Frozen yeast bank not freezing

I know this question has been asked before, but I could not find an answer. Is it ok for your frozen yeast bank to not freeze. I have put my yeast bank in my chest freezer at -5F. I used a 30% glycerin solution (I know it is more than is recommended) and my yeast won't freeze. Is this common/typical? Should I be looking to start over with a lower concentration of glycerin?

Thanks,
T-Hops

My samples never freeze - I mean, they are in liquid form, no ice crystals.

correct me if I'm wrong, but the whole point of using glycerin is to keep it from actually freezing (crystalizing) -ice crystals destroy cells.

The purpose of the glycerin is to make sure that the ice that does form is of the small crystal variety. When large ice crystals form, they can cut and puncture the cell walls and destroy the cells.

15% glycerol(glycerin) is a good concentration to use. Too much can be toxic to the yeast when you're warming them up prior to pitching your starter.

If your samples aren't freezing in a chest freezer, it could be because of the high glycerol concentration or it could be that your samples are supercooled. take one and give it a good rap against the side of your freezer, see if that makes it freeze up.

Cheers

Well I appreciate the input. I did the rap on the side of the freezer. That is pretty cool. Mine turn to slush, but then turned back to a liquid within a few seconds. I guess I will try less glycerin next time.

Thanks everyone for the replies.
T-Hops

Freezing solid is fine - as long as you add the proper amount of glycerin. In the lab, our yeasts are stored in 15% glycerin at -80 C. The tubes are quite solid.

I would be careful with tapping the vials to induce freezing. The point of freezing yeast is not to get solid ice crystals, but to lower the temperature. If the sample is at the low temperature of a freezer, then the biological processes are slowed which is why the yeast "survive" longer. Rapid freezing is what causes large ice crystals which damage the cell walls. Supercooling and then tapping the vials to induce crystallization is probably even worse then not using any glycerin at all. When laboratories are preserving living samples, they generally use refrigerators designed to slowly lower the temperature in order to prevent the large crystal growth I mentioned earlier (and what you see when you tap the supercooled vial).

If you are a fan of the effect you can see it on a bigger scale with a bottle of Corona in the freezer, just let it chill for a day or so then take it out and tap it on the counter. Its really cool to watch.