Author Topic: Yeast viability calculation  (Read 13726 times)

Offline mm658

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Yeast viability calculation
« on: September 25, 2012, 09:26:00 PM »
Using the Yeast Starter tool, I notice that the yeast viability and aging calculation works in ways that I wouldn't expect. 

For liquid yeast, the viability of 96% seems to match to Jamil's Mr. Malty app, and the aging rate of 21% seems reasonable (Jamil seems to use 21.5%, FWIW).  However, as you move the production date further back in time, things diverge greatly between the two calculations.  The calculations in BeerSmith seems to be nonlinear, while Jamil's is linear.

For example:
Production Date    Viability-Jamil    Viability-BeerSmith
Today                       96%                       96%
1 month ago              75%                       75%
2 months ago             53%                       59%
3 months ago             32%                       46.5%
4 months ago             10%                       36.5%       

The Mr. Malty calculation bottoms out at 10% and seems to stay at that number for anything older than 4 months (which seems suspect to me).  But in general, he seems to be subtracting 21.5 "percentage points" per month until reaching the 10% floor.

On the other hand, the BeerSmith calculation reduces the number by 21 "percent of the previous number" per month. 

With fresh yeast, the differences aren't significant.  But with older yeast, the differences are significant.  Jamil's formula takes only 4 months to reach 10% viability; with BeerSmith it takes over 9 months to get down to that number.

Is this an intentional difference?  Which is "more correct?"     


Offline jomebrew

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Re: Yeast viability calculation
« Reply #1 on: September 26, 2012, 10:39:09 AM »
Whitelabs advised the viability, after 30 days, is 75-85%.  The shelf life is 4 months.    For 0-2 months it really doesn't matter which one you use but I would lean towards Mr malty after 2 months based on the Whitelabs info. http://www.whitelabs.com/beer/homebrew_FAQ.html

Offline mm658

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Re: Yeast viability calculation
« Reply #2 on: September 26, 2012, 01:56:18 PM »
Thanks, jomebrew.  I agree that it seems the Mr. Malty approach is more appropriate for aged yeast -- except for the fact that the viability never goes below 10% with time.

Ideally, it would be great to see the BeerSmith calculation changed to mirror Jamil's approach (without the 10% floor). 

I have some other suggestions for the yeast tools, but I'll post them on the Suggestions board. 

Offline BeerSmith

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Re: Yeast viability calculation
« Reply #3 on: September 27, 2012, 11:15:49 AM »
Hi,
  The effect from a scientific view is clearly non-linear.  Think of it this way - if you start any given month with X amount of yeast cells, and at the end of the month have 21% less yeast cells or X(1-0.21) you get a classic exponential decay line.  This is true no matter how large X is.  Almost all natural decays occur in this way.

  I think Jamil choose to simplify things a bit with his calculator, but I do stand by my calculation for viability.  If you lose 21% of the cells per month it is not a straight line.

Brad
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Offline mm658

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Re: Yeast viability calculation
« Reply #4 on: September 27, 2012, 07:30:20 PM »
Thanks for the reply, Brad. 

I agree that you'll wind up with a non-linear decay using the approach of decrementing the previous month's value by 21% repeatedly.  And perhaps Jamil went for simplicity with his calc.  My concern is that if White Labs is saying that the shelf life is 4 months, it's a bit optimistic to have 36% viability calculated at that point, and 10% viability nine months after the production date. 

No worries, though.  What I might do is just bump up the aging rate in the settings to something like 30%.  Over longer periods, it may be more accurate.  Granted, it may underestimate viable yeast for newer packs/vials, but better to grow more cells than not enough. 

Offline tom_hampton

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Re: Yeast viability calculation
« Reply #5 on: September 27, 2012, 09:13:16 PM »
Hi,
  The effect from a scientific view is clearly non-linear.  Think of it this way - if you start any given month with X amount of yeast cells, and at the end of the month have 21% less yeast cells or X(1-0.21) you get a classic exponential decay line.  This is true no matter how large X is.  Almost all natural decays occur in this way.

  I think Jamil choose to simplify things a bit with his calculator, but I do stand by my calculation for viability.  If you lose 21% of the cells per month it is not a straight line.

Brad

The correct word is "mathematic" as opposed to scientific.  The real world quite frequently violates our nice pretty models.  Jamil has said on Brewstrong and various other shows on TBN that he and Wyeast did actual cell counts on aged refrigerated samples to determine the viability calculations used in his calculator.  THAT is the definition of scientific.

He's been questioned about it numerous times on forums (TBN, HBT, morebeer, etc) regarding the calculation.  He has reaffirmed the accuracy of it every time. 

Just because it isn't non-linear, like our pretty diff-eq models tell us it should be....doesn't mean its wrong. 

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