Mycelium is the vegetative body of a fungus, existing as a dense, branching network of thread-like structures called hyphae. This intricate network allows fungi to absorb nutrients from their environment. Harvesting is the procedure of separating this fungal biomass from the growth medium, whether liquid broth or solid substrate, for practical applications. The techniques used for separation must be tailored to the original growth environment to ensure a clean, high-purity yield.
Understanding the Applications of Harvested Mycelium
The purpose of harvesting mycelial biomass dictates the required purity and post-processing steps. One common application is using the harvested material as a starter culture, known as spawn or inoculant, to colonize new substrates for mushroom cultivation. This requires the mycelium to be alive and contaminant-free.
Another major use is the extraction of valuable bioactive compounds for medicinal or nutritional purposes. Fungal cell walls and metabolites contain polysaccharides, such as beta-glucans, and compounds like chitin/chitosan. The harvesting method must avoid processes that could degrade these temperature-sensitive molecules.
Mycelium is also harvested to create sustainable biomaterials, serving as a natural binder for products like packaging, construction insulation, or leather alternatives. In this context, the entire myceliated mass is often utilized, as the interwoven structure creates a dense, composite material. The final application determines the necessary scale and complexity of the harvesting operation.
Harvesting Mycelium from Liquid Culture
Mycelium grown in a liquid medium, often through submerged fermentation, forms a biomass suspended in the nutrient-rich broth. The goal of this harvesting method is to efficiently separate the fungal mass from the nutrient solution. Maintaining a sterile environment is paramount during this stage to prevent contamination.
Small-scale harvesting often employs filtration techniques. Simple gravity filtration can use materials like cheesecloth or fine mesh, allowing the liquid to drain while the mycelium is retained. For larger volumes, vacuum filtration utilizing a Buchner funnel and filter paper is more efficient. Applying a vacuum speeds up the dewatering process by drawing the liquid through the filter media, leaving a consolidated mycelial mass or “cake.”
Centrifugation is an alternative technique, useful for separating very fine or dispersed mycelial particles. Spinning the liquid culture at high speeds forces the denser biomass to the bottom, allowing the supernatant liquid to be decanted. After separation, the collected mycelium must be gently rinsed with sterile water to remove residual sugars and nutrients, ensuring higher purity.
Extracting Mycelium from Solid Substrate
Extracting mycelium from a solid substrate, such as grain, sawdust, or lignocellulosic waste, is challenging because the fungal hyphae are tightly interwoven with the substrate particles. This process, often called Solid State Fermentation (SSF), results in a myceliated composite material. The required degree of separation depends on the end product, as some biomaterials use the entire composite, while others require pure mycelial biomass.
For applications demanding high purity, the myceliated substrate must first be broken down mechanically. Techniques like agitation, grinding, or milling reduce the composite material into discrete particles. This mechanical action helps loosen the mycelium from the solid medium’s matrix.
A washing and sieving process is then employed to physically separate the fungal biomass from the substrate particles. Repeated rinsing with water, followed by passing the slurry through progressively finer sieves or screens, washes away lighter substrate fragments while retaining the denser mycelial clumps.
Advanced separation can involve enzymatic digestion, where specific enzymes break down non-mycelial components like cellulose or lignin, leaving the mycelium relatively intact. This technique yields a cleaner biomass but is reserved for high-value applications due to increased complexity and cost. High purity after SSF often involves combining mechanical disruption and thorough washing.
Post-Harvest Processing and Storage
Once the mycelium is separated from its growth medium, the post-harvest phase begins with thorough washing using sterile or distilled water. This removes residual nutrients or substrate fragments, prevents microbial growth, and ensures a clean final product. The washed biomass is then prepared for storage or immediate use, typically involving a drying step.
Drying methods are selected based on the intended use and the sensitivity of the desired compounds. Air or cabinet drying at moderate temperatures (55°C to 60°C) is a common, cost-effective way to reduce moisture content to a stable range (3% to 12%). However, higher temperatures can degrade heat-sensitive bioactive molecules, such as certain polysaccharides.
For maximum preservation of biological activity and nutritional value, freeze-drying (lyophilization) is the preferred, though more costly, method. This process involves freezing the biomass and reducing pressure, allowing frozen water to sublimate directly to vapor. Vacuum drying is an intermediate option, operating at a lower temperature (around 40°C) under reduced pressure, yielding better compound preservation than standard air drying.
The final step for many applications is grinding the dried mycelial mass into a fine powder. This increases the surface area for extraction or allows for easy incorporation into supplements or food products. For long-term preservation, the processed material should be stored in an airtight container, such as a vacuum-sealed bag, in a cool, dark environment. Refrigeration is suitable for short-term storage, while freezing is often employed for extended preservation.