Cultivating mushrooms is a challenging task. It’s an exceedingly delicate process that can take several weeks, and slow-growing species can take months to harvest. One detail you need to consider is whether you should sterilize or pasteurize your casing mix. Both are useful for keeping your casing—and by extension, your mushroom mycelium—safe and free of contaminants. So, which is better? Check out our short guide below on pasteurized vs sterilized. (New to casing? Start with the difference between substrate and casing.)
What Is Sterilization?
Sterilization is the process of killing all living organisms in a material. Sterilizing substrate in a pressure cooker or autoclave kills all microorganisms and spores in it. That's done at 15 PSI (about 250°F / 121°C), typically for 90 minutes to 2.5 hours depending on the volume and density of what you're sterilizing; boiling water or an oven can't reach that temperature. The trade-off is that sterilization kills off good and bad microbes alike, so a sterilized substrate has no defenses and must be handled cleanly afterward. That's why grain spawn is sterilized in bags with a filter patch, which lets the substrate breathe without introducing contaminants.
When Should You Sterilize?
You’ll want to sterilize when you prep media like liquid culture, agar, and grain spawn. They must be free of any contaminants to expand and create healthy cultures and spawn. You should also sterilize any substrate with heavy supplementation, since those extra nutrients feed contaminants as readily as they feed mycelium. This is when you should sterilize your casing mix rather than pasteurize it.
What Is Pasteurization?
Pasteurization is a gentler heat treatment, usually holding the material around 150–170°F, that knocks back most harmful microorganisms while leaving some beneficial ones alive. Those survivors help defend the substrate against competing bacteria and mold. Pasteurization won’t eliminate every competitor, but it reduces them enough for vigorous mycelium to take over. It's also easier for a beginner since it requires no pressure equipment.
When Should You Pasteurize?
You’ll want to pasteurize bulk substrates and casings that don’t have much supplementation. Coco coir and vermiculite casings, hardwood fuel pellets, straw, and sugar cane mulch are good candidates for this method. Keep in mind that the more supplementation your substrate has, the more likely contamination will develop after pasteurization, so heavily supplemented mixes call for sterilization instead. Our ready-to-use bulk substrates and manure-based casing mixes come pre-pasteurized, so you can skip this step entirely.
Overall, the choice isn't personal preference; it depends on the supplementation in your substrate. Neither method is better across the board. You have to decide what fits your situation. For more on casing layers, see our guide to mushroom casing soil, and if something goes wrong, our contamination identification playbook.