Crustacean and Mollusc Shell Proteins: Origin, Structure, and Functionality

Crustacean and molluscan processing as seafood generates a significant amount of shell waste annually, owing to the dense, mineral-rich composition of the exoskeleton. Although valorization efforts have mostly focused on chitin and chitosan, the protein fraction remains underutilized despite its prominent nutritional, functional, and sensory potential. Crustacean shells typically contain 30%-40% protein, enriched with bioactive and flavor-active amino acids, such as glycine, proline, alanine, and arginine. These proteins exhibit promising functional properties, including solubility, emulsification, foaming capacity, and stability. However, efficient recovery remains challenging due to the tightly bound chitin-protein-CaCO₃ matrix and structural complexity of the Bouligand-type structure of crab exoskeleton. However, mollusc shells are primarily mineralized structures composed of calcium carbonate (95%-99%), with a smaller organic fraction (1%-5%) consisting mainly of proteins, glycoproteins, and polysaccharides. Compared with crustacean shells, molluscan shells have lower protein content. This review critically evaluates the shell protein valorization and extensive data on composition and extraction strategies, such as green methods. The multifunctionality of shell-derived compounds also extends to pharmaceutical, biomedical, and environmental domains. Although research on shell valorization is expanding, critical gaps remain in techno-economic feasibility, allergenicity assessment, regulatory clarity, and the development of scalable processing techniques. Overcoming these barriers requires integrated efforts across food science, biotechnology, and sustainability practices. By integrating green extraction technologies, compositional profiling, and food system applications, this review advances innovative and sustainable pathways for crab shell valorization aligned with circular bioeconomy principles.