Thereís some information out there about freezing yeast, but Iím not sure how many people are actually doing it. Iíve read a lot of posts about yeast washing and harvesting from ferementors so I thought Iíd do a pitch (no pun intended) on yeast farming/freezing as an alternative.
Freezing yeast is actually quite simple. You mix water, glycerine (glycerol), and yeast slurry together and throw it in the freezer. Thatís all there is to it. Glycerine is the magic ingredient that protects the yeast during the freeze. The yeast will remain vaible like this for quite a long time. Iíve brought up yeast that was almost 2 years old and brewed with it. So hereís my pitch.
The number one reason that I freeze yeast is for convenience. Smack packs, vials, slurry in a mason jar all loose viability over time and eventually die while sitting in the fridge. Dry yeast lasts a lot longer but will also lose viability. A frozen jar of yeast has a much longer shelf life. This means you can brew whenever you want (with a little planning) and never have to worry about available, healthy yeast.
Another benefit is cost. It ends up costing me about the same price as a pack of dry yeast (or a little less) for a jar of frozen liquid yeast and thatís after pitching to a 5-gallon batch of beer.
There are a couple of different approaches that you can take. The main thing is making sure you use enough glycerine. Unfortunately, there is no set number out there on how much glycerine to use. I use a 20% by volume concentration of glycerine. Others report using anywhere from 15% to 50% or over. Some like to add so much glycerine that the mixture does not actually freeze at all. Bottom line, pick what you think is best and run with it. If it works, continue on.
What glycerine to use?
You want to use food grade vegetable glycerine. I get mine from Nutrition Geeks
. The brand I get is ďNOW
.Ē Itís made in the United States. Be careful and donít buy glycerine made in China because there have been reports of the Chineese selling Ethylene Glycol (antifreeze) and passing it off as glycerine. The two compounds have similar properties except Ethylene Glycol will kill you and glycerine wonít.
How much yeast to freeze?
Most people Iíve read about out there are freezing small portions of yeast in vials or something similar. I like to freeze estimated portions of 100 billion cells. This way I have more viable cells after the freeze and can start with a larger starter instead of having to do small step ups.
As always, there is more than one way to skin a cat, but hereís how I do my process.
I like Wyeast
so I always start with an activator smack pack. You can use whatever yeast you want. I buy one smack pack of every yeast strain I plan to use and then farm enough yeast of each to last me for about a year or a little over. I use the Yeast Calc
pitching rate calculator, but you can pick the calculator of your choice (I know a lot of people like Mr. Malty
). I make up an appropriately sized starter in steps so that the estimated final cell count is ~500 billion. I setlle out the yeast, siphon off the starter beer and add enough glycerine/water solution to bring the volume up to 500 milliliters (with a final glycerine contration of 20% by volume).
Then I split the yeast into 5 equal portions and refridgerate them for 48 hours before freezing.
If I want more jars than that of a particular strain I will take one of the portions and put it back through the process. You can do this over and over again until you have as much yeast as you want.
A few things that will help
A deep freezer.
You want your yeast to remain frozen without the temperature fluctuating. It will have a longer shelf life this way. The best thing for this is a deep freezer. I like to store mine at the bottom of my chest freezer buried under a bunch of food. You donít want to put the yeast in a self-defrosting freezer (like your refridgerator freezer). Self-defrosting freezers warm up once in a while and this is how they defrost. You donít want your yeast warming and cooling all the time. If you donít have a deep freeze, you can get around this by putting the frozen yeast in a small cooler with some ice packs and storing the whole cooler in the freezer. Freeze the yeast first and then throw the cooler in on the mix. Iíve read reports of this working well for folks but Iíve never done it myself.
A pressure cooker.
I like to sterilize what materials that I can by pressure canning them. After all, itís very important to practice excellent sanitation when farming yeast. The yeast has a long way to go before it gets to your prized wort and contamination from the start will have ample opportunity to magnify along the way. I pressure my starter wort, glycerine solution and the jars that I freeze the yeast in. Itís also more convenient this way because you can make up a bunch of this stuff all at one time and not have to worry about boiling/cooling/sanitizing every step of the way. I pressure everything at 15 psi for 15 minutes. For my freezer jars, I use 4 oz jelly jars. They hold about 120 milliters. I fill them up with tap water and pressure them. When itís time to freeze yeast I pop the seal, dump the water and fill. Put the lid back on and tighten down the ring nice and snug. If you donít have a pressure cooker, donít fret, you could still do this. You would just have to boil and cool as usual and soak your jars in star san or something before filling.
