Yeast is one of the most important components of a beer. It does most of the heavy lifting in the brewing process. A homebrewer really only does a couple hours of work for each batch of beer (making the wort, transferring it to a secondary fermenter, and bottling the beer). However, this process can take weeks or even months from start to finish (actually even north of one year for the case of some barley wines). Someone (or should I say something) is working during all of that time, and that is the yeast.
So yeast is important, and since it can also be a bit expensive (about $7 for a vial that would work or a typical batch, which adds up over time), it makes sense to save and reuse the yeast if possible. As it turns out, this is not all that difficult to do. However, it is also not as simple as chucking a vial of yeast into the freezer (if you do this you will end up with a vial of dead yeast). There are a few simple steps required to be able to freeze yeast without killing all of the cells. I'm going to walk you through the requirements and explain why each is important.
Let's start with the components you will need to accomplish this job.
Several small containers
Small containers of some sort are needed to store the yeast. These containers should be big enough to store about 40 mL of yeast slurry (suspension of yeast in water). The yeast itself often comes in a nice little vial that can be reused (one can be seen in the photo above and the photo at the bottom of this post).
Centrifuge tubes are great for this purpose if you can get your hands on them (working in a lab can make that easier).
Glycerin
Not just a song from the early 90s! Glycerin (also known as glycerine and glycerol) is an alcohol with three hydroxyls too. About 10% by volume glycerine is mixed with the yeast slurry prior to freezing. The reason this is important has to do with crystal formation in water.
When water freezes, some of its molecules are able to orient into regular arrays and form crystal. However, a large portion of water molecules are frozen in place before they can orient into crystals, instead forming an amorphous solid. When you put water into your freezer to make ice, what you end up making is a solid that is comprised of a mixture of crystal domains in an amorphous matrix. Hopefully this makes a little more sense after looking at this schematic:
The typical ice in your freezer is mostly amorphous with small crystalline or polycrystalline regions. Here's a microscopic view what it looks like:
The fibrous, snow-flake-looking things are crystalline domains. The rest of the ice (which just looks smooth and transparent) is amorphous.
What you want when freezing yeast is to have the cells trapped in a completely amorphous ice matrix. The formation of ice crystals causes yeast cells to lyse (their membranes are disrupted and the cells are destroyed). If we tried to freeze our yeast in pure water we would end up forming a lot of ice crystals and killing the yeast cells. This is where the glycerin comes into play. When a mixture of glycerin in water freezes, ice crystals cannot form. As the water molecules begin to orient, glycerin molecules get in the way and disrupt crystal formation. The result is ice that is almost completely amorphous, which is exactly what we want when freezing yeast.
Isopropyl alcohol
I honestly don't even know why I bothered to put a picture of isopropyl alcohol. I think everyone is familiar with it (if not as isopropyl alcohol than as rubbing alcohol - they are both the same thing). The reason I needed isopropyl alcohol to freeze the yeast is because the freezer I have is frost-free. Most modern freezers are frost-free. If you are unsure whether your freezer is frost-free there is a simple test you can do to determine this: Open up your freezer. Is there a layer of frost covering everything your freezer? If no - your freezer is frost-free. If yes - your freezer is not frost-free.
The reason a frost-free freezer is not ideal for storing yeast has to do with the mechanism by which the freezer gets rid of frost. A frost-free freezer will periodically warm up, which allows any frost that might have formed in it to dissipate. Food that you have stored in the freezer is not harmed by this process, but yeast won't survive these cycles of warming and cooling.
Where the isopropyl alcohol comes in handy is in keeping the yeast at approximately the same temperature during its time in the freezer. If you look back at the first photo in this post, you will notice that I filled the jar containing the yeast vials partially with liquid. That liquid is isopropyl alcohol (~70%). The isopropyl alcohol acts as a temperature reservoir that keeps the yeast frozen and shields it from the frost reduction cycles. Other liquids (or gel cold packs) would work too as long as they don't freeze at the ambient temperature of the freezer.
That's all you need to freeze the yeast. Now let's walk through the steps required to accomplish the job.
- Recover and wash yeast from bottom of fermenter. I am not going to go through the steps required to do this. If you look up washing yeast on the internet you will find plenty of guides on how to do this (maybe in the future I will write up a blog post on doing this myself).
- Pour off excess water so there is just enough for yeast to form a very dense slurry.
- Sanitize some water by boiling it for 15 minutes. Chill in refrigerator after sanitizing.
- Make solution of 20% glycerin in chilled sanitized water.
- Mix equal volumes yeast slurry and glycerin solution, pour into vials, and shake to mix thoroughly.
- Put vials into container that will hold isopropyl alcohol bath and fill with enough isopropyl alcohol to cover above level of yeast.
- Loosen caps on vials, and freeze (caps are loosened so pressure doesn't increase and break vials as water expands during freezing).
- Once completely frozen, tighten caps on vials.
- You are done!
Here's a photo of one of the vials I froze. I will write up another post in the future when I reactivate it to let you all know how that goes.
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