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I have a sour mash that is going on 48 hours, and I'm hoping to press it closer to 72 as I like my sours bitter enough to strip the enamel off my teeth. Problem is by 24 hours (last night), I went to go check on the temperature, and it was very clear that heterofermentative lacto had settled in. The moment the lid came off, I had a walloping butter smell that required every window be opened to air it out. It was the most intense smell of diacetyl I think I'll ever encounter.

I heated the mash up to 140-150, ensuring that I kill off any bad (and good for that matter) bacteria, pitched dregs from a few sours (was too late to go pick up a vial at the homebrew store, had to swing by the liquor store instead). I let the mash cool down to 120, pitched in the dregs, and maintained temp again. The smell is still present, although not nearly as intense, and it certainly isn't getting any worse since killing it off.

My question is, once I go to ferment this wort, I'll pitch a dry English Ale yeast (WLP007). Will this (or any) yeast clean up whatever diacetyl remains in the mash as well as the diacetyl generated by the yeast during fermentation? I assume boiling won't kill off the flavor?

Update: It has been nearly two weeks and I am still getting very obvious buttery diacetyl characters in both the the aroma and the flavor. At this point it is difficult to tell whether the diacetyl has decreased during aging, but if I had to guess, I would suspect that it hasn't by any noticeable degree. It is a very low gravity beer, sitting at 3% ABV, so these sorts of off flavors really stand out. I was given the the suggestion elsewhere to get a yeast starter going and to pitch it at high krausen. While I dont see the harm in trying (other than wasting time and effort) is it worth the attempt when the diacetyl is still so prevalent? Am I too impatient at two weeks despite the low gravity?

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You may have picked the wrong yeast. I read somewhere I think it was from Wyeast, that all yeasts produce Diacetyl, but some yeasts are better than others at cleaning it up. So whenever Wyeast recommends a 'Diacetyl rest' for a strain, you should not use it to clean up Diacetyl. Instead I would pick a non English yeast that does not require a D-rest, which means it cleans up nicely. So yes, pitch the new yeast at high Krausen. I've never seen Diacetyl leave just by aging.

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Ha! Imagine that! I'm using WLP007, dry English ale, so yeah, that would be a problem. I have noticed that over time the aroma has diminished, and the flavor is all but gone at this point. I haven't tasted it in over a week now though (never pitched the starter), so we'll see tonight how it's fairing. –  Scott Oct 24 '13 at 17:20
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I'm not speaking from experience here, since I don't brew sours, but the yeast should reabsorb the diacetyl, just as it does in a regular ferment.

The yeast will convert diacetyl into acetoin then 2,3-butanediol, which has much higher taste threshold than diacetyl. Both conversions provide energy, and so help sustain the yeast when other energy sources are exhausted.

You may need to condition the sour for longer than usual after primary is complete to give the yeast time to convert all the diacetyl, but I imagine you're conditioning the beer on the yeast anyway.

See

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Being that it's a sour and you pitched bacteria, it's entirely possible that the diacetyl is a result of your purposeful "infection". I don't think there are any guarantees, but I do think it's possible that it will be at least somewhat reduced by the yeast.

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I'm entirely convinced that the diacetyl is a direct result of the 8oz. of un-malted grain used that contained both homofermentative, and unfortunately heterofermentative lactobacillus, After this batch, I'm thinking I may forgo adding un-malted grains next time, instead pitching a WLP vial of lacto into the mash instead, assuming that the vial won't contain such diacetyl generating bacteria. –  Scott Sep 29 '13 at 18:32
    
AFAIK, diacetyl like you have would be produced by a pediococcus infection. If that's the case, you could probably avoid it by pitching a "manufactured" bacteria that might be "purer". –  Denny Conn Sep 29 '13 at 18:56
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