8 Packaging-flavour interactions J. P. H. Linssen, R. w.G. van Willige and m. dekker, Wageningen University, The Netherland 8.1 Introduction Interactions within a package system refer to the exchange of mass and energy between the packaged food, the packaging material and the external environment. Food-packaging interactions can be defined as an interplay between food, packaging, and the environment, which produces an effect on the food, and/or package(Hotchkiss, 1997) Mass transfer processes in packaging systems are normally referred to as rmeation, migration and absorption(Fig. 8.1). Permeation is the process resulting from two basic mechanisms: diffusion of molecules across the package wall, and absorption/desorption from/into the internal/external atmospheres Migration is the release of compounds from the plastic packaging material into the product(Hernandez and Gavara, 1999). The migration of compounds from polymer packaging materials to foods was the first type of interaction to be investigated due to the concern that human health might be endangered by the ching of residues from the polymerisation (e. g, monomers, oligomers olvents), additives(e.g, plasticisers, colourants, UV-stabilisers, antioxidants) and printing inks. Later, absorption or scalping of components originally contained in the product by the packaging material attracted attention. Product components may penetrate the structure of the packaging material, causing loss of aroma, or changing barrier and/or mechanical properties, resulting in a reduced perception of quality (Johansson, 1993) The fundamental driving force in the transfer of components through a package stem is the tendency to equilibrate the chemical potential(Hernandez and avara, 1999). Mass transport through polymeric materials can be described as a multistep process. First, molecules collide with the polymer surface. Then the
8.1 Introduction Interactions within a package system refer to the exchange of mass and energy between the packaged food, the packaging material and the external environment. Food-packaging interactions can be defined as an interplay between food, packaging, and the environment, which produces an effect on the food, and/or package (Hotchkiss, 1997). Mass transfer processes in packaging systems are normally referred to as permeation, migration and absorption (Fig. 8.1). Permeation is the process resulting from two basic mechanisms: diffusion of molecules across the package wall, and absorption/desorption from/into the internal/external atmospheres. Migration is the release of compounds from the plastic packaging material into the product (Hernandez and Gavara, 1999). The migration of compounds from polymer packaging materials to foods was the first type of interaction to be investigated due to the concern that human health might be endangered by the leaching of residues from the polymerisation (e.g., monomers, oligomers, solvents), additives (e.g., plasticisers, colourants, UV-stabilisers, antioxidants) and printing inks. Later, absorption or scalping of components originally contained in the product by the packaging material attracted attention. Product components may penetrate the structure of the packaging material, causing loss of aroma, or changing barrier and/or mechanical properties, resulting in a reduced perception of quality (Johansson, 1993). The fundamental driving force in the transfer of components through a package system is the tendency to equilibrate the chemical potential (Hernandez and Gavara, 1999). Mass transport through polymeric materials can be described as a multistep process. First, molecules collide with the polymer surface. Then they 8 Packaging-flavour interactions J. P. H. Linssen, R. W. G. van Willige and M. Dekker, Wageningen University, The Netherlands
Packaging-flavour interactions 145 Polymer Migrating Adverse Foodstuff substance PERMEATION OxvoCn (1)Oxidation Water vapour microbial growl arbon dioxide Mould growth (2) Dehydration Decarbonation MiGrAtion Monomers ABSORPTION Aroma compounds Loss of aroma int (SCALPING Fats Ir ganic acids Dainage to the package Fig. 8.1 Possible interactions between foodstuff, polymer film and the environment, together with the adverse consequences(Nielsen and Jagerstad, 1994) adsorb and dissolve into the polymer mass. In the polymer film, the molecules hop or diffuse randomly as their own kinetic energy keeps them moving from vacancy to vacancy as the polymer chains move. The movement of the molecules depends on the availability of vacancies or holes'in the polymer film. These holes' are formed as large chain segments of the polymer slide over each other due to thermal agitation. The random diffusion yields a net movement from the side of the polymer film that is in contact with a high concentration or partial pressure of permeant to the side that is in contact with a low concentration of permeant. The last step involves desorption and evaporation of the molecules from the surface of the film on the downstream side(Singh and Heldman, 1993) Absorption involves the first two steps of this process, i.e. adsorption and diffusion, whereas permeation involves all three steps(delassus, 1997) 8.2 Factors affecting flavour absorption As polymer packaging is more and more widely used for direct contact with foods, product compatibility with the packaging material must be considered Flavour scalping, or the absorption of flavour compounds, is one of the most important compatibility problems. The problem of aroma absorption by plastic packages has been recognised for many years ( Johansson, 1993). Several research groups throughout the world investigated flavour absorption phenomena extensively. It is a complex field, and several factors have been
