Mycelium is the vegetative body of a fungus, a sprawling, root-like network composed of thread-like filaments called hyphae. This structure anchors the fungus within its substrate, serving as the primary mechanism for decomposition and nutrient absorption. It breaks down complex organic materials, recycling elements like carbon and nitrogen back into the soil. For both environmental understanding and commercial cultivation, temperature regulation determines the growth, survival, and metabolic activity of this fungal network.
The Lethal Upper Limit
To definitively eradicate mycelium and all competing microorganisms, a rapid, high-temperature approach known as sterilization is employed. This method is common in laboratory settings and commercial cultivation when working with nutrient-dense substrates susceptible to contamination. The benchmark for sterilization using moist heat is 250°F (121°C) maintained under 15 PSI of pressure.
This temperature is maintained for an extended period, typically between 15 and 90 minutes depending on the substrate’s volume and density. The mechanism of death is the irreversible denaturation and coagulation of the mycelium’s intracellular proteins. This heat energy rapidly breaks down the structure of the proteins and enzymes necessary for life, effectively killing the organism.
For dry materials, such as glassware or metal tools, dry heat sterilization requires higher temperatures and longer exposure times. Dry heat, without the penetrating power of steam, typically needs temperatures around 320°F (160°C) to 340°F (170°C). This complete elimination of all microbial life, including highly resistant bacterial spores, ensures a sterile environment for the cultivated mycelium to grow without competition.
The Role of Time in Heat Death
While sterilization uses extreme heat for a rapid kill, the concept of thermal death time (TDT) demonstrates that lower temperatures can be lethal if applied over a prolonged duration. This principle forms the basis of pasteurization, a technique that selectively kills the majority of unwanted organisms without creating a completely sterile environment. Pasteurization typically involves heating the substrate to a moderate range of 140°F to 176°F (60°C to 80°C).
This temperature range is maintained for approximately one to two hours, giving the heat sufficient time to penetrate the material and inactivate heat-sensitive molds and bacteria. The goal is to reduce the microbial population by a specific factor. This selective process leaves behind some beneficial, heat-tolerant microorganisms that help suppress the re-growth of more aggressive contaminants.
The contrast between this method and sterilization is that pasteurization does not attempt to kill every single spore. The retained organisms help create a more naturally balanced, competitive microenvironment that the cultivated mycelium can typically overcome. If the mycelium is exposed to temperatures in this range for only a brief moment, it may suffer cellular damage and a temporary reduction in growth rate, but it is unlikely to be killed outright.
Survival in Freezing Temperatures
In stark contrast to the destructive power of heat, low temperatures do not reliably kill mycelium; they primarily induce a state of metabolic dormancy. When temperatures drop to the freezing point (32°F or 0°C), the mycelium’s growth and metabolic processes slow dramatically or cease entirely. Many species survive prolonged exposure to sub-zero temperatures by accumulating intracellular compounds like polyols and trehalose, which function as cryoprotectants.
The actual temperature that causes death varies widely by species, with some fungi exhibiting tolerance down to -80°C. For many common species, a lethal effect on 50% of the population can occur between -7.6°C and -13.7°C. Lethality in cold is often caused by the formation of ice crystals inside the cells, which physically rupture the membranes and organelles. Repeated freeze-thaw cycles also pose a significant threat, as the constant stress of freezing and thawing is more damaging than continuous, stable deep-freeze conditions.
Environmental Factors Modifying Heat Tolerance
The temperature required to kill mycelium is not a fixed number but is strongly influenced by the surrounding environmental conditions. The moisture content of the substrate is the most significant factor, as moist heat is considerably more penetrating and lethal than dry heat. Wet mycelium and spores are killed much faster at lower temperatures because water facilitates the transfer of heat and the denaturation of proteins.
Substrate density and the physical size of the material mass also modify the effective killing temperature. A large, densely packed substrate requires a longer total exposure time for the center to reach the target temperature. Finally, the species of fungus itself dictates its thermal tolerance; thermotolerant species possess a higher heat threshold than those found in temperate forests.