Eggshell Waste Valorization: Nutritional Composition, Functional Properties, and Industrial Applications

Eggshell waste, a byproduct of global egg production, is a calcium-rich secondary resource whose disposal creates environmental and resource recovery challenges. This review critically integrates current knowledge on the composition of the mineral shell and eggshell membrane, pretreatment and conversion routes, resulting material properties, and applications in food, biomedical, environmental, agricultural, and industrial systems. Eggshells predominantly consist of calcium carbonate (CaCO₃), whereas the membrane contains collagenous proteins, glycosaminoglycan, and other matrix proteins. Cleaning, heat treatment, grinding, calcination, sol-gel conversion, mechanochemical synthesis, microwave-assisted processing, and microbial bioprocessing can produce eggshell powder, CaO, calcium phosphate bioceramics, adsorbents, catalysts, and composite fillers with application-dependent properties. Evidence supports calcium fortification with appropriately processed eggshells, experimental use of eggshell-derived hydroxyapatite and β-tricalcium phosphate in scaffolds or drug delivery systems, and applications in adsorption, catalysis, mineral fillers, and soil amendment. However, most routes remain at the laboratory or pilot scale, and the environmental superiority of green processes is rarely established by life cycle assessment. Feedstock variability, sanitation, process standardization, energy and chemical demand, regulatory classification, clinical validation, and consumer acceptance remain major barriers. By linking composition, processing, material performance, evidence maturity, and translation requirements, this review identifies priorities for safe and scalable eggshell waste valorization.