When cultivating mushrooms on agricultural by products such as sawdusts, cereal straws, and oher forms of dried plant debris and vegetation, most growers choose to either pasteurize or sterilize the medium before adding the spawn. there are two reasons for doing this. if the medium is dryer, pasteurizing or sterilizing with a hot water bath or injecting live, wet steam can help hydrate it, softening up the substrate to make it more digestible for the mushroom mycelium. and the heat or chemicals used to pasteurize or sterilize can decrease or eliminate populations of potential competions that the spawn may encounter as it is coloniing the medium. in a sense, it buys time, enough time for the mycelium to capture as much of the substrate as possible.