Upcycling casein from expired pasteurized milk into films – Development and characterization of films made from sodium caseinate

The present study focuses on the upcycling of expired pasteurized milk into sodium caseinate (NaCas) that was used to develop sustainable packaging films. Three NaCas types were prepared from pasteurized milk with a different fat content and a commercial NaCas was used as a control. The film forming solutions were added with glycerol as a plasticizer and were treated with two different sonication modes, bath or probe, before casting and drying in ambient conditions. Results showed that NaCas with varying milk fat content led to notable differences in properties of the derived films. Films with higher fat content (25 %) were thicker and with a more irregular and heterogeneous microstructure, as observed by CLSM and SEM. Sonication, particularly probe sonication, reduced the size of milk fat structures in the derived films. Fat content also impacted on mechanical properties, such as tensile strength and elongation at break, as well as on oxygen permeability which showed higher values in films with high fat content. A significant improvement in the oxygen barrier was achieved when bath sonication was used. Thermal analysis revealed distinct transitions related to both fat content and processing, affecting melting and crystallization in the different films. Films with high fat content were hazier than low-fat films. By properly combining milk fat content and sonication conditions, NaCas obtained from expired pasteurized milk can become a raw material for upcycling this high-impact waste into neat films for green packaging applications.