Mushroom medium loves to grow in nutrient-rich, moist and dark environments. Unfortunately, these conditions are also ideal for things like molds and bacteria to thrive. These contaminants can often outperform mushroom mycelium in the race to colonize substrates, so something needs to be done to give the mushrooms a head start. This is the primary reason why people sterilize and pasteurize their mushroom substrate before inoculating it with spores.

Understanding Mushroom Medium: What It Means for Your Coffee

Pasteurization involves heating the substrate for one or two hours at a temperature of 150-180 degrees Fahrenheit. While it doesn’t kill all contaminants completely, it significantly reduces the population and gives the mushrooms a significant advantage. The other alternative, sterilization, is far more intensive and requires a pressure cooker or similar piece of equipment to reach temperatures of 250 degrees Fahrenheit or higher.

The specific composition of the growth medium used for mushrooms varies from grower to grower. Typical ingredients include hay, straw, poultry or horse manure, and crop residues such as corn cobs and cocoa shells. The mix is usually composted for 30 days at 160 degrees to kill weed seeds, pests and disease pathogens. It is then steam-pasteurized at about 140 degrees before being inoculated with mushroom spawn.

Supplementation can also be added to the sterilised substrate for extra nutritional sustenance. The most common supplements are bran or seed derivatives (oat and wheat bran, for example) at a ratio of 5-10% dry weight. Other popular supplements are sawdust, hardwood pellets (typically sourced from maple or oak), and cardboard.