146 Novel food packaging techniques proven to have important effects on the extent of absorption of different flavour compounds by various packaging materials(Nielsen and Jagerstad, 1994) An understanding of absorption between flavour compounds and polymeric packaging materials requires knowledge of the chemical and physical structures of both the flavour compound and the polymer. The properties of a plastic packaging material are the foremost important parameters that control the amount of flavour absorption. The properties of a polymer result from chemical nature, morphology, formulation(compounding with additives) rocessing, and even storage and conditions of use. Important parameters derived from the chemical structure, such as glass transition temperature rystallinity and free volume that have an effect on flavour absorption are essentially determined upon the selection of a particular polymer 8.2.1 Glass transition temperature (Tg) Figure 8.2 shows the behaviour of one of the many properties of an amorphous and semicrystalline polymer: the modulus of elasticity. There are two sharp breaks indicating phase transitions. At low temperatures the polymer is rigid and brittle: it drops dramatically. Many of the properties of the polymer change a little at this temperature. Above Tg the polymer becomes soft and elastic; it forms a rubber At high temperatures, the polymer may melt, to form a viscous liquid (Wesseling and Krishna, 2000). The polymers that we know as glassy polymers, such as the polyesters polyethylene terephthalate(PET), polycarbonate(PC)and polyethylene nafthalate(PEN), have a Tg above ambient temperature. At room temperature glassy polymers will have very stiff chains and very low diffusion coefficients for flavour molecules at low concentrations. Rubbery polymers, such as the polyolefins polyethylene(PE)and polypropylene(PP), have a Tg below ambient temperature. Rubbery polymers have high diffusion coefficients for flavour compounds and steady-state permeation is established quickly in such structures (Giacin and Hernandez, 1997). Stiff-chained polymers that have a high glass transition temperature generally have low permeability, unless they also have a high free volume(Miller and Krochta, 1997) 8. 2.2 Free volume The free volume of a polymer is the molecular void volume that is trapped in the solid state. The permeating molecule finds an easy path in these voids Generally, a polymer with poor symmetry in the structure, or bulky side chains, will have a high free volume and a high permeability(Salame, 1989) 8.2.3 Crystallinity The importance of crystallinity to absorption has been recognised for many years. All polymers are at least partly amorphous; in the amorphous regions theproven to have important effects on the extent of absorption of different flavour compounds by various packaging materials (Nielsen and Ja¨gerstad, 1994). An understanding of absorption between flavour compounds and polymeric packaging materials requires knowledge of the chemical and physical structures of both the flavour compound and the polymer. The properties of a plastic packaging material are the foremost important parameters that control the amount of flavour absorption. The properties of a polymer result from its chemical nature, morphology, formulation (compounding with additives), processing, and even storage and conditions of use. Important parameters derived from the chemical structure, such as glass transition temperature, crystallinity and free volume that have an effect on flavour absorption are essentially determined upon the selection of a particular polymer. 8.2.1 Glass transition temperature (Tg) Figure 8.2 shows the behaviour of one of the many properties of an amorphous and semicrystalline polymer: the modulus of elasticity. There are two sharp breaks indicating phase transitions. At low temperatures the polymer is rigid and brittle: it forms a ‘glass’. At the glass transition temperature Tg the modulus of elasticity drops dramatically. Many of the properties of the polymer change a little at this temperature. Above Tg the polymer becomes soft and elastic; it forms a ‘rubber’. At high temperatures, the polymer may melt, to form a viscous liquid (Wesselingh and Krishna, 2000). The polymers that we know as glassy polymers, such as the polyesters polyethylene terephthalate (PET), polycarbonate (PC) and polyethylene nafthalate (PEN), have a Tg above ambient temperature. At room temperature, glassy polymers will have very stiff chains and very low diffusion coefficients for flavour molecules at low concentrations. Rubbery polymers, such as the polyolefins polyethylene (PE) and polypropylene (PP), have a Tg below ambient temperature. Rubbery polymers have high diffusion coefficients for flavour compounds and steady-state permeation is established quickly in such structures (Giacin and Hernandez, 1997). Stiff-chained polymers that have a high glass transition temperature generally have low permeability, unless they also have a high free volume (Miller and Krochta, 1997). 8.2.2 Free volume The free volume of a polymer is the molecular ‘void’ volume that is trapped in the solid state. The permeating molecule finds an easy path in these voids. Generally, a polymer with poor symmetry in the structure, or bulky side chains, will have a high free volume and a high permeability (Salame, 1989). 8.2.3 Crystallinity The importance of crystallinity to absorption has been recognised for many years. All polymers are at least partly amorphous; in the amorphous regions the 146 Novel food packaging techniques