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:

  1. Elasticity, to ensure good handling properties and to prevent damage to the surfaces of the packaged goods. Stiffening of the films when exposed to cold is a typical characteristic of thermoplastics.
    Plasticizers are used to increase elasticity, especially low temperature flexibility, and to prevent brittle fracture caused by mechanical stress at film freezing temperatures.
  2. Water vapor and gas permeability of plastic films. It is particularly important with frozen goods for the films to be extensively water vapor tight, to prevent loss of quality and weight. Films 0.1 mm thick should display water vapor permeability which does not exceed 0.5 g/m² over 24 hours where used for fish, meat and poultry and 1 g/m² over 24 hours where used for fruit and vegetables. The above values also apply to the seal seams.

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%
PVC - rigid films 6 - 8
PVC - flexible films 8 - 12
High pressure polyethylene films 0.6 - 0.8
Low pressure polyethylene films 0.1 - 0.2

Table 10: Water vapor permeability
of certain packaging films [6]

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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