Food spoilage due to undesirable microbial growth is a common factor which shortens the shelf life of food products and can cause food borne illness outbreaks. Food spoilage is one of the main factors that causes food waste. According to United States Department of Agriculture, approximately 133 billion pounds and $161 billion worth of food in 2010 was wasted, which counts for 30-40 percent of the food supply. The addition of antimicrobial agents (active compounds that kill or prevent the growth of microorganisms) to foods or food surfaces during processing is an effective technique that is frequently employed to reduce food spoilage and eliminate food borne pathogens. However, the antimicrobial activity may be rapidly lost due to inactivation of the antimicrobials by components in foods or the antimicrobials are diluted below active concentrations due to migration into the bulk food matrix. Therefore, antimicrobial agents have been incorporated or immobilized to packaging materials, called antimicrobial food packaging (AM food packaging), to prevent the growth of microorganisms on the food surface and thus lead to an extension in shelf life or improved microbial safety of the food (Collins-Thompson and Hwang 2000; Suppakul and others 2003). AM food packaging has received great interest from the food industry attributed to the increasing consumer demand for minimally processed, preservative-free, “fresh” foods, because a major advantage arising from the use of AM packaging is that only low levels of preservative come into contact with the food, compared to the direct addition of preservative to the food (Suppakul and others 2003; Robertson 2006).
Types of AM packaging
Mechanisms that provide the antimicrobial action of packaging materials include incorporation or immobilization of antimicrobial agents into packaging materials or onto the material surface, or using polymers that are inherently antimicrobial (Appendini and Hotchkiss 2002; Suppakul and others 2003; Robertson 2006). Depending on the mechanisms and whether the antimicrobial agents are volatile or nonvolatile compounds, antimicrobial packaging can be divided into five different types including:
Antimicrobial packaging realized with volatile compounds
The volatile compounds, including chloride dioxide, plant extracts, essential oils, and allyl-isothiocyanate, etc, can be added into sachets, pads, gauze, or filter paper, incorporated into the polymer films, or coated on the film surface (Lucera and others 2016). The advantage of this type AM packaging is that the polymer does not necessarily need to be in direct contact with foods and volatile compounds can penetrate most of the food matrix (Lucera and others 2016). For all the other types of AM packaging mentioned below, physical contact between the packaging material and the food surface is required in order to achieve antimicrobial activities, such as which occurs in vacuum packaged, shrink-wrapped, or rigid bottle contained foods.
Incorporation of non-volatile antimicrobial agents
Direct incorporating antimicrobial agents into packaging materials, such as polymers and papers, is a convenient means by which AM activity can be achieved. While contacting with food surface, the antimicrobial agent migrates partly or completely into the food or in the space surrounding the food and exercises its antimicrobial activities. Various polymers have been studied as potential candidates for incorporation of an antimicrobial substance in food packaging applications, including synthetic polymers derived from petroleum, such as low-density and high-density polyethylene, polypropylene, polystyrene, polyethylene terephthalate, ethylene-vinyl acetate, poly(vinyl chloride), and poly(butylenes adipate-co-terephthalate); edible films, such as starch, cellulose derivates, chitosan, alginate, fruit-puree, whey protein isolated, soy protein isolated, corn zein, wheat gluten, egg albumen, gelatin, and sodium caseinate; and bioplastics, such as thermoplastic starch, polycaprolactone (PCL), poly (lactic acid) (PLA) and polyhydroxyalkanoates (PHA) (DeGruson 2014).
Coating or adsorbing antimicrobials onto polymer surfaces
Antimicrobials can be coated onto the packaging materials after forming or added to cast films to impart AM effectiveness. Coating is especially valuable if AM cannot tolerate the temperatures used in polymer processing. Cast edible films, for example, have been used as carriers for antimicrobials and applied as coatings onto packaging materials and/or foods. For example, fungicides incorporated into waxes to coat fruits and vegetables, shrink films coated with quaternary ammonium salts to wrap potatoes, and nisin using methylcellulose as a carrier coated onto polyethylene films.
Immobilization of antimicrobial agents
In this type of AM packaging, the antimicrobial agents are immobilized by the functional groups in packaging materials by ion or covalent linkages. While contacting with food surface, the antimicrobial agents in packaging materials will not migrate onto food surface and they act when target microorganisms come into contact with the surface of the packaging materials. Surface modification of polymers are often necessary to generate the functional groups to bond with the antimicrobial agents. Shin (2016) at University of Wisconsin-Stout modified the food cling wrap low density polyethylene (LDPE) using UV light treatment to graft natamycin.
Use of polymers that are inherently antimicrobial
Some polymers are inherently antimicrobial and thus can be uses as AM packaging in films and coatings. These polymers include chitosan, poly-l-lysine promote cell adhesion, and calcium alginate (Appendini and Hotchkiss 2002). Among them, chitosan has been studied widely to use in AM packaging in multiple ways. It can be formed into edible films and coatings which inherently have antimicrobial activity. The functional properties, such as mechanical and barrier properties, of the chitosan-based films and coatings can be improved by blending with other polymers or by incorporating micro- or nanoparticles to form composites or nanocomposites. Chitosan also shows a great potential to encapsulate bioactive compounds to enhance the antimicrobial activities. Another possibility to use chitosan in AM packaging is not only as matrix or main materials in a composite or nanocomposite approach, but also as nanostructured chitosan or particles as second phase in a multifunctional system with potential application at an industrial level (Fortunati 2016).
