Preparing Agar Media

The follow example details the process of making 1 litre of Potato Dextrose Agar (PDA) as this is the most accessible method. You will need to adjust the quantities depending on the size of the petri dish you are using 700ml is enough to enough to pour 20 x 80mm petri dishes.

Potato Dextrose Agar (PDA) 1 litre

  • 300g – 400g potato
  • 20g Agar agar
  • 10g dextrose powder
  • 2g yeast (optional)
  • .5g Gupsum (optional)
  • The broth should be about 1litre in volume once boiled and strained


  • Potatoes (Organic)
  • Agar agar
  • 2-3l water
  • Glucose Dextrose Powder
  • Yeast
  • Gypsum
  • cotton wool or polyester filling
  • Tinfoil


  • erlenmeyer flask / narrow, long neck bottle
  • pressure cooker
  • alcohol solution in spray bottle
  • infra red thermometer
  • 20 x 80mm petri dishes (just this example, you could us other sizes obviously)
  • latex gloves
  • face mask
  • tyvex sleeves or hazmat suit
  • Funnel
  • electronic scales
  • sieve


1. Cut potatoes into chunks or shred, add to pot with 2l of water this will be enough to make 1litre as it reduces; bring to boil and simmer for one hour.

2. Now prepare your flask/bottle. Place the flask onto the scales and using the funnel weigh out the ingredients.

3. When the potato broth is ready place a sieve onto the funnel and pour the strianed liquid into the flask.

4. Use cotton wool or polyfill as a bung in the top of the flask, lightly screw on the cap (if applicable) then cover the top with tinfoil; Place the flask ino the pressure cooker; once at 15 psi cook for 20 minutes.

5. Once finished Allow the pressure to return to to 0 psi and then move the pressure cooker to you sterile work environment.

6. Ensure you have eliminated/reduce all the vectors of contamination, open the pressure cooker and remove the flask with a folded paper towel (it’s hot!).

7. At this point you will either be using a still air box or working with a HEPA fFilter. Place the flask into the area where you will be working. Check temperature with infra red thermometer when it drops to 40c it’s time to pour. You can occasionally aggitate the mixture to encourage an even cooling of the mixture.

8. Your petri dishes will be ready in a stack. If using glass dishes you will have sterilsed these in the pressure cooker along with eth flask of agar. If plastic they will be in a sealed bag pre sterilsed.

9. Now you are ready to pour, carefully lift the lid from the petri dish at the bottom of the stack. Make sure your fingers just touch the top of the sides of the lid. Pour the Agar, replace the lid , lift up the lid of next petri dishes second from the bottom and so on until all dishes are filled. You Just need enough to fill half of the dish.


10. Wait until the dishes have cooled below 20c and then there a safe use. Or seal with para film (or micropore tape) and refrigerate for future use.

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