Diacetyl...where did it come from?

I'm trying to figure out how I ended up with diacytel in my American light lager. I used Wyeast 2007 which, according to the literature, is a low diacetyl producer. I built a 2L starter at 50 deg F and chilled my wort down to the same temp before pitching, so diacetyl production should have been minimal. I let the carboy rise to 70 deg F for a day after fermentation was complete before lagering. The beer spent about a month at fermentation temp and another month at lager temp.

So:
Low diacetyl producing yeast
Low fermentation temp
Diacetyl rest
Plenty of time on the yeast

Where did my diacetyl come from?

On the plus side, I now realize how off-putting diacetyl is in an otherwise clean lager.

Cell counts?

Did you bottle or keg the beer? I had a similar issue using san fran lager yeast in my latest brew. There was no diacetyl at the end of fermentation (and 2 days @ 65F), none after lagering 4 weeks, but in a tester bottle after 4 days of conditioning, there was massive diacetyl (I always test my bottle conditioning early). Searching around, it appears to be pretty common for this to happen. There can be plenty of diacetyl precursor (acetolactic acid) hanging around after fermentation/lagering, and oxygen will cause it to give off diacetyl. In my case, I think that bottle conditioning will take care of it. If you're in a keg, you may want to warm it up for a week or so, and also give it a good shake to rouse yeast.

Check the sensory science blog: there's a good entry on this ("Who put butter in my beer?")

By time you got to your diacetyl rest, the fermentation was probably mostly done and never had the chance to eat up those compounds. I like to slowly raise my temperature during the entire fermentation, to ensure it stays active, and I try and time it to reach the rest temp toward the end of the fermentation.

For instance, in a typical Lager, I do this profile:
1. Pitch at 45degs (or whatever the lowest recommended temp is for the yeast I'm using).
2. Raise temperature 1 degree every 12 hours.
3. After 7 days, the temp will be up to 59degrees, and I hold it there for a few days to continue the diacetyl rest- though it's likely all gone by that point anyway.
4. Lower temp to 50 for a few more weeks, or cold crash, or do whatever you normally do with your lagers from this point.

Doing this will ensure the diacetyl is gone, because it keeps the yeast active the entire way through. The main part of fermentation happens at the lower temps, and slowly raising it keeps it active, and the diacetyl will get consumed at the end of the cycle.

-Did you use at least 90 seconds of pure O2 to oxygenate?
-What was the date on your smack pack?
-Did you make the starter with a stir plate?

Waiting until fermentation was complete to raise the temp is too little too late. As others have noted, you need to raise it slowly throughout or at least when the beer is 3/4's attenuated.

Questions for the masses in followup...

My understanding is a low temperature fermentation with good temperature control shouldn't produce much/any diacetyl? Is this incorrect? Is the diacetyl production more dependent on yeast cell count then temperature?

Theoretically you should be able to brew a clean diacetyl free lager without a diacetyl rest...shouldn't you?

k1200rsvt said:

Questions for the masses in followup...

My understanding is a low temperature fermentation with good temperature control shouldn't produce much/any diacetyl? Is this incorrect? Is the diacetyl production more dependent on yeast cell count then temperature?

Theoretically you should be able to brew a clean diacetyl free lager without a diacetyl rest...shouldn't you?

Click to expand...

It WILL produce diacetyl, no matter what - but the yeast will eat it up toward the end of it's fermentation cycle. The issue is that toward the end of the cycle, the yeast isn't nearly as active as it was during the beginning when it first produced it, so you can never guarantee that it will clean it up at the end.

With lager yeast and cold temperatures, the yeast can often go dormant before it has a chance to finish it off. Even Ale yeast can do this at much warmer temps. Even though *in theory* it will clean it up at the end of fermentation, the only way to guarantee that it will is to raise the temperature and help it out.

It won't make the beer taste bad, or less smooth or whatever, by doing a rest for a few days - so it's just best practice to go ahead and do one.

Thanks for all the good info!

It's sounds pretty likely that my diacetyl rest was too late and the yeast weren't active enough to absorb what was produced. I might try some of your suggestions to clean up what I've got. It's not horrible but the buttery notes in the flavor really stand out.

darknazgul said:

Thanks for all the good info!

It's sounds pretty likely that my diacetyl rest was too late and the yeast weren't active enough to absorb what was produced. I might try some of your suggestions to clean up what I've got. It's not horrible but the buttery notes in the flavor really stand out.

Click to expand...

Well, FWIW - everyone has gone through this (or will go through this eventually). You make a great beer, no mistakes, you just KNOW it's going to be amazing and your friends will bow before the Beer God that you are .. then the yeast betrays you. DAMN!!

It's necessary though, because you may make 10 beers with no diacetyl rest and it works out great every single time, then that 11th one has that stupid buttery thing going on. After that you just adjust your practices and prevent it in the future.

darknazgul said:

Thanks for all the good info!

It's sounds pretty likely that my diacetyl rest was too late and the yeast weren't active enough to absorb what was produced. I might try some of your suggestions to clean up what I've got. It's not horrible but the buttery notes in the flavor really stand out.

Click to expand...

The batch I bottle two weeks ago that had horrible diacetyl at 4 days is now almost totally cleaned up, and is drinking really good. I suspect yours will go away with a bit of warm conditioning.