How to Make Potato Dextrose Agar

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Potato dextrose agar (PDA) is a good general-purpose semisolid medium used for culturing fungi. It is often used to isolate, grow and maintain mushroom-forming species. Ready-made petri dishes or test tubes containing PDA can be purchased, or PDA can be quickly made using a commercial premix. However, it is not hard to make a simple and effective version from scratch at little cost.

Things You'll Need

  • 300 g peeled, chopped potatoes
  • Saucepan
  • Kitchen strainer or cheesecloth
  • pH test paper or pH meter
  • 1 molar hydrochloric acid
  • 1 molar sodium hydroxide
  • 4 500-mL glass Ehrlenmayer flasks (or other narrow-mouthed autoclavable containers)
  • 10 g dextrose (glucose) powder
  • 20 g agar
  • Aluminum foil
  • Pressure cooker
  • Sterile petri dishes

Preparing Potato Extract

  • Place potatoes in a saucepan with approximately 1.2 liters of tap water and boil on a stove for 1 hour.

  • Remove saucepan from heat and let liquid cool until it can be comfortably handled (about 10 minutes).

  • Pour liquid potato extract through a strainer or cheesecloth and collect it in another container.

Preparing the Medium

  • Add dextrose and agar to 1 liter of the potato extract water.

  • Test the pH of the solution with pH test paper or a pH meter.

  • If the pH is above 6.5, adjust it down by adding one drop of the hydrochloric acid solution at a time and retesting.

  • If the pH is below 6, adjust it up by adding one drop of the sodium hydroxide solution at a time and retesting.

Sterilization

  • Thoroughly mix the medium and divide it into smaller batches of 250 mL poured into the four 500-mL flasks (Note: each flask must be no more than half full to avoid boil over).

  • Cover each flask with a piece of loosely crimped aluminum foil, so the foil will not fall off, but does not make a tight seal.

  • Place the flasks in a pressure cooker.

  • Operate the pressure cooker in accordance with the manufacturer's instructions to sterilize the PDA at 121 degrees C (250 degrees F) at a pressure of 103,421 Pascals(15 PSI) for 15 minutes.

  • Shut off the pressure cooker and let it return to atmospheric pressure, then remove the flasks and let them cool until they can be handled.

Pouring the Medium

  • Line up the sterile petri dishes on a flat surface.

  • Once the flasks have cooled to handling temperature, but before the medium begins to solidify, grasp a flask with one hand and remove and set aside its foil lid with the other hand.

  • For each petri dish, lift the lid with one hand, quickly fill halfway with liquid PDA, and replace the lid.

  • Repeat Step 3 above until all petri dished are filled.

  • If there is any PDA left over, loosely recover the flask with its original foil lid, let it cool and solidify, tighten the foil lid and store in a refrigerator.

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  • Photo Credit Zedcor Wholly Owned/PhotoObjects.net/Getty Images
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