One spawn recipe uses bran and perlite as a base. Again, many types of bran are acceptable. Rye grain is one the most preferred grains for spawn production because it does not tend to cake together as easily as does rice or wheat berries. It is also one of the least expensive grains and is quite suitable to mycelial growth.
Rice tends to form solid cakes when sterilized.
Another method used to avoid caking is to precook the grain, mix in the gypsum and then sterilize the blend. The lids are loosely placed on the jars and the filled jars are sterilized at 15 psi in a pressure cooker for 45-120 minutes. It is important when using standard Mason jar lids to invert the inner dome so that the lid does not seal when the jars cool. Sealed lids cause the inhalation of ambient contaminants upon opening. Stamets also advises soaking the grain for 12-24 hours prior to heat sterilization.