Tunable biodegradable films from fish gelatin and shark protein hydrolysate: Role of glycerol/hydrolysate ratio in controlling structural and functional properties

This study investigates the valorization of marine by-products by evaluating white-cheek shark protein hydrolysate (WCSPH) as a bio-based plasticizer to replace synthetic glycerol (0-100%) in gelatin films (20:0 to 0:20 ratios). Increasing the WCSPH content significantly enhanced barrier properties, reducing water solubility from 70.8% to 56.8% and water vapor permeability from 11.8 to 5.3 g·mm/(day·m 2 ·kPa). Surface hydrophobicity improved markedly, with contact angle rising from 68.4° to 96.0°, supported by increased surface roughness as measured by AFM. Mechanically, WCSPH increased tensile strength from 4.8 MPa to 20.5 MPa but reduced elongation at break from 182% to 6.9%. A 1:1 glycerol:WCSPH formulation (10G+10WCSPH) was optimal, balancing a tensile strength of 9.4 MPa with 90% elongation and a uniform microstructure. FT-IR analysis revealed enhanced hydrogen bonding, supporting improved matrix cohesion. WCSPH also provided potent antioxidant activity; the 20WCSPH film exhibited high DPPH (82.3%) and ABTS (82.4%) radical scavenging and a FRAP value of 12 μmol Fe 2+ /g. The films maintained high lightness ( L * = 90) and reduced opacity (0.34 to 0.20), though slight increases in yellowness and greenness were observed. In conclusion, WCSPH is an effective multifunctional modifier, and the 10G+10WCSPH film is a promising candidate for sustainable packaging, successfully valorizing marine by-products. • Gelatin films were developed using white-cheek shark protein hydrolysate (WCSPH) and glycerol. • WCSPH acts as either a co-plasticizer or anti-plasticizer, depending on its concentration. • Balanced WCSPH/glycerol ratio enhances film microstructure and overall performance. • WCSPH incorporation significantly improves moisture resistance and antioxidant properties. • FT-IR and AFM revealed stronger molecular interactions at higher WCSPH/glycerol ratios.