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Synthesis and Applications of Non-migratory Metal Chelating Active Packaging

Many packaged foods use synthetic chelators (e.g. ethylenediaminetetraacetic acid, EDTA) to inhibit metal promoted oxidation and/or microbial growth that may cause food spoilage. Consumer demand for foods without synthetic additives has prompted growing interest in alternative preservation methods. Our research group has previously developed non-migratory metal chelating active packaging materials by surface immobilization of polymeric chelators and demonstrated their ability to inhibit lipid oxidation in model food emulsions. The work presented in this dissertation investigates the synthesis, performance stability, and practical application of metal chelating surface modifications to optimize design of non-migratory metal chelating active packaging materials. Metal chelating active packaging materials were synthesized by grafting of metal chelating polymers from the surface of polypropylene (PP). Three metal chelating ligand chemistries were investigated for their known affinity for iron: carboxylic acids, hydroxamic acids, and catechols. Iron was chosen a target metal ion because it is a strong prooxidant and essential nutrient for spoilage bacteria. When utilizing photoinitiated graft polymerization to surface graft poly(acrylic acid) that contained carboxylic acid ligands, it was demonstrated metal chelating polymer chain length and density may be manipulated to tailor both overall material iron chelating capacity (chain length) and ligand to metal binding ratio (chain density on food contact surface). Carboxylic acid functionalized PP (PP-g-PAA) enhanced the antimicrobial activity of lysozyme against Listeria monocytogenes under conditions that minimized protein fouling onto the charged food contact surface (PP-g-PAA, pKabulk 6.45). Compared to PP-g-PAA, hydroxamic acid functionalized PP (PP-g-PHA) was hypothesized to have a higher performance stability due to its low effective charge (pKabulk 9.65) and high specific affinity for iron. PP-g-PHA retained iron chelating capacity over a wide range of pH (3.0-5.0) and viscosity (~1 to 105 mPa·s) conditions as well as in presence of competing ions (Na+, Mg2+, Ca2+) and hydrocolloids typically found in foods. In order to investigate the efficacy of catechol based surface modifications, a metal chelating material inspired by polyphenol chemistry was developed. Polyphenol coatings were fabricated by oxidative polymerization of catechol and catechin from the surface of PP. Application of coating onto chitosan functionalized PP prevented coating delamination in food simulants under standard migration test conditions. Polyphenol coated PP exhibited both iron chelating and radical scavenging capacity for dual antioxidant functionality. In accelerated storage studies, polyphenol coated PP extended the lag phase of lipid oxidation and inhibited lycopene degradation in oil-in-water emulsions (pH 4.0). This work demonstrates the synthesis of non-migratory metal chelating active packaging materials and their antimicrobial and antioxidant applications. Such technology may allow for the removal of synthetic additives from product formulations, while maintaining food safety, quality, and shelf life.
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