Agar is obtained from certain species of red algae and is commercially available in a powdered form. Its value in culture systems is:

  1. Ability to melt when heated
  2. Change to a semisolid gel at room temperatures, and
  3. Relative biological inertness.

Most agars are prewashed or purified USP grade.

Photo showing agar growing medium in a test tube.

Agar quality may vary among commercial producers. Agar performance may be affected by concentration and by pH. Usually a pH of 5.0 to 6.0 is obtained, but the pH of the medium may need to be adjusted through the addition of acids or bases.

Gelrite is a polysaccharide derived from a fungus that has similar properties to agar and can be used as a substitute.

Photo pointing out example of gelrite growing medium.

Food coloring may be added to the media to color the agar. This allows the propagator to color-code the media for different uses.

Photo showing an example of agar dyed red.