Capturing and Maintaining Wild Yeast Cultures
Fungus Facts for Selecting a Culture Input
Yeasts can be found everywhere around you — maybe even in places you wouldn’t expect. What will you try as a culture input for the capture of wild yeast? Remember, there are no rights or wrongs. Some stuff will be easier to grow yeast from than others, but that doesn’t mean that you shouldn’t try. Because every living thing exists in symbiosis with other life forms, any culture input you choose will likely be associated with other microbial organisms beyond wild yeast. There are definitely culture inputs that will be enriched in wild yeast (and wild yeast will out compete anything else around). And there are cases where the opposite is true. Just go for it and see what happens!
Keep in mind:
- Yeast feast on sugar, so you may be able to find wild yeast from things that are sweet like fruits and flowers. In fact, fruits or flowers that are decomposing might bring with it yeast and a lactic acid bacteria populations — this might be helpful for co-culturing experiments.
- The natural reservoir of wild yeast is thought to be oak leaves, oak bark, oak leaf litter, and soil around oak trees. The rest of the yeast you find is likely the result of human- or animal-dispersal.
- While it feels easiest to work with food or plants, consider insects — especially pollinators!
- Think about the industry uses of yeast — what if you were to grab a sample from your local wine store during a tasting? Live near a brewery or bakery? See if you can find something there. Seriously, yeasts are everywhere!
- Consider that yeast levels fluctuate with the environment. Might you explore seasonal differences comparing yeasts from a specific plant (or other culture input)?
Setting Up a Wild Yeast Culture
The first step in setting up your cultures is to choose and collect the items you need to get things started. As a sustainability note, first see what you may already have around your house or classroom — there are a lot of different vessels and medias that could serve you well! Once you have figured out the materials you will use, the second step is making sure that your selected culturing vessels and workspace are sterile and clutter free. Here are other considerations:
Label and Note Keep!
First rule of science: establish some way to label and note keep. Use a sharpie or similar marker to label and date each container used, being sure to label the actual vessel and not only the lid (label the lid, too!). In fact, you should label and set everything up before you start creating your cultures. Trust me.
As for note keeping, ensure that you are writing down everything about your setup in your laboratory notebook. This should include details like the date, number of samples, description of culture inputs, media type and volume used in each jar, time cultures were set up, general observations like the temperature of the room being used to set up cultures. Feel free to add any other details that may be relevant. You can never have too many observations!
Choosing your culture containers
We have been using 4oz ball jars as culturing containers with great success, however, there are no single “right” containers for culturing. When choosing a vessel, aim for a) something that is glass; b) something that can be covered, but still allow for airflow (ensuring that the cover, too, is clean and sterile); and c) is the right size for your intended use.
Choosing your media
Finding the right culture media for each cell type is a science in and of itself. As such, we are finding that pasteurized apple juice — given its nutritional makeup — is a more than suitable medium for culturing wild yeast. However, apple juice is merely a suggestion! Feel free to experiment with other possible media types to establish some baseline assessments. Some people will take dried fruits and culture them right in a jar of water. There are so many possibilities!
Always set up a “blank.”
We want to be sure that any growth we see in our cultures is actually from our culture input, and not because our media is contaminated. A blank — a jar that has only media and no culture input — will help keep you abreast of the dependability of your selected media!
Plan!
Make a plan to check on your cultures every day to measure, make observations, and just make sure things are moving along.
JUST DO IT
Once you’ve made the decisions required to get to this point, simply drop your culture input into the jar containing media. Cover the jar, but make sure that you are still allowing for gas exchange.
As the culture begins to “bloom,” you will see the media become increasingly cloudy. There might also be some bubbles, and a sweetish smell. After a while (days and beyond), you may see the accumulation of microbial cells at the bottom of your jar (it will become disturbed if you gently swirl the jar). Voila! You’ve created a microbial culture that (hopefully) contains wild yeast!
Maintaining a Wild Yeast Culture
To maintain your cultures in the short-term you should leave your culturing vessels on a flat surface, such as a table or dresser that is a relatively undisturbed location. At least once per day, look closely at each vessel for signs of yeast growth, that the lid is not too tight (gas exchange is taking place!), and for signs of possible contamination of other microbes (such as stuff that is green and fuzzy). If you do see contamination, add a little bleach to the culture vessel, then dump it down the drain while running the water for a minute to two.
You can keep wild yeast cultures in the refrigerator for a really long time — on the order of months. To maintain your cultures for the long-term, simply secure the lid of the culturing vessel and pop it into the fridge. Maybe figuring out the shelf life of your captured yeast can be your experiment!
You can also use your culture to expand how many wild yeast cultures you have. The ultra-science-y terms “passage” or “split” are used in labs to describe when a bit of culture is used to “seed” fresh media, and expand how many cells are present. To split your cultures, simply set up freshly sterilized and labeled vessels with fresh media. Add a small amount of your existing culture to the jars, and that’s it!
Get More Pointers from our Summer Program Videos
Listen to some ideas about selecting and setting up wild yeast cultures. These are videos that were created to guide high school student researchers through out Wild Yeast research track during the 2020 Summer Science Research Program.