Pasteurization vs. Sterilization — Which Do You Need for Mushrooms?

One of the first things new mushroom growers bump into is whether they need to pasteurize or sterilize their substrate. Both processes fight contamination, but they’re not the same — and using the right one saves time, money, and frustration.

What Pasteurization Is

Pasteurization = using moderate heat or chemicals to kill most, but not all, competing microbes while leaving some beneficial organisms. It’s quick, simple, and perfect for forgiving species like oysters.

Common pasteurization methods:

• Hot water bath: Submerge straw or similar materials at 160–170°F (71–76°C) for 1–2 hours.

• Cold fermentation: Soak straw or cardboard for 5–7 days; anaerobic bacteria naturally kill off competitors.

• Lime bath: Adding hydrated lime to water raises the pH and kills many contaminants — often used for straw or outdoor grows.

Pros:

• Less gear needed.

• Faster prep.

• Great for outdoor or large batches.

Cons:

• Not enough for picky or slower species.

• Still risk of contamination if done sloppy.

What Sterilization Is

Sterilization = using high heat and pressure to wipe out everything. This is what you do for grains, sawdust, and any nutrient-rich substrate that easily molds.

How to sterilize:

• Pressure cooker / autoclave: 15 PSI for 90+ minutes is common for grain spawn and sawdust blocks.

Pros:

• Extremely clean; ideal for gourmet species beyond oysters (lion’s mane, shiitake, chestnut, etc.).

• Gives a head start when contamination would otherwise explode.

Cons:

• Requires pressure cooker or autoclave.

• More time and fuel/power.

Quick Rule of Thumb

• Growing oysters on straw/cardboard? → Pasteurize.

• Working with grain spawn or nutrient-rich sawdust blocks? → Sterilize.

• Outdoor low-tech grows? → Pasteurize is usually fine.

Think of it like:

• Pasteurize = easy & cheap

• Sterilize = clean & controlled

Tools & Supplies I Recommend (Affiliate)

• Pressure cooker — reliable sterilization

• Food-safe bucket — for pasteurizing straw

• Hydrated lime — if you want to try lime pasteurization

• Mycology lids

Affiliate disclosure: I may earn a small commission if you buy through these links, at no extra cost to you.

Want to Go Deeper?

If you’re just starting and want to master substrates, grain spawn, and culture work, check out my Mushroom Substrate & Culture Guide. It’s written for beginners but covers the techniques I use every day.

FAQ: Pasteurization & Sterilization for Mushroom Growing

Do I need to pasteurize coffee grounds?
Coffee grounds are already pasteurized when brewed, but they can pick up molds quickly. If you’re mixing them with other materials, heat them again or mix into a pasteurized substrate for safety.

Can I sterilize without a pressure cooker?
Not really — you can boil or steam but it won’t reach the 15 PSI needed for true sterilization. Without a pressure cooker, stick to pasteurization-friendly species like oysters.

Is lime pasteurization safe for beginners?
Yes, if you can measure pH and use hydrated lime carefully (wear gloves). It’s popular because it doesn’t require heating but does need attention to pH levels.

Can I combine methods?
Some growers pasteurize bulk substrate and sterilize grain spawn separately — that’s very common.

Final Thoughts

Don’t overcomplicate it — pasteurization works great for simple, hardy species like oysters. When you step up to richer substrates or more sensitive mushrooms, sterilization is worth the extra effort.