Off flavor after Kegging

[Marcelo Braga]

Hi!
I Brewed a ESB, everything was going well, It fermented for 21 days, first 7 days 20 C (68 F), the other 14 days 22C (72F). After that I did a cold crash for 7 days cooling to 5 C (41F). I used London Yeast from Lallemand. I Brewed 10 Liters so I transfered for 2 mini kegs of 5 Liters. The Beer was perfect, no off flavors, malty, bitter... It had everything.
Sadly after 3 days of force cabonation when I tried there was something, I tasted what I suppose to be acetaldehyde.
Every time I Keg, I put CO2 in the empty keg and purge the air out. This time I did not do that.
What I don´t understand is that acetaldehyde reflect incompleted fermentation I got a attenuation of 73%, higher than the yeast average (65%). So I thought if I had a problem with my fermentation I could wait and maybe the other 5 Liters mini keg will be ok, I opened the lid and put 3 g of sugar in the beer at room temp. like the mini keg was a big bottle.
I would like to know if this happened with someone? Could this off flavor be something else? Can I save the other mini keg?

 

[VikeMan]

It's not just a byproduct of fermentation thing. When ethanol (the main alcohol in beer) oxidizes, it becomes acetaldehyde. As a first step, I would let the kegs come to room temperature and see if the remaining yeast can process it.

 

[Marcelo Braga]

It Makes sense, The first keg is now empty, I had a barbecue and people drank the whole mini keg, nobody notice even when I told about the green apples...
The second one is sitting in room temperature for a month now, when I put it on tap I will post the result.
Thank you for the answer!