Diacetyl and Bottle Conditioning

I revived this question in a thread that was in the beginner's forum, but I thought it would be more useful to post it here and elaborate on it.

Certain physical and temporal restraints led me to primary and secondary by brown ale for one week each before bottling. I tasted the beer before bottling and it was very nutty, with no detectable hint of diacetyl. I opened a bottle yesterday just to sample (10 days after bottling) and it had a noticeable buttery diacetyl flavor. I imagine that this will go away with a little more time, but it brought a few questions to my mind.

Is it normal for beer with little to no diacetyl at bottling to have significant levels 10 days later? I usually taste samples as the beer is conditioning, and after several dozen batches, this is the first time I've had that happen.

Under the constraints present during bottle conditioning (no oxygen, limited sugar, increasing pressure), what is the capacity of bottle conditioning yeast to reduce diacetyl levels?

How do these changes occur over time? Would most of this happen in the first couple of weeks? After a month? 6 months? I was always under the impression that after a couple of weeks in the bottle, any flavor changes that occur are not a direct result of any active processes from the yeast, as they are dormant by then.

If the yeast do process diacetyl during bottle conditioning, what is the byproduct(s) that results from this? I seem to recall reading that the resulting compounds are flavorless.

Diacetyl is typically produced by yeast on the earlier end of fermentation. If it wasn't there initially after primary fermentation finished and developed later, an infection is not out of the question.

Hopefully, it's just a nutty sweetness and young beer flavor that you're interpreting as diacetyl. Or it is diacetyl and you didn't notice it before when you tasted it due to it being masked by young beer flavors. In which case, it will likely subside given time in the bottle. Give it a few weeks and maybe it will go away.

Yeast metabolize diacetyl into... [blank]. (Drawing a blank here, but I know it's in one of my brewing books (Noonan?, Fix?) and I also recall the resulting compounds not having an impact on flavor.)

menschmaschine said:

Diacetyl is typically produced by yeast on the earlier end of fermentation. If it wasn't there initially after primary fermentation finished and developed later, an infection is not out of the question.

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This is what I was worried about. I do remember reading that as well, so that's why it seemed unusual to me to not have noticed it at bottling but to have it now. It's possible I just didn't notice it due to to it being so green then. It was only two weeks from brew day, so that is a distinct possibility.

I'm always so obsessive about sanitation. I hope it is not infected.

menschmaschine said:

Yeast metabolize diacetyl into...

Click to expand...

Found it!... acetoin and subsequently 2,3-butanediol

DUH! Can't believe I forgot acetoin and 2,3-butanediol.:cross:

They also have a high flavor threshold, so they're rarely tasted.

Diacetyl is produced when acetolactic acid is oxidized (generally enzymatically by yeast or bacteria in beer).

Fix describes a phenomenon whereby diacetyl flavors show up after packaging. He proposes that deficient yeast have left high levels of acetolactic acid in the beer (rather than oxidizing them to diacetyl and so on). Later, the acetolactic acid is oxidized to diacetyl by a non-enzymatic oxidizing agent (oxygen in this case).

Where the diacetyl the result on an infection, the bacteria involved will also sour the beer so this should be apparent (at least over time).