Sterilisation & Pasteurisation

The complete beginner's guide to killing contaminants before they kill your grow. Pressure cooker sterilisation and bucket pasteurisation explained from scratch.

Why This Step Matters

Mushroom mycelium grows slowly compared to mould and bacteria. If you put your mushroom culture onto grain or substrate that has not been properly cleaned of competing organisms, those competitors will take over before the mycelium gets a chance. Sterilisation and pasteurisation are the two methods we use to give our mushrooms a head start by removing the competition.

Sterilisation vs Pasteurisation — What Is the Difference?

These two words sound similar, but they do very different things. Understanding the difference will help you know which method to use and why.

Sterilisation

Sterilisation kills everything — every single living organism, including bacteria, mould spores, and even heat-resistant endospores. After sterilisation, your material is completely lifeless and sterile. This is necessary for nutrient-rich materials like grain, agar, liquid culture, and supplemented substrates (such as masters mix), because these materials are so nutritious that any surviving organism will rapidly multiply and ruin your grow.

Sterilisation requires a pressure cooker that can reach 15 PSI (pounds per square inch). At 15 PSI, the temperature inside the cooker reaches approximately 121°C (250°F) — hot enough to destroy even the toughest bacterial endospores that would survive normal boiling.

Pasteurisation

Pasteurisation kills most harmful organisms but deliberately leaves some beneficial ones alive. This is used for bulk substrates like CVG (coco coir, vermiculite, gypsum) and straw. These materials are low in nutrients, so they do not attract aggressive contaminants the way grain does. The surviving beneficial organisms actually help protect the substrate from contamination by occupying ecological niches that harmful moulds would otherwise fill.

Pasteurisation only requires hot water — no pressure cooker needed. You simply pour boiling water over your substrate (bucket tek) or soak it in hot water at 65–80°C (150–175°F).

Quick Comparison

Equipment for Pressure Cooker Sterilisation

If you are completely new to pressure cookers, this section explains everything you need and what it all means.

The Pressure Cooker

A pressure cooker is a sealed pot that traps steam to build up pressure inside. As pressure increases, the boiling point of water rises above 100°C, which is what allows us to reach the 121°C needed for sterilisation.

What Does PSI Mean?

PSI stands for "pounds per square inch" — it is a unit of pressure. Think of it this way: at normal atmospheric pressure (0 PSI gauge), water boils at 100°C. At 15 PSI of extra pressure above atmospheric, water boils at 121°C instead. That extra 21 degrees is what kills the tough endospores that survive normal boiling. You need a pressure cooker that can reach and maintain 15 PSI.

Recommended Pressure Cookers

Other Equipment You Will Need

• Mason jars (wide-mouth quart jars) — for grain spawn. Wide-mouth jars are easier to fill, shake, and empty than regular-mouth. Ball or Kerr brands are standard. You will also use half-pint or pint jars for liquid culture.
• Aluminium foil — used to cover jar lids during sterilisation. This prevents condensation and water from dripping into the jars through the filter or modified lid.
• Trivet or canning rack — a metal rack that sits in the bottom of the pressure cooker to keep jars elevated above the water. Jars sitting directly on the bottom of the pot can crack from direct heat contact. Most pressure cookers include one. If yours does not, use extra mason jar lid bands or a folded dish towel on the bottom.
• Modified jar lids — lids with a gas exchange hole (covered with micropore tape or a filter patch) that allow the contents to breathe during sterilisation and colonisation. See the Grain Spawn or Liquid Culture guides for lid modification instructions.

Step-by-Step: Pressure Cooker Sterilisation

This is the core technique you will use for grain jars, liquid culture, agar, and any supplemented substrate. Follow these steps exactly.

Before You Start

Step 1: Add Water to the Pressure Cooker

Pour 2–3 inches (5–7 cm) of water into the bottom of the pressure cooker. This water generates the steam that creates the pressure. Too little water and the pot can boil dry (dangerous). Too much and it can boil up into your jars.

Step 2: Place the Trivet in the Bottom

Set your trivet, canning rack, or improvised spacer in the bottom of the pot, sitting in the water. The trivet keeps your jars above the water line and prevents direct heat contact with the bottom of the pot, which can cause thermal shock and crack the glass.

Step 3: Load Your Jars

Place your prepared jars on the trivet. Each jar should have:

• Its contents already inside (grain, LC medium, agar medium, etc.)
• The lid placed on loosely — do NOT tighten the lid bands. Finger-tight at most, then back off a quarter turn. If the lid is tight, air inside the jar cannot escape during heating, and the pressure difference can crack the jar or blow the lid off.
• A piece of aluminium foil covering the top of the jar, crimped loosely around the sides. This keeps water and condensation from dripping into the jar through the lid.

