Solid-State Fermentation of Ganoderma lucidum on Brewer’s Spent Grain and Sawdust for Mycelium-Based Biomaterial Production

Ganoderma lucidum is a widely studied fungal species due to its potential for producing mycelium-based biomaterials. In mycotecture applications, fungal mycelium colonizes organic substrates, forming composite structures that can be used as sustainable biomaterials. Meanwhile, the management of agro-industrial and wood residues remains an environmental challenge, as these materials are often discarded or burned. This study aimed to evaluate the effects of substrate composition and inoculum load on the recovered dry fraction obtained during the solid-state fermentation (SSF) of G. lucidum using brewer’s spent grain (BSG), an agro-industrial by-product of beer production, and sawdust, a wood-processing residue. An exploratory approach based on a reduced central composite design (CCD) was used to evaluate the effects of inoculum load and substrate composition on the recovered dry fraction. The apparent specific growth rate (µ) during the observed apparent exponential growth interval (days 3–6) and the corresponding doubling time (td) were also estimated. The treatment associated with the highest recovered dry fraction contained 40.13% (w/w) sawdust and 59.87% (w/w) BSG, with an inoculum load of 2.69% (v/w). Growth kinetics were subsequently estimated under these conditions. The apparent specific growth rate over days 3–6 was 0.6346 day−1, corresponding to a doubling time of 1.0922 days. Substrate composition was the main factor associated with variation in the recovered dry fraction. These findings suggest that BSG and sawdust may be suitable substrate components for valorization through the SSF of G. lucidum. However, the recovered dry fraction is an operational measurement and should not be interpreted as a direct quantification of pure fungal biomass. The resulting specimens produced in this study exhibited a density of 0.24–0.27 g cm−3 and a compressive strength of 550–700 kPa.