Pressure cooker (sterilization)

Here is a DIY approach to sterilization of media, equipment, etc that can be done at the BioFabLab. This approach is simpler and usually faster than using the large autoclave machines in RUC's laboratories (also less scary if you're just starting off).

NOTE: Don't start the timing before the pressure have build up inside the pot, and steam starts coming out.


There are two types of pressure cookers in the BioFabLab: the smaller kitchen variety used for most of the video, and the larger Hawkins Big-Boy models. Specific information on the latter is given in the last section of the video, as well as in this manual (which can be found online or in the Kitchen Lab).

In general the pressure cooker sterilization procedure is as follows:

1. If you are sterilizing a sample in a container (eg: agar) make sure it doesn't fill the container by more than two-thirds. Put the lid on the jars, but don't close the lid has to be completely loose, if not the glass jar will explode. You can also cover jars and glassware with aluminium foil and keep it down with a piece of masking or autoclave tape.

2. Fill the bottom of the pressure cooker with water to the depth of 1-2cm (not enough to start covering the base of the sample). This should be de-ionised water, which can be gotten from the BioFabLab Laboratory, in the green taps that doesn't have hot/cold symbols. Place a separator in the bottom of the pressure cooker so that the sample sits just on top of the water layer and is not directly touching the bottom of the cooker. In the video, we just used a spoon.

3. Seal the pressure cooker and place it on a hot plate at high heat. Make sure the pressure valve is completely open.

4. When steam begins to come out of the pressure valve opening, close it fully and turn the heat down to medium (approx. half the max). When pressure have build up inside the pot, and steam leaves through the pressure valve you can start the timer. The required sterilization time is usually 20-30 min.

5. When the time is up, switch off the hot plate and remove the pressure cooker so it can cool down slowly for approx. 10-15min.
   Then, gradually open the pressure valve until steam has stopped coming out. With the Hawkins Big-Boy, this can be done by lifting the pressure cap slightly using a fork. If you release the steam to quickly, the sample will boil-explosively and spill the samples all over, and in worse case explode the glass containers.

6. Open the pressure cooker, close the lids of the samples. Careful, it will be hot.