How to Make Potato Dextrose Agar

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.

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Preparing Potato Extract

Step 1

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

Step 2

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

Step 3

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

Preparing the Medium

Step 1

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

Step 2

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

Step 3

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

Step 4

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

Sterilization

Step 1

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).

Step 2

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.

Step 3

Place the flasks in a pressure cooker.

Step 4

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.

Step 5

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

Step 1

Line up the sterile petri dishes on a flat surface.

Step 2

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.

Step 3

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

Step 4

Repeat Step 3 above until all petri dished are filled.

Step 5

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.