Yeast Culturing
Culturing yeast at home takes more work than buying commercial yeast, but may be worth it if you are planning on preserving yeast cultured from a bottle of beer or a strain that isn’t offered year-round.
Necessary Materials
Slants and plates are commonly used in all yeast and bacteria culturing. A slant is your “mother” culture, which should remain pure due to its infrequent usage. A blank slant is made by pouring a hot, liquid agar solution into a test tube and letting it cool at an angle, creating a sloping surface within the tube. A plate is a petri dish in which molten agar has been poured and cooled. Agar is a gelatin-like material that is liquefied at temperatures over 107 °F (42 °C) and forms a gel under 99 °F (37 °C). Yeast will grow on this Jello-like surface, and provide a means of medium-term storage.
Once inoculated with yeast, the mother culture should be used to create a “working” culture. A plate serves as your working culture — which in turn provides the yeast for the starter culture that you pitch into a batch of beer. The plate actually serves two functions. It’s a working store of your yeast culture, and it offers you a look at the purity of your yeast.
To prolong the life of your plates and slants, store them in the fridge. A plate will last several months, stored wrapped in parafilm with the agar side up. Slants are the best possible medium-term storage. They should be re-cultured every 4 to 6 months before they start to mutate and die. Healthy yeast should be a creamy white color. If the colonies on your slants or plates start changing color, this is most likely because they are dying.
You can buy sterile, individually-packaged test tubes and petri dishes — and all the other materials mentioned in this article — from most scientific supply houses. Some even offer pre-poured slants and plates.
Inoculation loops can be disposable or metal. They are used for transferring bacteria from an agar surface to either another agar surface or liquid media. Disposable loops are packaged sterile, and are easy to work with. Metal inoculation loops need to be sanitized by flaming prior to any transfer. In a lab, the flame would come from a bunsen burner. At home, an alcohol burner — or even a butane lighter — will suffice. When culturing, your work area should be clean and wiped down with sanitizing solution.
Streaking a Plate
Streaking an agar plate is a quick and easy way to isolate yeast, check for purity and re-culture yeast from aging plates or slants. First you should flame your loop (unless using a sterile-packed loop) and get it red-hot. Next, immerse the loop in 70% ethanol or touch it to an unoccupied agar surface to cool it. Open the slant and pass the test tube opening through flame before dipping your inoculation loop into it. Dip the loop into a sample of yeast and simply touch a colony. This will transfer plenty of bacteria to the loop, even if you don’t see any material clinging to the metal. Re-flame the opening before closing the slant. Open the petri dish just enough so you can insert the loop. Touch the loop lightly to the agar surface on the plate and move it back and forth so you make a long, zig-zagging streak on the agar surface. This will inoculate the plate, although all you will be able to see is a very slight indention in the media. Replace the cover on the petri dish as soon as you finish. The microscopic bacteria transferred to the plate will grow into isolated colonies that are easily visible to the naked eye.
Grow the plate for two to three days at room temperature (70 °F/21 °C), agar side up (in other words, so the bacterial colonies are hanging upside down from the agar surface). Dense yeast will grow in the initial contact point, getting more diluted towards the end of the streaks. If isolated colonies are not obtained, a new plate can be re-streaked.
Making a Starter Culture for Brewing
Yeast has a creamy white color. This is typical of a healthy sample. If you have colonies that are other colors, or have a different morphology, these are likely contaminants.
Each dot on a plate is a colony or group of colonies of yeast, any of which can be selected to make a starter culture. A starter culture is the yeast that you’ll pitch into a batch of beer.
To make a starter culture, first make a sterile wort starter by boiling dried malt extract and water, adding it to a test tube and letting it cool. Then pick either a single colony from a plate or a loopful from a slant. It’s better to select single colonies, grown from single cells, as they are the purest form of yeast identifiable. To pick the colony, all you need to do is touch the agar on one side of the colony then swipe the loop through the colony; you do not need to gouge a hole in the surface of the agar to get enough bacteria. A light swipe over the surface is all it takes.
Once you have the yeast on a sterile loop, transfer it to the 10-mL wort starter, being careful to open containers for the minimal time required. Flame the opening of the test tube. Then insert the loop into the test tube and shake the tip of the loop in the liquid to transfer the yeast. Re-flame the opening and cap the tube tightly. Shake starter to aerate and to mix yeast into suspension. Then loosen the cap enough to allow oxygen to get in the tube. Leave upright in a warm place (70–80 °F/21–27 °C). Your 10-mL wort starter is now inoculated and will grow over a period of 24 to 48 hours. It won’t look like a normal fermentation, because very little action will be seen at the top. A white sediment will appear on the bottom, assuring you that growth has occurred. You should use this starter after approximately two days. It will keep if refrigerated, after growth has occurred, up to seven days. Warm it to room temperature before proceeding to the next step.
When you are ready to grow your starter, boil 400 mL of water with four tablespoons of dried malt extract and some dry yeast nutrient. Add mixture to your sanitized Erlenmeyer flask, or other glass container, and cap with aluminum foil. (If you use a one-liter Erlenmeyer flask and a large microwave oven, you can boil the wort directly in the flask.) Cool the wort and then add the 10-mL starter to it. Flame the tip of the test tube and flask before dumping out the yeast and quickly replace the foil cap on your 400 mL starter. Aerate the wort with sterile air or oxygen. If you have a stir plate, stir the yeast while it grows. If not, simply swirl the container a couple times a day. This culture can then be stepped up to a pitchable starter for 5 gallons (19 L) or more of beer. From this point on in your yeast propagation, it is best not to expand the culture volume by more than 10 times per step. In other words, the largest volume you could step up to from the 400 mL starter would be 4,000 mL (1.05 gallons).
Your biggest challenge in yeast propagation is to keep your cultures free of contamination. Working quickly and recapping tubes or propagation vessels as soon as possible will help. If you are serious about trying this, it would be worthwhile to take a microbiology class from your local university as all of these techniques are standard for handling any microorganism.
