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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 âthe diffusional molecular exchange of gases, vapours or liquid permeants across a plastic material which is devoid of imperfections such as cracks and perforationsâ (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 mâ2 dayâ1 (for gases) and g mâ2 dayâ1 (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.