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

Mihaly Cozmuta, Anca, Apjok, Robert, Peter, Anca, Mihaly Cozmuta, Leonard, Nicula, Camelia, Baia, Monica, Vulpoi, Adriana
Cellulose 2018 v.25 no.9 pp. 5205-5225
air, cellulose, cellulosic fibers, chitosan, humidity, molds (fungi), nanocomposites, nanoparticles, paper, peroxide value, photocatalysis, scanning electron microscopy, seeds, shelf life, titanium dioxide, walnuts, water vapor, yeasts
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.