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Rigid polymer materials have become a popular choice for chilled products; these include polyethylene (PE), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polystyrene (PS). The need for lower-cost packaging materials such as the flexible polymers or packaging films consists of multiple layers of laminates. Ethylene vinyl alcohol (EVOH), for example, is the film of choice for improving the oxygen barrier properties. A good overview of chilled food packaging can be found in Reference 5.

Modified Atmosphere Packaging

Chilled products may be packaged under a modified atmosphere to allow extended shelf storage (6). This type of approach allows products to be shipped longer distances and enables a reduction in economic losses due to spoilage. Modified atmosphere packaging (MAP) depends on replacing air in a package with different gas mixtures to regulate microbial activity within the food. The composition of gases in the pack does not remain constant and will change during storage as a result of chemical, enzymatic, and microbial activity of the product. There are three parameters to consider for the successful use of MAP in chilled foods: the packaging film/container, the gas or gas mixtures being used, and the storage temperature. The film must have high barrier properties against moisture loss and low rates of transmission for the gas mixture being used. The role of the gases is quite distinct; carbon dioxide is inhibitory to both bacteria and yeast and molds and acts by extending the lag phase and, therefore, the generation time. The concentration of the gas and the age, type, and load of the initial bacterial population influence this effect. Low temperatures increase the inhibitory action of carbon dioxide. Use of high levels of this gas can affect sensory properties of a food, producing off-odors and pigment, taste, and texture changes. It may also cause package "collapse" as a result of the carbon dioxide being absorbed by the food. Nitrogen has little or no effect on bacterial growth unless the residual oxygen levels are extremely low (0.2%) and usually acts as a filler for the package. The presence of oxygen in MAP retards anaerobic growth and can improve or stabilize color within the pack. The application of MAP alone for chilled foods may not always provide shelf-life extension but should be considered as a part of a hurdle system. Some commonly used gas mixtures for MAP are shown in Table 2.

Advantages and limitations of MAP in chilled foods are listed in Table 3. Further discussions on MAP technology can be found in References 7 and 8. The desired atmospheres within a package can also be achieved by use of sachet technology, an alternative to gas flushing. This approach referred to as active packaging involves placing a sachet of a gas generator or absorber directly into the package before sealing. Currently there are two kinds of MAP sachets available—oxygen absorbers and carbon dioxide generators. The most common oxygen absorbers work on the principle of iron being converted to ferric hydroxide by oxygen and water vapor. This technology is easily applied during the manufacturing stage, and the atmosphere generated can be maintained during storage. Another application of sachet technology is the use of an ethanol pack. A sachet containing ethanol is placed in a package, and the released vapor acts as an antimicrobial agent.

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