This guide is for the preparation of 500ml’s of Agar, based on the recipes found here.
You will need the following…

  • Large Glass Bottle (500ml minimum)
  • Funnel
  • Aluminum Foil
  • Pressure Cooker (large enough to fit your bottle)

Step 1 : Prepare the mixture.

  1. Gather the ingredients from your chosen recipe.
  2. Using the funnel, add the dry ingredients into the glass bottle using the measurements listed on the recipe.
  3. Add 500ml of warm water to the bottle as listed in the recipe.
  4. Temporarily cap the the bottle in order to mix it.
  5. Shake the bottle until all ingredients are mixed thoroughly, and completely dissolved.
  6. Remove the cap of the bottle and re-cap with aluminum foil.

Step 2: Load the Pressure Cooker

  1. Place the riser plate in the bottom of the pressure cooker.
    If your pressure cooker does not have a riser plate, you will need to obtain or create one by improvising.
    The glass should not be in direct contact with the bottom of the pressure cooker. This fact is likely to be noted in your pressure cookers operating manual. Direct contact risks overheating and breaking of the glass.
  2. Place the bottle of agar mixture into the pressure cooker.
  3. Steady the bottle by surrounding it with mason jars.
    The mason jars should be empty, with ONLY THE RING portion of the cap screwed on to protect the edge of the glass.
    DO NOT FULLY CAP THE JARS. Capping the jars completely with air or water trapped inside, can cause them to explode.
  4. Pour 3 quarts of water into the pressure cooker.
  5. Make sure that the top of your agar bottle is at least few inches higher than water level in the pressure cooker.
    You do not want boiling water rising up above the top of the bottle and mixing into your agar bottle.

Step 3: Pressure Cooking / Sterilization at 15psi

Pressure cooker operating instructions may vary from model to model.
The instructions below are based on the use of a Presto 23 Quart Pressure Canner, Model #0178107.
Pressure Sterilization can be broken down into five stages.

  1. Initial Heating
  2. Venting
  3. Pressurization
  4. Sterilization
  5. Cooling

Each phase is important and care should be taken to make sure you respect the equipment you are working with, and understand what is happening at each phase.

  1. Initial Heating
    • With the pressure canner loaded and filled with water, place the lid on and twist until locked in place.
    • Place the canner on a level burner (see warning about 12,000 BTU gas ranges in operating manual.)
    • Heat on a high setting until a steady stream of steam can be seen, felt or heard escaping from the vent.
  2. Venting
    • Adjust heat if needed to maintain a steady, moderate flow of steam during venting.
    • Allow canner to vent steam for 10 minutes.
      Venting forces all the AIR out of the canner, replacing it with STEAM. Inadequate venting will leave air trapped in the canner which leads to uneven heat distribution and incorrect readings on the pressure gauge.
  3. Pressurization
    • At this time, place the weight over the vent allowing pressure to build.
    • Shortly after placing the weight on the vent, pressure will begin to build and the lid lock should pop up
    • Once the lid lock is up, within a few minutes, you should begin to see the pressure dial moving higher.
    • As the pressure builds, begin reducing heat slowly. The goal is to have the heat setting in a position that allows the canner to maintain 15psi for the duration of the sterilization period. It may take some time to figure out that position is, but the goal is to find the “sweet spot”, without large swing of rising and falling pressure.
    • Once 15psi is registered on the dial and the weight is rocking. Sterilization begins.
  4. Sterilization
    • Now that we have reached the sterilization stage, set a timer for 20 minutes.
    • Sterilize the contents of the canner for the full 20 minutes.
    • The pressure gauge should read 15psi, and the weight should be rocking for the entire duration of the sterilization process.
  5. Cooling
    • Turn off the heat source, and allow cooling to begin.
    • Cooling may take several hours. As it cools, the lid lock will eventually fall.
    • At this time, you may remove the lid from the pressure cooker to hasten the cooling process.
    • Agar must be poured while hot. So, keep a watchful eye and make sure remove the bottle from the canner and begin pouring your plates as soon as you are able to safely handle the bottle. If you have an infrared thermometer, the proper temp for pouring should be between 120-130°F (50-55°C).
    • You may find it useful to wear gloves when pouring to insulate yourself from the heat of the bottle. The temperature, while relatively safe for short durations of contact, will likely be very uncomfortable.

If you made it this far, congratulations. Your agar is ready to pour.
For more information on the procedures for pouring agar, click here.