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

  1. #1
    Join Date
    Mar 2017
    Location
    London
    Posts
    33

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

    Hello,

    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.

    Matthias

  2. #2
    Join Date
    Oct 2012
    Location
    Palatine, IL
    Posts
    104
    Who was the supplier? There's a certain big name yeast supplier that is well known for having wild yeast, specifically diastaticus contamination.

  3. #3
    Join Date
    Apr 2014
    Location
    AUS
    Posts
    148
    Yeast Bay?

  4. #4
    Join Date
    Aug 2018
    Location
    Hickory, NC
    Posts
    14

    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. #5
    Join Date
    Aug 2018
    Location
    St. Paul
    Posts
    2

    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+.

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