u/Secure_Craft5595

I've tried substrates like vermiculite with flour, millet, rye, and corn, and the only one that works for me is vermiculite with flour. I want to use corn or rye, but I don't know what I'm doing wrong; they always get contaminated. I hydrate them for 12-24 hours, boil them until they soften sufficiently, and sterilize them in a pressure cooker at 15 psi for about one hour. The contamination always breaks out from the inside of the grains, so I know I am processing and sterilizing the substrate incorrectly. Is 15 psi pressure insufficient? Can I crush the grains and mix them with a little vermiculite to make sterilization easier? Please advise.