DIYeast: Creating Plates

Creating Plates - Pouring Agar (icon)

Creating Agar Plates

So you’ve acquired some wild yeast and bugs from your backyard, fruit or barrel room. Maybe you’ve brewed with it and now have your own unique house culture. That’s awesome. You are a rock star.

But what do you really have? Is it wild yeast? Is it something funky, like Brett? Is it all of the above? What’s causing that sour fermentation, if at all? Mixed wild cultures are wonderful, and are as close as we’ll ever get to replicating ancient fermentation practices.

It’s not until you isolate individual pure strains that you’ll be able to get some kind of control over your microbes and be able to accurately reproduce similar batches. You’ll also learn what makes each strain tick, and what beer styles and ingredients they meld with best. Plus, once you isolate your own yeast strain, everyone will think you’re a whiz kid.

Before proceeding, checkout Bootleg Biology’s Backyard Yeast Wrangling Tool Kit. Everything the experimental homebrewer needs to capture wild bugs, create agar plates, and isolate wild yeast.



Agar is a gelatinous-like substance made from seaweed that when boiled with wort causes the mixture to set quickly into a solid, jelly-like medium, that individual microbe colonies can grow and feed on.

Creating a Simple Agar Plate:3

  • 300 mL of 1.040 (or lower) wort (If using extract, boil water and DME mixture before adding other recipe ingredients)
  • 5 grams of agar agar powder (Add more if liquid fails to set)
  • Dab of yeast nutrient (other sources of Amino Acid can be added due to their benefit to yeast metabolism, but aren’t required)
  • Of course, you can also skip the recipe and grab Bootleg Biology’s Wild Yeast Agar Blend
  1. Heat wort in a small pot up to near boiling and stir in agar.
  2. Bring to a boil. Agar and wort won’t successfully bind if not brought up to a boil.
  3. Let cool for several minutes until temperature is in the low-100’s Fahrenheit or approximately 5-10 minutes. Watch your pot closely, since agar can set very quickly; the medium still needs to be a thick liquid when poured into the petri dish.
  4. Creating Plates - WrapPour a small layer of liquid on to the petri dish and move to your draft-free storage area. It’s important to let the liquid cool before covering with petri dish lid to prevent condensation from forming on the lid, which helps reduce the likelihood of mold growth.
  5. If a good agar/wort ratio was used, the agar plates should set within a few minutes. If you find your plates aren’t setting, you may need to reboil your mixture with more agar added.
  6. Wrap the edges of the plate lid with stretchable plastic wrap or stretched 1/2 inch strips of Parafilm. This will help reduce agar plate contaminants and prevent the plate from drying out if you don’t use the plate shortly afterward.
  7. Leave your plates upside down (prevents condensation from falling on your agar medium) for a couple days in a warm, dark place to incubate. This will help determine which plates are contaminated with mold spores or other uninvited guests, making them unusable2. When you’re done, store upside down in a resealable plastic bag and refrigerate.

Remember, you’re not working in a clean room with a hood, you will get contaminated plates. So don’t be sad when some of the plates you’ve created look like this after incubation:

  Now that you’ve made your own agar plate, the last thing to do is to Isolate Your Yeast.

1. Agar can be cheaply procured from some international grocery stores, through Amazon or the good folks at Bootleg Biology. Many people who work in labs actually make agar plates by just boiling potatoes.
2. The only way to truly prevent other bugs and mold from growing on your agar plates is by using some form of radiation, like placing them under a UV light prior to incubation. You will likely lose quite a few plates without the light, so it may be better to use reusable glass petri dishes. I was able to purchase a cheap light fixture from Home Depot and an inexpensive antimicrobial UV light from If you do go the UV light route, don’t be a dummy and burn your eye balls, always use protective eye-wear and clothing when working with UV lights.
3. This recipe is about as simple as it can get using beer wort. If you want to experiment with different recipes/mediums to help you select for different types of microbes, I’d recommend visiting BKYeast’s blog.