Pressure cooker sizing and use

Cost

$80–250

Build time

n/a

Capacity

4–20 quart jars per run

Skill level

beginner

Sterilisation requires 121°C, and water boils at 100°C. The only way to get there is under pressure. There is no substitute, no workaround, and no method circulating online that achieves it another way.

If you intend to make your own grain spawn, you need one of these.

Pressure canner, not pressure cooker

The distinction matters and the words are used loosely.

Pressure canner — large, tall, holds jars upright, has a pressure gauge or weighted jiggler rated to 15 psi. This is what you want.

Pressure cooker (stovetop, small) — usually fine if it reaches and holds 15 psi and is tall enough for your jars.

Electric multi-cooker (Instant Pot and similar) — generally not sufficient. Most top out around 10–12 psi, which is roughly 115°C. That reliably kills molds and fails to kill Bacillus endospores, which is precisely the organism grain sterilisation exists to defeat. Some models reach 15 psi; most claiming to do not hold it. If you use one, expect wet spot eventually.

Sizing

Measure the internal height and compare with your jar height plus the rack.

Size	Holds	Suits
16 qt	4–7 quart jars	First setup, small runs
23 qt	7–9 quart jars, double-stacked pints	The common choice
41 qt	16–20 quart jars	Small commercial

23 quart is the size most home growers settle on. It fits a useful batch and still lifts.

Check the jar fit before buying. Wide-mouth quart jars are about 7″ tall; many smaller cookers will not close over them.

Times and pressure

15 psi, and the clock starts when the gauge holds 15 psi — not when the burner goes on, not when steam appears.

Load	Time at 15 psi
Agar bottles	30 min
Liquid culture	30 min
Pint jars of grain	90 min
Quart jars of grain	120 min
5 lb sawdust blocks	150 min

Longer does not hurt. Shorter does, and mistiming this is the most common cause of contamination in the entire hobby.

Running it

1. Add water to the depth the manual specifies — usually 2–3 inches.
2. Rack on the bottom. Jars must not sit on the base or they will crack.
3. Load with space between jars for steam to circulate. Do not overpack.
4. Lids on but loose, or use filter-patch lids. Sealed jars can burst.
5. Heat with the vent open for 10 minutes to purge air. Trapped air lowers the actual temperature at a given gauge reading, which is a silent failure.
6. Close the vent, bring to 15 psi.
7. Start timing when it holds pressure.
8. Hold steady. Adjust the burner to keep the jiggler rocking gently or the gauge steady. Wild swings are hard on jars and gaskets.
9. Cool naturally. Do not run it under water or force the pressure down.
10. Leave 8–12 hours — overnight — before opening. Jars must be completely cool before inoculation.

Safety

The most dangerous object in cultivation, and the one people are most casual about.

• Check the gasket before every run. Cracked, hardened or deformed means replace it. They are cheap.
• Keep the vent pipe clear. Hold it to the light and look. Do not assume.
• Never force the lid. If it will not open, it is still pressurised.
• Never modify the safety valve. People do this to hold higher pressure. It is the worst idea in the hobby and it is how people get hurt.
• Do not run it unattended overnight.
• Do not run it in a room where someone might reach for the handle without knowing what it is.
• Replace the overpressure plug on the schedule the manufacturer gives.

Common failures

Timed from heat, not pressure. The single most frequent error.

Ran at 10 psi. Kills molds, leaves Bacillus. Produces contamination that looks random but is not.

Air not purged. Gauge reads 15 psi, contents never reach 121°C.

Overpacked. Steam cannot circulate; jars in the middle run cooler.

Opened warm. Draws contaminated air in as contents contract, and kills culture on inoculation.

If you cannot have one

Some living situations make a pressure canner impractical. You can still grow:

• Buy pre-made grain spawn and go straight to spawn to bulk.
• Buy sterilised sawdust blocks ready to inoculate.
• Work with pasteurised substrates only — straw and CVG, which need nothing more than boiling water.

That covers oysters, wine cap and a good deal else. What it does not cover is making your own spawn, which is where the economics of scaling up live.