Sterilization and pasteurization are the two main ways growers prepare materials before introducing mycelium. Sterilization aims to kill everything, including heat-resistant bacterial spores. Pasteurization only reduces the population of competing organisms, leaving some harmless microbes behind.

Choosing the wrong one is one of the most common reasons new growers see contamination. The good news is that the rule of thumb is simple: the more nutritious the material, the more thoroughly it needs to be treated.

What Is Sterilization?

Sterilization uses high heat under pressure to destroy all living organisms in a material, including bacterial endospores that can survive boiling.

• Typically done in a pressure cooker or autoclave at 15 PSI, which brings the temperature to about 250°F (121°C)
• Grain usually needs around 90 minutes or more at pressure, depending on volume and container size
• Larger loads and denser materials need longer cycles so heat reaches the center
• Sterilized materials must be handled with clean technique afterward, since nothing is left to compete with contaminants

Boiling alone tops out around 212°F (100°C) at sea level, which isn’t hot enough to reliably kill heat-resistant spores. That’s why pressure is the key part of the process.

What Is Pasteurization?

Pasteurization uses lower, controlled heat to knock back most competing organisms without wiping out everything. The microbes that survive can actually help by occupying space that molds would otherwise take.

• Usually done at roughly 140 to 170°F (60 to 77°C) for 1 to 2 hours
• Can be done in a large pot, an insulated cooler with hot water, or a steam setup
• Some growers use cold-water methods such as hydrated lime for straw
• Pasteurized materials stay workable for a shorter window, so use them soon after they cool

Which Materials Need Which?

The deciding factor is how much food the material offers to competitors.

Sterilize nutrient-rich materials

• Grain such as rye, wheat, millet, or sorghum
• Supplemented hardwood sawdust (for example, with bran or soy hulls)
• Agar and liquid culture media

Pasteurize low-nutrient bulk materials

• CVG (coco coir, vermiculite, and gypsum)
• Straw
• Many unsupplemented bulk substrates

Nutrient-rich materials are a feast for bacteria and molds, so anything left alive will take over quickly. Low-nutrient bulk substrates are less attractive to contaminants, so pasteurization is usually enough and avoids over-processing the material.

Why Not Just Sterilize Everything?

It sounds safer, but sterilizing low-nutrient bulk substrates has drawbacks:

• It removes the harmless microbes that help defend pasteurized substrate
• A sterile bulk substrate with no competition can be colonized by whatever lands on it first
• Large volumes are difficult to sterilize evenly in home equipment
• It takes more time and energy without improving results for most bulk materials

Matching the method to the material is more effective than defaulting to the most aggressive option.

Signs Your Treatment Didn’t Work

• Sour, fermented, or rotten smells after the material cools
• Wet, slimy, or discolored patches before mycelium has had time to grow
• Green, black, pink, or orange growth
• Grain that’s split, mushy, or sitting in excess water

When in doubt, it’s better to discard a questionable batch than to risk spreading contamination through your workspace. Note what happened in your records so you can adjust time, temperature, or moisture next time.

Tips for Better Results Either Way

• Get moisture right first; overly wet materials are harder to treat and easier to contaminate
• Use a thermometer rather than guessing pasteurization temperatures
• Let materials cool fully before use, and keep them covered while they cool
• Work cleanly afterward, wiping surfaces and tools with 70% isopropyl alcohol
• Track times, temperatures, and outcomes for every batch

Conclusion

Sterilization and pasteurization aren’t interchangeable. Each one is the right tool for a different kind of material.

Sterilize nutrient-rich materials like grain and supplemented sawdust under pressure, and pasteurize low-nutrient bulk substrates like CVG and straw at controlled temperatures.

Match the method to the material, work cleanly afterward, and keep good notes, and you’ll see fewer surprises in every batch.

FAQ

Q: What is the difference between sterilization and pasteurization?
A: Sterilization kills all organisms, including heat-resistant spores, using high heat under pressure. Pasteurization uses lower heat to reduce competing organisms while leaving some harmless microbes behind.

Q: Do I need a pressure cooker to sterilize grain?
A: Yes. Boiling doesn’t get hot enough to reliably kill bacterial endospores. A pressure cooker at 15 PSI reaches about 250°F (121°C).

Q: What temperature is used for pasteurization?
A: Most growers hold bulk substrate at roughly 140 to 170°F (60 to 77°C) for 1 to 2 hours.

Q: Should CVG be sterilized or pasteurized?
A: CVG is a low-nutrient bulk substrate, so pasteurization is the usual choice.

Q: Why did my sterilized grain still contaminate?
A: Common causes include grain that was too wet, a cycle that was too short for the load size, or contamination introduced after sterilization during handling.