Making a Frozen Stock Yeast Bank

If this is your first visit to Homebrew Notes, read THIS first.

Most recent update: 2019-04-12 (See Updates)

Homebrew Notebook Page: Summary Homebrew Note
Status: In Testing
Source(s): Homebrew Talk Article

Contents

• Why This is Important and/or Interesting
• Items Needed
• Process Gallery
• Process

Why This is Important and/or Interesting

The paragraph below is copied directly from the source article above because frankly, I couldn’t have explained it better.

“Keeping frozen stocks of yeast cultures is the way to go for a homebrewer. 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, equipment, or valuable refrigerated space. The addition of a cryopreservative (food grade glycerin) is required 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.”

While this might seem super-scientific and overly complicated, it’s surprisingly easy and quick to do if you have the right equipment. There are also a few significant benefits that convinced me to take the plunge on yeast banking.

The first benefit is the cost savings. Yes, there is a one-time equipment expense but with liquid yeast packs costing $7-15 per pack, you’ll recoup the equipment costs quickly. Second, I got tired of wanting to brew and finding that my local homebrew shop didn’t have the yeast I wanted. I choose yeasts for specific characteristics so I’ve always cringed when I had to resign myself to finding the best possible substitute due to availability. I felt I was starting with a flaw and I hadn’t even brewed yet. With your own yeast bank, you’ll have your favorite yeast strains available with a quick walk to your freezer. Another benefit is the ability to bank rare yeasts. Occasionally I’ll get a rare yeast from White Labs’ Yeast Vault (waiting on WLP773 Scottish Cider Yeast Blend now) and being able to keep a supply of it for use once it goes out of production is pretty awesome. Yet another benefit is the ability to help out a friend. When you have a brew buddy or someone in your homebrew club who needs a yeast that’s not available, you can be the hero with a good deed from your yeast bank.

One final benefit is that these little mini-packets of yeast would be perfect for yeast and or recipe flavor experiments. Imagine that you brew a 5-gallon batch of a relatively neutral 1.050 OG beer and split that up into 1/2 gallon growlers. If you had a lot of strains in your yeast bank, you could add one tube to each 1/2 gallon of wort, let it ferment, and learn the flavor differences that various yeasts can impart. Likewise, you could take a 5-gallon batch and split it into 1/2 gallon growlers and add various adjuncts to find your ideal spice/flavor combination. Your mini-pitch tube will be near perfect for any 1/2 gallon batch. I plan to do the yeast experiment once I get more strains in my yeast bank and I’ll write it up as a project on HBN at a later date.

One final comment in this introduction – I think what I have below will be helpful and it’ll serve as a good process to follow. However, I’m not a microbiologist and I think the source article is a great primer for yeast banking and saving yeast in general. I’d encourage you to take a few minutes to read that article and others on this topic, once you’ve finished reading the rest of this Homebrew Note.

Now let’s start the process.

NOTE: This post contains affiliate links. In the event of a sale, I will be awarded a small commission (at no additional cost to you). Don’t worry, I’ll just use the commission to buy more brewing stuff anyway.

Overall Items Needed

• Pressure Canner (My Pressure Canner)
• Mason Jar (x 2)
• Sterile Test Tubes (15 ml Test Tubes – 50 Pack)
• Glycerin (Vegetable USP Grade Glycerin)
• Malt Extract (3 lbs of Dry Malt Extract)
• Sanitizer Spray Bottle (See: How to Make a Sanitizer Spray Bottle)
• Measuring Cup
• Hand Towel (optional)
• Test Tube Rack (optional) (Test Tube Rack – 50 Tube Capacity)
• Oral Syringe (optional) (6 ml Oral Syringes – 50 Pack)
• Insulated Box or Lunch Cooler (choose a size that will fit in your freezer)
• Reusable gel ice packs (another reason to stock up on gel ice packs!)

Process Gallery

Have a quick look at the gallery photos by clicking on the first image and reading the captions as you click your way through the images. Then read the [Process] section below for context.

NOTE: You’re making yeast samples that will be used to ferment future batches of beer. I hope it goes without saying that you should keep everything during the processes that follow as sanitized as possible to avoid infection.

Process: Making Your Cryopreservative

While many will keep yeast slurry from a previous batch, that yeast needs to stay in the refrigerator instead of the freezer. The reason is that freezing yeast causes ice crystals to form which will rupture yeast cell walls killing the majority of the yeast. For short term storage, keeping yeast slurry in the fridge works fine. It doesn’t work if you want to keep that yeast for a long time because the yeast continues to mutate at the lower temp and those jars take up a lot of valuable fridge space.

