Agar Plates

June 29, 2016

agar plates

The following Agar Plate technique is based on the instructions and equipment available at Bootleg Biology. I recommend reading through their notes as well, but for the sake of chronicling the experiments at the Microobservatory, I’ll provide my own recipes and tips here.

If you’ve followed one of the techniques for collecting wild yeast, you will end up with a small jar or vial of bubbling starter wort. After a few days you should see some bubbling and hopefully a thick krausen. In 1-2 weeks you should see the krausen subside, indicating that the fermentation has run it’s course and the yeast have settled out at the bottom of the fermentation vessel. At this point you should smell the liquid—it smells good, great! If it smells bad, throw it out and start over! Most of the wild yeast I’ve collected ends up smelling lemony and tart, usually funky to one degree or another.

A note on safety: At this point, the fermentation will have made the wort more acidic and produced a tiny bit of alcohol. If any unwanted microbes made their way into the wort, they will probably have been killed or rendered inactive by the harmful conditions. We can increase the odds of eliminating unwanted microbes by batching up the wort into a larger volume of beer (essentially a second fermentation that adds more alcohol and further lowers the pH), or desired microbes can be isolated from the teeming ecosystem of your starter wort by using agar plates.

Create Agar Plate

Additional instructions at Bootleg Biology


  • Sanitized petri dishes
  • Saran wrap
  • Paper clips
  • Lighter or butane torch (or sanitizer)


  • 300 mL (10oz.) filtered water
  • 35g dried malt extract (DME)
  • 5g agar powder (sometimes listed as “Agar Agar Powder”)
  • Pinch of yeast extract (optional)


  1. Bring water to boil in a small saucepan and add DME, stirring to be sure that it doesn’t boil over.
  2. Boil for 15 minutes
  3. Remove from heat and stir in agar powder.
  4. Let mixture cool to below 100°F (about 15 minutes), keeping an eye on it to make sure it doesn’t set completely. The mixture should be thick but still pourable.
  5. Carefully pour mixture into petri dishes (I usually lay out a bunch of them on a baking sheet).
  6. Let the mixture cool until completely set, storing in a draft-free environment. You can cover them to avoid contamination as long as you make sure the cover itself is clean.
  7. Once set, put the lids onto each petri dish and tightly seal them with saran wrap.
  8. Store the dishes upside-down so that condensation will collect on the lid.
  9. Store prepared dishes in a dark draft-free location for one day. If mold appears in any of the dishes, you should throw out the contents.

Up Next: Streaking Plates!