Quantification of fungal biomass in mycelium composites made from diverse biogenic side streams

Mycelium composite materials are comprised of renewable organic substrates interconnected by fungal mycelium, allowing full biodegradability after use. Due to their promising material properties, adaptability, and sustainable nature, these biomaterials are investigated intensively. However, one crucial aspect that has hardly been covered so far is the proportion of fungal biomass in the composites, which would be necessary to assess its contribution to the material characteristics. Since a complete physical separation of mycelium and substrate is not feasible, we approached this issue by isolating the fungal DNA and relating it to the mass of mycelium with the help of quantitative PCR. Overall, 20 different combinations of fungi and biogenic side streams were evaluated for their handling stability, and growth observations were related to the quantification results. Ganoderma sessile was able to form stable composites with almost all substrates, and a positive correlation between mycelial biomass and composite stability could be found. However, the amount of mycelium required for fabricating firm materials strongly depends on the combination of substrate and fungal species used. Less than five mass percent of fungal biomass can suffice to achieve this, as for example when combining Trametes versicolor with sugar beet pulp, whereas a mass fraction of twenty percent leads to crumbly materials when using Pleurotus pulmonarius on green waste. These results indicate that the mycelial biomass is an important factor for the composite's stability but that the properties of the fungal hyphae, as well as those of the substrate, are also relevant. The presented quantification method not only allows to estimate fungal growth during composite production but can also improve our understanding of how the mycelium influences the material.