154 Novel food packaging of different polymer packaging materials. They found that amorphous PET remained an excellent oxygen barrier even after storage in rapeseed oil for 40 days The polyolefins(PP and high-density PE)showed an increased oxygen transmission rate (OTR)after being in contact with rapeseed oil for 40 days his was attributed to swelling of the polymer matrix. However, the increase in OTR was not proportional to the amount of absorbed oil Sadler and Braddock(1990) showed that the oxygen permeability of LDPE was proportional to the mass of absorbed limonene. In another paper, they concluded that oxygen permeability of LDPE and the diffusion coefficients of citrus flavour volatiles in LDPE were related to the solubility of these compounds in the polymer (Sadler and Braddock, 1991). The increased oxygen permeability of LDPE could only be explained by absorption. Attachment of volatile molecules at the polymer surface(adsorption) might hinder oxygen permeation, which would lower the oxygen permeation, or leave it unchanged An increased oxygen permeability of LDPE indicated that absorption of volatiles must be responsible for structural changes in the polymer. Flavour absorption can have a major influence on the oxygen permeability of plastic packaging materials, and consequently on the shelf-life of a food product making it necessary to investigate this important aspect more thoroughly Van Willige et al,(2002b)investigated the influence of oxygen permeation on absorption of several flavour compounds(limonene, decanal, hexyl acetate and 2-nonanone) into LDPE, PP, PC and PET packaging materials. They measured the oxygen permeability of the exposed polymer specimens up based on the isostatic continuous flow technique. In the isostatic method the pressure differential across the test film remains constant during the total permeation process. Whereas the high-pressure side(oxygen chamber)remains constant at a certain value, the low-pressure side (nitrogen chamber)is maintained by sweeping the permeated molecules with a continuous flow of carrier gas(Hernandez and gavara, 1999) Figure 8.5 gives a good picture of the influence of the total amount of flavour absorption on the oxygen permeability of all four investigated polymers. PP and LDPE showed an increase in oxygen permeability after absorption of flavour compounds. This increase in oxygen permeability indicated that molecular changes occurred in the polymer network. Several researchers reported that swelling of a polymer by a permeant (i.e. plasticising)greatly increased the diffusivity. During the absorption process molecules are absorbed in the free volume (holes)which is always present in the amorphous regions. Diffusion and a slow relaxation of the polymer, reducing the intercatenary forces and even promoting polymer swelling control the rate of absorption. This further enhances the rate of diffusion, which further influences the relaxation. As a result, the permeation of one component affects the permeation of another component,i.e the plasticising effect within the polymer matrix becomes apparent(Halek, 1988; Hernandez-Munoz et al., 1999) Absorbed water has a similar effect on the permeability of some hydrophilic polymers, such as ethylene vinyl alcohol(EVOH) and most polyamides. Waterof different polymer packaging materials. They found that amorphous PET remained an excellent oxygen barrier even after storage in rapeseed oil for 40 days. The polyolefins (PP and high-density PE) showed an increased oxygen transmission rate (OTR) after being in contact with rapeseed oil for 40 days. This was attributed to swelling of the polymer matrix. However, the increase in OTR was not proportional to the amount of absorbed oil. Sadler and Braddock (1990) showed that the oxygen permeability of LDPE was proportional to the mass of absorbed limonene. In another paper, they concluded that oxygen permeability of LDPE and the diffusion coefficients of citrus flavour volatiles in LDPE were related to the solubility of these compounds in the polymer (Sadler and Braddock, 1991). The increased oxygen permeability of LDPE could only be explained by absorption. Attachment of volatile molecules at the polymer surface (adsorption) might hinder oxygen permeation, which would lower the oxygen permeation, or leave it unchanged. An increased oxygen permeability of LDPE indicated that absorption of volatiles must be responsible for structural changes in the polymer. Flavour absorption can have a major influence on the oxygen permeability of plastic packaging materials, and consequently on the shelf-life of a food product, making it necessary to investigate this important aspect more thoroughly. Van Willige et al., (2002b) investigated the influence of oxygen permeation on absorption of several flavour compounds (limonene, decanal, hexyl acetate and 2-nonanone) into LDPE, PP, PC and PET packaging materials. They measured the oxygen permeability of the exposed polymer specimens with a set￾up based on the isostatic continuous flow technique. In the isostatic method the pressure differential across the test film remains constant during the total permeation process. Whereas the high-pressure side (oxygen chamber) remains constant at a certain value, the low-pressure side (nitrogen chamber) is maintained by sweeping the permeated molecules with a continuous flow of carrier gas (Hernandez and Gavara, 1999). Figure 8.5 gives a good picture of the influence of the total amount of flavour absorption on the oxygen permeability of all four investigated polymers. PP and LDPE showed an increase in oxygen permeability after absorption of flavour compounds. This increase in oxygen permeability indicated that molecular changes occurred in the polymer network. Several researchers reported that swelling of a polymer by a permeant (i.e. plasticising) greatly increased the diffusivity. During the absorption process molecules are absorbed in the free volume (‘holes’) which is always present in the amorphous regions. Diffusion and a slow relaxation of the polymer, reducing the intercatenary forces and even promoting polymer swelling control the rate of absorption. This further enhances the rate of diffusion, which further influences the relaxation. As a result, the permeation of one component affects the permeation of another component, i.e. the plasticising effect within the polymer matrix becomes apparent (Halek, 1988; Hernandez-Mun˜oz et al., 1999). Absorbed water has a similar effect on the permeability of some hydrophilic polymers, such as ethylene vinyl alcohol (EVOH) and most polyamides. Water 154 Novel food packaging techniques