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).

  • 1.020FG seems very high from a 1.050OG beer. Can you check those figures?
    – mdma
    Jun 2, 2014 at 13:17
  • No, I moved it to a d-rest when it hit 1.020. It finished at 1.012, I believe.
    – Graham
    Jun 2, 2014 at 14:31

1 Answer 1


Given that it happens a long time after your beer has reached FG, I doubt it's due to insufficient oxygen in the wort at pitching time. If it were insufficient oxygen, that would lead to a poor ferment, with the CH3CHO being created as the penultimate step in fermentation (conversion of pyruvate into acetaldehyde and carbon dioxide) but the poor fermentation conditions preventing the completion of last step - reduction of acetaldehyde into ethanol. You would taste the acedaldehyde sooner rather than later.

That leaves two other options:

  • acetobacter contamination: Acetaldehyde is produced by acetobacter as a precursor to acetic acid. This can make the beer taste more cidery/tart, like bruised apples.

  • oxidation of ethanol: if the beer is exposed to oxygen, this can reverse the reduction and turn the ethanol back into acetaldehyde.

Given that you have this in several beers, then it seems to be an issue with process, which could be either of the above. It's worth double-checking sanitation procedures. But if we assume the sanitation is good, then really the culprit is most likely oxidation post-transfer. You can reduce the chances of this during transfers by purging the racking cane and receiving vessel with CO2.

To fix this in an already brewed beer, I would add more wort and fresh yeast (ideally with the yeast already fermenting the wort.) The yeast will continue to ferment, and will bring fresh reserves of Alcohol Dehydrogenase, the enzyme used to reduce acetaldehyde to ethanol.

  • Ok cool. The next time I'm making a starter with clean ale yeast, I'll make it double sized, and I'll pour a quart or so of starter wort + yeast at high krausen into the keg, does that sound correct? I'll off-gass the keg after a day or two.
    – Graham
    Jun 2, 2014 at 14:46
  • sounds good to me
    – mdma
    Jun 2, 2014 at 16:43

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