An autoclave is a sealed pressure vessel that sterilizes equipment and materials using saturated steam. Raise steam above atmospheric pressure inside a closed chamber, hold it there long enough, and you destroy every living organism in the load: bacteria, fungi, viruses, and the bacterial endospores that survive almost everything else.
That last part is what separates sterilization from every other cleaning method. Bleach, alcohol, and hot water all reduce contamination. An autoclave eliminates it.
For anyone running a mushroom operation, a tissue culture lab, or a small clinic, this is the single piece of equipment that determines whether the rest of your process works. You can build a perfect clean room and run a flawless transfer technique, and none of it matters if your substrate went into the bag carrying viable Bacillus spores.
How does an autoclave work?
Three things have to line up: temperature, pressure, and time. They are not independent, and the relationship between them is where most people get confused.
Water boils at 100°C (212°F) at sea level, and no matter how much heat you add, it will not get hotter than that at atmospheric pressure. The extra energy goes into turning liquid into vapor instead of raising temperature. That 100°C ceiling is a problem, because bacterial endospores routinely survive hours of boiling.
Seal the vessel, and the steam has nowhere to go. Pressure climbs, and the boiling point climbs with it. At 15 psi above atmospheric, water boils at 121°C (250°F). Now you have steam hot enough to denature the proteins inside a dormant spore.
Pressure itself does not kill anything. It is only the mechanism that lets you get steam above 100°C. This distinction matters when things go wrong, because a chamber can read 15 psi and still be nowhere near sterile.
Air is the problem
Steam sterilizes by condensing on a cold surface and dumping its latent heat into that surface. Air does not do this. Air is a poor thermal conductor, and worse, it is heavier than steam, so it pools in the bottom of the chamber and inside any container with an opening facing up.
A chamber with trapped air will show correct pressure on the gauge while the actual temperature in that pocket sits 20 or 30 degrees low. The cycle completes, the display says done, and the load is not sterile. Trapped air is the most common cause of a failed autoclave run, and it is almost always a loading error rather than an equipment fault.
Two designs deal with this differently:
Gravity displacement. Steam enters at the top and pushes the denser air down and out through a drain at the bottom. Simple, reliable, cheap. It works well for liquids, glassware, and open containers. It struggles with anything porous or bagged, because steam has trouble penetrating and air has trouble escaping.
Pre-vacuum (or pulsed vacuum). A pump actively evacuates the chamber before steam enters, often through several vacuum-and-charge cycles. This forces air out of porous loads and deep containers before sterilization begins. It costs more and adds cycle time, but for dense grain bags or tightly packed loads it is meaningfully more reliable.
Most growers running bags will get better results from a pre-vacuum or pulsed-vacuum unit. Most people sterilizing jars, tools, and media do fine with gravity displacement. You can compare both designs across our sterilizers, autoclaves and pasteurizers.
How hot does an autoclave get?
Standard operating temperature is 121°C (250°F) at 15 psi gauge pressure. Some units run a high-temperature cycle at 134°C (273°F) around 30 psi, which cuts required hold time substantially.
| Gauge pressure | Steam temperature | Typical minimum hold |
|---|---|---|
| 0 psi | 100°C / 212°F | Not sterilizing |
| 10 psi | 115°C / 240°F | 30+ min |
| 15 psi | 121°C / 250°F | 15 min (unwrapped tools) |
| 20 psi | 126°C / 259°F | ~10 min |
| 30 psi | 134°C / 273°F | 3–4 min |
Two cautions on these numbers. First, they assume pure saturated steam with no trapped air. Second, they are surface-contact times, not load times. A tray of unwrapped forceps reaches temperature in seconds. A 5 lb bag of hydrated grain does not.
Altitude also shifts things. Gauge pressure is measured relative to ambient, so at elevation you reach a lower absolute pressure and a lower steam temperature at the same gauge reading. Above roughly 3,000 feet you should be adding pressure or time.
How long should a cycle run?
The published minimums are for the item itself at temperature. Your cycle time is that number plus however long it takes heat to reach the center of the load.
Rough working figures at 121°C:
- Unwrapped instruments and tools: 15 minutes
- Wrapped instruments: 30 minutes
- Glassware and empty containers: 20–30 minutes
- Liquid media, small volumes: 20–30 minutes (with slow exhaust)
- Agar, 1 L flask: 30–40 minutes
- Grain spawn bags: 90–120 minutes
- Supplemented sawdust or masters mix blocks: 2–2.5 hours
Grain and sawdust need far longer because they are dense, insulating, and carry a heavy bacterial spore load from the raw material. A bag that hits 121°C on the outside may sit at 105°C in the core for another 40 minutes.
