Antimicrobial packaging is an active packaging strategy that can serve a variety of distinct barrier functions in MAP systems, such as package self-sterilization, sterilization of produce, or reduction of growth of spoilage organisms and/or pathogens on packaged produce. Synthetic polymer films are most commonly researched for this functionality. Antimicrobial packaging films can be grouped into two general categories, nonmigratory and migratory. Nonmigratory packaging incorporates antimicrobials into the polymer or immobilizes them on the surface of the film in such a way that the compounds are not released; food must be in direct and intimate contact with such films in order for antimicrobial activity to occur. Migratory antimicrobial packaging incorporates the antimicrobial into or on the surface of the film in such a way that migration can occur to the food product where activity then occurs; unlike nonmigratory antimicrobial films, the antimicrobial becomes part of the foodstuff that eventually is ingested. Migratory antimicrobials can be released in aqueous solution (e.g., nisin, organic acids), or as a vapor (e.g., allyliso-thiocyanate, chlorine dioxide). The latter method of release is perhaps most suited to MAP systems where headspace between package and product is maintained and intimate contact between package and product does not typically occur. Release of migratory preservatives must be finely controlled for useful and effective activity that persists over the course of a defined or desired package shelf life.
The development of antimicrobial films has been significantly restricted by the legal status of antimicrobial compounds available for food contact use or as food preservatives or additives; currently only a limited number of such approved compounds exist, and approval varies among countries. Silver-substituted zeolites, a broad-spectrum high-activity antimicrobial with low human toxicity, has been extensively used commercially in Japan as a thin laminant on packaging film surfaces; its use on food contact surfaces in the European Union and the U.S., however, is unclear . Some U.S. Food and Drug Administration (FDA) generally regarded as safe (GRAS) materials that have been considered for use as antimicrobials in synthetic polymer films include organic acids (benzoic, lactic, propionic, malic, succinic, tartaric, sorbic), enzymes (lactoperoxidase, lactoferrin, lysozyme, chitinase, glucose oxidase, ethanol oxidase), isothiocyanates (allylisothiocyanate), bacteriocinsc (nisin, pediocin, sakacin, subtilin, carnocin), and essential oils (thymol, cinnamic acid, eugenol) [6,24]. Natural plant extracts such as grapefruit seed extract have been shown to be effective against Staphylococcus aureus and E. coli on MAP lettuce and bell pepper, and when used in combination with imazalil could also provide protection against growth of molds, yeasts, and lactic acid bacteria . Antimicrobials that are volatile, such as chlorine dioxide and allylisothiocanates, have an advantage in that they can be distributed within the closed package.
Any antimicrobial must not only be approved for food use, it must also be compatible with the packaging material and the package/film manufacturing process as well as maintain activity in the particular food matrix and MAP system . Thus, different strategies may be employed in creating antimicrobial films and designing antimicrobial packaging systems from synthetic polymers versus more natural materials as are used in edible and biodegradable films.
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