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Wild yeast growth on fresh pitch (vermont ale?!)

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  • Wild yeast growth on fresh pitch (vermont ale?!)


    we ordered a new pitch vermont ale and streaked it on our standard copper sulphate agar plates and saw growth on them (>50 cfu) some smaller, some bigger colonies but morphology all the same. The beer tastes fine and nothing unusual. Cells look like a normal brewing yeast under the microscope. We plated from the container the yeast was delivered in and from the tank which has been cleaned very thoroughly (rinse - caustic - rinse - pasteurised (30min 80C) - acid - sanitised)

    It is not some weird Belgium strain that might be resistant to copper sulphate. Maybe the concentration of the copper sulphate is too low in the agar?

    We have brewed with that strain before and always had clean plates.

    This is very strange.

    I hope someone can help out here.


  • #2
    Who was the supplier? There's a certain big name yeast supplier that is well known for having wild yeast, specifically diastaticus contamination.


    • #3
      Yeast Bay?


      • #4
        Talk with your supplier

        In general I trust the media and if you have 50 cfu on a cupric sulphate plate that is normally clean then that's definitely a problem. I'd start by contacting the supplier and telling them what you have found, not accusing them of something but just as a way to start a conversation so that they can look back at their QC to help you figure this out. Next I'd do some sleuthing in the brewery. I'd start by plating the beer that was pitched and see if you find the yeast on cupric sulphate again; you said that you've had clean plates from beer fermented with this yeast, pull a bottle and replate it (if you still have one around). Are you plating negative controls? Are you plating on cycloheximide, it's informative to know if it's cycloheximide resistant as well.

        Good luck!


        • #5
          Plating Method

          What concentration of cupric sulfate are you using & how much yeast are you plating?

          It's possible for normal brewers yeast to overwhelm the cupric sulfate if too many cells make it on the plate. I use to use LCSM & LWYM, and both medias say to plate only 10^6 cells. In my experience, streaking from slurry usually results in growth on LCSM.

          Assuming it's a true positive, it might be worth your time to mix up some ferulic acid broth to see if it's POF+.


          • #6
            Was this plated directly from the yeast cream? If you plated 100ul, then 50 CFU corresponds to 500 (or 5x10^2) CFU per ml. If your yeast cream had 5x10^9 cells/ml then this contamination is around 1 cell in 10 million, which is below the specs tolerated by most yeast producers.

            As mentioned by others, you can get some background growth if you plate too many cells. Yeast cells make a good growth media, so if you put enough cells on the plate it creates a barrier on top of the selective media allowing some colonies to grow.

            Copper concentration is also important. We use 600ppm CuSO4 in our LCSM.

            Keep in mind too that yeast producers will talk about non-Sacch contamination specs, but they don't always have any specs about Sacch contamination. If this is a neutral Sacch contamination (POF-, killer -) then it may have passed QC.

            Contact the supplier to follow up and pick individual colonies for STA1 PCR or Durham tube analysis for diastaticus. There is a good chance though that you have nothing to worry about though, this is a very low level contamination and will likely not change the flavor profile of your beer if it is a neutral Sacch contamination.
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