adsorb and dissolve into the polymer mass. In the polymer film, the molecules ‘hop’ or diffuse randomly as their own kinetic energy keeps them moving from vacancy to vacancy as the polymer chains move. The movement of the molecules depends on the availability of vacancies or ‘holes’ in the polymer film. These ‘holes’ are formed as large chain segments of the polymer slide over each other due to thermal agitation. The random diffusion yields a net movement from the side of the polymer film that is in contact with a high concentration or partial pressure of permeant to the side that is in contact with a low concentration of permeant. The last step involves desorption and evaporation of the molecules from the surface of the film on the downstream side (Singh and Heldman, 1993). Absorption involves the first two steps of this process, i.e. adsorption and diffusion, whereas permeation involves all three steps (Delassus, 1997). 8.2 Factors affecting flavour absorption As polymer packaging is more and more widely used for direct contact with foods, product compatibility with the packaging material must be considered. Flavour scalping, or the absorption of flavour compounds, is one of the most important compatibility problems. The problem of aroma absorption by plastic packages has been recognised for many years (Johansson, 1993). Several research groups throughout the world investigated flavour absorption phenomena extensively. It is a complex field, and several factors have been Fig. 8.1 Possible interactions between foodstuff, polymer film and the environment, together with the adverse consequences (Nielsen and Ja¨gerstad, 1994) Packaging-flavour interactions 145
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 the
proven 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
Packaging-flavour interactions 147 glass transition 7e i--temperature modulus of 500 Fig. 8.2 Modulus of elasticity against temperature, showing the glass transition and melting temperatures(Wesselingh and Krishna, 2000) polymer chains show little ordering. However, polymers often contain substantial crystalline parts, where the polymer chains are more or less aligned. The crystalline areas are typically a tenth denser than the amorphous parts; for many permeants they are practically impermeable. So, diffusion occurs mainly in the amorphous regions in a polymer, where small vibrational movements occur along the polymer chains. These micro Brownian motions can result in hole formation as parts of the polymer chains move away from each other. It is through such holes' that permeant molecules can diffuse through a polymer (Johansson, 1993, Wesselingh and Krishna, 2000). Therefore, the higher degree of crystallinity in a polymer, the lower the absorpti 8.2.4 Concentration and mixtures of flavour compounds There are relatively few reports relating flavour absorption to the relative concentrations of the sorbants in a liquid or vapour. Mohney et al. (1988) eported that low sorbant concentrations will affect the polymer only to a ve limited extent and the amount of absorbed compounds will be directly proportional to the concentration of the sorbants. At higher concentrations, however, the absorption of compounds into a polymer material may alter the polymer matrix by swelling( Charara et al, 1992; Sadler and Braddock, 1990) Consequently, to avoid overestimation of the amounts of absor bed compounds or swelling of the polymer, it is advisable to use a mixture of compounds in the concentration range that can be expected to be found in a food application (Johansson and Leufven, 1997). However, to generate reliable and reproducable analytical data, experimental procedures are usually carried out with enhanced
polymer chains show little ordering. However, polymers often contain substantial ‘crystalline’ parts, where the polymer chains are more or less aligned. The crystalline areas are typically a tenth denser than the amorphous parts; for many permeants they are practically impermeable. So, diffusion occurs mainly in the amorphous regions in a polymer, where small vibrational movements occur along the polymer chains. These micro Brownian motions can result in ‘hole’ formation as parts of the polymer chains move away from each other. It is through such ‘holes’ that permeant molecules can diffuse through a polymer (Johansson, 1993; Wesselingh and Krishna, 2000). Therefore, the higher degree of crystallinity in a polymer, the lower the absorption. 8.2.4 Concentration and mixtures of flavour compounds There are relatively few reports relating flavour absorption to the relative concentrations of the sorbants in a liquid or vapour. Mohney et al. (1988) reported that low sorbant concentrations will affect the polymer only to a very limited extent and the amount of absorbed compounds will be directly proportional to the concentration of the sorbants. At higher concentrations, however, the absorption of compounds into a polymer material may alter the polymer matrix by swelling (Charara et al, 1992; Sadler and Braddock, 1990). Consequently, to avoid overestimation of the amounts of absorbed compounds or swelling of the polymer, it is advisable to use a mixture of compounds in the concentration range that can be expected to be found in a food application (Johansson and Leufve´n, 1997). However, to generate reliable and reproducable analytical data, experimental procedures are usually carried out with enhanced Fig. 8.2 Modulus of elasticity against temperature, showing the glass transition and melting temperatures (Wesselingh and Krishna, 2000). Packaging-flavour interactions 147