Two practical rules. Do not pack the chamber tight, because steam needs a path to every surface. And always use slow exhaust for liquids, since a fast pressure drop makes superheated liquid flash to steam and boil out of the container.
Sterilization versus pasteurization
These get used interchangeably and they are not the same thing.
Pasteurization runs at 65–80°C for one to two hours. It kills most competing molds and bacteria but leaves thermophilic spores and beneficial organisms alive. That surviving population competes against contaminants, which is why pasteurization works well for straw and other bulk substrates inoculated with vigorous species like oyster.
Sterilization at 121°C kills everything. It gives you a blank slate, which is what you need for grain spawn, agar, and supplemented substrates where any survivor will outgrow your mycelium. A sterilized substrate with a contamination breach has no defenses at all.
Choose based on the substrate and the species, not on which one sounds more thorough.
How do you know the cycle actually worked?
A completed cycle on the display tells you the machine ran a program. It does not tell you the load reached sterilizing conditions. Verification is a separate step, and in most operations it is the weakest link.
Chemical indicators change color when exposed to a specific combination of temperature, time, and steam. They are cheap, instant, and belong in every load. Class 1 indicators (autoclave tape) confirm only that a package was processed, not that it was sterilized. Class 4 through 6 strips respond to multiple variables and are what you want inside the load, placed in the hardest-to-reach position rather than on the outside. We stock sterilization indicator strips for exactly this, and we cover the classes and how to read them in how to read autoclave sterilization indicator strips.
Biological indicators contain live Geobacillus stearothermophilus spores, the most heat-resistant organism in routine use. You run one in a load, incubate it, and see whether anything grows. This is the only method that directly proves lethality. Run one weekly, or with every load if you are producing for sale.
Physical monitoring means recording temperature, pressure, and time for each cycle. Many commercial units print or log this automatically. If yours does not, keep a paper log. When a batch fails three weeks from now, the log is how you find out whether the autoclave was the cause.
How much does an autoclave cost?
Prices track capacity and automation more than anything else.
- Benchtop units, 18–30 L: $500–$2,500. Fine for tools, agar plates, and small liquid volumes.
- Vertical units, 50–100 L: $2,000–$5,000. The entry point for real spawn production.
- Vertical units, 150–200 L: $4,000–$10,000. Where most small commercial farms land.
- Horizontal and pass-through, 300 L and up: $10,000–$50,000+.
Budget for the things that are not the sticker price: electrical service (larger units often need 220V or three-phase), floor space and clearance, water quality, and consumables. Hard water will scale a chamber and shorten its life considerably, so distilled or deionized water is worth the ongoing cost.
The more useful question than price is throughput. Work out how many bags you need per week, divide by bags per load, and multiply by cycle time plus heat-up and cool-down. A unit that forces you into three cycles a day becomes a bottleneck that no amount of savings justifies. We walk through that calculation with worked examples in 150L vs 200L: how to size a sterilizer for your farm.
How to clean and maintain an autoclave
Maintenance is straightforward and skipping it is the usual reason a unit dies early.
Every cycle: Check the gasket for debris or nicks. Confirm the chamber drained. Do not leave the door sealed on a cool, wet chamber, since that grows biofilm.
Weekly: Drain and refill the reservoir with distilled water. Wipe the chamber with a soft cloth and mild detergent, never steel wool or chlorine cleaners, both of which will pit stainless steel. Clear the drain strainer.
Monthly: Descale if you are not using distilled water. Inspect the door gasket properly and replace it if it has flattened or cracked, since a failing gasket shows up as slow pressure build or a cycle that will not reach setpoint. Test the safety relief valve. Gaskets, valves and other wear items are in sterilizer parts.
Annually: Have the pressure vessel and relief valve inspected. In many jurisdictions this is a legal requirement for vessels above a certain size, and it is a good idea regardless.
Common questions
Can you autoclave plastic? Polypropylene and PTFE handle 121°C. Polycarbonate survives but degrades over repeated cycles. Polyethylene, PET, and most single-use plasticware will deform. Check for an autoclavable marking before assuming. Our mushroom bags are rated for full sterilization cycles.
Can a pressure cooker replace an autoclave? For small batches, yes. A quality pressure canner reaches 15 psi and 121°C. What it lacks is a verified hold at temperature, a controlled exhaust, and any cycle documentation. It is a reasonable starting point and a poor production tool.
Why did my sterilized bags still contaminate? In order of likelihood: trapped air in the load, insufficient hold time for the density, packing too tight, a compromised filter patch or seal, or contamination introduced after the cycle during inoculation. Indicator strips placed inside the load will tell you within one run whether the autoclave is the problem.
Does the autoclave need to cool before opening? Yes. Open below 80°C and below atmospheric-equivalent pressure. Opening hot risks a steam burn and pulls unfiltered room air across a still-warm load.