How to Use a Pressure Cooker to Sterilise Mushroom Substrate

If you want to move beyond kits and grow mushrooms from grain spawn or supplemented sawdust, a pressure cooker becomes the most important tool in your kitchen. Boiling water can’t make grain or sawdust sterile. A pressure cooker or pressure canner can, because steam under pressure gets hotter than boiling water.

This guide explains why pressure matters, how to choose a cooker, and how to run a sterilisation cycle step by step. It borrows heavily from home canning guidance, because the equipment is the same and extension services have published clear, tested advice on using it safely.

Why you need pressure, not just boiling

At sea level, water boils at 100°C. That kills many microbes, but not the heat-resistant spores of some bacteria and moulds. Grain and supplemented sawdust are rich in nutrients, so any spore that survives will race your mushroom culture and often win.

Steam under pressure is hotter. Michigan State University (MSU) Extension notes that pressure canning is the only USDA-recommended way to process low-acid foods because they need to reach 240°F (about 116°C) or higher, a temperature at which “pressurized steam” destroys the bacterial spores that cause botulism. Mushroom growers aim a little higher still. In a 2023 study in Mushroom Research, Ufitinema and colleagues sterilised grain spawn bottles at 121°C and 15 psi, and The Mushroom Guide, published on Lowimpact.org, describes the goal as getting all of the grain to “121C/15PSI”. Penn State’s mushroom spawn lab also runs its rye spawn at 121°C.

If you are deciding whether a material needs full sterilisation or just pasteurisation, see our article on sterilisation versus pasteurisation. Straw and other low-nutrient materials are usually pasteurised. Grain and supplemented sawdust are sterilised.

Choosing a pressure cooker or canner

Pressure cooker or pressure canner?

The two terms are often used loosely. In general, a pressure canner is a larger pot designed to hold jars, with a gauge or weight to show and control pressure. A smaller kitchen pressure cooker is designed for cooking food quickly. For mushroom work, what matters is that it can reach and hold 15 psi and that it fits your jars or bags. The Mushroom Guide lists “a pressure cooker that can reach 15 PSI” as essential equipment for making spawn.

Dial gauge or weighted gauge?

	Dial gauge	Weighted gauge
How it shows pressure	A needle on a dial	A weight on the vent that rocks or jiggles when pressure is reached
Accuracy checks	Must be tested for accuracy every year, according to Oregon State University Extension	Doesn’t need testing, because it can’t go out of calibration
Settings	Any pressure the dial can show	Usually fixed steps, such as 5, 10 and 15 pounds
Best for	Seeing exactly what is happening inside	Simple, set-and-forget operation

MSU Extension advises replacing a dial gauge that reads more than 2 pounds high. Oregon State notes that many extension offices will test a dial gauge for you.

Size

Think about what you’ll be sterilising. A couple of quart jars fit in a modest cooker. Grow bags of sawdust or grain need a large canner. Bigger loads also take longer to heat through, as the table below shows.

Preparing jars and bags for the cooker

The containers need to survive the heat and still let the mycelium breathe afterwards. The Mushroom Guide describes the common options:

• Jars: mason jars are popular, but any heat-proof jar works for beginners. The lids need a way to exchange air, such as a filter disc, micropore tape, a lid with an injection port (sometimes called an airport lid) or a plug of polyester stuffing.
• Bags: grow bags have a built-in filter patch. The Mushroom Guide suggests filling them so the grain sits no higher than 1 to 2 inches (2 to 5 cm) below the patch, leaving enough air to shake the bag later.
• Don’t block the filter. Grain pressed against the filter, or a filter soaked with water, stops air exchange.

Penn State’s spawn lab uses flasks plugged with cotton and covered with paper bags, while Ufitinema and colleagues used bottles plugged with cotton and covered with aluminium foil. If you’re new to spawn, our guide to growing oyster mushrooms is a good first project to use it on.

Step-by-step: running a sterilisation cycle

These steps combine the operating advice from MSU Extension and Oregon State Extension with mushroom-specific timings. Always read your own cooker’s manual first, because it overrides anything general.

1. Check the cooker. Inspect the gasket. MSU Extension warns that “nicked, dried or brittle gaskets” leak steam and should be replaced. Make sure the vent pipe is clear.
2. Add water and a rack. MSU Extension’s canning guidance uses 2 to 3 inches of hot water, following the manufacturer’s directions. The Mushroom Guide says the water should come above the bottom plate. Never let jars or bags sit directly on the base.
3. Load the jars or bags. Leave space between them so steam can circulate. Make sure filter patches or lid filters won’t be soaked.
4. Lock the lid and vent. Heat on high with the weight off until steam flows steadily, then let it vent for 10 full minutes. Venting drives air out. Air pockets stop the inside from reaching full temperature.
5. Bring it up to pressure. Put the weight on, or close the petcock. MSU Extension says the canner takes about 3 to 5 minutes to pressurise. Start timing only once the target pressure is reached.
6. Hold the pressure steady. Turn the heat down until the weight rocks gently or the dial stays at your target. Oregon State Extension warns that rocking too fast can boil the canner dry. If pressure falls below target, both MSU and Oregon State say to bring it back up and restart the timer.
7. Let it cool naturally. Turn off the heat and leave it. MSU Extension says “Do not force cool the canner”. A weighted-gauge canner needs at least 30 to 45 minutes. Once the pressure is down, remove the weight, wait another 10 minutes, then open the lid tilted away from you.
8. Let the contents cool fully before inoculating. Hot grain will kill your culture. Ufitinema and colleagues cooled their bottles for at least 3.5 hours.

