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

Abstract

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.