Valorization of marine by-products through solid-state fermentation for the production of microbial protein

The growing global demand for protein, together with the environmental impact of conventional agriculture and livestock production, highlights the urgent need for alternative, sustainable sources of protein. Marine by-products represent an abundant, low-cost, and underutilized resource that can be revalorized through biotechnological processes. This study evaluated six marine by-products from mussels ( Mytilus galloprovincialis ) and tuna ( Katsuwonus pelamis ), as well as macroalgae ( Ulva rigida and Saccharina latissima ), as substrates for the cultivation of Rhizopus oligosporus CECT20169 by solid-state fermentation (SSF). Preliminary substrate screening and assessment of maximum protein production identified mussel epibiota as the most suitable substrate. Optimization using a central composite design (CCD) and the desirability function indicated that an inoculum concentration of 20 × 10 6 spores·mL −1 , 54.95% substrate moisture, and 1.98% Ulva sp. supplementation resulted in a protein content of 70.24 ± 2.64%, closely matching the 71.79% predicted by the CCD. This represents an increase of more than 27% compared to the original biomass. Amino acid profiling also revealed notable changes in protein composition after fermentation, suggesting protein transformation during the SSF and potential modifications in sensory attributes. These findings demonstrate the feasibility of valorizing marine by-products for high-protein biomass production via SSF, while promoting sustainable waste management in the seafood industry.