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
Packaging-flavour interactions 149 8.2.7 Temperature Temperature is probably the most important environmental variable affecting transport processes. The permeability of gases and liquids in polymers increases with increasing temperature according to the Arrhenius relationship. Possible reasons for increased flavour absorption at higher temperatures are( Greml increased mobility of the flavour molecules hange in polymer configuration, such as swelling or decrease of crystallinity change in the volatile solubility in the aqueous phase 8.2.8 Relative humidity For some polymers, exposure to moisture has a strong influence on their barrie properties. The presence of water vapour often accelerates the diffusion of gases and vapours in polymers with an affinity for water. The water diffuses into the film and acts like a plasticiser. Generally, the plasticising effect of water on a hydrophilic film, such as ethylene-vinyl alcohol(EVOH) and most polyamides yould increase the permeability by increasing the diffusivity because of the higher mobility acquired by the polymer network (Johansson, 1993). Absorbed water does not affect the permeabilities of polyolefins and a few polymers, such as PET and amorphous nylon, show a slight decrease in oxygen permeability with increasing humidity. Since humidity is inescapable in many packaging situations, this effect cannot be overlooked. The humidity in the environment is often above 50%RH, and the humidity inside a food package can be nearly assus, 1997) 8.3 The role of the food matrix quality and the shelf-life of the packaged food depend strongly on physical chemical properties of the polymeric film and the interactions between food Con ponents and package during storage. Several investigations have shown that iderable amounts of aroma compounds can be absorbed by plastic kaging materials, which can cause loss of aroma intensity or an unbalanced flavour profile(Hotchkiss, 1997; Arora et al, 1991; Lebosse et al., 1997 Linssen et al., 1991a; Nielsen et al., 1992; Paik, 1992) factors,see Fig. 8.3)in determining the amount of flavour absorptions The composition of a food matrix is of great importance(besides ot plastic packaging materials. There is only limited information available in literature about the influence of the food matrix on flavour absorption by polymers. Linssen et al.(1991b)and Yamada et al.(1992) showed that the presence of juice pulp in orange juice decreased absorption of volatile compounds into polymeric packaging materials. They suggested that pulp particles hold flavour compounds(e.g, limonene) in equilibrium with the
8.2.7 Temperature Temperature is probably the most important environmental variable affecting transport processes. The permeability of gases and liquids in polymers increases with increasing temperature according to the Arrhenius relationship. Possible reasons for increased flavour absorption at higher temperatures are (Gremli, 1996): • increased mobility of the flavour molecules • change in polymer configuration, such as swelling or decrease of crystallinity • change in the volatile solubility in the aqueous phase. 8.2.8 Relative humidity For some polymers, exposure to moisture has a strong influence on their barrier properties. The presence of water vapour often accelerates the diffusion of gases and vapours in polymers with an affinity for water. The water diffuses into the film and acts like a plasticiser. Generally, the plasticising effect of water on a hydrophilic film, such as ethylene-vinyl alcohol (EVOH) and most polyamides, would increase the permeability by increasing the diffusivity because of the higher mobility acquired by the polymer network (Johansson, 1993). Absorbed water does not affect the permeabilities of polyolefins and a few polymers, such as PET and amorphous nylon, show a slight decrease in oxygen permeability with increasing humidity. Since humidity is inescapable in many packaging situations, this effect cannot be overlooked. The humidity in the environment is often above 50%RH, and the humidity inside a food package can be nearly 100%RH (Delassus, 1997). 8.3 The role of the food matrix The quality and the shelf-life of the packaged food depend strongly on physical and chemical properties of the polymeric film and the interactions between food components and package during storage. Several investigations have shown that considerable amounts of aroma compounds can be absorbed by plastic packaging materials, which can cause loss of aroma intensity or an unbalanced flavour profile (Hotchkiss, 1997; Arora et al., 1991; Lebosse´ et al., 1997; Linssen et al., 1991a; Nielsen et al., 1992; Paik, 1992). The composition of a food matrix is of great importance (besides other factors, see Fig. 8.3) in determining the amount of flavour absorption by plastic packaging materials. There is only limited information available in literature about the influence of the food matrix on flavour absorption by polymers. Linssen et al. (1991b) and Yamada et al. (1992) showed that the presence of juice pulp in orange juice decreased absorption of volatile compounds into polymeric packaging materials. They suggested that pulp particles hold flavour compounds (e.g., limonene) in equilibrium with the Packaging-flavour interactions 149
