Wild yeast spores naturally travel by air, but it is easier to collect them from a habitat they can grow, multiply, and thrive. The skins of fruit commonly contain the three requirements for this growth: warmth, moisture, and sugar. Local berries could be a great source of yeast, perhaps holding new and unexpected flavors right on the skin just waiting to be discovered. This guide is meant to provide an overview to the process of collecting and selecting a single strain for its potential use as a fermenter. Not every strain of wild yeast creates a desirable flavor profile, but when you find one that does it’s a uniquely exciting experience that can only be accomplished by giving it a try.
PART 1 – PREPARING STREAK PLATES
The agar streak plate is a powerful tool in a microbiologist’s arsenal. A thin gelatin nutrient layer is inoculated with a sample you’ve collected. Then, with a dragging motion of the inoculating loop the sample becomes spread across the plate. Each quadrant streaked should contain fewer cells than the one before it in a form of dilution. With this method (which will be explained in more detail in Part 2), single yeast cells can become deposited to grow their own pure, isolated colonies. One of these colonies can then be carefully selected and stepped up into a slurry.
Materials:
- Petri dishes with lids
- Dry Media (Powdered Agar)
- Erlenmeyer flask
- Aluminum foil
- Disinfectant wipes
- A Bunsen burner
- A hotplate
- Tongs or heat-resistant gloves
- An autoclave or a steam pressure cooker
Procedure:
Taking time to prepare your equipment and work area is very important. Contamination is the enemy, and the contaminants can’t be seen with your eyes. Using the disinfectant wipes, clean the area around your Bunsen burner. This will be your sterile field; nothing should be opened away from this field to avoid dust and the contaminants it carries. The open flame and the updraft created by its heat is the key here. Within a half-meter radius the risk of contamination by airborne particles is greatly reduced.
[figure 1: the sterile field approximated]
As important as a clean and sterile workspace is, it’s no use if the equipment or glassware isn’t sterile as well. Autoclaving is a popular option for Petri dishes, using the power of steam to quickly kill the bacteria and spores that would otherwise overtake the plates.
30 minutes of contact time with steam (121° C/250° F) is ideal. Don’t pack them in too tightly, and have a plan to retrieve them once the steam cycle is complete. Tongs or heat-resistant gloves should be sterile as well before touching the hot plates! Preventing contamination is important at every step.
[figure 2: Petri dishes in a steam cooker, ready to be autoclaved]
NOTE: If no autoclave is available, dry heat sterilization can be preformed in an oven 177° C (350° F) for 2 hours.
While the dishes are autoclaving, you can begin to boil the agar solution. Prepare the agar medium according to manufacturer instructions. In this case I am preparing enough Schwarz Differential Agar (SDA) for 5 plates of 20mL each by measuring 8.3 grams of the powdered agar into 100mL water in a clean sterile flask and capping it with aluminum foil. The solution must be mixed well by swirling, and then brought to a boil to completely dissolve. Keep swirling every few minutes while heating, otherwise some will burn to the bottom of the flask.
[figures 3 & 4: mixing and boiling the agar]
Once the Petri dishes are sterilized and the agar solution is boiled, the dishes drip dry while the agar is autoclaved for 10 minutes. Check manufacturer instructions, this step may not be required for all.
Set up the Petri dishes in your sterile field with lids on. Arrange them in a way that you can easily access each one. Flame the mouth of the flask and carefully pour a thin layer into each dish, removing only one lid at a time. Replace the lid immediately and once all are poured, leave them to solidify before moving forward. This can take several minutes depending on the temperature in the lab.
[figures 5 & 6: arranging the plates & poured plates cooling]
Once they have cooled enough, carefully turn them over without opening them. (This prevents condensation from gathering on the inside of the lid.)
[figure 7: the inverted plates]
After cooling to room temperature, the plates are now prepared. They can be streaked immediately or carefully stored for future use by refrigerating (still inverted) for up to several weeks in a clean, sterile, covered container. When you are ready to use one, simply remove it from the refrigerator and allow it to reach room temperature before streaking.
PART 2 – STREAKING THE PLATES – COMING SOON