Homebrewers often find they would like to have yeast at the ready, to defray costs or to be able to collect strains that are not readily available and grow them to a practical amount of yeast. I looked into yeast ranching with slants and decided the chore of sterilization and reculturing would be too much for me to handle. An excellent book on that subject is “First Steps in Yeast Culture” by Pierre Rajotte.
I decided to look for a different method and tried freezing yeast with glycerin based upon posts on rec.crafts.brewing. My results using a frost-free freezer were a total disaster. The freeze/thaw cycle of the fridge caused the yeast to die. The idea is when freezing/frozen the glycerin helps to keep the cell walls of the yeast from bursting. The freeze/thaw cycle can be the kiss of death depending upon how the yeast are stored.
A few years later a brewer suggested I try a non-frost-free freezer and gave me a protocol. He used a lab freezer at –80ºF and a 15% glycerin solution. I had a regular upright freezer at work that ran about –5ºF and decided to give it a whirl.
Here is the protocol I utilized (Many thanks go out to Brad Nicholson).
Necessary materials and equipment:
Yeast slurry (from bottom of starter)
Pipette (or other measuring method)
Test tubes and caps (sterile, or sanitized)
The #1 thing to do is to have surfaces and equipment sanitized. Take nothing for granted.
1) Mix glycerin and water in the ratio of 30 ml glycerin to 70 ml water. This will yield a 30% glycerine solution.
2) Microwave the mixture until boiling.
3) Remove to the counter, cover with saran wrap and allow to mixture cool to room temp.
4) Add equal amounts of the 30% glycerin solution and the yeast slurry to the test tube. (The amount will be dependent upon the volume of the tube.)
5) Cap the tube and shake.
6) Mark a piece of masking tape with the yeast type and label the tube.
Glycerin can be purchased at most local drug stores, but can sometimes be difficult to locate in the store. Be diligent and you will find it. If in doubt, ask the pharmacist and they can point you to it.
You now have a frozen tube that is 15% glycerine and yeast. When I get a new strain, I try to make at least 3 tubes from the starter slurry. My procedure is to hold my finger on the end of a pipette and stick into the slurry and remove my finger allowing the slurry to be sucked into the pipette. I transfer that into the test tube as many times as necessary.
My yeast samples were kept frozen in the freezer, but eventually I had to remove them from my employer to the house. I put them into a cooler with ice packs and brought home. I then placed them into a thick foam cooler with frozen ice packs to prevent the freeze thaw cycle of the frost-free freezer from killing the yeast. Recently I revived the yeast by putting them on a stir plate with a small amount of starter wort and gradually adding more wort over time. The yeast revived perfectly after more than a year of frozen storage. The only caveat is the yeast had to grow so the results took longer than yeast abuse.
Another great page on freezing yeast is Mike D’Brewer’s entitled Yeast Harvesting and Freezing. I was going to take pictures for this page, but his are excellent.