3D-printed structure design enhances solid-state fermentation of soybean meal–okara substrates

Conventional solid-state fermentation is often limited by heterogeneous mass transfer and poor process controllability. 3D printing offers a novel strategy to optimize fermentation by enabling precise structural design of the substrate. In this study, Neurospora intermedia was used to ferment soybean meal (SM) and okara (OK) substrates fabricated via 3D printing, systematically evaluating the effects of printed structures on hyphal growth and metabolite accumulation. By adjusting model height and infill density, the 3D-printed matrices exhibited distinct microstructural characteristics that significantly influenced hyphal spatial distribution and fermentation activity. Compared with conventional SSF, the 3D-printed structures not only promoted uniform mycelial colonization and enhanced metabolic activity—raising soluble protein content to (57.39 ± 1.6) mg/g and ergothioneine accumulation to (391.82 ± 4.3) mg/kg—but also delayed spore formation, resulting in lower early accumulation of carotenoids. Morphological and microstructural analyses indicated that N. intermedia preferentially grew along the printed scaffold within the matrix, enhancing gas exchange, improving nutrient availability, and forming interconnected mycelial networks that reinforced structural stability and improved the texture of the fermented products. These findings demonstrate that rationally designed 3D-printed structures can effectively regulate oxygen diffusion pathways and the spatial organization of nutrients and microbes, providing a tunable platform for the valorization of agro-industrial by-products such as SM and OK. This study highlights the potential of 3D printing to optimize SSF performance and develop structurally guided, sustainable food bioprocessing systems.