How long to sterilise

Load	Time at 15 psi	Source
A couple of grain jars	60 to 90 minutes	The Mushroom Guide
Several small bags of grain	At least 2 hours	The Mushroom Guide
A cooker full of large bags	About 3 hours	The Mushroom Guide
250 ml flask of rye grain (lab)	35 minutes at 121°C	Penn State spawn lab
1,000 ml flask of rye grain (lab)	45 minutes at 121°C	Penn State spawn lab
Grain spawn bottles (research study)	2 hours at 121°C, on two days in a row	Ufitinema et al. 2023

The wide range has a simple explanation. The Mushroom Guide notes that the centre of a bag can take a long time to heat up. Small lab flasks heat through fast. Dense bags of grain or sawdust heat slowly. If in doubt, go longer, and treat contamination as a sign that your cycle wasn’t long enough.

Altitude

Water boils at a lower temperature as you go higher. The University of California’s Master Food Preservers explain that “as elevations increase, atmospheric pressure decreases, which results in water boiling at a lower temperature”, and their canning guidance raises the pressure on a dial gauge canner above 1,000 feet. For example, they give 11 pounds at 1,001 to 2,000 feet, rising to 15 pounds at 8,001 to 10,000 feet, while a weighted gauge canner is run at 15 pounds at all of these altitudes. These figures are for canning food, not for sterilising substrate. But the principle is the same: at high altitude, the gauge reading needed to reach a given temperature is higher. If you live well above sea level, check your cooker manufacturer’s guidance and lean towards longer cycles.

Safety

• Never open a cooker under pressure. Wait until the pressure has fully dropped and follow your manual’s opening procedure.
• Don’t let it boil dry. Start with enough water for the full cycle. Oregon State notes that for long processing times, adding extra water does no harm.
• Keep vents clear. Loose grain or filter material must not block the vent pipe.
• Look after your gauge. Test dial gauges every year, and replace gaskets that are damaged.
• Handle hot jars carefully. Use a jar lifter or heat-proof gloves, and keep hot glass away from cold surfaces and draughts, as MSU Extension advises for canning jars.
• Use the right equipment. If your cooker can’t reach or hold 15 psi, don’t use it for sterilising grain or supplemented sawdust.

After sterilising

Once the jars or bags are cool, they need to be inoculated in a clean space. Our comparison of a still air box and a laminar flow hood explains your options. Work quickly, keep everything wiped down, and label each jar or bag with the date. Our guide to mushroom spawn explains what you’ll be making.

Troubleshooting

Problem	Likely cause	What to try
Contamination appears in most jars within a few days	Incomplete sterilisation	Lengthen the cycle, vent fully before pressurising, and don’t overload the cooker
Contamination in only one or two jars	Contamination during inoculation	Improve your clean workspace and technique
Grain burst and mushy after cooking	Grain was too wet going in	Drain longer before loading. The Mushroom Guide warns that over-hydrated grain breaks apart in the pressure cooker
Cooker won’t reach pressure	Leaking gasket or lid not seated	Check and replace the gasket, and reseat the lid
Cooker ran dry	Not enough water for a long cycle	Add more water next time, and keep the heat low enough for a gentle rock

If you keep seeing mould, our guide to contamination will help you work out what it is and where it came from.

Sources and further reading

• McGarry, J. and Messing, L. (2023). Pressure Canning Basics. Michigan State University Extension
• Rhoades, B. (2022). Using a Weighted Gauge on a Dial Gauge Canner. Oregon State University Extension Service
• UC Master Food Preservers of Central Sierra. Preserve it! Series: Know Your Elevation When Canning. University of California Agriculture and Natural Resources
• Noel, B. (2019). The Mushroom Guide Part 7: preparing mushroom spawn (2). Lowimpact.org
• Noel, B. (2019). The Mushroom Guide Part 6: preparing mushroom spawn (1). Lowimpact.org
• Penn State Department of Plant Pathology: Spawn preparation (mushroom spawn lab procedures)
• Ufitinema, B., Hakizimana, M. and Niyomugabo, J. D. D. (2023). Mycelial growth of oyster mushroom in spawn made using different cereal grains, sawdust and vermiculite as substrate. Mushroom Research 32(1): 35–40

Featured image: jars being loaded into a pressure canner, by Colorado State University Extension via Flickr (CC0 1.0).