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I've seen plenty of data and apps that help you estimate the viability of a liquid yeast culture. But I haven't seen a lot of data about dry yeast. I like using dry yeast for 1 gallon test batches and experiments, and it would be great to understand how much healthy yeast I'm actually pitching.

For the sake of argument, lets assume I'm asking about yeast that has been properly handled and stored at 40 degrees F. Lets also assume i properly rehydrate with water before using.

Mr Malty Pitching Calculator seems to estimate that liquid cultures are < 1% vaible at one year. But it looks like dry yeast is still 68% viable when a year old.

What are some good references on this subject, and how can I refine my control of dry yeast pitching rate?

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FWIW, a lot of people would disagree with the concept that liquid yeast is completely dead after a year. It might take a 2-stage starter, but you can revive some pretty old yeast if it was handled properly. –  Graham Dec 20 '11 at 13:32
    
I agree Graham, I should not have written "dead". In terms of pitching viability would you agree with the tool that less than 1% of the yeast are alive? If so i will edit my post to clarify. –  dana Dec 20 '11 at 19:54
    
Not sure I'd commit to a specific percentage. I'd just put a caveat that liquid yeast is rarely ever 100% dead (unless you leave it on your car's dashboard all summer in Texas). –  Graham Dec 20 '11 at 21:00
    
@Graham: Updated to at least reflect what the web tool is communicating. –  dana Dec 20 '11 at 21:15
    
IMHO, if you are ever concerned about viability of the dry yeast then just use a new pack. Alternatively make a starter with low gravity wort. If you pitch what is in suspension when it's at high krausen, then you'll get >90% viability, as good as a fresh liquid yeast. Estimates for viability of dry yeast will have a large degree of error due to the significant variables that are outside of your control (you usually don't know the history and how well the yeast has been handled). Using a starter, or using dry yeast that's at most a few months old removes the guesswork. –  mdma Dec 22 '11 at 8:05
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2 Answers

I've been poking at this a little today, There's a site located Here

Which suggests dry yeast loses 4% viability per month.

Reverse Engineering the Mr. Malty calculator suggests that dry yeast is only 90% viable when packaged, and loses 2% viability per month stored.

EDIT: It would seem that each strain actually handles the dehydration (dessication) process differently. I pulled this out a publication on the Fermentis website:

enter image description here

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This article states

Active Dry Yeast looses about 20% of its activity in a year when it is stored at 75 F and only 4% when refrigerated.

As well as knowing the viability of dehydrated cells, a number of cells can be lost during rehydration. The article also states that if the rehydration temp is 95-104F, then all viable cells (at the at point) are retained. Using 60F water, as much as 60% are lost. Quite an eye-opener, since I usually pitch the yeast directly into the fermentor.

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What is the source of this article? –  dana Dec 21 '11 at 22:17
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