The way to get around this is to mix your yeast slurry with a cryopreservative that will minimize the formation of ice crystals so the yeast samples can be frozen. Making your own cryopreservative is an easy thing to do if you have a pressure canner and one jar’s worth will last you for quite a while. So let’s get to it.

For this stage of the yeast banking process, you’ll need:

• Pressure Canner
• Glycerin
• Measuring Cup
• Mason Jar (preferably a short/wide one with a wide mouth)

With the above items in hand, making a cryopreservative solution is pretty simple.

Create a solution that’s 25% glycerin and 75% water in your pint jar. I recommend 2.5 oz (75 ml) of glycerin and 7.5 oz (225 ml) of water. Now pressure can the jar for 10 minutes.

Once the jar of solution has been pressure canned, take it out of the pot to cool to room temperature slowly via ambient temperatures. If you run the jar under cool water or put it in a water bath, there’s a reasonable chance the jar will crack or shatter and you could get injured. Be patient. The solution will cool in a couple of hours. Once cooled, it’s ready to use. If kept closed except for when being used, this solution can be used as needed until empty. It should be enough for six batches (60 tubes).

Put your canner, glycerin, and measuring cup away since you’re finished with this step in the process.

NOTE: Using the 25% glycerin to 75% water does allow you to scale up to quart or larger volumes of the solution, if desired. I’ve just never had a need.

Process: Making a Yeast Starter

For this stage of the yeast banking process, you’ll need your yeast starter gear and ingredients (malt extract, etc.).

To build up enough yeast for banking, you’ll need to make a starter of the yeast you want to save. If you don’t know how, pause here and learn how to calculate the yeast you need and how to make a yeast starter. Now, make a 1 liter, 1.040 gravity starter of the yeast you plan to bank. When the time comes, make sure to decant the solution so that almost all of the excessive wort has been poured off. Leave just enough to allow the yeast to get swirled into solution. You want as little wort as possible and as pure of yeast as possible.

Process: Making a Yeast Bank

For this stage of the yeast banking process, you’ll need:

• Mason Jar (preferably a short/wide one with a wide mouth)
• Sterile Test Tubes (15 ml Test Tubes – 50 Pack)
• Freezer-safe, Leak Proof Plastic Bin (Bin Package)
• High Purity (>90%) Isopropyl Alcohol (91% Alcohol)
• Sanitizer Spray Bottle (See: How to Make a Sanitizer Spray Bottle)
• Ultra-Fine Tipped Sharpie (Dual-Tipped Sharpie)
• Masking Tape (optional)
• Hand Towel (optional)
• Test Tube Rack (optional) (Test Tube Rack – 50 Tube Capacity)
• Oral Syringe (optional) (6 ml Oral Syringes – 50 Pack)
• Insulated Box or Lunch Cooler (choose a size that will fit in your freezer)
• Reusable gel ice packs (another reason to stock up on gel ice packs!)

After making and decanting your yeast starter, it’s time to bank that yeast. Begin by setting up your station. A 3′ by 5′ space should give you plenty of room. You’ll want a chair to sit on for stability. I use the top of my fermentation freezer but your dining room table or kitchen counter will work just fine. Spread out a hand towel so your drips are easier to clean up and place 10 test tubes in a tube stand so they’re easy to handle (see the gallery photos if you haven’t already). Leave all of the tubes closed so they remain sterile. Get your sanitizer spray bottle. Get your cryopreservative and leave the lid on it.

Sanitize your short, wide mouth mason jar and pour your well-decanted yeast into it. Transferring your decanted yeast into a smaller vessel makes it much easier to work with. Spray sanitize the jar’s lid and cover the jar.

IMPORTANT: Some oral syringes come individually vacuum-sealed in plastic and some don’t. Those that are sealed are likely sterile but those that are loose in a box (like these) are not sterile. You’ll want to sanitize the inside of that syringe before you use it. What I do is pour a small amount of sanitizer from my spray bottle into a half-pint mason jar and use the syringe to suck the sanitizer in and squirt it back out a few times. Then I proceed as described below.

ALSO IMPORTANT: The next steps should go quickly because you’re exposing your yeast to the open air (i.e. bacteria). Be methodical, mindful, and quick but don’t get yourself overly worked up about a few seconds of air exposure. Just don’t walk away and leave things uncovered for minutes at a time.

