That’s a very clean strain, and it sounds like you did everything right regarding starter and fermenting temperature. The fermentation temperature might have risen a bit, but probably not to levels where off-flavors were that much of a problem. Anyhow, acetaldehyde isn’t generally associated with high temperature fermentation.
It’s just possible that you overpitched. The dry yeast packets have a lot of cells and your mild wasn’t that big a beer. On the other hand, if the packet was old and you didn’t rehydrate the yeast first, it’s possible that your cell count was too low. Either case can result in off-flavors.
The recipe looks solid, so no problems there. Actually, for an authentic mild, you could have used a bit of caramelized sugar or syrup, but since you added the smoke malt I can tell you’re not a slave to style categories. BTW, while it’s not recognized by the BJCP, mild ale actually comes in different strengths. Remove the smoked malt and you’ve got what the Brits would call a “best mild” or “strong mild” which overlaps into English brown ale or brown porter territory. (Mild is often the draught product, brown ale the bottled product. Take the beer a bit darker and roastier, and it’s a brown porter.) Keep the smoke in, and you’ve potentially got something that would taste like an authentic 17th or 18th century “brown beer” or “running porter.”
This is a potential problem. If you’re getting oxygen into your wort by stirring or agitating, you need to aerate for at least 10 minutes to get anything like the oxygen levels you need (about 6 ppm max. using just air). To get proper O2 levels in a strong wort you almost need to use oxygen and a sintered airstone to get your magic 10 ppm dissolved O2. I can’t comment on O2 levels achieved using an aerating cane/hose since I’ve never used one.
Sympathy. I’m stuck with the same problem. But, I don’t think that a temperature spike would have made the yeast flocc prematurely. Instead, it’s more likely a temperature crash that knocked them out of solution. A potential situation for this to occur might be a hard cold snap for a couple of days, coupled with serious drafts through your fermenting area.
Alternately, the fermentation temperature started unusually high or slow so the yeast were under stress or otherwise produced lots of off flavors, then went cooler so that they didn’t have time scavenge up the acetaldehyde and diacetyl they produced.
Speaking of which, did you get any buttery or slick notes in your beer, or are you not particularly sensitive to diacetyl?
The combo of “green apple and butter” would make it a slam dunk case for a fermentation problem, whereas acetaldehyde aroma on its own might be due to some other things.
Since you’ve got a draught system, it couldn’t hurt to clean your lines, especially if you’re getting off-flavors from your other beers. Some bacteria will produce acetaldehyde plus other off-flavors and they love to live in dirty beer lines.
Finally, it’s also possible that you’re just mistaking apple esters, which are really common in ales, for acetaldehyde, so there’s actually no problem. Sometimes apple esters plus a bit of roast can be interpreted as acetaldehyde. The smoke malt might be tricking your sense of smell/taste.
