I recently brewed up an English Best Bitter. I enjoy these session style pale ales quite a bit. English bitters in general have plenty of malt character balanced by earthly slightly spicy English hops. My favorite is the classic East Kent Goldings. Brewing English beers is fairly straightforward the trickiest part is managing the fermentation. English ale yeasts are well know for a couple off flavors if not treated right. And that’s what we discovered when we tasted my latest brew.
This beer has a subtle but noticeable acetaldehyde character. Namely it comes across as an apple quality. At lower levels acetaldehyde may seem like another fruity ester. Esters are actually a pleasant and welcomed flavor common to English ale yeasts.
So where did this acetaldehyde flavor come from. Well production of acetaldehyde is a normal part of the fermentation life cycle of all yeasts, not unique to just English strains. Normally at the end of fermentation yeast will re-absorb acetaldehyde and convert it to ethanol. They metabolize it as a carbon energy source after the sugars are all consumed. However this takes some time.
In my case, I think the garage was starting to chill down as late fall started to transition into pre-winter like temps. English ale yeast is notoriously finicky to changes in temperature. Especially a drop in temperature. If the yeast sense a drop in temperature they begin to flocculate. English yeast strain are normally the highest flocculators of most brewers yeasts. This creates a situation where the lower temp signals a time to go dormant, flocculation happens and you end up with either incomplete fermentation or a lack of cleanup.
I suspect that’s what happened here. I wasn’t using active temp control and my yeast flocculated out too soon leaving me with some unused acetaldehyde. Thankfully I didn’t get a double disaster with some diacetyl as well! To prevent this in the future, or in your own beers, I would ferment cool for the first three days then I’d start ramping the temp two degrees or so a day until I got it to 72F. Then I’d hold the beer there for another 7 days or so. Of course this changes somewhat strain to strain,wort to wort, and recipe to recipe.