Ultrasound-enhanced bio-based active packaging films derived from passion fruit waste pectin: structural evolution, property correlations, and antioxidant activity through eggshell and oleuropein integration

Passion fruit peel, rich in natural pectin, is often discarded as agricultural waste, resulting in a significant loss of its potential value. In this study, pectin was extracted from discarded passion fruit peels and used to fabricate a multifunctional composite film Via an ultrasound-assisted casting method, incorporating eggshell powder and oleuropein as functional additives. Ultrasound treatment not only promoted the uniform dispersion of fillers, but also facilitated molecular interactions, thereby enhancing film formation and structural integrity. The molecular structure, mechanical strength, thermal stability, barrier properties, and antioxidant activity of the resulting films were systematically investigated. Through optimization experiments, the best-performing film was found to contain 2 % (w/w) eggshell powder and 1 % (w/w) oleuropein. This formulation provided the highest overall performance, achieving a tensile strength of 2.68 MPa, a water vapor transmission rate of 11.66 g/m²/24 h, and a DPPH radical scavenging activity of 80 %. (1) FTIR analysis revealed that Ca²⁺ ions from eggshell powder formed stable ionic cross-linking networks with pectin molecules, as evidenced by the redshift of hydroxyl stretching peaks (from 3300 to 3259 cm^-1), indicating the formation of coordination bonds. (2) When the concentration of eggshell powder ranged from 1.5 to 2.5 %, the tensile strength of the film increases of 1.77-2.68 MPa, while the water vapor transmission rate decreased of 14.31-11.66 g/m²/24 h, representing an 18.4 % enhancement in barrier performance. (3) The incorporation of oleuropein significantly improved the antioxidant capacity of the film in a dose-dependent manner. Specifically, 2,2-diphenyl-1-picrylhydrazyl (DPPH)radical cavenging activity increased markedly from 16 % to 80 % as the oleuropein concentration was elevated from 0.1 to 1.5 %. This progressive enhancement reflects the strong electron-donating and hydrogen-atom transfer ability of oleuropein, which can effectively neutralize free radicals and terminate chain reactions of oxidative processes. The polyphenolic structure of oleuropein provides multiple hydroxyl groups capable of interacting with reactive species, thereby amplifying the overall antioxidant response of the composite films. These findings demonstrate that the developed composite film holds great promise for active food packaging applications, providing a sustainable approach for the high-value utilization of agricultural by-products and promoting the development of eco-friendly packaging materials.