A pressure cooker is a necessity for every mushroom cultivator. A pressure cooker is used to sterilize substrate and instruments by exposing it to high temperatures under high pressure. Sterilisation destroys all micro organisms.
Although pressure cookers don't come cheap, it remains of the few somewhat more expensive items a mushroom cultivator needs. All other needed materials are relatively inexpensive.
After sterilisation the cooker needs time to cool down. Remain patient, don't try to open the lid too fast. Too much pressure can render the substrate and jars to become useless. It can also lead to dangerous situations.