Sterilisation vs pasteurisation

This is the decision underneath every other substrate choice, and it is decided by one variable: how nutritious the substrate is.

The two treatments

Pasteurisation — 65–75°C for 60–90 minutes. Kills most competing organisms but deliberately leaves a population of benign thermophilic microorganisms alive. Those survivors occupy the substrate and actively resist colonisation by contaminants. Pasteurised substrate is not sterile; it is selectively populated.

Sterilisation — 121°C at 15 psi, 90–150 minutes depending on volume. Kills everything, including bacterial endospores. Sterile substrate is a blank slate, with no resident population defending it.

Which to use

The rule:

Low nutrition → pasteurise. High nutrition → sterilise.

Substrate	Treatment	Why
Straw	Pasteurise	Low nutrition, poor contaminant medium
Coir / CVG	Pasteurise	Nutritionally inert by design
Manure-based	Pasteurise	Moderate nutrition, benefits from surviving microflora
Plain hardwood sawdust	Either	Marginal — pasteurise for outdoor, sterilise indoors
Supplemented sawdust	Sterilise	Bran and soy hull make it a superb bacterial medium
Master’s Mix	Sterilise	Same reason, more so
Grain	Sterilise	The most nutritious thing in the process

Why pasteurising grain does not work

The most common and most expensive mistake in cultivation.

Bacillus species — the organisms responsible for wet spot — form endospores that survive boiling comfortably. Pasteurisation temperatures do not touch them. Sealed inside a nutritious, moist, warm jar with no competition, they have ideal conditions.

Grain needs 15 psi, which reaches 121°C, held long enough for heat to penetrate the centre of the jar. There is no shortcut, and every alternative method that circulates online produces the same sour grey result eventually.

Why sterilising straw is often worse

Less obvious, and genuinely counterintuitive.

Sterile straw is a blank slate. It has no resident microflora, and the first organism to reach it — through the air, off your hands, from the room — has it entirely to itself. Since straw is usually handled openly, that first organism is frequently not your mushroom.

Pasteurised straw arrives already occupied by heat-tolerant organisms that compete with arrivals. Combined with a healthy spawn rate, that gives your mycelium the advantage.

If you want to sterilise straw, you must then handle it with grain-level sterile technique, which for most people is more trouble than it is worth.

Doing it properly

Pasteurising

Hot water: submerge in water held at 65–75°C for 60–90 minutes. Above 80°C you are effectively sterilising and losing the benefit. Use a thermometer; guessing produces one failure or the other.

Cold lime bath: submerge in water with hydrated lime at pH 12–13 for 12–18 hours. No heat, no fuel, works at scale. Lime is caustic — gloves and eye protection, and add lime to water, never water to lime.

Cold ferment: submerge in plain water for 5–7 days and let it go anaerobic. It works, it is free, and it smells genuinely awful. An outdoor method.

Sterilising

15 psi, timed from pressure. The clock starts when the gauge holds 15 psi, not when the burner goes on. This one detail causes more failures than anything else.

Quart jars of grain: 120 minutes
Pint jars of grain: 90 minutes
5 lb sawdust blocks: 150 minutes
Agar: 30 minutes

Leave space between jars for steam to circulate. Do not overpack.

Cool completely before opening — 8–12 hours. Opening warm draws contaminated air in as the contents contract, and kills culture if you inoculate immediately.

See the pressure cooker page for sizing and the safety points, which are not optional.

Getting it wrong

Under-treated: contamination, usually within a week. Wet spot in grain, Trichoderma in bulk.

Over-pasteurised straw: slow, patchy colonisation and higher contamination than expected, because you sterilised without the technique to match.

Under-hydrated before treatment: you cannot add water afterwards without compromising sterility. Get the moisture right first — see field capacity.