Modeling Yeast Die Off Rates

Discussion in 'Homebrewing' started by VikeMan, Jan 23, 2014.

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  1. VikeMan

    VikeMan Grand Pooh-Bah (3,043) Jul 12, 2009 Pennsylvania
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    Recently, there was a discussion about whether it was more appropriate to assume that dormant, refrigerated yeast tend to die off as a function of the original count (of freshly minted yeast) or as a function of the remaining cells. So I emailed Wyeast. Here's the question and response. (If anyone wants the name of the Wyeast Microbiologist, Beermail me. I'm not going to post it here, because I didn't ask for or receive permission.)

    THE QUESTION:
    "Hello,
    I know that all other things being equal, a two month old package of yeast will have more viable cells than a 4 month old package, etc.
    My question is... When ballpark estimating the amount of viable cells left, would it be better to assume some daily die-off rate as a function of the remaining cells each day, or as a function of just the original count? If the former, a smaller number of cells would be assumed to be dying on each subsequent day (because the pool of living cells is smaller each day). If the latter, a constant number of cells would be assumed to be dying each day.
    Thanks for your help!"

    THE ANSWER:
    "I think it would be best to base your rate of die off as a function of the original count.
    Let me know if you have further questions."

    This, of course, would mean an increasing percent change (gasp!) in viable cell count per day (or whatever period) as the yeast get older.

    But hardly definitive based on the wording of the response. I think I will add a user selectable parameter in BrewCipher, so that each user can decide which die-off model floats their boat.
     
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  2. FATC1TY

    FATC1TY Pooh-Bah (2,502) Feb 12, 2012 Georgia
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    Interesting to note.

    I've got a pack of Dec 17th Wyeast 1217 PC West Coast IPA, in the fridge that I want to use soon. A bit lazy to make a starter, and don't want to brew an IPA just yet, so thinking about using it in a small 1.045 Blonde first, and pitching on some slurry/cake for the bigger IPA.

    I'm using the assumption that the yeast is at 100B and has dropped in vitality via the mr. malty and beersmith assumptions.
     
  3. JackHorzempa

    JackHorzempa Grand Pooh-Bah (3,351) Dec 15, 2005 Pennsylvania
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    “…dropped in vitality via the mr. malty and Beersmith”

    Well, I will mention names. Dr. Chris White stated that the die off rates of the Mr. Malty yeast calculator is conservative. Feel free to use what the Mr. Malty yeast calculator yields but recognize that Dr. Chris White thinks you will have more viable yeast cells.

    Cheers!
     
  4. ChrisMyhre

    ChrisMyhre Initiate (0) Sep 15, 2013 Massachusetts

    I would love to hear more about what they think about appropriate die off percentages, if you do ask follow up questions I'd love to hear about anything you are able to learn. It seems to me this functionality in your spreadsheet should satisfy all ranges of thought on die off rate being conservative or not and allow us to dial in the correct starter size based on our own assumptions. As always thanks for the contributions.
     
  5. VikeMan

    VikeMan Grand Pooh-Bah (3,043) Jul 12, 2009 Pennsylvania
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    It seems odd to tell people not to use a particular model when you don't have an alternative to offer.
     
  6. inchrisin

    inchrisin Pooh-Bah (2,013) Sep 25, 2008 Indiana
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    With all the microscope talk we've done on the forums nobody's plated yeast on a weekly basis to see how well they hold up in the fridge? :slight_smile:
     
  7. VikeMan

    VikeMan Grand Pooh-Bah (3,043) Jul 12, 2009 Pennsylvania
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    I think LeeryLeprechaun is planning to do something similar to this.
     
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  8. hopfenunmaltz

    hopfenunmaltz Pooh-Bah (2,623) Jun 8, 2005 Michigan
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    Counting viable cells would the way to test this.
     
  9. inchrisin

    inchrisin Pooh-Bah (2,013) Sep 25, 2008 Indiana
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    Is there a way to stain viable cells--or rather, how do you tell the difference between viable and dead?
     
  10. hopfenunmaltz

    hopfenunmaltz Pooh-Bah (2,623) Jun 8, 2005 Michigan
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    Yes you use a dye. I am not an expert, but it is outlined in Yeast by White and Jamil Z.
     
  11. mattbk

    mattbk Savant (1,099) Dec 12, 2011 New York

    Hemacytometer using methylene blue or violet dye. Living cells have an active membrane that reject the dye, dead cells do not and the dye is taken up by the cell. Count cells with and without the dye and you have a count.

