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Now that I'm all grain I've done many experiments regarding differences in grains, trying to make 2 batches identical in every way except for one variable with the grain.

However I just learned via brewersfriend that a yeast starter with a month older yeast than another yeast starter results in drastically different cell counts.

I.e, with a 1.6L starter at 36 gravity using the Braukaiser equation

75 days = 278 billion 
105 days = 257 billion 

which is a 7.5% decrease relative to the younger yeast.

In general, does a cell count difference of 10-20% have a material effect on the outcome of the beer for tasting purposes, and therefor invalidate any tasting experiments one may do?

I can always mix the yeast together before adding them to the starters, of course.

  • I'd think this would depend a lot on the strain. With a neutral strain, probalby not. With a particularly estery or phenolic strain, it might. Could you point me to the equations on BF & BK you're referring to? Also, I think you mean a 7.5% decrease, right? – Franklin P Combs Feb 27 '15 at 15:06
  • Thanks, corrected. This brewers friend link the definitions you requested: brewersfriend.com/yeast-pitch-rate-and-starter-calculator – Matthew Moisen Feb 27 '15 at 21:20
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I've done a lot of experiments (hence, the book Experimental Homebrewing) and I've found that for valid results you must tightly control all the variables, including the amount of yeast you pitch. For that reason, unless yeast is the subject of the experiment, you're almost always better off using dry yeast if possible. If you get 2 packs with the same date code, you can be pretty much assured they'll have the same amount of viable yeast. since you don't need to do a starter with dry yeast, you don't need to worry about throwing off that cell count.

  • In other words, if I pitch two separate liquid yeasts with the same expiration date into two flasks and pour 1.6L of 1.036 gravity wort in and aerate on stir plates at the same speed, each flask may have a different cell count? – Matthew Moisen Feb 27 '15 at 21:22
  • No, in that case you'd likely be pretty close to the same. But likely not as close as 2 packs of dry yeast from the same date. The last experiment I did I got 4 packs of US05 that were produced within 15 min. of each other. Dry is less susceptible to problems caused by shipping than liquid, so I feel more confident in getting equal cell counts from it. My experience has borne that out. – Denny Conn Feb 28 '15 at 22:45
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"I can always mix the yeast together before adding them to the starters, of course."

That's the answer.

If your experiment was testing something obvious, like the taste of cocoa vs coffee, that would be one thing. But for subtler differences every known source of variability will bring doubt to the result.

Everything that can be identical between the batches should actually be identical.

And when you compare your results, triangle testing will help keep the tasters honest (I think Denny can comment on how triangle testing works).

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