How to Modify a Pressure Canner for Mushroom Sterilization
Any wet and nutritious media (grain, substrate, agar, liquid culture) needs to be sterilized. We sterilize it to kill all the unwanted mold spores and bacteria that occupy every square inch of open air. In order to sterilize we put the medium into a container that runs under pressure. Either a pressure canner, pressure cooker or autoclave. They all do the same thing and that is heat up water until it creates steam, and hold that steam in to create pressure.
Pressure x time = sterilization.
Once the nutritious media is sterilized, only our mushroom culture will have access to the nutrients required to fruit mushrooms. However we still can't expose sterilized media to open air without inviting contamination until the substrate is fully colonized and ready to fruit. At that point the mycelium has claimed the nutrients and while mold and bacteria can still grow on it, it's effectively immune until it finishes fruiting.
Pressure canners are a great way to open up the ability to cultivate larger quantities for cheaper. Pressure cookers (like an instant pot) also work. They're the same thing. They're just typically smaller and they also run at a lower psi so they need to run longer. The benefit is how well they hold water and they don't need to be monitored or adjusted, just set and forget. Pressure cookers do not need to be modified.
A really commonly used pressure canner for mushroom cultivation is this Presto 23 Qt. I have 3 of them but I typically just use 2 at the moment for cultures and grain because I also have a 200L autoclave.
If you decide to get the Presto or if you have another pressure canner they typically come with a trivet. That's a circular metal piece that holds stuff above the bottom of the canner so that your stuff isn't touching the bottom which is directly touching heat. It also allows water to steam without your stuff being submerged.
The issue with pressure canners is that they use more water than pressure cookers (like instant pots) do so we need to raise the trivet higher, in order to allow more water so they can run for longer.
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Measure the size of the holes in your particular trivet and make sure the washer is large enough. Then just screw them in. Space them close enough together that putting pressure on one side doesn't tilt the trivet. I used 6 for mine.
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