Mycelium is the vegetative structure of a fungus, analogous to the root system of a plant, and it represents the true organism being cultivated. This dense network of thread-like cells, called hyphae, is responsible for secreting enzymes that digest nutrients from its environment. Cultivating mycelium is the necessary first step in growing mushrooms, as it creates the “spawn” which is then introduced to a larger substrate for eventual fruiting. Supporting this initial growth phase is the foundation for any successful cultivation project.
Essential Requirements and Sterile Preparation
The single most determinative factor in successful mycelium growth is maintaining a clean environment, as the nutrient-rich materials used for cultivation are also attractive to competing microorganisms. A pressure cooker or autoclave is necessary to achieve the high temperatures and pressures required to eliminate bacterial endospores and mold spores. For grain substrates, such as rye or millet, sterilization typically requires holding the material at 15 pounds per square inch (PSI) for 90 to 120 minutes to reach 250°F (121°C) throughout the core mass.
Sterilizing the growth medium, often hydrated grain or a sawdust mixture, ensures that the mycelium faces no competition when it is introduced. These sterilized substrates are typically contained in glass jars or specialized bags with filter patches that allow for gas exchange while preventing airborne contaminants from entering. All tools, including scalpels, inoculation loops, and syringes, must be sterilized, usually by heat or alcohol, before they touch the growth medium.
The actual inoculation process must occur within a sterile workspace to protect the freshly sterilized substrate from the ambient air. A still air box (SAB) or a laminar flow hood provides a localized, cleaner environment by either allowing airborne particles to settle or by actively filtering the air.
Inoculating the Growth Medium
Once the substrate and environment are prepared, the next step involves introducing the fungus culture, or “spawn,” into the sterile medium. Cultivators typically source this initial culture in a few forms, each with different advantages. Spore syringes, containing microscopic spores suspended in sterile water, are an accessible starting point, but they can sometimes lead to varied growth characteristics and a higher contamination rate.
Liquid culture (LC) syringes contain living, suspended mycelium and offer a faster colonization time because the fungus is already actively growing. A grain-to-grain (G2G) transfer, where fully colonized grain is used to inoculate fresh, sterile grain, is the fastest method. Regardless of the source, the transfer must be executed quickly and deliberately within the sterile workspace.
When transferring the culture, tools like scalpel blades are often sterilized by briefly heating them until they glow orange-red, then allowing them to cool before use. The injection port or lid of the prepared container is wiped with an alcohol solution immediately before the culture is introduced. After the spawn is introduced, the container is immediately sealed, with filter patches or syringe ports remaining in place to facilitate gas exchange.
Incubation Conditions and Growth Monitoring
After inoculation, the containers are moved to an incubation area where environmental conditions are carefully controlled. Temperature is a primary factor during this phase, with most commonly cultivated species thriving in a steady range between 70°F and 78°F (21°C to 25.5°C). Consistent temperatures promote rapid mycelial growth, while temperatures above 80°F (27°C) can stress the mycelium and favor bacterial contaminants.
The mycelium requires gas exchange, primarily oxygen intake and carbon dioxide release. This air exchange is facilitated by the filter patches on specialized bags or the modified lids of jars, which prevent the build-up of metabolic gases that could stall growth. Light is generally not required for colonization, and the incubation area is often kept in darkness.
Healthy growth manifests as white, fluffy, or rope-like growth spreading across the substrate. Mycelium may exhibit a soft, cottony appearance called tomentose growth, or it may form thick, aggressive, root-like strands known as rhizomorphic growth. To accelerate the colonization of larger containers, growers often employ a “shake and break” technique once the substrate is about 30% colonized, physically breaking up the mycelial network and distributing the spawn throughout the uncolonized material.
Identifying and Preventing Contamination
Despite the best efforts at sterile preparation, contamination is a common issue that can quickly derail a cultivation project. Contaminants often present visually as colors other than white. Green is the most common indicator of mold, specifically the aggressive Trichoderma species. Blue or blue-green patches often indicate Penicillium mold, while black or gray spots can signal Aspergillus mold.
Bacterial contamination, often referred to as “sour rot” or “wet spot,” typically appears as slimy, wet patches, frequently accompanied by a foul or sour odor. These bacterial endospores are particularly difficult to eliminate because they can survive the standard sterilization process unless the grains are properly hydrated before pressure cooking, which encourages the endospores to germinate.
Prevention relies heavily on quality control, starting with the use of laboratory-grade culture sources and rigorously adhering to sterilization procedures for both the substrate and all tools. Regular monitoring of the incubation environment ensures that temperatures and humidity levels do not unintentionally favor contaminant growth. Any container showing signs of contamination should be immediately removed from the incubation area and disposed of to prevent the release of spores that could infect other cultures.