Desenvolupament, caracterització, i modelització de nous materials polimèrics destinats a l'envasat actiu alimentari

This dissertation deals with the development, characterization, and modelling of new polymeric materials intended for active food packaging applications capable of exerting effective antimicrobial action on the preserved product. These materials are essentially composed of ethylene – vinyl alcohol copolymers (EVOH) with carvacrol, citral or other essential oils functioning as natural antimicrobial agents, which allow the release of their active ingredients towards the packaged foodstuffs when the ambient humidity generated by them is able to substantially modify their barrier characteristics. They are either in the form of stand-alone films or as coatings over other conventional polymers, such as polypropylene (PP) or polyethylene terephthalate (PET). The relation between the active properties of the new materials and the ambient conditions to which they are subjected has been studied in the present work through the measurement of the diffusion and solubility coefficients of the mentioned agents, which were studied in their matrix as a function of the temperature and the relative humidity. In parallel to these studies, a complete functional characterization of the new materials has also been carried out through the determination of their main morphological, mechanical, thermal, optical, surface, and barrier properties in order to evaluate their possible alteration or deterioration due to the potential physicochemical interactions between the incorporated compounds and their carrier matrices. These new materials were employed in the construction of active packages for the preservation of fresh fish and minimally processed vegetables, designed to release the antimicrobial agents into the headspace in sufficient concentration so as to inhibit the growth of pathogens or food altering microorganisms on their surface. However, it also became evident that the activity exerted by these packages was only effective during the three first or last days of their storage, as a function of the specific location of the active layer within the multilayer structure characteristic of the constituent films. For this reason, the present dissertation subsequently addressed the potential improvement of their antimicrobial performance through two different approaches or strategies. On the one hand, mathematical models of both packages, based on the finite element method (FEM), were developed with the aim of revealing the diverse structural parameters and / or ambient conditions that mostly governed their behavior through computational simulations, and thus ultimately find the existing ways towards their optimization. On the other hand, two new chemical and / or structural modifications were introduced in the matrix of the carrier material with the aim of increasing its retention capacity for the active agent and of further controlling its release. This consisted of the incorporation of bentonite nanoparticles into the polymeric matrix as inorganic loading, and its substitution by other ethylene copolymers of different polarity in the form of aqueous dispersions (latex). The improvement attained in the antimicrobial performance of such a polymer, after these chemo-structural modifications of its matrix, was experimentally evaluated with new measurements of its active and functional properties. In the particular case of bentonite incorporation however, a new mathematical model was also developed in order to theoretically estimate it and thus compare the results found through both methodologies.