Acetaldehyde due to headspace in bucket buddy?

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Making sales and brewing ales.
Apr 30, 2018
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Springfield, Oregon
I ferment in 9 gallon Bucket Buddy fermenters--usually a 5.5 to 6.0 gallon batch size. The fermenters stay shut throughout fermentation, minus drawing a couple gravity samples toward the end of fermentation via the spigot. They are ferm-wrapped and have glycol coils in them, so temperature control has never been a problem.

I've noticed what I think is acetaldehyde in numerous beers. I always aerate well after chilling, splashing wort like crazy (no oxygen, though), and a vast majority of my batches are 1.065 or under. I always make sure to pitch the recommended yeast amount or starter via brewersfriend's calculators, too. My equipment is clean, and I ditched my plate chiller a few batches ago for an easier-to-maintain counterflow.

Could the extra headspace between the top of the wort and the lid of the bucket buddy be causing oxygen ingress and promoting acetaldehyde in some batches?

I don't get stale cardboard flavors, and some of my beers show no signs of it. I made a Kolsch a few months back that tasted like apple juice, just kegged a french saison last night that hinted at its presence, and had a weizenbock turn out completely undrinkable. Any ideas? Should I be brewing 8 gallon batches instead?

What's the least amount one could safely brew in a 9 gallon fermenter?

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Unless there’s a poor seal somewhere on the FV, I doubt the extra headspace is the problem. It’s fine to ferment half batches in an FV. Does it occur in both FVs? Otherwise more likely yeast related. Do you add yeast nutrients containing zinc? What’s your pitching practice? Do you prep fresh yeast starters? How long does it take for your fermentations to finish? If it’s insufficient aeration of the wort, and you’re doing all you can physically to oxygenate, try pitching more yeast. Say, 50 to 100% more.
Surely the headspace isn't a problem. Otherwise open fermentation would always lead to a high acetaldehyde production.

Comparing to my brews, I only got acetaldehyde, fusels and some nasty phenols when added simple sugars. Temperature plays a hole in it, but it isn't your issue, since you have temp control.

Other thing is timing, do you let yeast reabsorb fermentation byproducts? AKA how much fermentation time?
I think it’s exceedingly unlikely that headspace has anything to do with it.

First thing to do is confirm that there’s actually an acetaldehyde problem. Get a second opinion if you can. Know any judges or Cicerones? Particularly well informed brew club members? Or you could always buy a spike kit.

Kolsch esters lean pomme, anyhow, and “apple juice” sounds like it might be a particularly estery batch. Acetaldehyde, to me, is less pleasant: more like raw pumpkin than apple juice.

I’ll echo the advice to look at fermentation process, especially length.

Otherwise, I suppose infection is a possibility. You say your equipment is clean, but an OCD-level cleaning and sanitation spree of everything … well, that can’t hurt, can it?
Acetaldehyde can be a yeast issue. Staining your yeast with too high a temp, over/under pitching, over/under oxygenation of wort, not leaving it on the yeast long enough, repitching too many times, etc. Have you changed the water you brew with? This can also stress the yeast. Try some yeast nutrients or are you using too much? If your fermenters have good seals I would say head space isn't an issue. Sure sign of a leak is the airlock is not bubbling. Is there anything else new or changed in your process. Just some thoughts and I know you've addressed some of these.

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