Tom, I have a neighbor who is a retired biologist who has offered my a very large container of glycerin. He apparently used it for preserving bugs and stuff. Tell me, please, exactly how to use it for freezing yeast samples.
I trust with an amply and free supply, I can freeze fairly large samples which would help speed up the stepping up process.
Oops, sorry i missed this. It's really easy. All you need to do is first, determine the concentration of glycerol you have. It will say it on the bottle, and it's probably 100%.
If you plan to freeze it solid (10-20% glycerol), the next thing you want to do is chill your yeast sample for 3 days or so. This will maximize trehalose in the cells, which serves as further cryoprotection. It is not strictly necessary, but it should help more cells survive freezing. You also want to chill your glycerol, because within a few minutes of warming up the yeast will lose the trehalose. Mix the glycerol with the yeast sample (keeping it all chilled) and stick it in the freezer. The slower it freezes the better. To figure out how much glycerol to add, use 25% of the volume of the yeast sample - so for 20 mls, you'd use 5 mls of 100% glycerol, this will give you 25 mls at 20% glycerol. It's that easy. I don't recommend mixing them and then leaving it in the fridge for a few days, because healthy yeast can use the glycerol as a carbon source and will break it down. Slower at cold temps obviously, but it might still be a problem (I haven't tested it).
If you are mixing it to get 50% glycerol final, then don't worry about chilling anything, the sample won't freeze anyway so the trehalose isn't important. Just mix the sample 50/50 with 100% glycerol you'll end up with a sample at 50% glycerol concentration.
I wouldn't go with large volumes, you'd be better off making starters from small samples because glycerol can affect the mouthfeel of the final beer. Doing 30 ml samples or so should be fine though.
I think the other thing to worry about is the sanitization of the glycerol. I don't know what it had previously been used for, but you could just end up contaminating your yeast. If you have a way to sterile filter it that would be good. We've never autoclaved it so I don't know what that would do, but I imagine it would darken and chemically change it somewhat, so sterile filtering is preferred.