The Secrets to Freezing Yeast
At the bottom of my freezer lies a homebrewer’s treasure trove: Dozens of small vials containing frozen yeast. These vials are organized in groups of five or six, each cluster representing a different yeast strain. My collection includes popular varieties like Chico and seasonal favorites such as British Bedford Ale.
At any time, I can select a vial or two, revive the yeast in a starter, and have it ready to pitch into a fresh batch of wort. The best part? Some of these frozen vials are over five years old and still perform well when reactivated.
The secret lies in proper freezing techniques. When managed correctly, frozen yeast can maintain its viability for an impressively long time, offering homebrewers a convenient and cost-effective way to always have their favorite strains on hand.
Understanding the Basics
Freezing yeast is a relatively straightforward way to keep various strains on hand without taking up much space. The basic process involves creating a protective solution called a cryopreservative using food-grade glycerin and water. When combined with yeast, this solution helps prevent ice crystals from damaging the yeast cells during freezing.
After sterilizing the solution, you mix it with a small amount of yeast (either fresh yeast or from a slurry) and store it in small vials in the freezer. This method slows down the yeast’s biological activity, keeping it viable for years. With careful handling, you can easily revive the yeast by thawing out a couple of vials and adding them into a starter a day or so before brew day.
The Benefits of Frozen Yeast
Purchasing fresh yeast for every batch can get expensive, and since the viability of yeast cells decreases over time, it requires careful planning to ensure you have fresh yeast just in time for brew day. For years, my favorite way to address these concerns was to overbuild starters.
Here’s how overbuilding starters works:
A day or so before brew day, take a packet of fresh liquid yeast and pitch it into a yeast starter that’s about 50% larger than needed for the intended batch.
After a day or two of spinning the starter on a stir plate, collect a third of the starter in a sanitized vessel — such as a pint-size Mason jar — and store it in the fridge.
The rest of the starter is pitched into your wort as usual.
When it’s time to use that yeast again, take it out of the fridge a couple of days before your next brew day, overbuild another starter, and again split that starter between your new brew and another sanitized vessel.
This method allows a single packet of yeast to be used multiple times without the added steps of harvesting yeast from the bottom of your fermenter post-fermentation. However, freezing yeast overcomes several limitations of the overbuilt starter approach:
Extended Viability: A yeast slurry stored in the fridge can retain some viability for months, though it is often recommended to use it within a week or two. With each passing day, more yeast cells die, and after a few months, the slurry may contain very few viable yeast cells. By freezing yeast, the yeast cells’ biological activity is significantly slowed down, preserving their viability, potentially for years.
Reduced Generational Mutation: Freezing yeast also reduces generational mutation. With the overbuilt starter approach, each use of the yeast slurry results in a new generation of yeast. Repeating this process more than a handful of times can lead to significant mutations not present in the fresh yeast received from the yeast lab. In contrast, freezing yeast allows you to split a single starter or packet of fresh yeast (let’s call it generation 0) into 10 vials. When you have only a few vials left, you can build a starter with the remaining vials to create another 10 vials of what is now generation 1 yeast. This cycle means many more batches of yeast are available from each generation, resulting in fewer mutations over time.
Step-by-Step Guide
The process to freeze yeast takes a bit of preparation but with some minimal equipment is easy enough to follow and is something I’ve repeated dozens of times.
Step 1: Prepare a cryopreservative solution
Yeast cannot simply be stored in the freezer as ice crystals will form and rupture the cell walls, killing the majority of the yeast. A cryopreservative is needed to protect the yeast cells during freezing. Here is how to make the cryopreservative:
Required Equipment
Pressure canner
Food-grade glycerin
Measuring cup
Mason jar (preferably with a wide mouth)
Instructions
Measure out 2.5 fl. oz. (75 mL) of glycerin and 7.5 fl. oz. (225 mL) of water. I’ll typically use filtered or bottled water. This creates a 25% glycerin, 75% water solution.
Combine the glycerin and water in the Mason jar.
Place the jar in a pressure canner and process for 10 minutes following your canner’s instructions. This will sanitize the cryopreservative and make it shelf stable.
After processing, remove the canner from heat and allow it to depressurize naturally.
Once it’s safe to open the canner, remove the jar and let it cool to room temperature. Do not rush this process by running it under cool water or placing it in a water bath, as rapid temperature changes could cause the jar to shatter.