    I am planning to get around to this sometime in the next few months.
     
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  12. JackHorzempa

    JackHorzempa Grand Pooh-Bah (3,351) Dec 15, 2005 Pennsylvania
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    Matt, counting viable yeast over a period of time (e.g., one month old pack, two month old pack, three month old pack, etc.) is indeed a great idea!

    One other aspect to consider is that from my conversation with Dr. Chris White at the 2013 NHC he impressed on me that yeast die off rates are strain specific. If possible, it would be worthwhile to perform your experiment with a number of yeast strain varieties (maybe a few ale strains and a few lager strains).

    I also wonder if the method of yeast production and packaging has an effect on yeast die off. Maybe a comparison of Wyeast 1056 vs. WLP001 would be helpful?

    Cheers!
     
  13. mattbk

    mattbk Savant (1,099) Dec 12, 2011 New York

    This sounds like a great idea! Please forward along an application for the JackHorzempa grant so I can acquire all the necessary resources to begin this study!:wink:

    In all seriousness - I'll likely start with one strain, see how that goes, and go from there if its worthwhile. I would expect this trend to be (at least slightly) strain specific.
     
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  14. FATC1TY

    FATC1TY Pooh-Bah (2,502) Feb 12, 2012 Georgia
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    I like to think I have less than I do, so I'm covered if all else fails. I'd rather have more to go around than not enough to get it done.. Catch my drift?
     
  15. NiceFly

    NiceFly Initiate (0) Dec 22, 2011 Tajikistan

    Trypan Blue and count the bright cells.

    I am going to take a wild guess and say the curve will be an exponential decay, opposite direction of exponential growth.

    edit: the media yeast are stored in will be a factor. I suspect good media will delay the onset of the decrease in viability, but once it starts it will follow the same curve every time. This is just a guess. I hope you guys post your results here.
     
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  16. VikeMan

    VikeMan Grand Pooh-Bah (3,043) Jul 12, 2009 Pennsylvania
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    Careful. Some might find that result abhorrent. Just imagine how the percent change in viable cells would get bigger every day. Chaos.
    Kidding aside, I am looking forward to seeing some data after a couple folks get their microscopes fired up.
     
  17. inchrisin

    inchrisin Pooh-Bah (2,013) Sep 25, 2008 Indiana
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    That guy is you, when you can find some time aside the Am Stout. :slight_smile:
    (TY BTW!)
     
  18. rocdoc1

    rocdoc1 Savant (1,191) Jan 13, 2006 New Mexico

    I'm lazy-I just make a starter every time and don't worry about the numbers. My method has made beer every time.
    It is an interesting experiment and I would be willing to bet wyeast and White Labs have done it, but for whatever reasons they don't see the need to share that info with us. I'd also like to see how dry yeast holds up.
     
  19. pweis909

    pweis909 Grand Pooh-Bah (3,176) Aug 13, 2005 Wisconsin
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    .[/quote]


    Vikeman, ecological population models frequently use exponential models (exponential decay, in this case) for this purpose: N(t)=N(0)e^(kt)

    In this case, the constant k in the exponent is negative. I suspect I am not telling you anything you don't know, but this sounds like the model the Wyeast rep was probably alluding to? Other considerations are positive and negative density dependence, and this could be what Chris White was referring to? Negative density dependence implies a negative feedback between population size and growth rate; positive density dependence implies a positive feed back between population size and growth rate. Or maybe White just felt like the data Mr. Malty used were flawed and biased the model towards conservative?

    Jack, in your conversation with Chris White, did he suggest how conservative the model was? (e.g.,hypothetically, you will probably over pitch by 10% if the yeast are young; you might over pitch by 30% when using the model to pitch yeast that are close to the expiration date). FWIW, I have emailed White Labs in the past with geeky yeast questions and have gotten nice responses. Perhaps someone there could shed more light onto the issue?
     
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  20. pweis909

    pweis909 Grand Pooh-Bah (3,176) Aug 13, 2005 Wisconsin
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    For what it's worth, sometime after I move in the next couple weeks and eventually get settled in, I'll start brewing again. I have a pretty nice microscope form a grad school research grant that I no longer use for my work. I have half a mind to start doing my own set of yeast counts and such to satisfy my own curiosity. Of course, if one you guys comes out with a nice article in Zymurgy before that happens, I'd save myself some tedious labor. Go for it. Someone? Please?
     
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