Guide to Making a Frozen Yeast Bank
I really like the variety and quality of liquid yeasts that are available today, but I sure donít like paying $10 for a smack pack of yeast that I will only use once. It is by far my most expensive ingredient. Also, fresh liquid yeast can be tough to find at my LHBS, so I really wanted to have a bank of yeast on hand for whenever I needed it. Some quick research on the internet revealed that building a yeast bank in your freezer is a pretty easy thing to do, and the equipment required is minimal. The advantage is that you can easily cut the cost of liquid yeast by at least 90%, and you can have a good variety of yeast on hand whenever you need it. Below is the method that I have been using with great success.
Here is a picture illustrating the items I use (clockwise from top):
- 2000 mL Pyrex flask and stir-plate for propagating yeast (optional)
- pressure cooker (poor man's autoclave) for sterilizing equipment
- 500 mL beaker for holding glass vials
- 15 mL (1/2 oz) flat-bottomed glass vials with autoclavable screw tops
- small glass graduated cylinder
- autoclavable pipette (not shown)
- spray bottle filled with Star San solution (not shown)
Step 1 Ė Prepare a yeast starter
Obviously, the first step in the process is to prepare some yeast for freezing. It is best to freeze concentrated yeast from the bottom of a starter, so I have been building a big (1.5 Ė 2 quart) starter on my stir plate from a new pack of liquid yeast. If you donít have a stir plate, make your starter in two rounds. For example, start with 3 cups of wort, ferment it out, and then add another 3 or 4 cups of wort to really build up the yeast. Aerate the heck out of the wort at each addition to ensure the yeast get lots of oxygen required for reproduction. An oxygenation system is handy for this, but not necessary.
Once your starter is built up and the yeast have fully fermented the beer, crash cool your yeast in the fridge to drop the yeast out of suspension. If you like, you can also pour off the beer on top, but this isnít necessary. In Step 4 you will be drawing yeast off the bottom of the flask, and you want it to be as concentrated as possible, so make sure that the yeast have sedimented well to the bottom of the flask.
Step 2 Ė Use glycerine for freeze protection
To freeze yeast successfully, you need to do so without killing the yeast. Yeast can recover from sub-zero temperatures, but frozen water can kill them. Frozen water crystallizes and can puncture yeast cell walls, but two things can be done to prevent this. First, use a small volume of water (hence the small vials) and leave a bit of room for expansion. Second, a bit of glycerine, also known as glycerol (but NOT glycol!!) helps prevent freeze damage. I buy glycerine from my LHBS or pharmacy. Just be sure to get the type that is safe to ingest (it is harmless at these quantities).
I use a small graduated cylinder to measure and pour glycerine into the test tubes. You want less than one-third of the solution in the test tubes to be glycerine, so I shoot for about 2-3 ml in each tube (15%). I do 10 to 16 tubes at a time, so I just measure my total amount necessary in the graduated cylinder, and distribute it approximately evenly among all the tubes. You don't need to be absolutely precise.
Step 3 Ė Sterilization
It is essential to be as sanitary as possible when storing yeast for a long period of time. I use my pressure cooker (doubles as my steam infusion system, see here) as a home autoclave to sterilize my equipment. Pressure cookers are great because they will heat up to about 250 degrees, which is enough to kill everything inside. Boiling water works pretty well, too, but it only gets up to about 212 degrees (depending on your elevation), which kills all living things but not wild yeast spores.
Very loosely cap your vials (now partially full with glycerine) and put them in a Pyrex beaker or other heat-resistant container inside the pressure cooker. Also put in your pipette, eye dropper, stainless steel turkey baster, etc. that you will use to extract yeast from your starter flask. Note that in this picture I am using a SS turkey baster because it can be sterilized easily, but I have since switched to a pipette and bulb. Donít put the bulbs in the pressure cooker unless you are certain they can withstand the heat! (I just sanitize mine with Star San.)
Heat the pressure cooker to steam pressure and hold for 15 mins. Turn off the cooker and let it cool without opening the lid (will take at least an hour or two).
If you donít have a pressure cooker, just boil your items for at least 20 minutes to sanitize them. Alternatively, if you start with clean vials, a no-rinse sanitizer like Star San should also do an acceptable job and would be much quicker.
Step 4 Ė Fill the vials
Once the pressure cooker and contents have cooled, clean your counter-top really well and spray with a no-rinse sanitizer like Star San, if you have some. Working quickly, open the pressure cooker and carefully remove your vials and line them up on your counter. Remove your sterile beaker and use it to hold your pipette/dropper/turkey baster. Put your yeast starter in easy reach.
Now, one at a time, uncap your vial and extract a sample of yeast slurry from the bottom of the flask. Inject enough into the vial to fill it about 80%. If the yeast is really thick at the bottom, you may have to fill it about 60% full and top off with some beer from the flask. You donít want the mixture to be too viscous or the glycerine wonít mix in very well. Cap the vial immediately after filling, and move on to the next one.
Once all the vials are full, vigorously shake each one to thoroughly mix the glycerine with the yeast. You should still have plenty of yeast left in the flask to pitch into a batch of beer.
Step 5 Ė Freeze the yeast
Your vials are now prepared and ready to be frozen. I use a small cooler lined with ice packs to store my yeast inside my freezer. The cooler and ice packs are actually essential for keeping the frozen yeast at a relatively constant temperature while in the freezer. This becomes especially important if you have a self-defrosting freezer: the defrost cycle will warm the fridge and potentially thaw and re-freeze the yeast repeatedly, which could kill them.
Be sure to label each vial very carefully. I like to use the Avery 06504 removable white ID labels. As a minimum, the following information should appear on each label:
- brand and strain of yeast (e.g., Wyeast 1028 London Ale)
- date of preparation
- generation # (e.g., 1 if frozen from a fresh pack of yeast, 2 if re-cultured from a vial, etc.)
EDIT: Just found a great tip here -- refrigerating your vials for 48 hours prior to freezing can double your yeast viability. I'll be doing this on my next batch.
Step 6 Ė Reviving and propagating the yeast
When you are ready to use a vial of yeast, leave about 3 – 4 days lead time before you brew. First, you need to gently warm the yeast to room temperature. I usually take a vial out of the freezer in the morning, wrap it in a dry hand towel, and set it in the fridge to gently warm. That evening, I take it out of the fridge and warm it to room temperature while I prepare a small (1 cup/250 ml) starter. Allow the starter to cool to about 105 F/40 C, and then aerate it really well to ensure good yeast propagation.
When you pitch the vial, you want to be as sanitary as possible because you aren’t starting with much yeast and there is a real risk of infection. Open the cap and use a butane torch to flame the mouth of the vial to kill any nasties. Alternatively, spray it well with Star San. Pitch the yeast immediately.
The next morning, the small starter should have fermented out (look for a krausen ring as proof). Now use this yeast to build up a big starter (usually 1 to 2 quarts) the same way that you would use a fresh pack of liquid yeast. Give the big starter 1 or 2 days to propagate and ferment out completely. Before you pitch it, smell the starter or (using sanitary techniques) pour off some of the beer and give it a taste. If it smells or tastes bad, it may have developed an infection so don’t use it. Otherwise, you should have a healthy culture of yeast ready to work its magic for you in your next brew.
You are my hero!!!! That's fantastic! Thanks for the writeup. :mug:
Very nice and thorough job! I'll be maintaining a few yeast stocks myself; I just can't justify $7 a pop for yeast. DIY stirplate here I come (electrical engineering student and computer geek so I only need to buy a stirbar :p).
Excellent. Thanks for taking the time to detail the whole process.
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