Step 4: Close the Lid and Build Pressure

1. Close and lock the pressure cooker lid according to your model's instructions.
2. If your model has a pressure regulator weight (like the Presto), leave it OFF the vent pipe for now. If your model has a valve, make sure it is set to the "vent" or "open" position.
3. Turn the heat to high.
4. Wait for steam to begin venting from the vent pipe. You will see a steady stream of steam shooting out. Let it vent for 10 minutes. This step purges cold air from inside the cooker — air pockets prevent proper sterilisation.
5. After 10 minutes of venting, place the pressure regulator weight on the vent pipe (Presto) or close the valve.
6. The pressure will begin to build. Watch the pressure gauge (if your model has one) or listen for the weight to start rocking.

Step 5: Start Your Timer When Full Pressure Is Reached

This is the most important timing detail in the entire process:

Once you are at full pressure, reduce the heat to the lowest setting that maintains pressure. You want the weight to rock gently and slowly — not violently. Violent rocking means the heat is too high, which wastes water and can cause jars to bounce around.

Sterilisation Times

Step 6: Turn Off Heat and Let Pressure Drop Naturally

When your timer goes off:

1. Turn off the heat. Remove the pot from the burner if using an electric stove (electric elements retain heat).
2. Leave the pressure cooker completely alone. Do not touch the weight. Do not open the valve. Do not move it. Just let it sit.
3. Wait for the pressure to drop to zero on its own. This takes 30–60 minutes depending on the model and how full the cooker is.

Step 7: Wait After Pressure Reaches Zero

Even after the pressure gauge reads zero (or the weight stops rocking), the jars inside are still extremely hot. Wait an additional 30 minutes before opening the lid. This allows the jars to cool gradually and reduces the risk of thermal shock (cracking from sudden temperature change when cold air hits the hot glass).

Step 8: Remove Jars and Tighten Lids

1. Open the pressure cooker lid carefully — tilt it away from you so any residual steam escapes away from your face.
2. Using oven mitts or jar lifters (the jars are still very hot), remove each jar.
3. Immediately tighten the lid bands. As the jars cool, they will create a vacuum seal that keeps the contents sterile. You want the lids snug before they cool.
4. Set the jars on a towel or wire rack to cool to room temperature. Do not remove the foil until you are ready to inoculate.
5. Let the jars cool completely (usually overnight or at least 8–12 hours) before inoculating. Injecting spores or LC into a warm jar will kill the living culture.

Step-by-Step: Bucket Tek Pasteurisation (for CVG)

This is the simplest preparation method in mushroom growing. You are making CVG substrate (coco coir, vermiculite, gypsum) by pouring boiling water over the dry ingredients in a bucket. No pressure cooker needed.

1. Boil your water. You need approximately 4 litres of water at a rolling boil. Use a kettle or large pot.
2. Combine dry ingredients in a clean bucket. Place the coco coir (broken into chunks), vermiculite, and gypsum into a clean 5-gallon (20 L) bucket. See the Substrate Preparation guide for exact amounts.
3. Pour the boiling water over the dry ingredients. Pour evenly to saturate all the dry material as much as possible.
4. Seal the bucket with the lid immediately. Close it tightly. The trapped heat will pasteurise the substrate — the temperature inside will remain above 65°C for hours, which is all that is needed to kill harmful organisms.
5. Wait 4–8 hours. Overnight is best. The longer the substrate sits sealed in the hot bucket, the more thorough the pasteurisation. Most growers prepare their substrate the evening before they plan to spawn.
6. Open, mix, and check field capacity. After the wait, open the bucket, mix everything thoroughly with clean hands or a large spoon, and perform the squeeze test to check moisture. See the Substrate Preparation guide for the field capacity squeeze test.

Step-by-Step: Hot Water Bath Pasteurisation (for Straw)

Oyster mushrooms grow excellently on pasteurised straw. This method uses a sustained hot water soak rather than boiling water in a bucket.

1. Chop your straw into 5–10 cm (2–4 inch) pieces. A garden shredder works well, or run the straw through a lawn mower on a tarp. Shorter pieces colonise faster and pack more evenly.
2. Heat a large volume of water to 65–80°C (150–175°F). Use a large pot, stock pot, or even a clean metal drum. You need enough water to fully submerge the straw. A thermometer is very helpful here.
3. Submerge the straw in the hot water. Push it down and hold it under with a weight, a lid, or a mesh bag with something heavy in it. Straw floats, so you need to keep it submerged.
4. Maintain the temperature at 65–80°C for 60–90 minutes. Check with a thermometer periodically. If the temperature drops, add more hot water or increase the heat. Do not let it go above 82°C — temperatures that are too high will start to break down the straw structure.
5. Drain the straw thoroughly. Remove the straw and let it drain in a clean colander, mesh bag, or on a clean wire rack until it stops dripping freely. The straw should be damp but not waterlogged — similar to a wrung-out sponge.
6. Cool to room temperature before mixing with grain spawn. Spawning onto hot substrate will kill your mycelium.

