Wow, you are really generous with the info and time. Thanks
One last question. Is the slurry thickness just for desired pitch count, or does it have something to do with making it work? My idea would be a lot thinner. Maybe I should decant that 1L down to about 300ml before swirling, and just make 3 jars?
No worries. If I have information to share then I'll gladly do it, unlike some other folks (cough **rabeb** cough). I also recognize that I'm not as knowledgeable as Mark when it comes to maintaining lab-quality yeast cultures, and while the world of yeast isolation and culture purifying intrigues me, I'm just not willing to take the necessary steps to implement it in a way that I would be content with. I'm certain, Mark's methods of yeast handling are far superior to mine. I just look at my methods as "no worse than pitching slurry from one batch into another" (and probably better is some regards).
The slurry thickness I aim for is just to try to end up in the ballpark of 100billion cells ± 50billion
. Sometimes I just want a single step starter and starting with near 100billion can get me where I want to be in one step. There is nothing magic about the thickness of the slurry that makes it work/fail. The glycerin content will keep the ice crystals from forming sharp points thus rupturing the cell walls. I have a few vials (propagated from bottle dregs) that I estimate at much lower cell count (i.e. thinner slurry).
As far as what is "best" in terms of slurry thickness, decanting, etc..., I think it just comes down to your own wants and requirements. For me, it was about space savings and keeping many strains on hand. If you're only ever planning on a few frozen jars then there's no harm in using larger jars with thinner slurry, IMO.
If you want to test the freezing idea/method out before "pot committing" yourself then you can easily do so with a little bit of yeast slurry from any batch of beer. You don't even need to keep the process sterile since it's just a "proof of concept" test. Just pull enough from a yeastcake after racking your beer and put in a clean but non-sterile canning jar. Add some premix glycerin solution to it just like you would if you were doing it for real (again, no need to sterilize). Freeze it. Wait a week or four. Make up a small batch of starter wort and use your frozen yeast. Judge how it performs after being frozen (taking into account the lack of sterilization of the process). Again, just a proof of concept test if you will.