Freezing Yeast

[Jeremy Dunn]

Equipment:
Pressure canner (just about any one will do, but this is what I use. You can also find these at Rural King):
http://www.amazon.com/Presto-01781-23-Q ... ure+canner
50 mL storage tubes:
http://www.amazon.com/Karter-Scientific ... ge_o02_s00
Food-grade Vegetable Glycerin:
http://www.amazon.com/Duda-Energy-vg950 ... ge_o03_s00
This will hold 4 of the above tubes upright while processing in a pressure canner (optional):
http://www.amazon.com/Corning-1000-600- ... ge_o02_s00
Flask and stir plate for making a yeast starter (optional):
http://www.amazon.com/2000ml-Narrow-Mou ... iter+flask
(Just about all homebrew shops offer stir plates now which seem to be cheaper than what you can find on Amazon.)

Procedure:
1. Make a yeast starter from a pure culture (I use a 2 liter flask, but only fill it to about 1.6 liters)
(I typically make 2 starters from a single pure culture, 1 for brewing within a week and 1 for freezing)
2. After your starter has fermented completely, cold crash for a couple of days (this makes it easy to decant in order to get a thick yeast slurry. It also prepares the yeast for dormancy so they produce carbohydrates reserves.)
3. Fill your 50 mL tubes with 30 - 50% glycerin (I usually do 30% [15 mL])
4. Loosely put the caps on and put in the pressure cooker.
5. Place the container holding your tubes on the rack that came with your pressure canner. Fill the bottom of the pressure cooker with just enough water to touch the bottom of the container holding your tubes. (You want enough water that you can process it for 30 minutes without all of the water evaporating)
6. Process in the pressure canner for 30 minutes at 15 PSI and then allow it to depressurize and cool on it's own.
7. Once depressurized, move the glycerin tubes to the fridge until they are the same temp as your yeast starter.
8. Decant your yeast starter so you have a thick slurry.
9. Fill up each tube with yeast slurry using a sanitized funnel and recap the tubes. (You can optionally light a bunsen burner (or maybe a candle?) to create an updraft so airborne particles don't land in your tubes while you're filling them)
10. Place your tubes in the fridge overnight.
11. Now you should have 3 layers in each of your tubes (glycerin in the bottom, thick yeast slurry in the middle, starter wort in the top). Decant the top wort an leave behind the thick slurry. Now you have closer to a 50/50 ratio of glycerin to thick yeast slurry.
12. Label them, shake them well and pop them in the freezer.


Creating a starter from "frozen" yeast is just about the same as before, but here are a few things I do that may make a difference.
- I always make my starter wort using some leftover hops (I put a few pellets in a tea ball and toss it in my boiling wort). The idea is since we're handling yeast more, there's more chance for infection. Hopefully the antibiotic properties of hops can mitigate this risk a little?
- Move your yeast vial from the freezer to the fridge the night before making a starter (don't toss your yeast directly from the freezer into your starter wort).
- Be prepared for a longer lag time. You may not see signs of fermentation for 24 hours.
- I store my yeast vials in a chest freezer. If you keep yours in a freezer with a defrost cycle, it may be a good idea to keep them insulated with some ice packs.


Let me know if you have any questions!