Soak oat or wheat straw in a clean bucket for one hour. Add room-temperature water to cover the straw. Leave the straw undisturbed while soaking.
Drain the straw and rinse thoroughly with cool, fresh water. Drain the rinsed straw and place it in a clean 5-gallon bucket.
Add 1 gallon of fresh water to the bucket and mix in 1 liter of hydrogen peroxide. Stir once to mix the water-hydrogen peroxide solution. Allow the straw to soak in the solution overnight.
Drain the straw and rinse thoroughly with fresh water at least three times. Place the pasteurized straw in sterile glass jars and add mycelia to the straw substrate. Add 1 cup of mycelium spawn for every 1/2-gallon of pasteurized straw substrate.