Spawn can be transferred to bags and mixed with the bulk substrate in open air, because it is pasteurized, not sterilized. Published August 21, 2016 at 2592 × 1936 in Pasteurizer from a Food Steamer Spawn can be transferred to bags and mixed with the bulk substrate in open air, because it is pasteurized, not sterilized. Spawn can be transferred to bags and mixed with the bulk substrate in open air, because it is pasteurized, not sterilized.