Reuseing yeast

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zrule

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I brewed a cream ale on January 31st and collected a lot of yeast. I have three 20oz bottles that are 1/4 full of yeast and then filled to the top with with sterilized water. I want to use it but it is two months old. Can I rewash this to get rid of the dead cells??
 
You might also be interested to know that 5 oz of slurry contains about 200 billion cells and that the viability drops very little over a two month period. If this didn't come out of a high gravity beer you are probably still near 90% viability.
 
Thanks guys. I am worried about the dead cells giving off flavors
 
Thanks guys. I am worried about the dead cells giving off flavors

With two month old yeast there should be very few dead cells. Washing doesn't remove the dead cells. You could start with a smaller amount of slurry and build a larger stater. 1ml of slurry for every 10g of DME is about as low as you would want to go. This will grow 10 fold over the course of a few days.
 
You might also be interested to know that 5 oz of slurry contains about 200 billion cells and that the viability drops very little over a two month period. If this didn't come out of a high gravity beer you are probably still near 90% viability.
The idea that a refrigerated slurry would still be at ~90% viability after two months is surprising to me. It's certainly far off from the estimate mrmalty.com and other calculators give us. Would you be able to walk us through an example of how you calculate a viability estimate? I see your formula on your 'Simple Yeast Storage' article, but I'm still not sure I fully understand it.

Btw, I really appreciate the work you've done on this topic. It's nice to see someone giving careful scrutiny to all the common notions people have about yeast rinsing.

And just to pass something along, I’ve been repitching unrinsed slurries for a couple years and while I had been getting overall better results than before when I rinsed, I still noticed that by generation 3 or 4, my ferments would become less and less vigorous. I very recently started using Wyeast yeast nutrient and it’s made all the difference. I’m on generation 5 with 1056 and each ferment so far has been super consistent in terms of lag, vigor, length, size of krausen and attenuation. I just wanted to share a solution to something that had been driving me nuts for years.
 
The idea that a refrigerated slurry would still be at ~90% viability after two months is surprising to me. It's certainly far off from the estimate mrmalty.com and other calculators give us. Would you be able to walk us through an example of how you calculate a viability estimate? I see your formula on your 'Simple Yeast Storage' article, but I'm still not sure I fully understand it.

Viability can vary quite a bit. My swag of 90% is probably overly optimistic. I was counting viability in a sample that had been refrigerated for 65 days and it came in at 87% viability, and had that number in my head. Using the equation from the simple storage procedure it comes in at 67%.

V = exp(-60/150) = 0.67 or 67%

Btw, I really appreciate the work you've done on this topic. It's nice to see someone giving careful scrutiny to all the common notions people have about yeast rinsing.

And just to pass something along, I’ve been repitching unrinsed slurries for a couple years and while I had been getting overall better results than before when I rinsed, I still noticed that by generation 3 or 4, my ferments would become less and less vigorous. I very recently started using Wyeast yeast nutrient and it’s made all the difference. I’m on generation 5 with 1056 and each ferment so far has been super consistent in terms of lag, vigor, length, size of krausen and attenuation. I just wanted to share a solution to something that had been driving me nuts for years.


This is a good point. Wort contains some plenty of hydrocarbons for yeast, but limited amounts of other nutrients such as nitrogen. Typical wort only contains about 0.5% nitrogenous compounds per gram of solids,[1] but healthy yeast growth requires 3%.[2]

[1] Barrett, J., and B. H. Kirsop. "The relative contributions to wort nitrogen of nitrogenous substances solubilized during malting and mashing." Journal of the Institute of Brewing 77.1 (1971): 39-42.

[2] http://braukaiser.com/blog/blog/2013/07/07/yeast-growth-on-malt-and-sugar/
 
It's probably one of those things that is recognized by some but not all. Exp is a notation used mostly by programmers for "e raised to the power" ( e being Euler's number or about 2.718281828459)
 
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