Remove the caps from five of your test tubes. Open your yeast slurry jar. Using your 6 ml oral syringe, extract 5-6 ml of your yeast slurry from the yeast jar and then gently squirt it into the first tube. Repeat the process until all five tubes have yeast samples. Sit your syringe down and cover each of those open tubes. Open the other five tubes. Spray your syringe with sanitizer and fill the remaining test tubes with 5-6 ml of yeast slurry. Sit down your syringe and cover the open tubes.

Open your cryopreservative jar and the first five test tubes. Spray your syringe with sanitizer. Extract 5-6 ml of cryopreservative from the jar and gently squirt it into the first tube. Repeat the process until all five tubes have cryopreservative. Sit your syringe down and cover each of those open tubes. Open the other five tubes. Spray your syringe with sanitizer and fill the remaining test tubes with 5-6 ml of cryopreservative. Sit down your syringe and cover the open tubes. Cover your cryopreservative jar. Shake up each of the tubes to mix your yeast and the cryopreservative. You’ve just created yeast samples in about 10 minutes!

OPTIONAL: If you don’t have syringes, you can pour the yeast directly into the tubes. It’ll be messier, slower, and you’ll lose some yeast so I think it’s worth the $15-20 investment for 50 sterile oral syringes. That many syringes will last a LONG time. Similarly, the tube rack isn’t mandatory but I think it’s worth the $5-7 investment to have a proper stand and tidy storage rack.

Freezing your yeast samples without damaging the yeast cells is a two-stage process. The first step is mixing the yeast with a cryopreservative, which you’ve already done. The second step is to slow the freezing process for your yeast samples. You do this by submerging your yeast sample test tubes in plastic bin and put all of your test tubes in it. Now pour your isopropyl alcohol into the bin until the yeast samples are fully submerged. Cover the bin. I like to put a piece of masking tape on the bin lid and using my Sharpie, write: yeast number (e.g. WLP001), yeast name (e.g. Cali Ale), date the samples were created (2019-03-31), and the yeast generation (e.g. 1). I do this in case I forget the samples for a while. If I have to guess at what’s in there, it becomes useless. This is also the same information I’ll transfer to the tubes once they’re frozen.

Put your plastic bin of yeast in the freezer and leave it for a minimum of 24 hours, preferably 48. Clean up your yeast banking station. You’re finished for today.

After a couple of days, pull your bin of alcohol and test tubes from the freezer along with your test tube rack. Also grab a hand towel and your Sharpie. Pull each tube from the alcohol bath one at a time, wipe them dry with the hand towel. Using the Sharpie’s ultra-fine tip, write on the white label area: yeast number (e.g. WLP001), yeast name (e.g. Cali Ale), date samples were created (2019-03-31), and the yeast generation (e.g. 1). Once labeled, put the tube in your test tube rack. Once you’ve done all of the tubes, reseal your alcohol bath so you can use it again for your next batch.

Now, place your new yeast bank inside an insulated box or lunch cooler along with some ice packs. The reason this is important is because frost-free freezers will increase their temperatures as part of the frost prevention process. As Wikipedia states, “The temperature of the freezer contents rises during the defrosting cycles, especially if there is a light load in the freezer. This can cause ‘freezer burn’ on articles placed in the freezer, from partially defrosting, then re-freezing.” By keeping your yeast in an insulated box (or small lunch cooler) with some gel ice packs, you minimize the impact of these defrosting cycles. You can also place your plastic bin of alcohol back into the freezer. It does not need to be in the cooler box.

That took you 5 minutes and you’re finished creating your yeast bank! Because they are frozen with cryopreservative, these yeast samples are good for several years.

IMPORTANT: Each of these yeast samples/tubes should have approximately 20 billion yeast cells. The most common yeast producers, White Labs and Wyeast, provide 100 billion yeast cells per pack at the time of production. While that may seem like your samples don’t contain nearly enough, that’s not quite true. Those commercial yeast packs are losing viability at a rate of nearly one billion yeast cells per day and that’s assuming they’re staying refrigerated. Your yeast cells are frozen so they aren’t losing viability in that way. A commercial yeast packet only needs to be four months old (pretty common) to be down to 16 billion yeast cells. That’s one of the reasons that making a yeast starter for liquid yeasts is so important. So with your 20 billion yeast cell sample, you can build up your own starter without issue. You can always use two samples/tubes if you want to start with more cells. Even if you use two each time for a 40 billion starting cell count, you’re still getting five uses from each banking of yeast.

