Introduction: In the food industry, the growing demand for sustainable food packaging has drawn attention towards active biodegradable forms of food packaging. In the presented work, alginate-based films incorporating zinc oxide nanoparticles (ZnO NPs) were prepared and evaluated for extending the shelf life of packed black grapes.

Methodology: ZnO NPs were biosynthesized using a green microalgae strain, Dictyosphaerium HSM, isolated and identified via ITS sequencing. Synthesis was confirmed through UV-Vis, FTIR, XRD, and TGA, revealing hexagonal nanoparticles (<100 nm) via SEM and AFM. Alginate–ZnO films with different ZnO ratios were prepared via solvent casting. XRD and SEM analysis confirmed uniform NP distribution and surface morphology.

Results: The ZnO NPs exhibited strong antibacterial, antifungal, and antibiofilm effects and minimal cytotoxic effects. Mechanical testing showed enhanced tensile strength, reduced moisture content, and increased hydrophobicity. Antimicrobial and antibiofilm activities were evaluated using disc diffusion and SEM, using E. coli as a model organism. The films significantly lessened the microbial load on inoculated grapes, extending their shelf life from 7 to 9 days at room temperature.

Conclusion: These findings strongly indicate the potential of alginate–ZnO nanocomposite films as active, eco-friendly food packaging materials able to reduce the spoilage caused by microbial contamination. Their antimicrobial potential and their enhanced mechanical characteristics make them an effective solution for the long-term preservation of food.