The Substrate Effect: Rice Husk Versus Cotton Gin Trash in Mycelium-Based Composite Production

Mycelium-based composites (MBCs) are emerging as low-carbon materials for converting agricultural residues into construction-oriented panels. However, the specific role of substrate selection remains difficult to define because fungal species, nutrient systems, and processing conditions often vary between studies. Here, rice husk and cotton gin trash were directly compared as locally available Australian feedstocks for heat-pressed MBC panels under the same cultivation and fabrication framework. Four white-rot fungal isolates, Amauroderma sp., Ganoderma sp., Pycnoporus coccineus, and Trametes hirsuta, were cultivated with wheat grains as a nutrient supplement, enabling the substrate effect to be isolated more clearly. The panels were assessed through density measurements, microstructural imaging, tensile, compressive and flexural testing, and thermal characterisation by TGA and DSC. Rice-husk panels formed denser, more compact board-like structures, with densities of approximately 250–259 kg m−3, supporting their potential for compact, non-load-bearing interior panel applications. Cotton gin trash panels were lighter, with densities of approximately 138–149 kg m−3 but showed improved deformation tolerance and strong flexural performance in selected panels. Mechanically, CGT-based panels reached peak tensile, compressive, and flexural stresses of approximately 0.70, 0.90, and 3.04 MPa, respectively, compared with maxima of approximately 0.35, 0.80, and 1.31 MPa for the corresponding RH-based panels. Thermal analysis further confirmed substrate-dependent behaviour. This study demonstrates that substrate choice can be used as a practical design tool to tailor MBC panels for either compact board-like products or lightweight functional panel systems.