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:
Glycerin
Water
Yeast slurry (from bottom of starter)
Pipette (or other measuring method)
Test tubes and caps (sterile, or sanitized)
Masking tape
Permanent marker
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.
7)
Freeze
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.