Grain Spawn

Preparing, sterilising, and colonising grain spawn — the foundation of every successful mushroom grow.

What Is Grain Spawn?

Grain spawn is sterilised grain that has been colonised by mushroom mycelium. It serves as the "seed" for your grow. When mixed with bulk substrate in your monotub, the mycelium spreads from the grain into the substrate, eventually colonising the entire tub and producing mushrooms.

The quality of your grain spawn directly determines the success of your grow. Well-prepared, fully colonised grain spawn gives you a massive head start against contamination.

Choosing Your Grain

Rye Berry Preparation

Proper grain preparation is about achieving the right moisture content — wet enough for mycelium to thrive, but dry enough on the outside that the grains do not stick together or harbour bacteria.

Step 1: Rinse

Place your rye berries in a large pot or bucket and rinse them under running water. Stir the grain and pour off the cloudy water. Repeat 2–3 times until the water runs mostly clear. This removes dust, chaff, and surface debris.

Step 2: Soak (12–24 hours)

Cover the rinsed grain with water, at least 5 cm (2 inches) above the grain level — the grain will absorb water and expand. Soak for 12–24 hours at room temperature.

• 12 hours is the minimum for adequate hydration.
• 18 hours is ideal for most situations.
• 24 hours is fine but do not exceed this — longer soaks encourage bacterial growth.

Step 3: Simmer (15 minutes)

Drain the soaked grain and place it in a large pot. Cover with fresh water and bring to a boil, then reduce to a simmer. Cook for exactly 15 minutes.

• You want the grain to be hydrated through to the centre but not split or burst open.
• Test by cutting a grain in half — it should be uniformly hydrated with no dry white centre, but the outer hull should be intact.
• If grains are splitting and showing their starchy insides, you have overcooked them. Starchy, burst grain dramatically increases contamination risk.

Step 4: Drain and Dry

Drain the grain thoroughly in a colander. Then spread it out in a single layer on clean towels or a wire rack. The goal is to remove surface moisture while keeping the internal hydration.

• Stir and turn the grain every 10 minutes.
• You can use a fan to speed up drying.
• The grain is ready when the surface feels dry to the touch but the grain still feels heavy and hydrated when you squeeze it.
• A grain should not leave a wet mark on a paper towel when pressed against it.
• This drying step typically takes 30–60 minutes.

Step 5: Add Gypsum

Sprinkle gypsum (calcium sulphate) over the dried grain at a rate of about 1 tablespoon per litre of grain. Toss to coat evenly.

Gypsum serves two purposes:

• Prevents grains from clumping together, keeping them loose for better air exchange.
• Provides calcium and sulphur, which support mycelial growth.

Mason Jar Preparation

Wide-mouth quart (1 L) mason jars are the standard for grain spawn. You need modified lids for inoculation and gas exchange.

Modified Lid Components

• Self-healing injection port: A blob of high-temperature RTV silicone (or a pre-made injection port) that allows you to push a syringe needle through and have it seal behind you. Drill a 10 mm hole in the lid and fill with silicone, or glue a pre-made port over the hole.
• Filter patch or synthetic filter disc: Allows gas exchange (mycelium needs oxygen) while keeping contaminants out. Use a 0.2 μm or 0.5 μm adhesive filter patch over a second 10 mm hole. Micropore tape over a hole also works but is less reliable.

Filling the Jars

• Fill each jar to about 3/4 full with prepared grain. Leave headspace — the grain needs room for shaking during colonisation.
• Wipe the jar rim clean.
• Screw on the modified lid. Do not overtighten — finger-tight is enough.
• Cover each lid with a square of aluminium foil. This protects the filter patch and injection port from water during pressure cooking.

Pressure Cooking (Sterilisation)

This is the most critical step. Proper sterilisation kills all bacteria, mould spores, and other organisms in the grain, giving your mushroom mycelium a clean start.

1. Place a trivet or rack in the bottom of your pressure cooker. Jars must not sit directly on the bottom.
2. Stack your jars inside. You can stack jars on top of each other — they do not need to be upright during sterilisation.
3. Add water to the cooker according to its instructions (usually 2–3 cm deep).
4. Seal the pressure cooker and bring to 15 PSI (1 bar / 121°C / 250°F).
5. Once at full pressure, maintain for 90 minutes. Start your timer only after reaching full pressure.
6. After 90 minutes, turn off the heat and let the pressure drop naturally. Do not quick-release the pressure.
7. Leave the jars inside the cooker until they cool to room temperature. This can take 8–12 hours. Many growers pressure cook in the evening and let jars cool overnight.

Inoculation

Once your jars have cooled to room temperature, inoculate them using either a spore syringe or liquid culture (LC).

Follow the inoculation procedures described in the Spore Syringes or Liquid Culture guides. Always work in a SAB with proper sterile technique.

Incubation and Break-and-Shake

After inoculation, store your jars at 24–27°C (75–80°F) in a dark location. The mycelium will begin colonising the grain from the inoculation points outward.

Break-and-Shake Technique

Break-and-shake is a technique where you shake the jar to break up colonised grain clumps and redistribute them throughout the jar. This dramatically speeds up full colonisation by creating many new growth points.

First Break-and-Shake: At 30% Colonisation

1. Wait until approximately 30% of the grain is visibly colonised (white with mycelium).
2. Firmly shake and rotate the jar to break up the colonised clumps and mix them with uncolonised grain.
3. The jar will look messy and mixed up — this is normal and expected.
4. Return to incubation. Within 2–3 days, you will see many new growth points throughout the jar.

Second Break-and-Shake: At 70% Colonisation (Optional)

1. Some growers do a second shake at 70% colonisation to speed up the final stretch.
2. This is optional but can shave several days off total colonisation time.
3. Be gentler than the first shake — the mycelium network is well-established by this point.

Signs of Healthy Grain Spawn

• Bright white mycelium growing uniformly through the grain.
• No off colours — green, black, orange, pink, or yellow spots are contamination.
• No sour or unusual smells when you open the jar. Healthy mycelium has a pleasant, earthy, mushroomy smell.
• Grains remain individual — they should not be mushy or clumped into a solid wet mass (this indicates wet rot).
• Metabolic moisture — small water droplets on the glass are normal. This is moisture released by the mycelium as it grows.

When Is Grain Spawn Ready?

Your grain spawn is ready to use when the jar is 100% colonised — every grain should be covered in white mycelium with no visible uncolonised grain remaining. Using spawn that is not fully colonised increases the risk of contamination when you expose it to the non-sterile bulk substrate.