MAP generally utilizes an internal package atmosphere of something other than air (air can be approximated as <0.1% CO2, 21% O2, 78% N2) in a hermetically sealed package of suitable permeability in order to extend product shelf life and maintain food safety. While other gases have been explored for use in MAP systems, O2, CO2, and N2 are most commonly employed; O2 levels are commonly reduced below and CO2 increased above atmospheric levels (with a balance of N2) in order to reduce the commodity respiration rate, retard ripening and senescence, and reduce microbial activity. Microorganisms are affected indirectly by reductions in ripening and senescence and directly by restriction of O2 and antimicrobial activity of CO2; superatmospheric O2 has also been shown to be antimicrobial but is not currently commonly employed.
MAP is a dynamic process where environmental and packaging characteristics and the contained product interact to create an equilibrated internal atmosphere (EMA). The EMA is achieved when the rate of O2 consumption and CO2 generation as a result of respiration by a particular commodity equals the rate of gas transmission through the packaged material. Generally, an EMA of 3 to 6% O2 and 2 to 10% CO2 achieves microbial control and extension of shelf life for a wide variety of whole and MP produce, although other atmospheres are also used with commodities that are not physiologically sensitive to high O2 or CO2. Package EMA can be created actively, where a target internal atmosphere is established initially upon packaging by actively flushing with the desired atmosphere, or, more commonly, passively, where the package atmosphere is allowed to reach the desired gas mixture around the commodity during the course of storage at a particular temperature; a longer time period is required to achieve a target EMA for passive than for active MAP.
The EMA is dependent upon both extrinsic and intrinsic factors, including product respiration rate, packaging film permeability to gases and water vapor, package dimensions, and fill weight. The intrinsic respiratory activity of produce is in turn influenced by the particular commodity, the cultivar, stage of maturity at harvest, type of tissue, mass, condition, and whether the product is whole or minimally processed. The mass or produce size indirectly influences respiratory activity by affecting O2 diffusion rates into tissues, which subsequently directly influence respiration rates; alternatively, the stage of maturity or age has a direct impact on metabolic activities and rates. The extent of film permeability to gases per unit thickness, the effect of relative humidity on this permeability, the package surface area, seal integrity, free volume inside the package, and relative humidity around the package will also affect the EMA achieved. Temperature is the most influential extrinsic factor to consider as it affects both commodity respiration rate and film gas and water vapor permeability. MAP systems will generally be exposed to a dynamic environment during distribution, storage, display, and consumer purchase. Thus, while a particular MAP system should be optimized for a particular storage temperature, the effect of significant temperature changes on the system should be considered .
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