All microbiology textbooks state that wet heat sterilization requires a temperature of 250°F for 20 minutes. Therefore, before inoculation, supplemented sawdust fruiting blocks and mushroom grain spawn must be sterilized entirely. This requires exposing the substrate or grain to high pressure and temperature for a long duration to ensure the core temperature is reached and proper sterilization is achieved.
The best method is to use a stovetop pressure canner large enough to contain the material and produce 15 PSI. The Presto 23L is the go-to for most of us. Sterilization time for supplemented sawdust fruiting blocks should be 2.5 hours, although grain spawn can typically be sterilized in 120 minutes. I prefer to sterilize my grain jars for 120 minutes and bags for 180 minutes.
Steam Temperature at Pressure Guide
5 PSI 108°C or 226°F
10 PSI 115°C or 239°F
12 PSI 117°C or 242°F
15 PSI 121°C or 250°F
Substrate Sterilization
When sterilizing substrates, grains, and agar mixtures, search for a pressure cooker that can hold a pressure of 15 PSI, which allows for a steam temperature of 250°F. Hot enough to eliminate any competing mold or bacteria, giving the desired mushroom species the best chance of survival.
Can I use lower pressure?
Is 12 PSI sufficient?
Is it possible to get away with 5 PSI?
If the pressure is lower, complete sterilization can still be achieved by increasing the sterilization time.
Suggested Sterilization Times @ 15 PSI
- Liquid Agar 30 minutes
- Quart Size Grain Jars 120 minutes
- 5lb Grain Bags 180 minutes
- 5lb Sawdust Block 180 minutes
Hope this helps. Leave a comment and let us know what works for you!