Sorry to be the thousandth person to ask about acetaldehyde here; I don't think that I'm making any of the normal mistakes that cause a green apple flavour, but I have an ale brew that's hit the correct FG for the kit and it tastes like a (perfectly decent) cider.

As I understand it, the main causes of the acetaldehyde off flavour are:

  • Not leaving it to brew for long enough. It's been 22 days since I pitched the yeast.
  • Oxygenation. The FV (a bucket) has a good seal -- lots of healthy bubbling -- and I didn't open it to sample until day 18. I've opened the lid for a maximum of a minute at a time, and taken the samples using a sanitised turkey baster, making sure that all air was expelled before putting the tip in to take out the samples.
  • Low pitching rate. Maybe -- this is from a kit with a 6 gram yeast packet. But fermentation was very busy initially and the gravity has gone down from the 1.043 to 1.014 as the kit instructions expect. And if anything, the apple flavour has got worse from the first sample four days ago to today, so it doesn't sound like it could be incomplete fermentation.
  • High fermentation temperatures. It's been a pretty steady 20 Celsius from pitching to today, never more than 22, no sudden temperature changes.
  • Sanitation problems. I don't think I'm messing this up. I've had this problem in three kits now; I've used three different sanitisers, made sure everything gets properly soaked for the right amount of time, and so on. This time I'm using Star San. I've confirmed that the pH is somewhere between 2 and 3.

Some things that feel suspicious:

  • There was some apple flavour when I tasted the sample (gravity just over 1.015) on the 18th day, but it was just a hint. There were a lot of 0.5-1cm sized "broccoli" shaped yeast floaters then.
  • When I took the second sample on day 21 the floating yeast was gone but there were scattered islands of floating white foam. Kind of like what you might see if you poured a glass and let the head settle completely. The apple flavour was stronger.
  • Today the floating foam was still there but no worse. The apple flavour was stronger.

It's not actually a bad flavour -- if I bought a pint of cider at a pub that tasted like it I'd be quite happy with it... But if I wanted cider I'd be brewing cider :-)

Any suggestions much appreciated!

  • What were the ingredients? If a significant portion (i.e. > 20%) of the fermentable comes from simple sugar, this could create cider flavour. Feb 12, 2016 at 19:36
  • @Giles Thomas did you ever get to the bottom of this? I'm having this same problem in a lot of my beers. Dec 30, 2019 at 21:21
  • Not sure, I worked on improving sanitation and the problem went away, so I guess it was that. Dec 30, 2019 at 22:46

2 Answers 2


Several causes and solutions, in this case I would say a combination of premature flocculation, lack of oxygen pre pitch, under pitched and possibly post fermintation o2 exposure.

Solution so you don't get a 4th batch like this.

  • 1)Areate wort before pitching yeast, consider direct o2 gas.
  • 2) Use a yeast starter to solve the underpitch and premature flocculation. Using fresh healthy yeast, and maybe a less floculant strain.
  • 3) Minimize post fermentation o2 exposure. Buckets are really bad you will lose your c02 blanket by removing the lid. If you must pull a sample, consider using a pipet 30ml or so that can sneak in the airlock hole.
  • Thanks! A couple of follow-ups: (1) when I put the wort into the bucket initially, I poured it from some height and got lots of froth -- about 15-20cm. Is this not a sign of sufficient aeration? (3) Interesting point. How does anyone ever successfully brew in a bucket if that's the case? Do people who use buckets just not tend to test gravity to see if fermentation is finished? Feb 9, 2016 at 14:32
  • @GilesThomas 1) in most cases that is fine, nothing really replaces a pure o2 injection though. 3) I don't think any manufacturer recommends completely removing a lid if at all, many have a plug hole on top for a racking cane. I think post fermintation o2 exposure was the least contributer of acetaldehyde. This usually manifests as a cardboard taste. Feb 9, 2016 at 14:38
  • [update] Just bought one of these: amazon.co.uk/Kitchen-Home-Brew-Winemaking-Large-Pipette/dp/… Feb 9, 2016 at 19:35

If your problem really is the O2, I would get something like this: http://www.amazon.co.uk/Water-Butt-Homebrew-Fermenter-Black/dp/B00R6GJY7G/ No more removing the lid and it makes bottling so much easier.

What exactly are you trying to brew?

  • It's a two-can kit brew (St Peter's Golden Ale made by Munton's). I actually have a tap like that -- I've got two buckets, one for fermenting with no tap, then I syphon to a separate one with a tap near the bottom (having added conditioning sugar first) for bottling. No tap in the fermenting bucket because I worry it would draw off trub and either get clogged or put it in the bottles. Feb 14, 2016 at 15:42

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