Natural antimicrobial nanomaterials for food packaging applications

Consumer demand for fresh-like, sustainable, minimally processed, and clean label food products has been increasing significantly over recent years. However, removing chemical preservative and additives from food products significantly reduces their shelf-life. To address this challenge, natural antimicrobial nanomaterials (NAMs) can be used to inhibit food spoilage microorganisms and extend the shelf-life of food products. Several of the most effective NAMs include: antimicrobial polysaccharides (chitosan (CS)), plant essential oils (oregano essential oil (OEO)), and organic acids (sorbic and benzoic acid). To this end, chitosan nanoparticles (CS NPs) were synthesised and the effects of the initial concentrations of chitosan (CS) and sodium tripolyphosphate (TPP), the CS:TPP mass ratio, the CS molecular weight (MW) and pH on the CS NP properties was assessed. Self-assembled monodisperse CS NPs with a particle size of 90 nm were successfully synthesised using solutions of low molecular weight CS at pH 4.6 and 3:1 (CS:TPP) mass ratio. However, when higher concentrations of CS were used, the application of external forces (tip sonication or Ultra-Turrax™) was necessary to induce monodispersity and reduce the particle size. While both native CS and CS NPs showed antimicrobial activity, no significant antimicrobial enhancement was observed for the NP form. Subsequently, native CS, CS NPs, and commercially-available carnosolic acid nano-solubilisates (CASB) solutions were used as part of an aerosolisation based coating method with vacuum skin packaging as additional hurdles to extend the shelf-life of highly perishable European Hake (Merluccius merluccius) fillets. Results indicated that aerosolised solutions of CS, CS NPs, and CASB on hake fillets were found to extend the shelf-life by up to 40, 50 and 55 %, respectively compared to untreated control samples. Moreover, physiochemical properties of hake fillets such as pH and colour were also found to improve with respect to storage. Regarding OEOs, the effects of different surfactants (Tween® 20 and Pluronic™ 127), the surfactant to oil ratio (2:1, 1:1, and 1:2) and physical treatments (untreated, IKA ultra-turrax or tip-sonication) on the physical properties of the nanoemulsions was evaluated. The antimicrobial activity of the monodisperse nanoemulsions was assessed and OEO nanoemulsions made from Tween® 20 in a surfactant to oil ratio of 1:1, an average droplet diameter of 131 nm and a zeta potential of 39.2 mV showed the greatest antimicrobial activity. Therefore, these nanoemulsions were applied via aerosolisation treatment on fresh European Hake (Merluccius merluccius) fillets. Aerosolisation treatment of OEO nanoemulsions on hake fillets extended the shelf-life by up to 50% compared to untreated control hake fillets. Moreover, the physiochemical properties of hake fillets such as pH and colour were also found to improve. In another approach, a simple method to attach an antimicrobial essential oil support material to a functionalised surface was investigated. SBA-15 was modified with 3-aminopropyltriethoxysilane (APTES) and was then bound to an epoxy modified surface using 3-glycidoxypropyl-trimethoxysilane (GPTS). These materials were then loaded with oregano essential oil and their physical properties and antimicrobial activity against food spoilage microorganisms assessed. Finally, the antimicrobial effects of benzoic and sorbic acid salt and their commercially-available nano-solubilisates against planktonic and biofilm cultures of Staphylococcus aureus, Pseudomonas fluorescens and chicken microflora was assessed. The organic acid nano-solubilisates were more effective compared to their organic acid salt counterpart with respect to both planktonic and biofilm cultures. However, biofilms of Staphylococcus aureus, Pseudomonas fluorescens and chicken microflora were more resistant to both organic acids salts and nano-solubilisates compared to the planktonic cultures. Overall, this thesis extends our knowledge of nanomaterials for antimicrobial control of foodstuffs and shows the potential that they may have in developing active packaging systems