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How to Melt Agar

How to melt agar being demonstrated in a hot water bath.

Mushyluv’s guide to our 3 favorite ways to melt agar.

Hot Water Bath:Using hot water bath to melt agar. Agar bottle sitting in 2 inches of water.

  • Fill a pot with 1-2″ of water.
  • Heat the water until it is gently boiling.
  • Loosen the cap on your agar bottle and place it into the hot water bath.
  • Monitor the bottle’s progress until it has completely melted.
  • Do a swirl test to see if you’re lump free.

Swirl Test

Using protective gloves, tighten the cap and swirl the bottom of the bottle to check for lumps. If your agar needs additional melting time, loosen the cap and heat for 5-10 additional minutes.  Repeat until completely melted.

Mushy IconWhen your bottle comes out of the microwave, it will be around 200°f.  Cool to 120°f or below before it’s safe to pour agar plates.

Microwave Melting Agar:

  • Loosen the cap to your bottle (very important) Never microwave a sealed container with liquid in it.
  • Place your agar container on a plate and microwave 6 minutes at power level 3.
  • Watch for boiling  to occur.
  • Verify the agar’s heated evenly with a swirl test.

Wearing hand protection, tighten the cap and swirl to check for lumps. If your agar requires additional de-lumping, loosen the cap and microwave for  1 minute at medium power.  Repeat until everything is completely melted.

Mushy IconWhen your bottle comes out of the microwave, it will be around 200°f.  Cool to 120°f or below before pouring agar plates.

Autoclave/Pressure Cooker:

Loosen the cap on the agar bottle and pressure cook the bottle at 15 psi for five minutes. While wearing protective gloves, carefully remove the hot bottle and cool the molten agar to 120°f before pouring plates or slants.

At what temperature does agar liquify?

Agar liquifies around 104°f.  To keep agar in a liquid state for an extended period of time, setup a water bath by filling a pan with an 1″-2″ of water and set the burner temp to the lowest setting (Lo).


You don’t have to use the entire pre-sterilized agar bottle at one time.   Pour as many plates as you need. When finished, make sure the cap is on tight to prevent contamination and drying between uses.  The sides of your bottle may draw in as it cools.  Re-heat as many times as needed. Bottled sterilized agar can be stored at room temperature for months. Never put agar in the freezer as It will cause the agar to breakdown and become unusable. To prevent contamination, keep all bottles and petri dishes sealed until ready to use.

bottle of pre-sterilized-agar.


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