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!