150 Novel food packaging techniques TEMPERATURE STORAGE TIME FOOD MATRIX POLYMER FLAVOUR RYSTALLINITY MOLECULAR SIZE ABSORPTION POLARITY POLARITY AFFINITY Fig8.3 Factors influencing flavour absorption by plastic polymers (Van Willige watery phase, which could be responsible for the decrease of absorption of these compounds by the plastics Fukamachi et al. (1996) studied the absorption behaviour of flavour compounds from an ethanolic solution as a model of alcoholic beverages. The absorption of a mixture of homologous volatile compounds(esters, aldehydes and alcohols with carbon chain length 4-12)into LDPE film first increased with a maximal absorption at 5-10%(/v)aqueous ethanol and then decreased remarkably with increasing ethanol concentration. EVOH film showed similar absorption behaviour, with maximal absorption at 10-20%(v/v) aqueous ethanol. Nielsen et al. (1992) investigated the effects of olive oil on flavour absorption into LDPE. Olive oil and, thereby, the flavours dissolved in the oil were absorbed in large amounts by the plastic. The partition coefficients for alcohols and short-chained esters in an oil/polymer system were higher than in a water/polymer system, while the partition coefficients for aldehydes and long- ined esters were lower in an oil/polymer system than in a water/polymer rstem. Not only the type of plastic used is of importance for the uptake of aroma compounds, but also possible interactions between flavour and food components. Flavour components may be dissolved, adsorbed, bound entrapped, encapsulated or retarded in their diffusion through the matrix by food components. The relative importance of each of these mechanisms varies with the properties of the flavour chemical(functional groups, molecular size shape, volatility, etc )and the physical and chemical properties of the components in the food(Kinsella, 1989, Le Thanh et al. 1992) Knowledge of the binding behaviour of flavour components to non-volatile food components and their partitioning between different phases(food omponent/water and water/polymer) is of great importance in estimating the rate and amount of absorption by polymers. Because many food products are emulsions of fat and water, such as milk and milk products, the fat content is an important variable in the food matrix. Fat/oil content is often reduced in order to
watery phase, which could be responsible for the decrease of absorption of these compounds by the plastics. Fukamachi et al. (1996) studied the absorption behaviour of flavour compounds from an ethanolic solution as a model of alcoholic beverages. The absorption of a mixture of homologous volatile compounds (esters, aldehydes and alcohols with carbon chain length 4-12) into LDPE film first increased with a maximal absorption at 5–10% (v/v) aqueous ethanol and then decreased remarkably with increasing ethanol concentration. EVOH film showed similar absorption behaviour, with maximal absorption at 10–20% (v/v) aqueous ethanol. Nielsen et al. (1992) investigated the effects of olive oil on flavour absorption into LDPE. Olive oil and, thereby, the flavours dissolved in the oil, were absorbed in large amounts by the plastic. The partition coefficients for alcohols and short-chained esters in an oil/polymer system were higher than in a water/polymer system, while the partition coefficients for aldehydes and longchained esters were lower in an oil/polymer system than in a water/polymer system. Not only the type of plastic used is of importance for the uptake of aroma compounds, but also possible interactions between flavour and food components. Flavour components may be dissolved, adsorbed, bound, entrapped, encapsulated or retarded in their diffusion through the matrix by food components. The relative importance of each of these mechanisms varies with the properties of the flavour chemical (functional groups, molecular size, shape, volatility, etc.) and the physical and chemical properties of the components in the food (Kinsella, 1989; Le Thanh et al. 1992). Knowledge of the binding behaviour of flavour components to non-volatile food components and their partitioning between different phases (food component/water and water/polymer) is of great importance in estimating the rate and amount of absorption by polymers. Because many food products are emulsions of fat and water, such as milk and milk products, the fat content is an important variable in the food matrix. Fat/oil content is often reduced in order to Fig. 8.3 Factors influencing flavour absorption by plastic polymers (Van Willige, 2002c). 150 Novel food packaging techniques
