Active papers coated with chitosan and containing TiO₂ and Ag/TiO₂ nanoparticles for increasing the shelf-life of walnut kernels

Three active cellulose-based papers coated with chitosan (P–CH), chitosan–TiO₂ nanocomposite (P–CH–TiO₂) and chitosan–Ag/TiO₂ nanocomposite (P–CH–Ag/TiO₂), respectively, were subjected to real-time ageing for 6 months at 25 °C and 45% re humidity and characterized from a physical–chemical–antimicrobial perspective in relation to plain paper (P). Scanning electron microscopy pictures indicate a smooth morphology in the active papers, while the structure of P is rough with large pores. The fresh papers containing nanocomposites displayed enhanced barrier properties against water vapor, air and grease. The photocatalytic action of TiO₂ during ageing results in the deacetylation of the chitosan layer and the degradation of cellulosic fibers with a reduction of barrier properties. Fresh P–CH–Ag/TiO₂ displayed the highest value of inhibition index against yeasts and molds (95.65%), its value decreasing to 79.25% after 6-month storage. Walnut kernels were stored for 6 months at 25 °C and 45% RH in packages made of investigated papers and analyzed. It was found that P–CH–Ag/TiO₂ is the most effective in the prolongation of the shelf-life of kernels. They are assessed to be safe in terms of peroxide value and yeasts and molds count for 11.6 and 15.1 months when stored in P–CH–Ag/TiO₂ filled with 25 and 50 g kernels, respectively, as compared to 1.8 and 3.4 months, respectively when they are stored in P. The appearance of packages containing nanocomposites was clean without grease spots in opposition with that of P and P–CH and proves the self-cleaning property of P–CH–TiO₂ and P–CH–Ag/TiO₂. Cluster diagrams indicate a strong correlation between the quality and safety of kernels and properties of paper sheets in a paper type-depending manner.