Pasteurise or Sterilise Substrate? A Plain Answer

The two treatments are not interchangeable, and which one your mix needs depends on one ingredient: how much supplement is in it.

Why any heat treatment at all

Substrate is food. Whatever you do not eat, a hundred other organisms will, and most of them grow faster than mushroom mycelium does. Heat is how you clear the field first. It is not about sterility for its own sake, and it is not about being clinical: it is about giving your culture a two-week head start.

Pasteurisation is a bath around 60 to 80 °C for 45 to 90 minutes. It knocks the population down, kills most moulds and their spores, and deliberately leaves heat-tolerant bacteria alive. Those bacteria are largely harmless to a vigorous oyster running through plain straw or unenriched pellets.

Sterilisation is steam under pressure at about 121 °C for 90 minutes or more. It kills essentially everything, which is what an enriched, high-nitrogen mix needs, because that mix is a better bacterial culture medium than it is a mushroom one.

The rule, in one line

<b>Pasteurise plain, unenriched substrate. Sterilise anything with supplement in it.</b>

Straw and 100 % hardwood pellets: pasteurise. Any blend with soy hull, wheat bran, bran pellets or Master's Mix in it: sterilise under pressure. Getting this backwards is the most reliable way to grow green mould, and it is the mistake that makes people conclude that growing at home does not work.

How each one is actually done at home

Pasteurising in a stock pot is the beginner route for oysters: heat water to about 65 to 70 °C, submerge the chopped straw or pellets in a pillowcase or mesh bag, hold it there for an hour with the lid on and the pot off the burner, then drain until no water runs out. Cool to the touch before spawn goes anywhere near it.

Sterilising needs a pressure cooker rated for it: bags filled two-thirds, folded and taped, not touching the pot's walls, brought up to 11 to 15 psi and held 90 minutes for a 5 lb bag. Then – and this is the part nobody enjoys – cool overnight inside the turned-off pot or a clean room. Inoculating a warm bag cooks the spawn.

Field capacity, the test that matters more than the timer

Squeeze a handful of hydrated substrate hard for 30 seconds. Correct: the ball holds together, your palm is damp, but no water drips. Too wet: drips, and the block will sour and colonise poorly from the bottom up. Too dry: it crumbles apart, and mycelium cannot run through it.

Hydration is where most home batches are actually lost. Getting moisture right makes a pasteurised batch forgiving; getting it wrong makes a sterilised batch fail anyway.

What the failures look like

• Bright green patches – Trichoderma, almost always from supplement plus pasteurisation instead of sterilisation, or from work done in moving air.
• Sweet, sour or fermenting smell with no growth – bacterial bloom from waterlogged substrate.
• White growth that stays exactly where you left it – the bag was not hydrated enough for the mycelium to run, not contamination.
• Colonisation that stalls at 70 % – usually cold. Move it up two degrees before you blame the culture.

Write the diagnosis down next to what you did. After three batches you will have a better method card than any printed one, and it will be specific to your kitchen.

The short version

1. Chop straw or hydrate pellets to field capacity. Prepare
2. Pasteurise 60–80 °C for 45–90 min, or sterilise 121 °C / 90 min if supplemented. Heat
3. Drain until a squeezed handful shows no drips. Drain
4. Cool fully to the touch, then mix spawn at 5–10% of wet weight. Inoculate
5. Fold, tape with micropore and incubate in the dark. Seal and wait

Kit list for this guide

SUBSTRATEKiln-Dried Hardwood Fuel Pellets, 40 lb Bag$26.00SUBSTRATEMaster's Mix Substrate Blend, 5 lb Dry$15.00SUBSTRATESoy Hull Supplement, 40 lb Bag$29.00EMPTY FILTER-PATCH CULTIVATION BAGEmpty Filter-Patch Grow Bags, 5 lb, Pack of 10$24.00

Written by the MycoGrow fruiting room. No claim here guarantees a harvest: a grow is a living thing in a room you control, and the room is the variable.