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Yeast viability

In calculating my starter volume I was using the “calculate viabilty by date” function on Mr Malty. Does a properly stored, refrigerated liquid yeast really lose almost 1% viability per day? That seems to me quite a large amount for such a tenacious organism as our best friend. Does anyone know where this number comes from? Has this rate of decay actually been measured or is it a conservative estimate?

Not sure, I’ve often wondered that when typing numbers into that calculator but with the most important part of your beer it never hurts to be conservative.

I have a smack pack that is about 18 months old. If the theory is correct, it would have nothing left. I am going to smack it and make a starter soon mad see what happens.

I use 25% viability loss per month and it seems to be well on track.

Flag porter uses a yeast strain that came from a bottle of beer that sat on the bottom of the English channel for well over 100 years.

My point exactly. And the trouble with being “conservative” and pitching a larger quantity of yeast is that some beers benefit from the growth phase where the yeast increase greatly in number in the beer. This is true of one of my favorite styles, saison. If I pitch too much I’ll lose some of that funky farmhouse character that makes saisons so interesting. Then again, if I way underpitch I’m risking low attenuation and potential off flavors. I suspect the viability loss over time is overstated in the calculator, but I admittedly have no proof. Does anyone have anything more concrete?

My calculations puts it still around 0.57% viable.

I have recultured yeast from dregs of a 5 year old bottle with almost no problems (separating the individual strains was the problem). So yes, 25% per month seems more or less reasonable to me.

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