Goal: To explain how to make a yeast bank, how it works, and it's limitations for home brewers.
Liquid yeast can become very expensive to buy often. If you have a particular favorite, you can make a stock to put in your freezer for innoculating a starter. Buy once, have the strain forever!
Yeast slurry or washed yeast
Screw cap 2 ml tubes (sterile culture tubes)
Air tight container big enough for the above tubes
70% isopropanol (rubbing alcohol)
Glycerol (glycerin & glycerine are the same thing)
Some styrofoam or equivalent
1. Put enough 70% isopropanol into your large container to submerse the bottom half of your culture tubes. You can cut some styrofoam for a make shift holder. Place this in your freezer overnight.
2. The next day, add glycerol to yeast slurry to a final concentration of 25% glycerol. Fill vials halfway with yeast using a sanitized pipette or medicine syringe (no needle). Cap tubes and mix well by inversion.
3. Place tubes in your pre-chilled isopropanol container such that the yeast slurry/glycerol mix is below the isopropanol level.
4. Cap the large container and put it in the freezer.
5. Have a glass of mead.
How it works in layman's terms and limitations
Freezing yeast in water will kill them. Why? When water freezes, it crystallizes. If the water is in the cell membrane of the yeast when freezes, it crystallizes and causes a freeze fracture of the membrane. Once the membrane is no longer intact, the yeast explode and die.
So, how to prevent this? Glycerol mixed with water prevents the crystallization of water enough that most yeast survive. There are some catches though. In order to keep the yeast indefinitely, you need 2 things:
1. a non-frost free freezer
2. freezer must be below -55 C constantly.
Most of us don't have either of those things at home. So, how do we get around these issues?
1. Frost free freezers are bad because they temporarily warm up to defrost. Every time your culture goes through a freeze/thaw cycle, a significance portion of the yeast die. The way to get around this is to submerse the tubes in a liquid that will remain cold during this defrosting process and prevent the cultures from thawing. Hence, submerging your culture tubes in isopropanol serves this purpose.
2. Temperatures above -55 C, but still below freezing point will keep yeast for a long time...just not indefinitely. Temperatures of a normal household freezer is generally -20 C. At this temperature, cultures will only remain healthy for about one year. So that means you will need to thaw and make new cultures once a year or risk the cultures being damaged beyond repair. If it is a yeast you use a lot, this is no big deal.
Hope that helps those who are interested. I also welcome any ideas that can improve the design or efficiency as my bank is ever growing.