So I'm experiencing an Acetaldehyde problem. Some of my beers start off seeminging fine, but develop the classic "green apple" flavor of Acetaldehyde in the keg. My most recent one is a Munich Helles lager, a 1.050 beer into which I pitched 2 packets of 34/70 dry lager yeast (should be sufficient cells). I've had Acetaldehyde in Pale Ales previously, and perhaps in my darker beers, it is present but masked.
Anyway, my lager was spot-on from the brew day up until about 1 month of lagering. My fermentation temps were 50-52F with a 65F d-rest as the beer neared 1.020FG, and I even hit my mash pH on brew day, a first. The first month of lagering, the beer tasted like a young, rough lager, which was expected. At about a month in, I pulled a sample and said 'UGH!' as the dreaded green apple aroma came off glass before I could even taste it.
So I am not positive why I am having these issues, but I suspect it was poor aeration. I was just stirring the crap outta my wort after it chills down to 100F or so. I'm now borrowing a buddy's pure O2 setup to do oxygenation, so I'm hoping that's the fix.
Regardless, I'd like to know if there's a way to fix Acetaldehyde in a finished beer (a light colored lager in this case), given that the Acetaldehyde might have come from poor oxygenation at pitching, or perhaps a touch of oxidation during kegging (not sure).