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A curious thing has happened, twice now, and I was wondering if anyone has either experienced something similar, or has any suggestions as to a possible why.

I have done two (almost) identical full mash brews in succession, same equipment and, crucially, the same dried yeast - Safale S04. It's a traditional English bitter of about 3.7% ABV. In both cases I have primed the fermented wort with rehydrated malt extract and then split the batch between 12 pint bottles and a barrel to condition. Conditioning for bottle and barrel was controlled at 20 C (68 F) for a week before cooling to 12 C.

Both drink well, bright, nicely carbonated, good balance of flavours except that, from both batches, the barrelled beer has a very slight apple taste - assume the presence of acetaldehyde. It is in the background, kind of on the swallow, if you get my meaning, but is completely absent in the bottled version.

Cleanliness springs to mind; however, I feel that I have been thorough. Aside from cleanliness, could there be any other reason?

[EDIT] In response to @Pepi comment. The barrel is a 25 litre plastic pressure barrel. The malt extract is added to the barrel and then the fermented wort is syphoned in, submerging the tube in the beer. Once the barrel is full (and mixed without agitation), I then use a bottle wand attached to the barrel tap to fill a few bottles. The caps are plain crown corks, nothing special. Finally, I seal the barrel and inject CO2 into it, leave it to stand, before slowly releasing the pressure to expel air, leaving the CO2 behind.

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  • Your problem is related to oxygen exposure at packaging. More info about the barrel would be helpful, especially: whether it was served with artificial CO2 or air was allowed in, whether air was purge with CO2 when it was filled. Also, oxygen absorbing caps on the bottles?
    – Pepi
    Jun 1, 2015 at 4:21
  • Thanks for the comment @Pepi I have added an EDIT, I am doubtful that it is oxygen exposure.
    – iWeasel
    Jun 1, 2015 at 9:06

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Acetaldehyde after bottling is a classic sign of oxygen exposure, and is the main reason we wait a week or more before drinking bottled homebrew. Noticeable CO2 shows up in just a few days, but the acetaldehyde (normal byproduct of glycolysis) only gets converted to alcohol later, when there is no more oxygen around.

From your procedure, I think you are getting extra oxygen into the barrelled beer:
When you pressurize the barrel to purge it, the vent/pressure relief should be open at all times, otherwise you are actually forcing the the oxygen in the head space into the beer.

If you are able to gently put some CO2 in the barrel the beer during filling, the air will be lifted out before it has any chance to meet the beer. If that's not possible, do the 'inject CO2 ... release" to the empty barrel, removing oxygen before the beer goes in at all.

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  • I think you've nailed it there @Pepi. Marked as answer. I think the latter option of inject first followed by careful fill is going to be my best option. There is, too, always a temptation to drink home brew 'young' (I suffer from classic homebrew impatience), and a little extra conditioning may drop it to levels below threshold. The ale is fairly mild in strength and flavour, so any acetaldehyde present would be noticeable even at low level.
    – iWeasel
    Jun 2, 2015 at 10:21

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