« on: May 04, 2011, 07:11:04 AM »
This is a question that I see all the time on homebrew forums and nobody seems to have a definitive answer for yeast count per unit volume of settled yeast. Or at least the ones that claim to don't agree. We have yeast count but then we also have viability and non-yeast percentage numbers and everybody seems to be guessing at all three. It seems the yeast count of settled yeast part should be straightforward enough. Guessing at two variables is better than guessing at three.
There was a pretty good back-and-forth on the NB forum a couple years ago. http://forum.northernbrewer.com/viewtopic.php?f=26&t=86590&start=30
By a couple different methods, I concluded that a fully packed slurry will be about 4.3 billion cells per mL. Jamil Zainasheff, who's probably as much an authority in this as anyone, says 4.5 billion/mL. So I think you could take that to be fairly accurate. Similarly, if you're rinsing the yeast with sterile water, it seems to me - at least visually - that the non-yeast portion drops pretty darn low even after a single cycle. So figure 0-10% on that, make your guess at viability (25% loss per month, refrigerated and stored under beer/water is a pretty well-established figure) and you'll probably get your overall cell counts within ±20% or so. That actually compares pretty favorably with typical use of a plate hemocytometer, and is good enough for our purposes IMHO. It's only beer.