Wed Mar 24, 2010 8:44 pm
On the few occasions where I have experienced acetaldehyde in beer at a level worthy of commenting on in a score-sheet, there has been an assault of aldehydic qualities i.e. more like rotting apples with a hint of formaldehyde than the faint hint of apple one experiences with Budweiser. Remember that fermentation goes sugar --> pyruvate --> acetaldehyde --> ethanol i.e. acetaldehyde is a normal intermediate metabolite and thus there is bound to be some residual in any beer but expose it to reducing* conditions and eventually it should all (or nearly all) wind up as ethanol. Ordinary lagering conditions where there are live yeast cells present even though they are on the edge of dormancy do maintain this low ORP and indeed one of the reasons for lagering is the reduction of acetaldehyde. If you separate the yeast from the beer by filtration or centrifugation the ORP might rise to the point where acetaldehyde isn't completely reduced and you might have enough acetaldehyde residual to taste (think Budweiser again) but if you simply rack to the lagering vessle, even though there are globs of yeast cells in the fermenter cone, you are transferring billions of yeast cells, ceratinly enough to hold the ORP low enough that the acetaldehyde should evanesce with time.
If the "green apples" impression is more than subtle I always suspect zymomonas infection and in such cases I don't think there is much you can do though, of course, you have little to lose in trying.
I would just let it continue to lager. Reduction of nasty volatiles in beer is one of the main reasons for lagering and as lagering has been demonstrated to give better overall results at low temperature I wouldn't raise the storage temp. in the hopes of getting rid of acetaldehyde until convinced that normal lagering isn't going to be effective.
Acetaldehyde is very volatile (boils at about 20 °C) so as the beer has been lagering under pressure you might try depressurizing it to allow bubbles of CO2 to form to scrub out some of the stuff. Repressurize for a few days and bleed again and perhaps again after that.
How clear is the beer? Aldehydes tend to form adducts with proteins which would then be expected to settle out during lagering so if you still have protein haze that may be a good sign for eventual recovery.
If further lagering and scrubbing don't improve the situation you can try adding a Campden tablet to 5 gallons. This will reduce the acetaldehyde to ethanol but, naturally, leave a fair amount of SO2 in your beer which may make it worse tasting than with the acetaldehyde though the excess SO2 should eventually fly off as well. Nonetheless I only suggest this as a desperation measure. If there is so much acetaldehyde that this action is required the beer is probably a lost cause anyway.
Which yeast strain did you use?
*"Reduce" and "reducing" in this context refer to the supplying of electrons to acetaldehyde to produce ethanol: CH3CHO + 2e- + 2H+ --> CH3CH2OH
Of course it is true that the concentration of acetaldehyde is also reduced when it is reduced in the redox sense.
