Below is a text from a talk given on August 28th, 2017 by Noah Pierce. These notes are given here for educational purposes only. You can see Noah's WHOTalk in the video section
Maintaining A Healthy Yeast Bank Long Term
One of the most important things you can do to brew great beer is ensure you have a healthy, unstressed yeast population. From pitching rates, fermentation temperature, avoiding contamination by competitive organisms, all the way through bottle conditioning; an unhappy yeast culture will kill a batch faster than you can drop a 5 gallon glass carboy on a concrete patio.
The best way to ensure you have the happiest yeast possible is to completely control the cold-chain of your yeast supply. By that I mean yeast ranching, and doing it the right way to ensure you have a real, pitchable amount on brew day. Add in the ability to archive rare strains, as well as save you a few bucks per batch, and you’re now wondering “why haven’t I done this sooner?” You don’t have to own a certified lab to do this either (ie, my “laboratory”, is my kitchen table with a few candles to make an updraft). All you need is a little patience and a penchant for cleanliness and detail.
Storing Yeast Under Beer
Let’s say you brew a beer using a sachet or liquid pack of yeast from the homebrew store. At the end of primary fermentation, you’re left with a boat load of cells (usually around 2 trillion in a 5 gallon batch) in a cake after racking. Using this trub is the easiest way to keep a fresh stock of a favorite yeast strain. Fresh is the key word here. Yeast will continue to metabolize and be otherwise fairly biologically active in any environment above -20°C, so the term “fresh” is time dependent. A good rule of thumb is to store yeast in the fridge under beer no longer than 2 months before taking a portion of it and making a new starter to restart the clock.
Method
Take the cake, swirl it around in the bucket or carboy in the little beer left at the bottom to re-suspend, and pour into mason jars sanitized with your favorite solution. Keep the caps loosened for a few days and store them in the refrigerator. If used within a couple of weeks, 50ml of this compacted slurry at the bottom of the jar is good enough to inoculate a new 5 gallon batch. Viability will be reduced, and sluggishness will increase with time, so take that into consideration or use one of many online calculators (MrMalty, BrewersFriend, etc) to estimate the volume to use.
Notes
This is a very quick and dirty method for maintaining a yeast bank. While being quite easy and well suited for brewing many beers back-to-back with the same strain, it will quickly become difficult to maintain many strains or keep mutations to a minimum. That being said, there are also some requirements to keep yeast cultures this way. The batch used to create the initial cake must be very low impact to the yeast. It should be low gravity, reasonably low temperature (no hot and fast ferments), must be free from bacterial and wild yeast contamination, and it must not be a blend of strains that are used in many commercially available cultures. Also, it seems as if there has been a shift in the opinion in “water washing” the yeast prior to storage in a jar. Though logic suggests that a yeast population absent of hop debris and coagulated proteins would be beneficial for storage, the general thinking these days is that more collection steps would only increase the possibility of infection. I agree with this argument, and I suggest just dumping the whole lot into a sanitized jar for collection. K.I.S.S., always a good mantra in brewing.
Frozen Stocks
In my opinion, keeping frozen stocks of yeast cultures is the way to go for a homebrewer (and also quite simple to do). While you don’t get that warm, fuzzy feeling of watching a single cell grow into a colony and knowing exactly how pure your culture is, with careful aseptic technique you can keep dozens of strains ready to go for pitching in a couple of days, and it doesn’t take a lot of maintenance or equipment (or valuable refrigerated space). The addition of a cryopreservative (food grade glycerin) is required as mentioned above to keep ice crystals from rupturing the cells, but everything else is pretty much the same as keeping slurry jars. An added benefit of storing cells at such a low temperature is that biological activity slows way down, essentially halting the chance of spontaneous mutations that may occur from fridge temperatures.
Method
My method for keeping frozen stocks of yeast is focused on creating single-pitch tubes to make fresh starters for 5 gallon batches. You can also make much smaller tubes to steal a small quantity without thawing the stock and step the culture up for a starter. That takes more time, and time is valuable, so I don’t do it, but it is perfectly viable. The equipment needed for frozen banking is simple: pressure cooker, mason jars, sterile tubes, and glycerin (drug store).
Make a 25% v/v stock solution of glycerin in water (30ml glycerin, 70ml water) and pressure cook for 10 minutes.
Make a 1000ml starter from a fresh yeast culture and allow to settle or cold crash in the fridge
Decant the beer off the top and swirl the yeast up
Divide the yeast into 10 vials
Double the volume of each vial using the cooled 25% glycerin solution (for example, after decanting you are left with 50ml of yeast slurry. Add 5 ml slurry to each vial and add 5ml 25% glycerin to each vial. This will give you a final concentration of 12.5% glycerin).
Mix the tubes well and place in a container filled with isopropyl alcohol to the liquid level in the tubes to reduce the freezing rate in the freezer (this can be skipped, but cell viability will be reduced if placed directly into the freezer. Cells prefer to freeze at a rate of 1°C per minute).
After 24 hours the frozen tubes can be removed from the alcohol and stored for several years.
To inoculate a 1000ml starter for a brew, quickly unfreeze the tube to the starter wort temperature and dump it in. That will create a 1:10 dilution from the earlier split. Using a stir plate will have the starter ready and settled usually within 48 hours.
Once you are down to one tube in the freezer, make a starter as in Step 8 using the tube, and freeze into more tubes as in Step 3. So you theoretically from one smack pack and some DME have made enough cells for almost 30 batches in just 3 controlled generations of culture (or over 700 batches if you expanded each tube out 3 generations).
Notes
If you increase the final concentration of glycerin in the frozen culture to around 50%, viability will be reduced, but the cells will not freeze solid, and you can dip a sterilized loop in there to inoculate a small starter, very much like inoculating from a slant. This way you can cut down the storage volume of your stocks to a few 1.5ml tubes instead of larger vials. But keep in mind it takes a week or more to grow a starter from that instead of a couple of days.
Regarding the storage time, it is also assumed that a chest freezer or manual defrost freezer is used. If a fridge/freezer is used that has an automatic defrost cycle, the tubes should be placed in a styrofoam box with ice packs inside to maintain temperature while the defrost cycle works.
Keep Records
Whichever method you choose to maintain your yeast bank, keeping detailed records is mandatory. Typical details are yeast ID, strain, source, date commercially manufactured, storage date, number of generations from packaging, parameters of beer obtained from (if slurry), date to re-plate or refreeze, among others. These records will be the living document that tells you how viable the stocks may be, when routine culturing is necessary, and depending on how detailed you get, how changes in your protocol can increase cell viability. These are all good things, and just as necessary as keeping a brewing log book to note how a beer turned out. If you eventually build up a stock of over 40 different strains, you’ll definitely need a spreadsheet with tasting notes, fermentation parameters, and ideal beer styles in order to plan your next brew.