[KCguy]

Newbie to the harvesting of yeast, and question I have is about how much yeast is in the cake/trub/sludge that sits on bottom of jar, as opposed to the beer/liquid above that? 

My process is pretty normal I think - rack off beer on bottling day and I try to save as much of the trub and last 1/2 inch or so of beer, and it commonly fills up a good 4 mason jars, which is filling up the fridge fast.  Am I right in thinking after a few days in the fridge, the separation that occurs will allow me to save the top liquid layer and dump the remaining trub, without sacrificing a major portion of that yeast?  I notice the trub bubbling as the jars begin to warm up when Ive pulled them in preparation for this procedure; is this just CO2 coming up or does it indicate yeast activity in the trub?  Both? 

Is there enough yeast in the trub layer to worry Im throwing out a good portion?   Is there further separation that can occur if I drain off the top liquid layer and put them back in the fridge? 

Thanks fellas...

[Robert]

That layer of solids at the bottom IS the yeast,  with a little trub mixed in that won't hurt anything.  The beer above may be cloudy with suspended non yeast matter, proteins, etc., and dead yeast.  The proper way to store the yeast is just like that, under the protective environment of the beer it made, in the fridge, loosely covered with foil or a lid that's not screwed on -- some gas will still be produced and could explode the jar if sealed. As you've said, you noticed bubbling.  Don't discard the beer, and whatever you read in outdated and misinformed homebrew books,  NEVER rinse the yeast with water.  This is very damaging to the yeast, invites infection, and there is no need to remove the trub anyway.  You'll love reusing your harvested yeast.  Not only saves you money, but as the yeast adapts from lab conditions to actual brewery conditions over a couple of generations, you may find it performs better.   And one-fourth of the yeast left after fermentation is about the right amount to pitch in the next batch, but I wouldn't recommend saving all four jars.  Just use one to start the next fermentation,  then harvest one-fourth of that, and keep doing this, serially repitching the yeast.  It will be freshest that way, and again, if your sanitation is good, it will only improve; you can get at least five generations out of it if your practices are sound and sanitary and you use it again fairly soon.

[a10t2]

I'll second everything Robert said and add that if you're trying to quantify how much slurry to pitch, just after harvest it will generally be 1-2 billion cells/mL, and after separating out in the fridge more like 3-4 B/mL.

[KCguy]

woops, yet another fundamental misunderstanding...ok so the stuff to save is the sludge, and not the liquid layer above.  got it. 

The part of me that is my father and grandpa wants to save it all, but I understand just grabbing one jar.  I condensed four into two and that has freed up fridge space immensely.  loose caps. 

I read on brulosophy that about 20% die off every month, store bought yeast.  Can I safely assume same is true of harvested yeast?

[a10t2]

I read on brulosophy that about 20% die off every month, store bought yeast.  Can I safely assume same is true of harvested yeast?

Safely, yes, that's a pretty conservative estimate. In reality it's more of a shallow exponential decay and for slurry used within a couple months I've never seen less than ~85% viability (methylene blue, so potentially not so accurate FWIW). After a while I just stopped staining and simply assume 90%. If you aren't actually counting, it's an order of magnitude below what you need to be worried about.