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The course › Part Three — The Spawn
Module 10

Sterilisation: the science of heat

You are doing two different things in two different machines, and not knowing the difference is a common cause of failure.

Lessons in this module

16 lessons · 26 min

0 of 16 lessons done
06-1  Mixing the dried sorghum with lime0:3806-2  Bottling, step by step3:3506-3  Preparing the pressure cooker3:4306-4  Getting ready to cook the bottled spawn0:2906-5  What happens while the spawn cooks1:0306-7  Removing the spawn bottles4:2406-6  Taking the bottles out of the pressure cooker1:2608-2  Setting up the cooker1:0908-3  Putting water in the steel drum0:4008-4  Loading fruiting bags into the drum — Part 11:4608-5  Loading fruiting bags into the drum — Part 21:1608-6  Sealing and locking the steel drum2:0108-7  Lighting the fire1:4408-8  Why fruiting bags must be steamed0:3408-9  The temperature needed during cooking0:3308-10  Taking the fruiting bags out after 8 hours1:24
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The explanation

Two levels of heat

The drum is not a pressure vessel. It never exceeds the temperature of boiling water, so it is not true sterilisation.
SterilisationPasteurisation
MachinePressure cookerSteel drum with steam
HeatAbove 100°C, because of the pressureAbout 100°C only — no hotter than boiling water
What diesAlmost everything, including tough sporesMost things, but not all
What it is forThe spawn bottlesThe fruiting bags
Why that waySpawn is defenceless and takes a long time to coloniseThe number of bags and the cost of fuel make pressurising them all impossible
Why this matters to you

Because the fruiting bag is never fully sterile, you are in a race. Some microbes survive inside. Your aim is for the mycelium to claim the substrate before they can recover.

That is why the amount of spawn you add matters, and why the bag must be sealed quickly.

Sealed lidFruiting bagsRack — clear of the waterThe water that makesthe steamSteam — needs a pathbetween the bagsFire underneath
The steel drum steriliser. The water at the bottom makes the steam; the bags must never sit in it.

The rules of the drum

  1. Enough water

    If the water runs out before the time is up, the steam stops and the bottom scorches. Measure it before you start, not halfway through.

  2. A path for the steam

    Do not stack too tightly. Steam is what carries the heat — if it cannot get through, there are cold spots in the middle of the stack.

  3. No leaks

    A lid that leaks never reaches the right temperature, however long you cook. Check the seal before loading.

  4. Count the time from real steam

    Not from lighting the fire. The count starts when the steam is steady.

  5. Let it cool on its own

    Do not open it while hot. Air rushing in carries spores into hot, damp substrate — the finest environment they could ask for.

The two machines. Left: the steel drum, fired from below — it will only ever reach about 100°C. Right: the pressure cooker, loaded with bottles — this is the only place true sterilisation happens.
In the book — still being written
The exact temperature, pressure and time

How many hours at what heat in the drum; what pressure in the cooker; how many bags per drum; how much fuel; and how long it takes to cool.

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Preparing the sorghum