Film Permeability and CO2O2 Permselectivity

The specific gases employed, the gas permeability coefficients of the film, and the EMA desired will in part dictate the packaging films utilized. Concentrations of the two most commonly metabolically active gases employed in MAP, O2 and CO2, will impact produce quality and levels of both should be optimized within the package. CO2/O2 permselectivity, the ratio of CO2 to O2 permeation coefficients, will vary for different films and can be selected or altered to concurrently optimize levels of both CO2 and O2 in MAP systems [16]. Most commercial packaging films available have CO2/O2 permselectivities between 4 and 8, allowing greater diffusion of CO2 than of O2 [17]; anaero-biosis and less than optimal CO2 levels can result, depending upon the commodity and MAP system employed. Nitrogen is metabolically inert but can also be important as a filler gas to prevent package collapse. MAP produce subjected to temperature abuse or temperature changes along the normal distribution chain may result in increased respiration and depletion of in-package O2; subsequently, the effective target EMA will not be maintained, and premature spoilage or increased food safety risks may occur [18]. High-barrier films that further restrict diffusion of CO2 can result in excessive buildup of CO2 as well as anaerobiosis, altered EMA, and lowered product safety and quality. Thus, there is a demand for films with CO2:O2 permselectivities closer to 1, or engineered packaging systems that use novel technologies or films to increase effectively or finely manipulate the O2 flux. Particularly for higher respiring produce, permselectivites >2 increase the likelihood that the atmosphere will rapidly become anaerobic [16]. This is exacerbated as storage temperature increases. Polymer technology has been developed (Landec Corporation, Inc., CA) that allows the O2 transmission rate of films to increase more rapidly than the CO2 transmission rate in response to temperature, resulting in an adjustable CO2/O2 permeability ratio [19].

Use of a composite film comprising ethylene vinyl acetate (EVA), low-density polyethylene (LDPE), and oriented polypropylene (OPP) can enhance or improve gas permeability characteristics. Shredded cabbage and grated carrot stored in this composite material achieved an extension of shelf life of 2 to 3 days over that achieved in OPP alone [20]. Films have been developed with pores (micro-perforations) or holes (macro-perforations) to increase the O2 transmission rate, resulting in more equal rates of movement of O2 and CO2 between the internal package atmosphere and external atmosphere, achieving permeability ratios of CO2 to O2 near 1. Micro-perforated films may be appropriate for products with a high respiration rate, such as strawberries, where finer control of package atmosphere is desired, where internal package O2 depletion is a concern, or where temperature fluctuations may be anticipated.

The number, perimeter, and total effective area of perforations affects the rate of gas exchange [21,22]; the application and level of atmosphere control will determine these factors. Lee and others [21] developed a model to describe and predict changes in atmosphere and humidity in micro-perforated packages, verifying the model on refrigerated MAP peeled garlic. The number, cross-sectional area, and placement of perforations as well as the thickness of the film affect the EMA attained and alter gas and moisture exchange rates across package film. Long, narrow channels are more difficult for the gases to move through than wide, short-path perforations. Perforated films can be applied as an overwrap to a nonpermeable formed container or attached as a patch or label to a selectively permeable or nonpermeable bag. Used as a patch or label, only a small percentage of the package area is utilized to achieve the desired effects, creating a wide range of gas atmospheres, particularly when used in combination with a selectively permeable film.

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