148 Novel food packagt concentrations. Interactions between different flavour compounds may also affect the absorption of low molecular weight compounds into polymer food packaging materials(Delassus et al, 1988; Kwapong and Hotchkiss, 1987; Letinski and Halek, 1992). Some flavour compounds exhibit a lower absorption ate in mixtures compared to systems containing the individual flavour compounds. This may be due to a competition for free sites in the polymer and/or alteration of the partitioning between the solution and the polymer due to an altered solubility of the compounds in the solution. Therefore, the use of single compound model solutions may cause an overestimation of the amount absorbed in an actual food packaging application (Johansson and le elven 8.2.5 Polarity The polarities of a flavour compound and polymer film are an important factor in the absorption process. The absorption behaviour of different classes of flavour compounds depends to a great extent on their polarity. Different plastic materials have different polarities, hence their affinities toward flavour ompounds may differ from each other ( Gremli, 1996). Flavour compounds are absorbed more easily in a polymeric film if their polarities are similar (Quezada Gallo et al., 1999). Polyolefins are highly lipophilic and may be inconvenient for packaging products with non-polar substances such as fats, oils, aromas etc, since they can be absorbed and retained by the package(Hernandez Munoz et al, 2001). The polyesters, however, are more polar than the polyolefins and will therefore show less affinity for non-polar substances 8. 2.6 Molecular size and structure The size of the penetrant molecule is another factor. Smaller molecules are absorbed more rapidly and in higher quantities than larger molecules. Very large molecules plasticine the polymer, causing increased absorption into the newly available absorption sites(Landois-Garza and Hotchkiss, 1987). Generally, the absorption of a series of compounds with the same functional group increases with an increasing number of carbon atoms in the molecular chain, up to a certain limit. Shimoda et al. (1987) reported that absorption of aldehydes, alcohols and methyl esters increased with increasing molecular weight up to about ten carbon atoms. For even larger molecules the effect of molecular size overcomes the effect of the increased solubility of the compounds in the polymer, and the solubility coefficient decreased. Linssen et al.(1991a)reported that compounds with eight or more carbon atoms were absorbed from yoghurt drinks by HDPE, while shorter molecules remained in the product. They also bserved that highly branched molecules were absorbed to a greater extent than linear molecules
concentrations. Interactions between different flavour compounds may also affect the absorption of low molecular weight compounds into polymer food packaging materials (Delassus et al, 1988; Kwapong and Hotchkiss, 1987; Letinski and Halek, 1992). Some flavour compounds exhibit a lower absorption rate in mixtures compared to systems containing the individual flavour compounds. This may be due to a competition for free sites in the polymer and/or alteration of the partitioning between the solution and the polymer due to an altered solubility of the compounds in the solution. Therefore, the use of single compound model solutions may cause an overestimation of the amount absorbed in an actual food packaging application (Johansson and Leufve´n, 1997). 8.2.5 Polarity The polarities of a flavour compound and polymer film are an important factor in the absorption process. The absorption behaviour of different classes of flavour compounds depends to a great extent on their polarity. Different plastic materials have different polarities; hence their affinities toward flavour compounds may differ from each other (Gremli, 1996). Flavour compounds are absorbed more easily in a polymeric film if their polarities are similar (Quezada Gallo et al., 1999). Polyolefins are highly lipophilic and may be inconvenient for packaging products with non-polar substances such as fats, oils, aromas etc., since they can be absorbed and retained by the package (HernandezMun˜oz et al., 2001). The polyesters, however, are more polar than the polyolefins and will therefore show less affinity for non-polar substances. 8.2.6 Molecular size and structure The size of the penetrant molecule is another factor. Smaller molecules are absorbed more rapidly and in higher quantities than larger molecules. Very large molecules plasticise the polymer, causing increased absorption into the newly available absorption sites (Landois-Garza and Hotchkiss, 1987). Generally, the absorption of a series of compounds with the same functional group increases with an increasing number of carbon atoms in the molecular chain, up to a certain limit. Shimoda et al. (1987) reported that absorption of aldehydes, alcohols and methyl esters increased with increasing molecular weight up to about ten carbon atoms. For even larger molecules the effect of molecular size overcomes the effect of the increased solubility of the compounds in the polymer, and the solubility coefficient decreased. Linssen et al. (1991a) reported that compounds with eight or more carbon atoms were absorbed from yoghurt drinks by HDPE, while shorter molecules remained in the product. They also observed that highly branched molecules were absorbed to a greater extent than linear molecules. 148 Novel food packaging techniques