Preparing Grain Spawn at Home

Preparing your own grain spawn is an essential step in mushroom cultivation. This guide will walk you through how to soak, cook, strain, bag, and sterilize grain for spawn using a pressure cooker or Instant Pot.

What You'll Need:

  • Whole grains (rye, wheat, millet, or oats work well)

  • Water

  • Large pot

  • Strainer or colander

  • Clean towels or trays for drying

  • 3 lb mushroom spawn bags (or other heat-resistant bags)

  • Pressure cooker or Instant Pot

  • Gypsum (optional, to improve grain quality)

Step 1: Rinse the Grains

Start by thoroughly rinsing your grains under cold water to remove dirt and excess starch. Continue rinsing until the water runs clear. This step ensures cleaner grains and reduces the risk of clumping during the sterilization process.

Step 2: Soak the Grains

Place the rinsed grains into a large pot. Add enough water to cover the grains by about 2 inches. Soak the grains for 12 to 24 hours. Soaking hydrates the grains and allows endospores (which can survive sterilization) to germinate, making them easier to kill during the pressure-cooking process.

Optional: Add gypsum at a ratio of about 1 teaspoon per cup of grain. This helps the grains remain loose and reduces clumping later on.

Step 3: Cook the Grains

Once the grains have soaked bring the pot to a boil and cook the grains for about 10-15 minutes. You want to hydrate the grains fully, but avoid cooking them to the point where they burst or become mushy. Properly cooked grains will remain firm but hydrated. When approximately 5% of the grains have burst, they’re ready.

Step 4: Strain and Dry the Grains

After boiling, drain the grains using a colander or strainer, occasionally shaking it to release steam. Let the grains sit overnight until their surface is dry to the touch. Ensuring the grains are not overly wet is crucial, as excess moisture can lead to spoilage or contamination in your grain spawn.

Step 5: Bag the Grains

Once the grains are sufficiently dry, fill your mushroom spawn bags about two-thirds full with the prepared grain. Fold the bags so the filter patch faces outward to allow for gas exchange, but do not seal them before sterilization. This setup ensures that air can escape safely during the sterilization process.

Step 6: Sterilize the Grain Bags

Place the unsealed grain bags into your pressure cooker or Instant Pot. If you’re using a pressure cooker, sterilize the grain bags at 15 PSI for 90 minutes to 3 hours. If using an Instant Pot, use the sterilization or pressure-cooking function for the same duration (ensure it reaches sufficient pressure).

Ensure the bags are elevated slightly above the water level to avoid submerging them. You can use a trivet or jar lids to keep the bags above the waterline.

Step 7: Cool and Store the Bags

After sterilization, allow the pressure cooker or Instant Pot to cool naturally. Do not release the pressure and wait for it to reach room temperature overnight. Once cooled, store the bags in a clean area until they are ready to be inoculated with mushroom spawn.

Tips for Success:

  • Cook the grains until firm, and dry them thoroughly on the surface to prevent bacterial contamination.

  • Sterilize at 15 PSI for 90 minutes to 3 hours, ensuring the bags remain above the waterline to avoid contamination.