Packaging-flavour interactions 155 ,R2=08201 EE 6 R2=0.9894 E R=0.1664 0 Total amount of absorbed flavour compounds (mg/g LDPE, PP, PC 10 and PET*100) Fig 8.5 Influence of total flavour absorption on oxygen permeability of PP, LDPE, PC and PET at 25%C (Van Willige et al, 2002b) molecules absorbed at high relative humidities are believed to combine with hydroxyl groups in the polymer matrix and weaken the existing hydrogen bounds between polymer molecules. As a result, the interchain distances increase and thus free volume, facilitating the diffusion of oxygen and perhaps other gases. The presence of water in the hydrophilic polymer matrix not only influences how a permeant is sorbed and diffused, it also leads to depression of the glass transition temperature(Tg)of the polymer due to the plasticising effect of water. When the Tg drops below storage temperature, a substantial increase in oxygen permeability is expected(Zhang et al, 1999, Delassus et al, 1988) Krizan et al.(1990) reported that free volu her is the dominant factor in determining the permeation properties. a plot of the log of the oxygen permeability coefficients versus the reciprocal of the specific free volume showed a good linear relationship. Also Sadler and Braddock(1990) reported that the oxygen permeability was proportional to the mass of absorbed limonene The specific molecular composition of a flavour compound seems to play a more important role than the mass of absorbed flavour compounds. Each individual absorbed flavour compound caused swelling of PP; i.e. increased the specific free volume. Rubbery polymers(LDPE and PP) have very short relaxation times and respond very rapidly to stresses that tend to change their physical conditions. Glassy polymers(PC and PET) have very long relaxation times Penetrant(molecules)can therefore potentially be present in holes'or irregular cavities with very different intrinsic diffusional mobilities( Stern and Trohalaki 1990 Hernandez- Munoz et al.(1999)reported that there are two possible effects of sorbed flavour compounds on oxygen mass transport: (i)flavour compoundmolecules absorbed at high relative humidities are believed to combine with hydroxyl groups in the polymer matrix and weaken the existing hydrogen bounds between polymer molecules. As a result, the interchain distances increase and thus free volume, facilitating the diffusion of oxygen and perhaps other gases. The presence of water in the hydrophilic polymer matrix not only influences how a permeant is sorbed and diffused, it also leads to depression of the glass transition temperature (Tg) of the polymer due to the plasticising effect of water. When the Tg drops below storage temperature, a substantial increase in oxygen permeability is expected (Zhang et al., 1999; Delassus et al., 1988). Krizan et al. (1990) reported that free volume in a polymer is the dominant factor in determining the permeation properties. A plot of the log of the oxygen permeability coefficients versus the reciprocal of the specific free volume showed a good linear relationship. Also Sadler and Braddock (1990) reported that the oxygen permeability was proportional to the mass of absorbed limonene. The specific molecular composition of a flavour compound seems to play a more important role than the mass of absorbed flavour compounds. Each individual absorbed flavour compound caused swelling of PP; i.e. increased the specific free volume. Rubbery polymers (LDPE and PP) have very short relaxation times and respond very rapidly to stresses that tend to change their physical conditions. Glassy polymers (PC and PET) have very long relaxation times. Penetrant (molecules) can therefore potentially be present in ‘holes’ or irregular cavities with very different intrinsic diffusional mobilities (Stern and Trohalaki, 1990). Hernandez-Mun˜oz et al. (1999) reported that there are two possible effects of absorbed flavour compounds on oxygen mass transport: (i) flavour compounds Fig. 8.5 Influence of total flavour absorption on oxygen permeability of PP, LDPE, PC and PET at 25ºC (Van Willige et al., 2002b) Packaging-flavour interactions 155