Agar is obtained from certain species of red algae and is commercially available in a powdered form. Its value in culture systems is:
- Ability to melt when heated
- Change to a semisolid gel at room temperatures, and
- Relative biological inertness.
Most agars are prewashed or purified USP grade.
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.
Food coloring may be added to the media to color the agar. This allows the propagator to color-code the media for different uses.