Packaging-flavour interactions 151 decrease caloric intake to make food healthier. Removal or reduction of lipids can lead to an imbalanced flavour, often with a much higher intensity than the original full fat food (Widder and Fischer, 1996, Ingham et al, 1996) De roos(1997) reported that in products containing aqueous and lipid phases, a flavour compound is distributed over three phases: fat(or oil),water and air. Flavour release from the oil/fat phase of a food proceeded at a lower rate than from the aqueous phase. This was attributed, first to the higher resistance to mass transfer in fat and oil than in water and second to the fact that in oil/water emulsions flavour compounds had initially to be released from the fat into the aqueous phase before they could be released from the aqueous headspace. Kinsella(1989)reported that several mechanisms might be involved in the interaction of flavour compounds with food components. In lipid systems, solubilisation and rates of partitioning control the rates of release Polysaccharides can interact with flavour compounds mostly by non-specifi adsorption and formation of inclusion compounds. In protein systems adsorption, specific binding, entrapment, encapsulation and covalent binding count for the retention of flavour Oil and fatty acids can also be absorbed by polymers(Arora and Halek, 1994 Riquet et al, 1998)resulting in increased oxygen permeability (Johansson and Leufven, 1994)and delamination of laminated packaging material(Olaffson and Hildingson, 1995; Olaffson et al., 1995) However, the availability of data about the influence of oil on the absorption of flavour compounds by plastic packaging materials is limited. Nielsen et al.(1992) found that some apple aroma compounds added to and stored in pure olive oil were lost to a greater extent to LDPE than from an aqueous solution, probably due to differences in polarity of the aromas, polymer and solutions. Thus, oil/fat has a major influence on flavour compounds (perception, intensity, volatility, etc. ) and on the properties of ackaging material Van Willige et al (2000a, b)did a more detailed study on the influence of the composition of the matrix on food products. These authors used a model system consisting of limonene, decanal, linalol and ethyl 2-methylbutyrate to study flavour scalping in LLDPE from different models representing differences in food matrices. The proteins, B-lactoglobuline (B-1g) and caseine were able to suppress absorption of decanal and limonene, because B-lg interacted irreversibly with decanal and caseine was capable of binding limonene and decanal by hydrophobic and covalent interactions. Dufour and Haertle(1990) and Charles et al. (1996)reported that B-lg does not bind terpenes as limonene and linalol. The behaviour of ethyl 2-methylbutyrate could not be fully The presence of carbohydrates also affected the absorption of flavour compounds by LLDPE. Absorption rates of limonene and to a lesser extent of decanal were decreased in the presence of pectine and carboxymethylcellulose lowed down diffu f fla ds from th matrix to LLDPE. Roberts et al.(1996) also reported that thickened solutions of similar viscosity did not show the same flavour release. Their results showed an
decrease caloric intake to make food healthier. Removal or reduction of lipids can lead to an imbalanced flavour, often with a much higher intensity than the original full fat food (Widder and Fischer, 1996; Ingham et al., 1996). De Roos (1997) reported that in products containing aqueous and lipid phases, a flavour compound is distributed over three phases: fat (or oil), water, and air. Flavour release from the oil/fat phase of a food proceeded at a lower rate than from the aqueous phase. This was attributed, first to the higher resistance to mass transfer in fat and oil than in water and, second to the fact that in oil/water emulsions flavour compounds had initially to be released from the fat into the aqueous phase before they could be released from the aqueous phase to the headspace. Kinsella (1989) reported that several mechanisms might be involved in the interaction of flavour compounds with food components. In lipid systems, solubilisation and rates of partitioning control the rates of release. Polysaccharides can interact with flavour compounds mostly by non-specific adsorption and formation of inclusion compounds. In protein systems, adsorption, specific binding, entrapment, encapsulation and covalent binding may account for the retention of flavours. Oil and fatty acids can also be absorbed by polymers (Arora and Halek, 1994; Riquet et al., 1998) resulting in increased oxygen permeability (Johansson and Leufve´n, 1994) and delamination of laminated packaging material (Olaffson and Hildingson, 1995; Olaffson et al., 1995) However, the availability of data about the influence of oil on the absorption of flavour compounds by plastic packaging materials is limited. Nielsen et al. (1992) found that some apple aroma compounds added to and stored in pure olive oil were lost to a greater extent to LDPE than from an aqueous solution, probably due to differences in polarity of the aromas, polymer and solutions. Thus, oil/fat has a major influence on flavour compounds (perception, intensity, volatility, etc.) and on the properties of packaging material. Van Willige et al. (2000a, b) did a more detailed study on the influence of the composition of the matrix on food products. These authors used a