A sterilised grain bag is the quiet workhorse of mushroom growing. It looks like a bag of wheat, and it is, but it has been hydrated to a precise moisture, enriched, sealed behind a filter and run through a full autoclave cycle, so that nothing in it is alive until you put your culture in. From that moment the bag has one job: let the mycelium eat every kernel while keeping the rest of the world out.

The anatomy of a grain bag

The grain

Wheat, oats, corn or milo, hydrated so each kernel is plump but not split, and enhanced with nutrients chosen for fast, dense colonisation. Which grain matters less than people think, but each has a character.

The filter patch

A 0.2 micron patch that lets the mycelium breathe out CO2 and breathe in oxygen while blocking mould spores and bacteria. Never cover it, never puncture it.

The injection port

A self-healing rubber disc. A needle goes through, you inject, the needle comes out and the rubber closes behind it. It is how you inoculate without opening the bag.

The seal

Heat-sealed above the filter. The bag is sterile until the seal is broken.

How to inoculate a grain bag

1. Prepare

   Wipe down a table, your hands and the outside of the bag with 70% isopropyl alcohol. Have your liquid culture syringe, a lighter and alcohol wipes within reach. A still air box is ideal but not essential for a port injection.

2. Flame the needle

   Fit the needle to the syringe, hold it in a flame until the tip glows, then wait ten seconds for it to cool. A hot needle kills the culture; a cold, unflamed needle carries contaminants.

3. Wipe the port

   Alcohol wipe, firm pressure, let it flash off.

4. Inject

   Push through the port and inject 2 to 5 ml. If you can angle the needle to reach two or three spots in the grain, do; more points means faster colonisation.

5. Label and store

   Species, date, grain. Then into a dark cupboard at 20 to 26°C with the filter patch facing up and uncovered.

What happens next

For the first few days nothing visible happens; the mycelium is recovering from the injection and finding the grain. By day four to seven you will see white fuzz around the injection points. From there it spreads outward across kernels, and at about a third colonised you can break and shake: squeeze and knead the bag gently to distribute colonised kernels through the uncolonised ones, which can halve the remaining time. Then leave it. Oysters finish a 1kg bag in 10 to 14 days; lion's mane and shiitake in three to five weeks.

Reading a healthy bag

• Good: white, even growth that advances a little every day and smells clean, earthy or faintly mushroomy.
• Watch: patches that stop advancing, kernels that look wet or grey, or a sour smell at the filter.
• Bad: green, black, yellow or pink growth, slimy kernels, or a smell that makes you step back. That bag goes in the bin, sealed. The contamination guide shows each one.

What to do with a colonised bag

A fully white bag is grain spawn. From here you have two choices. Mix it through a bulk substrate at 1:2 to 1:4 by weight to grow a flush (ratio guide), or expand it by dropping a few handfuls into fresh sterilised grain bags in a still air box so one bag becomes several (colonised spawn guide). Either way, use it while it is young: spawn is at its most vigorous in the week or two after it finishes.

Storage before and after

A sealed, uninoculated bag keeps for a few weeks somewhere cool and dark; the fridge is fine, just let it warm before injecting. A colonised bag keeps two to three weeks in the fridge with slowly falling vigour. The full storage guide covers both.

Written by the Aus Myco Kingdom crew. Everything here is general growing information for gourmet and medicinal species; your room, your climate and your culture will have the final say.