White Labs PurePitch Yeast . . . some numbers

Discussion in 'Homebrewing' started by PortLargo, May 18, 2018.

  1. PortLargo

    PortLargo Devotee (461) Oct 19, 2012 Florida

    Just bought a packet of White Labs PurePitch yeast and read the fine print on the packaging. There's a lot of fine print and it directed me to a web site that showed the analytics of the actual batch my humble packet was taken from. You can check it out, my batch number is 1054308 :


    First, it has an interesting looking Avatar for the site:
    While "Yeastman" probably isn't as cool as VikeMan, I like the little guy on his shoulder . . . like a little yeast cell trying to "bud out". But to the point; my packet was released by QC on Mar 31, 2018 and the density is 2.18 billion cells per ml. The range of density varies by strain/batch from 2.0 - 2.8 billion cells/ml. My packet is clearly marked 40ml, so I'm looking at 84.8 billion cells (call it 85) as of the release date.

    Using the Mr Malty yeast calculator (also White Labs) shows a 63% viability rate (Mar31 to May17, or 47 days). Using Brewer's Friend calc gives 67%, yeastcalculator shows 66% . . . all essentially the same. But White Labs says PurePitch is different, viability decreases at a much slower rate. This is what they post about shelf life/viability which I've copied below:

    Average viability of PurePitch® pure liquid yeast over a period of six months:

    Time - - - Viability

    1 month...99.21%

    2 month...98.05%

    3 month...90.26%

    4 months...84.28%

    5 months...79.35%

    6 months...71.59%

    In the link there is a narrative on why PurePitch has reduced the aging process, but these figures are drastically different. Using this, my viability is ~98.5%. So do I have 83 billion cells (85B x 98.5%) or am I looking at 56 billion (85B x 66%)?

    I applaud White Labs for providing these details, but it is spread out over several web sites and is different enough from what they have published in the past (and from their own yeast calculator) to create significantly different results on aging of a packet.

    So are my fellow Zymurgists treating WL PurePitch yeast differently from say Wyeast? I believe all the yeast calcs default to 100 billion cells/packet (my current packet is 85B) and most use a 21% monthly decrease in viability. Mr. Malty shows 10% viability for liquid yeast at six months (many of us would toss it), where the figures from PurePitch at six months shows 71%, a respectable figure (if true). Somewhere there's a geek with a hemocytometer that could shed some light on all of this. Comments from any of those?
    #1 PortLargo, May 18, 2018
    Last edited: May 18, 2018
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  2. VikeMan

    VikeMan Poo-Bah (1,560) Jul 12, 2009 Pennsylvania

    The next update to BrewCipher will likely have an option to treat WLP strains differently, based on White Labs' new data. The trick though is that their data is presumably based on test samples that have been handled/stored under ideal conditions. So by default, there will be a penalty applied in my calculations to represent "typical" less than ideal conditions. Not sure yet how big that penalty will be yet.
  3. PortLargo

    PortLargo Devotee (461) Oct 19, 2012 Florida

    Storage instructions are pretty straightforward: "store at 40┬░F" . . . which is about where my LHBS had it and is exactly how I stored it.

    BTW, I inserted some random lot numbers in the "Yeastman" web site and number of cells really did vary from as low as 2.0 billion/ml up to 2.77 b/ml. I've always suspected that, White Labs seems to confirm you don't really know how many cells are active until you count them. I bet Wyeast cells (and others) are in the same boat here.

    So, how big a starter would you make for 5.2g of 1.060 wort?
  4. VikeMan

    VikeMan Poo-Bah (1,560) Jul 12, 2009 Pennsylvania

    Well, if you think the yeast has been handled and stored perfectly, then using White Labs numbers, you'd have 84.8B cells x 98.5% viability, or 83.5B viable cells. Assuming your target pitch rate is 750,000 cells/ml/degree P, you'd want ~218B cells. Using BrewCipher's growth curve, that would be a 1.2 quart stir plate starter, or a 1.8 quart periodic shaking starter.