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Diacetyl -

I made a basic American lager. No hint of diacetyl prior after primary fermentation and prior to lagering. Diacetyl taste was mild after lagering before kegging. 4 weeks later when carb’d it tastes like something Orville Redenbacher would be proud of. I hate to pitch it but doubt I came make myself drink it.

I’m not so concerned about why it happened, but want to know if can it be saved. My first thought was warm it and repitch some yeast to eat it up. Since then I’ve read ( didn’t write down source) that if diacetyl occurs after primary fermentation, that it isn’t reversible.

Thoughts ?


My thought here is that it could be an infection, since it was not detected much earlier, and has become more prominent with time.

I would wait for a second opinion before dumping it out, but if this is the case, I don’t think there would be much you can do at this point.

Diacetyl is certainly reversible. You need to warm the beer to upper 60s and add a very active yeast starter. Then leave it warm for about a week. The diacetyl might not disappear immediately, but with fresh yeast in there again it should clear up within about 3-4 weeks if you keep it at 50 to 65 F. Don’t chill it down again to 30s or 40s for a month. Should be gone by then.

You most certainly should care why it happened. Only way to avoid repeating your mistakes is to understand why they occurred. That said, if the diacetyl was due to incomplete processing of intermediate compounds by the yeast, it is reversible as Dmtaylo says. If it was due to an infection, I suspect it is a goner. Follow Dmtaylo’s advice. You’ll either come out with a fixed batch that is drinkable, or a better understanding that you need to improve your sanitation process.

I have heard that you must warm up the sample you are pulling when you are checking for diacetyl. So, if you pulled a sample at fermentation temp, it may not have been perceptible, but may have been there. I have only made a couple lagers, but have done a diacetyl rest each time for 5 days with great results.

Thanks. I’ll give it a go. I’ll just leave it in the keg, repitch the starter, bleed the C02 on occasion, hold at 60F +/- for a month or so.

For clarification to one comment on the original post. This purpose of this post was to find an answer as to clearing the dicaetyl, not the cause. The batch was an afterthought to making Classic American Pilsener. I reused the yeast and took a few short cuts when handling it.

Please let me/us know if this technique works. It should work, but to be honest, I’m not 100% certain, since there’s no fermentables left in the beer anymore besides the diacetyl (yes - yeast can eat diacetyl). If they aren’t hungry for diacetyl, they might just settle out right away. This is why it’s important to pitch an active starter, because they’re actively eating every morsel they can find and want to keep on eating. Yeast are strange beasts… they eat like pigs and don’t save any for later. They want all their food gone immediately.

I’ll repitch at roughly the same cell count as for the original beer. Throw it at the height of activity.

Count on the update in a few months…

I will finally update this post and report that the keg diacetly was eliminated. I made a new 2L starter and dumped it all directly in the keg. Fermentation did begin so I bled the pressure very few days.

Since this beer was an afterthought from another batch, I didn’t bother lowering the temp to 50F. I let it go a basement temp which was around 62F.

I gave it four weeks and put the keg back into the fridge. No racking. After another 8 weeks it cleared up and tasted like the boring bland american lager it was intended to be, sans the butter flavor. It wasn’t a good beer but it was drinkable.

Short story is that keg diacetly can be recoved with a repitch. I don’t plan to do it again, but if so, I’d rack to a carboy and lower to 50F for proper lager fermentation. I suspect that a 1L starter woulkd be adequate.

bwmac, thanks for the update. Believe me other people are taking notes on this. The fact that it happened to an ordinary CAP isn’t important as the valuable lesson you just learned, even tho it was such a long ordeal. That’s what you will always remember, and we all move forward because of that experience.
Thanks again.

Yes, bwmac2, thank you for the update. Glad to see that it worked out.

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