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How to Prepare Agar

Agar is a component of the cell walls of several red algae species. Once dissolved in boiling water and cooled, it looks like gelatin. However, it is firmer and stronger than gelatin and bacteria won't eat it. Although agar is mainly used as a culture medium for bacteria and other microorganisms, it is also used as a thickening agent for soups, sauces, jellies, ice cream and cosmetics. Preparing agar for a Petri dish is done in a few simple steps.

Things You'll Need

Bottled agar
Bunsen burner
Petri dish

Instructions

- 1
  Place the agar bottle in a container of hot water until the agar becomes liquid. Loosen, but do not remove the bottle cap.
- 2
  Hold the cap firmly against the bottle top and gently shake the bottle a couple of times.
- 3
  Remove the cap from the liquid agar and pass the neck of the agar bottle through the Bunsen burner flame to sterilize it. Set the cap aside temporarily.
- 4
  Open the Petri dish lid very slightly. Hold the dish at an angle, keeping the Petri lid directly over the dish.
- 5
  Pour 1/8-inch melted agar into the bottom of the sterile, plastic Petri dish, following the liquid agar manufacturer's instructions. Cover the Petri dish immediately. Gently swirl the plate to spread the liquid agar on the surface.
- 6
  Set the Petri dish, or agar plate, in a safe, room temperature spot for the agar to cool and set. The agar medium is cooled when it looks like stiff gelatin.
- 7
  Pass the neck of the bottle of prepared agar through a flame again. Secure the cap to the bottle top.

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