Thermoplastic Biopolymer Films Based on Lupin Proteins

Protein-based films can be produced by either wet or dry processing. While the wet process involves protein solubilization in an adequate solvent, the dry process is based on the thermoplastic behavior of proteins when water or plasticizers are present. In this comparative study, biopolymer films based on lupin protein isolate (LPI) from bitter lupins were produced by solvent-casting and compression-molding of manually mixed and extruded molding compositions. We focused on evaluating the effects of the processing methods as well as varying glycerol contents (200 to 500 mg g-1) on the mechanical and functional properties of the resulting films. Glycerol had a similar effect on all LPI films, where higher concentrations increased the ductility while decreasing the tensile strength (Rm) of the films. Cast films were highly homogeneous with high elongation at break values up to 140%. Compression-molded films exhibited a more irregular appearance but demonstrated increased water stability. Films produced by compression molding of extruded molding compositions exhibited significantly increased Rm values compared to the other investigated films. At a glycerol concentration of 300 mg g-1, Rm reached 9.6 MPa, approximately twice that of the other film types. This increase may be associated with the higher determined content of β-sheet structures, temperature-induced cross-linking, and the presence of a microfibrillar morphology, which was formed during the extrusion process.