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diacetyl reduction in kettle sours

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  • diacetyl reduction in kettle sours

    We've been having some issues with diacetyl reduction in recent kettle sours. We have been CIPing our kettle thoroughly, purging with an obscene amount of CO2 while cooling the wort, and we get very clean-tasting sour wort at knock out ot the fermenter. We knock out the wort at 3.25-3.4 pH, oxygenate as normal, and ferment at 68-70F with our house ale yeast (Wyeast 1728 - Scottish Ale). We tend to get pretty rapid attenuation, similar to our non kettle-soured beers. But even after a 5-7 day diacetyl rest at 70+ degrees, our force D test is positive. We've experimented with different sources and strains of lacto - no change. On our last batch, once we reached terminal gravity, we dumped the yeast and pitched some fresh, active yeast from another tank...our hypothesis being that the original yeast was too stressed out after low-pH fermentation to reabsorb all of the diacetyl, and that a fresh pitch would be able to get to the finish line. But alas, there is still a small amount of diacetyl.

    We are considering buying some more pH-tolerant white wine yeast (one that will not kill the sach) in hopes that it will be able to complete the fermentation and absorb the diacetyl.

    If anyone has had similar experiences or has more knowledge of low pH fermentations or diacetyl-destroying yeast strains, we'd love to get some input. I'm about ready to abandon kettle souring and stick to mixed fermentation with brett, and longer aging.
    Kevin Drake
    Alibi Ale Works
    North Lake Tahoe

  • #2
    Are you using yeast nutrients during the boil? I would bet that the lacto is eating up what valine is there during the souring phase. We have never experienced the problem using a similar process, though we finish with 001. Try using some yeast nutrient after souring in the boil. Hopefully this will keep your yeast from producing a ton of diacetyl.


    • #3
      Does your culture contain pediococcus? Is it contaminated with pediococcus?
      Some Pedio strains generate diacetyl. Perhaps that's your problem?


      • #4
        This is an interesting one as there are not a ton of info out there regarding off flavours in kettle sours. With the perceived diacetyl, is it in the smell, taste, or both. Could you describe exactly what you perceive as diacetyl? I'm trying to see if this is also something we are getting in our kettle sours.

        Baldrick is also right, pedio can also cause this. What is the source of your lacto strain?



        • #5
          I'd guess for a kettle sour he is using L. Brevis or L. Plantarum. If its not a pure culture (some use yogurt or probiotic drinks) then its possible you have a diacetyl forming bacteria present. If you don't taste any prior to fermentation, I would not suspect this is the cause.

          English strains of yeast are known to drop quicker (generally less attenuation) and leave behind diacetyl. To mop up diacetyl you need the yeast in suspension so this could be a part of your issues. Since you've said you even tried re-pitching fresh yeast, the suspension issue would have been overcome. The second half in my guess is that you are performing the rest too late. If there is not sugar left for the yeast to metabolize, it will not reduce diacetyl. D-rest should be performed with about 1*P (or more) left before final gravity. If you wait too long, then you wont have anything left for the yeast to metabolize, and therefore no breakdown of the diacetyl. The reason we raise the temperature for a D-rest is to incite more metabolic activity from the yeast. What gravity are you beginning your rest? What final gravity are you seeing? pH can have an impact on formation of diacetyl, however many people are souring in the range you mention. 5 days should be more than needed if done at the right time.

          I would suggest you try an earlier rest, or even a different yeast initially before abandoning the process. I would avoid a wine strain because I don't like the flavors they put off personally, but it certainly could work. I still put more emphasis on the timing of the rest. Like jebzter said, we also used a similar process with a west coast ale yeast and have had no signs of diacetyl present. I am also very sensitive to diacetyl, to the point I don't even like much caramel malt in my beers.

          IF all else fails you can always consider alpha acetolactate decarboxylase as an additive (although personally I don't add any enzymes to my beers).