Hobby gourmet mushroom grower here. For gourmet mushrooms you use about 60 percent water. For this purpose it is probably lower. Many products require many time

Hobby gourmet mushroom grower here. For gourmet mushrooms you use about 60 percent water. For this purpose it is probably lower. Many products require many times their weight in water; this is not that bad. Mushrooms care more about competing bacteria and fungi than impurities... I imagine pasteurized groundwater or rainwater would be good enough for this. Drying and killing the mycelium could be energy intensive depending on the local climate. Maybe they recapture most of the water during the bake... It would already be sterile.