(E)-2-nonenal.
From The Oxford Companion to Beer
Formerly known as “trans-2-nonenal,” this is a compound claimed to be a principal determinant of beer staling, affording a wet paper/cardboard note to beer. It is also associated with a cucumber aroma and is generally considered an oxidative off-note. E-2-nonenal has the formula C9H16O and is a degradation product from the oxidation of unsaturated fatty acids. However, it can be produced in other reactions, notably the aldol condensation of acetaldehyde and heptanal.
There is no absolute agreement that this is always a molecule involved in beer staling. It is argued that some beers that display distinct aged character do not display particularly high levels of (E)-2-nonenal, and vice-versa. It is particularly prevalent in beers that have been aged warm, including those that are force-aged in laboratories.
One test sometimes applied is that of “nonenal potential”: samples of wort or beer are heated and the level of nonenal produced is taken as an index of propensity to stale.
This definition is from The Oxford Companion to Beer, edited by Garrett Oliver. © Oxford University Press 2012.