exactly the same, I mean everything down to the wort composition and the yeast culture.
Forty years of research have proven that lipoxygenase is responsible for the production of 2-nonenal (wet cardboard). Lipoxygenase enzymatically oxidizes linoleic acid to 2-nonenal precursors in the mash. Barley breeders and maltsters are actively engaged in research to limit lipoxygenase in malted barely. Low mash temperature and higher than ideal mash pH activate lipoxygenase. Lipoxygenase is activated at 50C (122F) and denatured at 70C (158F).
Here's a link to a Lipoxygenase-related patent: http://www.google.com/patents/US20080193593
"BACKGROUND ARTBarley lipoxygenase-1 (hereinafter, “LOX-1”) is an enzyme present in malt, which oxidizes malt-derived linoleic acid to 9-hydroperoxyoctadecadienoic acid during mashing for production of malt alcoholic beverages (Kobayashi, N. et al., J. Ferment. Bioeng., 76, 371-375, 1993).
9-Hydroperoxyoctadecadienoic acid is further converted to trihydroxyoctadecenoic acid (THOD) by peroxygenase-like activity (Kuroda, H., et al., J. Biosci. Bioeng., 93, 73-77, 2002). It is known that THOD reduces beer foam stability, imparts an astringent flavor and impairs smoothness of beer flavor (Kobayashi, N., J. Am. Soc. Brew. Chem. 60: 37-41. 2002; and Kaneda, H. et al., J. Biosci. Bioeng., 92, 221-226. 2001), resulting in lower quality of malt alcoholic beverages. In addition, 9-hydroperoxyoctadecadienoic acid is converted to trans-2-nonenal which is the substance responsible for an unpleasant cardboard flavor in aged malt alcoholic beverages
(Yasui, Journal of the Brewing Society of Japan, 96:94-99 (2001)).
As a strategy for inhibiting production of trans-2-nonenal in order to improve flavor stability of malt alcoholic beverages, there"
In closing, I am not saying that cold-side oxidation does not occur, especially after the yeast has been separated from the beer. I am merely bringing to light the fact that cold-side oxidation is not responsible for the development of 2-nonenal. This stale flavor is developed in the mash.