Sterilization is a essentially a process of killing everything that is alive in a certain container. It allows you to clean up your media, substrates and instruments from all possible contamination. One of the most common and frugal ways to sterilize something is by applying extremely hot temperatures. That can be achieved using a pressure cooker, which allows us to raise the temperature of steam inside to about 121° Celcius (250° Fahrenheit).

Supplies

  • Pressure Cooker with Trivet1 and a Pressure Gauge
  • Assorted Rocks (find on the ground outside)
  • Tap water
  • Material (what you want to sterilize)
  • Glass Jar / Bottle (with a lid the closes tightly)
  • Stove (Gas/Electric will work, Induction is not validated)
  • 2-3 hours of time

Procedure

  1. Make sure you Pressure Cooker is fully functional and safe to operate2

  2. Add Assorted Rocks to the bottom of the Pressure Cooker and put the Trivet on top of the rocks. Bottom of the Trivet should be 3-5cm above the bottom of the pot.

    This will protect your material from being exposed to direct heat when the Pressure Cooker is running. If you put glass jars/bottles to the bottom of the pressure cooker, they will explode from the heat. This step might be optional with an electric pressure cooker or a yoghurt maker but we don’t have any validation data.

  3. Add hot tap water to the bottom of the Pressure Cooker.

    Normally it’s about 15% of the total volume of the Pressure Cooker. You should consult the “HOW TO PRESSURE CAN FOODS” section of your Pressure Cooker manual to determine exact amount of water2. Theoretically, it is fine to use more water, althought it will make this protocol slower than it should be. Avoid using less water than required, you might not have enough water to fill the Pressure Cooker with hot water vapor.

  4. Place your Material (or tool, tools can be sterilized like this too!) in a Glass Jar or a Bottle.

    Make sure that the lids are NOT closed tight, otherwise the glass might break under pressure. If you are sterilizing substrate in bags, there is no need to put it in Jars, as bags will do just fine. Make sure the bags are made of polypropylene or other heat-resistant material that can sustain 121° Celcius (250° Fahrenheit)3

  5. Put your Glass Jars/Bottle on top of the Trivet.

    It is fine for Jars/Bottles to be partially submerged in water.

  6. Close the Pressure Cooker lid, and remove the pressure regulator (the vent pipe cover)

  7. Turn on the heat on the Stove. Wait for the water to boil and a steady stream of steam start to come out of the vent pipe.

  8. Set a timer for 15 minutes.

    Let the steam push the air out of the Pressure Cooker. To reach sterilization temperature of 121° Celcius (250° Fahrenheit) at 15 Pounds per Square Inch (PSI) or 105 kilopascals (kPa) of pressure most of the air inside the Pressure Cooker should be replaced by steam, so these 15 minutes will help enough air escape.

  9. After the timer is ended, close the vent pipe with the pressure regulator, and watch the Pressure Gauge closely as it rises.

  10. When the Pressure Gauge reads 15 PSI or 105 kPa, reduce the heat so that the pressure is steady.

    Anywhere between 15-20 PSI or 105-180 kPa is safe and good for sterilization. The trick is to find a Stove setting where the pressure stays stable. Make sure it doesn’t drop below 15 PSI/105 kPa.

  11. Start your sterilization timer. If pressure drops below 15 PSI/105 kPa, bring pressure back up and start the timer all over again.

    Depending on how big of a load you have, the amount of sterilization time will vary. Here are some examples:
    • Agar bottle and some dishes: 40 minutes
    • Jar with scalpels or innoculation loops: 40 minutes
    • Full load of substrate Jars/Bags: 120 minutes

  12. When the timer runs out, turn off the heat completely. Let the Pressure Cooker fully depressurize on its own.

    Wait until the Pressure Gauge goes back to 0, and you are able to open the lid. Do not force the release of pressure because that might damage the glass inside the Pressure Cooker, as well as the Cooker itself. It’s best to leave to vent pipe cap on during this process.

  13. Open the Pressure Cooker lid and remove contents. Warning: the contents are hot and sterile!

Validated Equipment

Pressure Cookers:

  • Presto 23qt Pressure Cooker (model no. 01781)

Glass Jars/Bottles:

  • Kerr® Wide-Mouth Mason Quart Jar
  • Kerr® Regular-Mouth Mason Quart Jar
  • Kerr® Wide-Mouth Mason Half-Pint Jar

Notes

  1. metal disk that is usually found at the bottom of any pressure cooker. ↩

  2. you can find the manual for your Pressure Cooker by googling numbers engraved into the lid or body of the cooker. ↩ ↩2

  3. link to where you can find how to identify plastic types around the world. ↩