A common problem when sterilizing agar in a pressure cooker is boil over. This caused when the water outside the bottle cools off faster than the agar inside the bottle, which forces the agar to boil over through the filter, making a mess in your cooker. A way to remedy this is by adding lots of water to your pressure cooker so that the cool off rates will be similar. I usually place the bottle in the pressure cooker at an angle so that most of the agar will be at the same level as the surrounding water. Boil over rarely ever occurs if using this method.

Step 3: Cool down

Once the agar is sterilized, it will need to cool down sufficiently to be able to handle it, but be warm enough so that it remains in its liquid form. This usually takes at least a couple hours. This is a good time to prepare the area where you will pour your plates and washing up for sterile work.

Using agar requires an extremely high level of cleanliness and attention to potential contaminates. Any mold spore or other contaminant in the air, from your skin or fingernails or breath could ruin your project. If a contaminate land on your petri dish at any time, it will likely sprout into action, ruining your potential projects and cultures. Wearing hands and arms, wearing freshly laundered clothes, wearing nitrile gloves and even surgical masks can greatly increase your chances of success.

You will also need to use a “still air box” ( SAB for short) or even better- laminar flow hood. These tools will provide a clean area to pour your plates, and without using either of them, it is almost certain that your plates will contaminate. A SAB can be as simple as a clear tote with a lid, and holes cut for arms. Use alcohol to clean the inside of the SAB and make sure that everything that goes in an out of the both is clean.

If using a flow hood, turn it on a couple hours before intended use to scrub the air. Wipe the surface of the counter with alchohol and set up everything you think you will need.