Once cooled, your cryopreservative is ready for use. This solution can last for a good while — typically enough for six batches or about 60 15-mL tubes.
Step 2: Prepare your yeast
You have two options for preparing your yeast: Using a fresh packet or creating a yeast starter. Both methods are valid and have their own advantages.
Required Equipment
Sanitizer spray bottle
Wide-mouth Mason jar
Option A: Using a fresh yeast packet
Purchase a fresh pouch of your desired yeast strain. I’ve only ever used liquid yeast for this process, although dry yeast should work as well. Check the manufacture date on the yeast pouch — the fresher, the better to maximize viability.
Sanitize the outside of the pouch thoroughly using your sanitizer spray bottle.
Carefully open the packet and pitch the yeast into a sanitized Mason jar. This will make it easier to extract in the next step.
Option B: Creating a yeast starter
Alternatively, prepare a yeast starter 24–48 hours before you plan to freeze your yeast, following your preferred starter recipe.
Once fermentation is complete (usually 24–36 hours), place the starter in the refrigerator for a few hours to cold crash. This helps the yeast settle to the bottom.
Carefully decant most of the liquid from your starter, leaving behind the yeast slurry at the bottom.
Transfer the remaining yeast slurry to a sanitized wide-mouth Mason jar.
Step 3: Mixing yeast with cryopreservative
Now we are ready to combine the yeast and cyropreservative into test tube vials. Each vial will contain approximately 5–20 billion yeast cells, depending upon the viability of the source yeast.
Required Equipment
Sterile test tube vials (15-mL capacity, 10 per batch)
Oral syringes (6-mL capacity)
Sanitizer spray bottle
Instructions
Sanitize both the oral syringe and test tubes using a sanitizer spray bottle.
Using a sanitized 6-mL oral syringe, extract 5–6 mL of yeast and gently squirt the yeast into each test tube. One pouch of fresh yeast or a yeast slurry should easily be able to fill 10 vials.
Sanitize the syringe again and then extract an equal 5–6 mL of cryopreservative solution and add to each test tube.
Once all tubes have yeast and cryopreservative, close each tube and shake to mix the yeast and cryopreservative thoroughly.
Be sure to label the test tubes with the yeast strain added, the date, and the yeast generation.
Step 4: Freezing the yeast samples
We are now ready to freeze the yeast vials, but rather than just tossing the samples into the freezer, we’ll need to use an isopropyl alcohol bath. This will slow down the freezing process, which helps prevent damage to the yeast cells from ice crystal formation.
Required Equipment
Freezer-safe, leak-proof plastic bin
High purity (>90%) isopropyl alcohol
Freezer
Instructions
Take a freezer-safe plastic bin and place all of your prepared yeast sample test tubes in it.
Pour high purity (>90%) isopropyl alcohol into the bin until the yeast samples are fully submerged.
Securely cover the bin with its lid.
Place the sealed bin containing the alcohol and yeast samples into your freezer. Leave the bin in the freezer for a minimum of 24 hours.
The yeast samples can be removed from the alcohol after this period and stored in the freezer in a container of your choosing.
Step 5: Using your frozen yeast samples
When your next brew day arrives, the process of thawing and putting the yeast to work is straightforward.
Required Equipment
Yeast starter calculator
Yeast starter equipment (wort, flask, stir plate)
Instructions
Use a yeast starter calculator to determine your pitching needs. Enter your wort details (original gravity, volume, target pitch rate, and enter 20 billion as the starting yeast count for each tube you plan to use).
Once you’ve determined the correct number of tubes and starter specifications, remove the required number of yeast sample tubes from your freezer. I will typically only use one or two vials. Let them thaw slowly at room temperature.
Prepare your yeast starter following standard practices.
Use the thawed yeast samples as you would a commercial yeast packet when making your starter.
Once you are down to three tubes in the freezer of a given yeast strain, use them to make a new starter of this yeast to replenish your stock. Then follow the previously outlined steps again to create the next generation of your frozen yeast bank. I will typically go through three generations before purchasing new yeast, which is enough for dozens of batches of beer.
And that’s all there is to establishing your own private yeast bank, allowing you to keep your favorite yeast strains viable and on-hand whenever you need them.
Sources:
• www.homebrewnotes.com/making-a-frozen-stock-yeast-bank/
• www.homebrewtalk.com/threads/maintaining-a-healthy-yeast-bank-long-term.678997
