Production of a functionalized film for non-food applications from casein-based powders obtained from expired pasteurized milk

This study* explored sustainable strategies for upcycling expired pasteurized milk by valorising its protein fraction, particularly casein, for non-food applications. Although casein is widely used in the food industry, its functional properties also make it a promising material for the development of sustainable alternatives to synthetic polymers to be processed into coatings, films, or 3D-printed objects. Four different casein-rich powders (acid casein, rennet casein, sodium caseinate, and co-precipitate) were produced at lab scale from unsold pasteurized milk after expiration date. These powders were characterized for chemical composition, water solubility, and rehydration behaviour, which are key factors influencing potential applications. Water solubility was assessed under different conditions, varying protein concentration (3.5% and 10% w/v), pH (native and 9), and addition of calcium-chelating salts (trisodium citrate or disodium phosphate). The results revealed significant differences among the powders: acid casein and sodium caseinate exhibited high solubility in presence of salts, while rennet casein showed limited rehydration. Sodium caseinate was selected to produce film-forming solutions, which were then functionalized using two green crosslinking strategies: (i) thermal treatment at alkaline pH to induce lysinoalanine (LAL) formation, and (ii) riboflavin-mediated photo-oxidation to promote the formation of di-tyrosine (di-Tyr) bonds. Casein crosslinking aimed at improving mechanical and structural properties of the derived films obtained by casting. Formation of LAL crosslinks resulted in enhanced tensile strength and stiffness of the derived film, while formation of di-Tyr improved film ductility. This study highlights the potential of expired milk as a source of casein that can be used as sustainable alternative to synthetic polymers in non-food applications. The proposed approach is fully green, and the neat films of casein are more easily biodegraded. Overall, it demonstrates how milk waste can be transformed into value-added materials with improved functional properties, contributing to the circular economy. *This study has been funded by “Progetti di ricerca di rilevante interesse nazionale (PRIN) – bando 2022- PROT. 20224P7H7H” - “FROM EXPIRED MILK TO GOODS: upcycling milk protein to biodegradable 3D-printable objects for packaging application”