Antimicrobial edible films and coatings based on modified chitosan containing essential oil nanoemulsions

The interest towards the use of edible films and coatings in food preservation has been sustained in the recent years by the development of active biopolymeric matrices incorporating natural antimicrobial compounds, in replacement of synthetic additives. In this work, we present our recent activities addressed to the development and optimization of edible films and coatings based on modified chitosan containing different types of essential oil nanoemulsions. The essential oil nanoemulsions have been formulated to maximize the antimicrobial activity, developing significant synergies with the intrinsic antimicrobial properties of modified chitosan and its barrier properties, while preserving the desired material characteristics, and in particular transparency and hydrophobicity. The developed antimicrobial films and coatings have been tested in vitro (disk inhibition zone) and in product (broccoli florets and green beans) against different microorganisms of interest for food preservation, such as Escherichia coli, Listeria innocua, and Listeria monocytogenes. In addition, this work discusses also the possible synergies and technological drawbacks of the combination of the antimicrobial edible films and coatings with non-thermal preservation technologies, such as high intensity pulsed light and high hydrostatic pressure, in view of food microbiological stabilization during its shelf life, as well as of the impact on product color and texture. Results showed that emulsion formulations significantly affected the intrinsic antimicrobial activity of edible films and coatings. However, also the interaction of the nanoemulsions structure with the modified chitosan matrix impacted film properties and resulting bactericidal action. In addition, in comparison to the use of pure essential oils, the encapsulation into nanoemulsions also fostered the production of more homogeneous films with better appearance. The combination of the coating application with high hydrostatic pressure resulted as the most promising approach, causing a significant microbial reduction over the entire storage period analyzed, owing to the development of a significant synergism of antimicrobial effects; however, it also had a strong impact on product firmness. In contrast, the combination of the coating application with high intensity pulsed light exhibited a slight antagonistic effect, and had a slight detrimental impact on color properties. This work hence contributes to promote the incorporation of nanoemulsions of essential oils in edible films and coatings for integrated food preservation strategies