Bi-layer PVA/sodium alginate active films reinforced with clays and functionalized with Nigella sativa essential oil: enhanced stability, controlled release, molecular interaction insights, and fresh beef shelf-life extension

Abstract

In this study, active bilayer films based on PVA-SA, activated with Nigella sativa essential oil (NSEO) and stabilized with nanoclays (bentonite and LDH), were successfully developed and characterized. The incorporation of the essential oil-clay systems significantly improved the physicochemical properties of the films. The developed films exhibited satisfactory hydrophobicity, with a water contact angle of 53.8°, while water solubility and water absorption capacity were significantly reduced. Mechanical properties were also improved due to the plasticizing effect of the essential oil, as confirmed by DSC analysis, which showed a decrease in the glass transition temperature to 48.93 °C. Improved thermal stability was also evidenced by TGA analysis. The developed films exhibited strong antioxidant activity, with a DPPH radical scavenging activity of 59%, as well as significant antibacterial activity against Escherichia coli and Staphylococcus aureus , with inhibition zone diameters of 21 mm and 20.5 mm, respectively. Furthermore, the barrier properties were enhanced by improved UV-shielding ability and a reduced water vapor transmission rate of 45 g m -2 ·h -1 compared with the PVA-SA-EO film. The films also enabled a controlled release of essential oil bioactive compounds was observed up to 14 days. Molecular docking, rheological, FTIR, and XRD analyses suggested the existence of interactions between the polymer matrix, nanoclays, and the major constituents of the essential oil. The developed films were subsequently applied as spray coatings for the preservation of refrigerated beef. The coatings effectively preserved beef quality by limiting the increases in pH, lipid oxidation (TBARS), protein degradation (TVB-N), color deterioration, and weight loss during 10 days of storage at 4 °C. Overall, the developed PVA-SA-EO-clay coatings represent a promising, sustainable, and environmentally friendly strategy for extending shelf life, reducing food waste, and improving the safety of protein-rich foods.