Process: Using Your Yeast Bank Sample

REMINDER: Each tube of yeast has approximately 20 billion yeast cells.

You’ll need to use a yeast starter calculator to determine how many cells your upcoming beer needs but because you’re using a tube from your yeast bank instead of a commercial packet, the info you’ll enter into the calculator is a bit different.

In Part 1 of the calculator linked above, enter the Wort Gravity (OG), Wort Volume, and select Target Pitch Rate for the beer you plan use this yeast in. The other field values – Yeast Type, Packs, and Mfg Date – don’t matter so leave them with their default values. Click the Update button and make note of the Target Pitch Rate Cells value since that’s your new target number.

EXAMPLE: Let’s assume you’re going to brew a Pale Ale with a starting gravity of 1.055 and you’re going to do a 5.0 gallon batch. As noted in the Making a Yeast Starter homebrew note, I tend to go up one level on the Target Pitch Rate so in this example, it would be Pro Brewer 1.0. If you enter all of those values and click Update, you’ll get a Target Pitch Rate Cells goal of 257 billion cells.

In Part 2 of the yeast starter calculator, enter a Starting Yeast Count of 20 per tube you plan to use (20=1 tube, 40=2 tubes, etc.). Enter your Starter Size (e.g. 1L, 2L) and Gravity (should be between 1.030-1.040). Select the appropriate Growth Model and Aeration and click Update.

If your Ending Cell Count value is greater than your Target Pitch Rate Cells value, you’re good to go and you’ll see a message that Starter meets the desired pitching rate! Otherwise, increase the Starting Yeast Count (i.e. tubes), and/or your Starter Size, and/or your Gravity (keep it between 1.030-1.040) until the Ending Cell Count value exceeds the Target Pitch Rate Cells value. Using the example above with a stirplate, you would need a Starter Size of 1.5 L at a Gravity of 1.040 or a 1.7 L starter at 1.035 gravity.

Once calculation is complete, note the tubes needed (reminder: 1 tube=20 billion cells of Starting Yeast Count), the Starter Size, and the Gravity. Take out the correct number of sample tubes and let them slowly warm to room temperature. Using the tube(s) as your “yeast pack,” you can now follow the instructions in the Making a Yeast Starter homebrew note to create your yeast starter.

Check your yeast bank stock for this particular yeast. If you’re down to three or fewer tubes of this particular yeast, you should consider making another starter to rebuild your stock of yeast (the final process below). If you have more than three, no need to make more quite yet.

Process: Rebuilding Your Stock of Yeast

Once you are down to three or fewer tubes in the freezer, make a new starter of this yeast to replenish your stock. If you have a stirplate, you can do that with a 1 liter, 1.040 gravity starter using two of the tubes. If you’re using the occasional shake method, you’ll need three tubes with a 2 liter, 1.040 gravity starter. This will create another ~200 billion cell starter which can be split into 10 more tubes.

However, despite your best efforts, yeast will still mutate over time so it’s good to limit the number of generations used. The number of generations you can create from an original pack of yeast (“generation 0”) varies from yeast strain to yeast strain. As a general rule of thumb, I suggest using it for up to three generations. You can go longer if that’s your preference but you may start noticing unintended flavors. Below is a summary of the generations:

• Generation 0 = original smack pack used for making generation 1
• Generation 1 = first set of 10 sample tubes, 2-3 of which is used to make generation 2
• Generation 2 = second set of 10 sample tubes, 2-3 of which is used to make generation 3
• Generation 3 = third set of 10 sample tubes

After generation 3, just buy a replacement smack pack and start over with a first generation yeast. Of course, if the yeast is no longer available, you can try to keep pushing it for another two or three generations.

You’re now saving money, keeping your favorite yeast strains at your fingertips, and impressing your friends with your super cool yeast bank! Now save and/or print the summarized Homebrew Notebook Page for use in the future.

Updates:
– 2019-04-12 – Added paragraph to keep yeast tubes stored in an insulated box or lunch cooler with ice packs to prevent warming of the yeast during frost-free freezer cycling
– 2019-04-04 – Added note to sterilize oral syringes if the are not individually vacuum-sealed at purchase

Now that you’ve read this Homebrew Note, let me know if you have a question, recommended improvement, or other thoughts in the comments below. As I mention in About Homebrew Notes, these are living documents and your feedback is appreciated!