A stir plate.
You can grow a lot more yeast a lot quicker and with less wort with a stir plate. However, a simple starter would work just fine as long as you make one big enough to hit your target cell count. What this means is that you will probably have to make more starters to get the same job done.
Make sure you freeze healthy yeast.
I go the extra mile when Iím farming yeast to be frozen. I make sure the yeast have plenty of nutrients and give shots of pure oxygen at every wort addition. When the starter is done I settle out the yeast as fast as possible and refridgerate for 48 hours before freezing.
I want to make sure that the yeast have healthy cell walls and a full belly of reserves before I freeze.
How to mix up your glycerine solution.
I've been doing mine in % by volume.
You have 150 ml of yeast slurry. You want to know how much glycerine to add so that the final concentration is 20% by volume. Working with percents is kind of weird. We're shooting for 20% glycerine, so we have to divide the volume of slurry we have by 80% to find out what the final volume would be with 20% glycerine mixed in. If we wanted 30% glycerine, then we would divide by 70%, Here is the equation.
Volume of Yeast slurry / 0.8 = final volume. We have 150 ml of slurry. 150 / 0.8 = 187.5 ml. Now that we know the final volume, we can subtract the amount of slurry we already have (150 ml) to determine how much glycerine we need. 187.5 - 150 = 37.5. So, you add 37.5 ml of glycerine to the 150 ml of slurry. Final concentration of glycerine - 20%. 37.5 / 187.5 = 0.2
You have 150 ml of slurry. You want to use this slurry to fill (5) 100 ml vials. You want a final concentration of glycerine of 20% by volume. In this instance, we know what the final volume is going to be, 500 ml, becasue that is what we need to fill all the vials. Here is the equation for this one.
Desired final volume * desired glycerine concentration % = glycerine needed. 500 ml * .20 = 100. So, we add 100 ml of glycerine to the yeast slurry we already have and then add water to top it off at 500 ml.
You have 150 ml of yeast slurry. You want to use this slurry to fill (5) 100 ml vials. You want a final concentration of glycerine of 20% by volume. You have a pre-sterillized jar of 60% by volume glycerine solution. You want to know how much of the 60% solution to add to your yeast slurry. Here is the equation.
(Desired final volume * desired glycerine concentration %) / concetration of existing glycerine solution = glycerine solution needed
Our final volume is going to be 500 ml because that's what we need to fill the vials. We want 20% glycerine. Our pre-sterilzed glycerine solution is 60% by volume. (500 ml * 0.20) / 0.60 = ~167 ml. So, we add 167 ml of glycerine solution to the 150 ml of yeast slurry that we already have. Then we top off with water to get 500 ml.
Thawing the yeast and brewing with it
Ahh, the fruits of your labor. Thaw the yeast as quickly as possible with a water bath and pitch strait into a starter. It appears that viability after the freeze is on the low side (around 25%).
Again, I freeze an estimated 100 billion cells. When I thaw out a jar I use the Wyeast pitch rate calculator, select the propagator pack and then make an appropriately sized starter for the gravity beer that Iím brewing. Since propagators have 25 billion cells this works out nicely since 25 billion cells should make it through the freeze. Sometimes the frozen yeast exhibit an unusually long lag time on the first step of a starter. This is likely due to a low viable cell count.
After the first step is complete, the yeast chew through whatever I throw at em in 12 hours or less. At any rate, once the starter gets going, the yeast act as normal as ever, so be patient your first couple times doing this and donít be alarmed by an extended lag time on the first step. I recently updated the orginal post to include some process changes. I made the edits in red. I will be getting some lab equipment in the near future and running some tests on cell counts and viability after the freeze. As soon as I have some consistent results I will be sure to post them and update the post accordingly.
Well, there you have it. If you decide to give this a try, I think youíll find farming yeast and freezing it to be very convenient, cost effective, and relatively easy to accomplish. Anyone who makes yeast starters on a regular basis can do it. Youíll always have ample yeast on hand to brew with and youíll never have to wash yeast again. For further reading Iíve posted some sources at the bottom.