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If I'm creating a yeast starter from previously harvested yeast I'm supposed to know how many cells are in my slurry to effectively calculate how much yeast to use in my next batch.

To me common sense suggests that a fully-settled layer in the refrigerator will be pretty consistent in the number of yeast cell per unit volume - some may not be viable, yes, but a consistent density non-the-less.

However, as far as I understand it, and according to Wyeast (http://www.wyeastlab.com/com-yeast-harvest.cfm) a yeast slurry is not the fully-settled layer but the yeast in suspension prior to it settling out (settling out is simply a mechanism to allow the brewer to gauge how much yeast is in a slurry). Therefore, isn't this purely down to how much water I've added whilst harvesting? How do I know how dense the yeast population is in a watery slurry like that? Isn't it best to work from the settled layer?

By my reckoning, and reading the Wyeast page, there are approximately 2 billion yeast cells per millilitre of fully-settled yeast cake. Is this not a more sensible way to gauge your slurry?

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2 Answers 2

I'm not so sure I'd agree that full-settled yeast is going to have a consistent density. Certainly there's an order of magnitude size difference between Sacc. and Brett. cells, but even different flocculation rates/behavior between various Sacc. strains might affect settled yeast density. I really don't know, but it's not "common-sense"-obvious, to me.

A slurry is the yeast in some liquid suspension to provide it fluidity; a fully settled yeast cake does not really want to move from the jar to the carboy. :)

Wyeast's method of taking a sample of the slurry, letting it settle, and using the percentage volume and 2bn/mL estimate is a reasonable one. Even better, of course, is to count the cells of a serially-diluted sample under a microscope with a hemocytometer. You can also test for viability during this count as well.

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"yeast in some liquid suspension to provide it fluidity"... considering how tiny fluctuations in your brewing method can produce significantly different results, this seems a bit random? So basically, you're saying not quite yeast cake, but yeast with just enough water in to help it move around? –  Doug Jan 24 at 14:56
    
Yes, the conventional definition of "slurry". It is going to be random, based on how much liquid is there, how compacted the yeast cake is, &c. If you don't want random, then you need to actually count the cells in a specific representative volume of your pitch, and do the math. If you take 1ml of a uniform pitch of say 1600mL, dilute it 100× and count X cells, you can easily compute how many cells are in the overall 1600mL. Anything else is going to be highly variable with large error bars. –  jsled Jan 24 at 17:04
    
Coincidentally, I came across brewersfriend.com/yeast-pitch-rate-and-starter-calculator today, and near the bottom is an interesting line re Yeast Slurry: "Default slurry density is 1 billion cells / mL, but you can adjust it. Some sources say it is as high as 5 B / mL.". Quite a range! –  jsled Jan 25 at 15:01

In the homebrew world, slurry refers to the settled solids in the bottom if your fermenter or storage container. I have no doubt that Wyeaast or others may define it differently, but when homebrewers speak about slurry it's not the suspended yeast.

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