Common Mistakes

These are the errors beginners make most often. Avoiding them will dramatically improve your success rate.

1. Starting the Timer Before Full Pressure

This is the single most common sterilisation mistake. The sterilisation time only counts once the pressure cooker is at a full, steady 15 PSI. The heat-up period (which can take 20–40 minutes depending on your cooker and stove) does not count. If you start your 90-minute timer when you turn on the stove, you are effectively sterilising for 50–70 minutes instead of 90 — and your grain will likely contaminate.

2. Quick-Releasing Pressure

Quick-releasing (lifting the weight or opening the valve to dump pressure fast) causes the liquid inside your jars to flash-boil. This can:

• Crack or shatter glass jars
• Blow lids off jars
• Suck unsterile air into the jars as pressure equalises
• Spray boiling water and substrate inside the cooker

Always let pressure drop naturally. There are no shortcuts here.

3. Over-Filling Jars

Jars should be filled no more than three-quarters full. During sterilisation, the contents expand and the moisture boils. Over-filled jars will push their lids off, and liquid or grain can overflow into the pressure cooker water. For grain, this also means the grain becomes waterlogged and clumpy — bad for colonisation.

4. Not Using Foil Over Lids

During pressure cooking, steam condenses on the inside of the cooker lid and drips back down. Without a foil cover, this water drips directly into your jars through the gas exchange holes or loose lids. Water getting into grain jars makes the grain too wet. Water getting into LC jars dilutes the medium and can introduce contaminants. Always cover the top of each jar with a piece of aluminium foil.

5. Tightening Lids Before Sterilisation

A jar with a tight lid is a sealed container. When you heat a sealed container, the air and moisture inside expand with nowhere to go. The result is one of three things: the lid buckles, the jar cracks, or the lid blows off violently. Always leave lids finger-tight and then back off a quarter turn before pressure cooking. You tighten them after sterilisation, when you remove the jars.

6. Letting the Pressure Cooker Boil Dry

For long runs (90 minutes or more), make sure you start with enough water. If the pressure cooker boils dry, it can overheat, warp, and trigger the safety plug. More importantly, without water there is no steam — the temperature drops and your contents are not sterilised. Use 3 inches of water for runs over 60 minutes.

Safety

Pressure cookers are safe kitchen tools when used correctly, but they operate at high temperatures and pressures. Follow these safety rules every time.

Pressure Cooker Safety Rules

• Never leave a pressure cooker unattended while building pressure. Stay nearby (in the same room) for at least the first 15–20 minutes until pressure is stable and you have reduced the heat. Once stable at low heat, you can move around your home but check in every 15–20 minutes.
• Never force the lid open. If the lid does not open easily, there is still pressure inside. Wait longer.
• Always open the lid tilted away from you. Even at zero pressure, residual steam inside is very hot. Tilt the lid so steam vents away from your face and arms.
• Check the gasket before every use. The rubber gasket (sealing ring) should be flexible, clean, and free of cracks. A damaged gasket can fail under pressure. Replace it if it looks worn — gaskets are cheap and available from the manufacturer.
• Make sure the vent pipe is clear. Before each use, hold the lid up to a light and look through the vent pipe. If it is blocked (by food residue or mineral buildup), clean it with a pipe cleaner or toothpick. A blocked vent can cause dangerous over-pressurisation.
• Never fill the pressure cooker more than two-thirds full. There must be space for steam to circulate. Over-filling can block the vent and safety valve.
• Use the pressure cooker on a stable, flat surface. A heavy, pressurised pot falling off a stove is extremely dangerous.
• Keep children and pets away from the kitchen while the pressure cooker is in use.

Handling Hot Jars

• Always use oven mitts, silicone gloves, or jar lifters when handling jars after sterilisation. They are much hotter than they look.
• Set hot jars on a towel, wooden cutting board, or wire rack — never on a cold stone or metal surface. The sudden temperature change can crack the glass (thermal shock).
• Do not run cold water over hot jars to speed up cooling. This will crack them.
• If a jar has a crack, even a tiny one, discard it. Cracked jars can fail catastrophically in the next pressure cooker run.

Steam Burns

Steam at 121°C causes instant, severe burns. Be especially careful when:

• Opening the pressure cooker lid (even after depressurisation)
• Removing the foil from hot jars
• Handling the pressure regulator weight while it is still hot

If you receive a steam burn, immediately run cold water over the affected area for at least 10 minutes and seek medical attention for serious burns.

Quick Reference: Which Method for What?