Developing active packaging solutions through incorporation of organic/inorganic active components in renewable materials.

The development of new antimicrobial active packaging systems have been gaining a raising interest due to its potential to increase product shelf-life and provide food quality and safety benefits allied to society demands on food-related health risks, multi-drug resistance and environmental problems. A great variety of organic substances such as phenolic compounds and essential oils as well as inorganic metal oxide nanoparticles such as Ag+, ZnO and TiO2 have been intensively studied for having antimicrobial properties, although their efficiency is highly dependent on the target microorganisms, the material or media where they act as well as the surrounding environment are relevant (Burt, 2004; Suppakul et al., 2003; Visai et al., 2011). Recent research in active packaging is mostly focused on the use of natural renewable material resources including preservatives to develop biodegradable and recyclable packaging products. Accordingly, the NEWGENPAK project was funded within ITN-Marie Skłodowska-Curie EU program with the aim “to take wood cellulose based material a significant step forward by replacing petroleum-based additives used in paper and board packaging materials in order to achieve the barrier and other crucial properties needed for competitive, low carbon footprint, packaging materials”. NEWGENPAK, the acronym for New Generation of Cellulose Fibre Based Packaging Materials for Sustainability, just finished in December 2015, was an interdisciplinary research training network (ITN) constituted by 8 European universities, 3 research institutes and 6 enterprises from all over Europe, with13 researchers working full time developing their own individual researcher projects, making collaborations and receiving training on the field. This PhD thesis was carried out within this project and developed mainly at Innovhub SSI – Paper Division, Milan, Italy. The main target of this work was to attain antibacterial cellulose-based materials for food packaging applications, following two approaches based in the incorporation of active organic components or active nanoparticles as active agents. Besides, it was studied the possibility to develop an antibacterial packaging for medical applications, in order to prevent medical cross contamination. The fate of the nanoparticles in the recycling process and their effect on the biodegradability of the packaging was initially assessed as an important part of environmental aspects related to the end of life of packaging products. The first approach, described in the chapter 2, aimed to explore the possibilities to extract polyphenols from black tea brewing residues and use them as active compounds for the development of active cellulosic-based surfaces. Therefore, the chemical characterization of black tea residues as well as the antioxidant and antimicrobial properties of their extracts were addressed. The best infusion conditions, considering the yield of extraction, the antioxidant activity and the total phenolic content, were found to be at 80ºC for 7.5 minutes for an infusion of 2.5 g of tea residue in 100 mL of water, and just 1.1 mg of these extract where enough to provide a bactericidal effect. The resulting paper coated with 3.8 g/m2 of polyphenols-based coating formulation attained a complete killing effect against S. aureus. In the second approach, several papers were functionalized with formulations based on photo-active TiO2 NPs by dip-coating and compared regarding their antibacterial activity. The results presented in the chapter 3 have shown that both handsheets of bleached Kraft pulp (BK) and chemithermomechanical pulp (CTMP) displayed a bactericidal effect against gram-positive Staphylococcus aureus and gram-negative Pseudomonas aeruginosa, even after three weeks of storage either in light and dark conditions, while pre-coated recycled paper (PCR) and bleached pre-coated Kraft (BPK) paper samples did not shown any antibacterial activity. The effect of TiO2 NPs against S. aureus was inhibited in: i) PCR samples due to the presence of considerable amounts of inorganic compounds, such as calcium carbonate, that shielded the effect of active nanoparticles; and ii) BPK samples, most likely due to their high hydrophobicity that did not permit a good retention of the NPs and homogenous coating distribution. Accordingly, different preparation methods and deposition techniques were considered for hydrophobic surfaces and compared regarding the amount of TiO2 incorporated in nanofibrillated cellulose (NFC) loaded and finally retained on the BPK paper surfaces. Under the best conditions with the polyelectrolyte-assisted deposition 90% of nanoparticles retention was attained against only 25% for the direct-mixture formulations. The antibacterial activity of the paper samples reached approximately 2 log bacterial reduction of S. aureus showing the possibility to achieve a contact active surface based on layer-by layer assembly NFC-TiO2 formulation. Moreover a scale-up pilot demonstration of an over-print varnish based on ZnO nanoparticles was performed to be loaded by flexographic printing at industrial scale for medical packaging applications. The SAFEBOX packaging demonstrator produced was loaded with only 5.6 mg/m2 of ZnO NPs based varnish, due to some technical production constraints and restrictions, therefore it presented a slightly bacteriostatic effect with less than one log reduction. However, with the possibility to increase the amount of NPs loaded on the paper surface, promising results can be achieved. Preliminary results obtained at lab scale showed a bactericidal effect, up to 4 log reduction, for papers with about 1,5g/m2 of ZnO NPs on the surface. Regarding the preliminary studies on environmental impact of NPs, towards packaging end-of-life options presented also in the chapter 3, laboratory tests have shown only marginal effect of active ingredients on biodegradability performance whereas recyclability tests have shown a reasonable good retention of TiO2 nanoparticles (approximately 90%) in the recycled fibres after one recycling loop.