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316
FOOD PACKAGING TECHNOLOGY
transported across a plastic package material by a mass transfer process called
permeation. Permeation is defined as ‘the diffusional molecular exchange of
gases, vapours or liquid permeants across a plastic material which is devoid of
imperfections such as cracks and perforations’ (Hernandez, 1996). Essentially,
the gas molecules sorb into one surface of the plastic, are transported through the
material by a process of diffusion and desorb on the opposite surface. This
process is shown diagrammatically in Figure 10.1.
The driving force for gas permeation through a polymer film is the difference
in gas concentration between each side of the film. A concentration gradient
drives a flow of permeant molecules from the high concentration side to the
low concentration side of the film. In MAP, a gas concentration gradient exists
between the pack headspace and the surrounding environment. In order to
maintain the gas composition within the pack, the packaging material must be
impermeable (a barrier) to gases.
Transmission rate is a measure of the gas or water vapour barrier of a packaging material. The transmission rate T is defined as the quantity Q of gas (or
other permeant) passing through a material of area A in time t.
Q
T = ----At
Commonly used units for transmission rate are cm3 m−2 day−1 (for gases) and
g m−2 day−1 (for vapours including water vapour).
Movement of permeant molecules
Molecule sorbs
into packaging polymer
Molecule desorbs
from packaging polymer
Permeant molecule
diffuses through
polymer structure
High concentration
of gas molecules
Low concentration
gas molecules
Figure 10.1 Permeability model for gases and vapours permeating through a plastic packaging film.