|22.214.171.124 Use of plastic films|
|When plastic films are used for frozen goods, this packaging material must display adequate strength and elasticity. Both the films and their closures must be able to withstand the stresses which arise during filling, freezing, storage, transport and thawing. Elasticity must be sufficient even at low temperatures. The requirements placed in plastic films as a packaging material center on two aspects:
For fresh fruit and vegetables, on the other hand, the plastic films must display moderate water vapor and gas permeability in order for their metabolic processes to continue in chilled storage. This is generally achieved with perforations. Table 10 illustrates the water vapor permeability of certain packaging films.
Water vapor permeability at 20°C in g/m² over 24 hours
Film thickness: 0.1 mm
Humidity gradient: 85%
Table 10: Water vapor permeability
of certain packaging films 
In the case of goods which are additionally provided with outer packaging (cartons etc.), high pressure polyethylene films or low pressure polyethylene films are wholly adequate. Where stress is extreme, three-ply films must be used.
Three-layer films are used in the absence of outer packaging.
For even tougher use or greater stress, a polyamide coating is advisable. It is possible, however, to manage without film, e.g. in the case of fish fingers, if the board stock used for the cartons is polyethylene-coated.
Packaging has an effect on surface diffusion. The lower the water vapor permeability of the packaging material, the lower the evaporation level and consequently the lower the weight loss. If the packaging is absolutely impermeable to water vapor, the microclimate inside the package becomes increasingly saturated with water vapor, which is associated with an increase in microbiological spoilage.
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