Challenges of AM packaging by direct incorporation
Among the above five different types of AM packaging, direct incorporating antimicrobial agents into packaging materials is a convenient means by which AM activity can be achieved and has been widely studied. However, two potential problems may occur by directly incorporating AM into polymeric matrix: possible negative effect on properties of packaging materials and unpredicted release of AM from the polymeric matrix.
The properties of food-packaging films are influenced by the incorporation of antimicrobial substances, such as mechanical, gas barrier, thermal, and morphological properties (DeGruson 2014). The mechanical properties of polymeric films for food packaging applications are important since the packaging materials are under various stresses that occur during the processing, handling, and storage of packaged foods. A significant change can be obtained in the tensile properties of polymeric films after the incorporation of antimicrobials. Many studies have found that a decrease in film strength and resistance with an increase in concentration of the antimicrobial incorporated in the polymer. A few cases have found that the tensile properties were not significantly affected or even improved by the addition of antimicrobial agents.
Besides the possible negative effect on properties, the incorporation of low molecular weight antimicrobial agents into the polymer matrix has another disadvantage, the migration and the release of the AM cannot be easily predicted or controlled. Due to the polymeric structure and the interaction between the polymer and antimicrobials, the antimicrobials are either completely bound in the polymeric matrix or suddenly released into the aqueous system.
To overcome these problems, a method of loading antimicrobial-nanoparticle systems into polymeric matrix is developed in DeGruson’s packaging lab at University of Wisconsin-Stout. The mechanism of the method is explained below.
Use antimicrobial molecule-nanoparticles system for AM packaging
In particular, nanoparticles, layered double hydroxide (LDH), are used as an active molecule delivery vehicle to carry antimicrobial agents and the antimicrobial-nanoparticles systems are then incorporated into polymeric matrix to form packaging films. The LDH nanoparticles in the nanocomposite films play dual functions from two parts of the LDH, the inorganic nano-scale part improves the mechanical and barrier properties, and the loaded antimicrobial agents on LDH brings a functional group capable of providing antimicrobial activity (Fig. 1). Furthermore, the proper selection of antimicrobial agents to modify the nanoparticle can increase the compatibility between the nanoparticle and polymer, and thus further improve mechanical and barrier properties of nanocomposite films.
On the other hand, the impermeable nanolayers mandate a tortuous pathway for a permeant to transverse the nanocomposites (Fig. 2). The enhanced barrier properties and the controllable release of antimicrobial agents of nanocomposites can benefit from the hindered diffusion pathway through the nanocomposites (Sorrentino and others 2007).
Figure 1. Structure of nanoparticle LDH
Figure 2. Schematic illustration of the tortuosity for a diffusing penetrant introduced on exfoliated solid layered in a polymer matrix. A. Filled polymer and B. Unfilled polymer.
A., L., A., C., and Del, N. M. A. (2016). Volatile compounds usage in active packaging system. In J. Barros-Velazquez (Ed.), Antimicrobial food packaging (1at ed., pp. 319-327): Academic Press.
Appendini, P., and Hotchkiss, J. H. (2002). Review of antimicrobial food packaging. Innovative Food Science & Emerging Technologies, 3(2), pp. 113-126.
Collins-Thompson, D., and Hwang, C. A. (2000). Packaging with antimicrobial properties. In R. K. Robinson, C. A. Batt, and P. D. Patel (Eds.), Encyclopedia of food microbiology (pp. 416-418): Elsevier Science.
DeGruson, M. L. (2014). Synthesis of bio-based nanocomposites for controlled release of antimicrobial agents in food packaging. The Pennsylvania State University.
E., F. (2016). Multifuntional films, blends, and nanocomposites based on chitosan: Use in antimicrobial packaging. In J. Barros-Velazquez (Ed.), Antimicrobial food packaging (1at ed., pp. 467-477): Academic Press.
Robertson, G. L. (2006). Active and intelligent packaging. In G. L. Robertson (Ed.), Food packaging: Principles and practice (2nd ed., pp. 286-312): Taylor & Francis.
Sorrentino, A., Gorrasi, G., and Vittoria, V. (2007). Potential perspectives of bio-nanocomposites for food packaging applications. Trends in Food Science & Technology, 18(2), pp. 84-95.
Suppakul, P., Miltz, J., Sonneveld, K., and Bigger, S. W. (2003). Active packaging technologies with an emphasis on antimicrobial packaging and its applications. Journal of Food Science, 68(2), pp. 408-420.
Min L. DeGruson, Ph.D., is Assistant Professor at University of Wisconsin-Stout
We are proud to announce the project with AMETEK MOCON and UW Stout was a success and the joint paper has been accepted by the IAPRI World Packaging Conference to present at their event in China, June 2018! If you would like to attend, please visit the conference website at: http://www.2018iapriconference.org/