Freezing yeast can cause the cell walls of yeast to rupture, reducing the viability of the yeast. Obviously, this isn't preferable. However I did read an exbeeriment in which they froze yeast for one half of a batch or beer and used fresh unfrozen yeast for the other half of the batch. The frozen yeast took longer for the fermentation to begin. However, in the end the beers tasted exactly the same. So, it certainly isnt the end of the world either way.
I've been freezing yeast for several years. It can be done quite successfully, but does take some extra steps and special precautions to keep yeast cells from rupturing from initial freezing or from freeze/thaw cycles.
Once you have a clean yeast you want to save (like a rare strain or infrequent special release) you need to place a sample in a suitable container. I use cleaned and sanitized White Labs yeast tubes. Better yet would be borosilicate glass that could be autoclaved, but you could also use the plastic "forms" used to make 2 liter soda bottles. Those "forms" look like test tubes with screw-on caps and can be bought online.
Then make a solution of 70% distilled water and 30% glycerin. Put 10 ml of the yeast in the 'test tube', fill with the water/glycerin mix, tightly cap it and put it in the refrigerator for a day of two to pre-chill it. Once chilled, vigorously agitate to disperse the glycerin which will coat the yeast cells and prevent them from bursting when frozen. Place the samples in a deep freezer until you are ready to use them.
Here's the tricky part. If you use a frost free freezer you'll have to make sure the samples don't thaw during defrosting cycles of the freezer. Non- frost free freezers are O.K., but not your side-by-side kitchen unit. What I do is put the test tubes in an insulated lunch box along with freezer packs that don't thaw during defrosting. I surround the lunch box on all sides with additional freezer packs. So far I've never had any thaw.
When it comes time to use the yeast, simply take it from the freezer and let it slowly thaw in the fridge. You can decant the water/glycerin or just dump it in an Erlenmeyer flask to propagate a starter. It's food grade, non-toxic. You'll need at least two steps to build the starter up, because the saved samples are small (smaller samples freeze more successfully).
It's a good way to save some of those 'one off' yeasts. The oldest one I've revived was over three years old and worked fine. There's probably a dozen samples in my freezer, mostly 'Vault' samples from overbuild starters, that complement the 'house' strands in the beer fridge that get used on a regular basis.
Brooo Brother
