Culturing and Freezing Yeast for Brewing

Yeast in Beer

The influence yeast has on a beer is unmistakable. Yeast does more to your brew than make alcohol from sugar. Beer yeast gives character to your beer because they impart flavor. A certain yeast strain will have it’s own blend of aromatics from earthy flavors to fruity notes.

So, when you come across a beer with just the right yeast character, why not culture this yeast for your own brews! I will tell you just how to do just that in this post.

To culture yeast from your favorite beer, the beer in question has to meet certain characteristics. There has to bee viable yeast in the beer itself so it should be unfiltered and unpasteurized. Bottle-conditioned is a good sign but watch out for beers that use a different yeast strain for bottling then they do for the actual brew (Hefe-Weizen comes to mind). It’s also highly possible to culture yeast from your own home-brews!

First off I will give you a list of things you will need to culture and freeze your own yeast strains.

  • A culture flask, Erlenmeyer or other to culture your yeast.
  • Growing medium for the yeast, like malt extract.
  • Some kind of yeast nutrient (not absolutely necessary but advised).
  • Glycerol (or glycerine) for freezing the yeast.
  • Sterile containers that can be frozen.
  • StarSan or some other sanitizing agent.
  • A stove, oven, fridge and freezer
  • Optional: Magnet stirring table

Culturing Yeast

To start your culture you will need a container to culture your yeast. I use a Borosilicate glass Erlenmeyer flask, but any glass container that can be sterilized is fine (although you should be able to cover it and let gas escape from the fermentation safely). I sterilize my Erlenmeyer by stuffing the neck with cotton-ball, covering it with tinfoil and putting it in a hot oven for about 20 minutes. Sterilizing Culture Flask.#homebrew #homebrewing #yeast #yeaststarter #brew #beer #craftbeer
Figure 1. Erlenmeyer Flask being sterilized in the oven.

For growing the yeast I make a solution of 10% malt extract and 1% yeast nutrient. Ghetto tip for yeast nutrient: baking-yeast, just boil the heck out of it along with the malt extract. For my cultures I made 300 ml water with 30 grams of malt extract and 3 grams of baking-yeast.

Put the ingredients in a pan and boil it down for about 10 minutes. Then pour the hot culturing medium into your flask (carefully). Let it cool down to room temperature.

Immediately after pouring your beer of choice, pitch the dregs into the culturing flask. Make sure the malt extract is cooled before you pitch the yeast. Your flask should now look somewhat like mine in Figure 2. Also if you are using a magnetic-stirrer now is the time to add the magnet to your flask (sanitized with StarSan). Culturing Yeast from Dregs. #brew #brewing #homebrew #homebrewing #beer #beergeek
Figure 2. Once the malt extract has been cooled, pitch the dregs of your beer and start the culture.

Now if you let your culture grow without a stirring plate, just give it a good swirl every now and then, you will notice the malt extract fermenting because it will form bubbles and foam. I usually let my culture go until there’s Krausen on the edges of the flask. In my experience it will take a day or two. After this I will put the flask in the fridge and cold crash the culture until the yeast has formed a sediment on the bottom of the flask, as shown in Figure 3. Cold Crashing Yeast Culture. #brewing #homebrewing #brew #homebrew #beer #yeast #beergeek
Figure 3. A sediment has been formed at the bottom of the flask. Note the Krausen in the flask.

Of course it’s entirely possible to use the culture you made as a starter for a brew directly. But the second part of this post is to share with you how I save yeast strains for future use so I’ll describe that below.

Freezing Yeast Culture

To freeze the yeast for future brews we need glycerol, or glycerine (which is the same) and sterile containers. The idea is that glycerol prevents water-crystals from forming with freezing. The water-crystals are responsible for destroying the yeast cells. So the glycerol protects the yeast from frost damage!

Now, when the culture has been left to settle for long enough, I will decant the fluid from the flask. Make sure to leave the sediment intact as much as possible. It should look somewhat like Figure 4. Yeast sediment is rich in yeast cells. #brewing #yeast #brew #homebrew #homebrewing #beergeek #craftbeer
Figure 4. After decanting the liquid from the flask, the sediment is left. This contains most of the yeast-cells.

Now measure out how much sediment you have (let’s say 30 ml for this example). You will want to dilute this 1:1 with a glycerine solution. The glycerine mix is 1:1 glycerine to water. So if you have 30 ml of yeast suspension you will need 30 ml of glycerine solution, that means measuring out 15 ml of glycerine and 15 ml of water.

Boil off the glycerine solution for a minute or so. Cover the solution and let cool to room temperature. Once the glycerine solution is cooled, mix it 1:1 with the yeast suspension. Make sure everything you do is with sanitized materials as any contamination will ultimately be in your yeast-starter later.

As soon as possible, get your glycerine – yeast mixture into the sterile containers. I’m using 10 ml sterile syringes available at pharmacies. I close the syringes by melting the nozzle at the stove and pinching it shut with a set of tweezers. #beer #yeast #brew #brewing #homebrew #homebrewing  

Now that you have your yeast in glycerine, put the containers in the freezer at -18 degrees Celcius for storage!

When you want to use the yeast again, get it from the freezer and pitch it into a starter culture like I described in the first part of the post!