Preparing and Pasteurising Mushroom Substrate
Match substrate to species, then pasteurise straw or sterilise rich sawdust correctly. A practical guide to substrate choice, heat treatment, and moisture.
Choosing a substrate
Match the substrate to the species. Oysters fruit on pasteurised straw or supplemented sawdust, while wood-lovers such as shiitake, lion's mane, and reishi prefer hardwood sawdust with a bran supplement.
Pasteurising straw
Pasteurisation knocks back competing moulds while leaving some helpful microbes, which suits plain straw.
- Chop the straw so it packs well.
- Hold it in hot water at 65 to 80°C for one to two hours, or soak it in high-pH lime water for 12 to 18 hours for cold pasteurisation.
- Drain until it holds moisture but does not drip, around 65 to 70%.
- Cool it, then mix in grain spawn.
Sterilising rich substrate
Nutritious substrates such as Masters Mix, which is hardwood sawdust and soy hull, must be sterilised, not just pasteurised. Hydrate to about 60%, bag it, and pressure sterilise at 15 psi. Rich food contaminates easily without full sterilising.
Getting moisture right
Aim for about 60 to 70% moisture. A firm squeeze should release a few drops, not a stream. Too wet turns anaerobic and sours, and too dry stalls the mycelium.