model system, consisting of limonene, decanal, linalol and ethyl 2-methylbutyrate to study flavour scalping in LLDPE from different models representing differences in food matrices. The proteins, -lactoglobuline ( -lg) and caseine were able to suppress absorption of decanal and limonene, because -lg interacted irreversibly with decanal and caseine was capable of binding limonene and decanal by hydrophobic and covalent interactions. Dufour and Haertle´ (1990) and Charles et al. (1996) reported that -lg does not bind terpenes as limonene and linalol. The behaviour of ethyl 2-methylbutyrate could not be fully explained and needs further investigation. The presence of carbohydrates also affected the absorption of flavour compounds by LLDPE. Absorption rates of limonene and to a lesser extent of decanal were decreased in the presence of pectine and carboxymethylcellulose. Increasing viscosity slowed down diffusion of flavour compounds from the matrix to LLDPE. Roberts et al. (1996) also reported that thickened solutions of similar viscosity did not show the same flavour release. Their results showed an Packaging-flavour interactions 151
152 Novel food packaging techniques influence of both viscosity and binding interactions with the thickener on the elease of flavour. Binding interactions with carbohydrate-based thickeners are often due to adsorption, entrapment in microregions, complexation, encapsulation and hydrogen bonding between appropriate functional groups Kinsella, 1989, Damodaran, 1996). The presence of disaccharides, lactose and saccharose was able to bind water and cause a salting out effect of the lesser polar flavour compounds, linalol and ethyl 2-methylbutyrate, resulting in an increased absorption in the polymer. Also Godshall (1997)reported that disaccharides can lower the amount of bulk water due to hydration, which increases the effective concentration of flavour compounds and therefore can enhance their absorption into polymers The main effect of the influence of the food matrix on flavour scalping, owever, is the presence of oil or fat. Even a small amount of oil (50g/f)had a major effect on the amount of flavour absorption. Absorption of limonene and decanal is reduced to approximately 5%. A quantity of oil as low as 2 g/l results in a decrease of about 50% of absorption, meaning that the presence of oil very strongly influences the level of absorption of flavour compounds in polymeric packaging material(Fig. 8.4) The composition of the food matrix plays a major role in the absorption of flavour compounds by LLDPE. Several studies have already revealed that flavour compounds interact with oil, carbohydrates and proteins, but the Limonene Linalool 2MB E08 Concentration oil (g/l) Fig. 8.4 Influence of oil on the relative absorption of limonene, decanal, linalool and ethyl-2-methylbutyrate(E2MB) by LLDPE after one day of exposure at 4(Van Willige et al., 2000a)
influence of both viscosity and binding interactions with the thickener on the release of flavour. Binding interactions with carbohydrate-based thickeners are often due to adsorption, entrapment in microregions, complexation, encapsulation and hydrogen bonding between appropriate functional groups (Kinsella, 1989; Damodaran, 1996). The presence of disaccharides, lactose and saccharose was able to bind water and cause a salting out effect of the lesser polar flavour compounds, linalol and ethyl 2-methylbutyrate, resulting in an increased absorption in the polymer. Also Godshall (1997) reported that disaccharides can lower the amount of bulk water due to hydration, which increases the effective concentration of flavour compounds and therefore can enhance their absorption into polymers. The main effect of the influence of the food matrix on flavour scalping, however, is the presence of oil or fat. Even a small amount of oil (50g/l) had a major effect on the amount of flavour absorption. Absorption of limonene and decanal is reduced to approximately 5%. A quantity of oil as low as 2 g/l results in a decrease of about 50% of absorption, meaning that the presence of oil very strongly influences the level of absorption of flavour compounds in polymeric packaging material (Fig. 8.4). The composition of the food matrix plays a major role in the absorption of flavour compounds by LLDPE. Several studies have already revealed that flavour compounds interact with oil, carbohydrates and proteins, but the Fig. 8.4 Influence of oil on the relative absorption of limonene, decanal, linalool and ethyl-2-methylbutyrate (E2MB) by LLDPE after one day of exposure at 4º (Van Willige et al., 2000a) 152 Novel food packaging techniques
Packaging-flavour interactions 153 influence on flavour absorption by plastic packaging materials in different food matrices has been unclear for a long time. Van Willige et al(2000a, b) showed that food components can affect the quantity of absorbed flavour compounds by LLDPE in the following order: oil or fat > polysaccharides and proteins disaccharides. Because of the lipophilic character of many flavour compounds food products with a high oil or fat content will lose less flavour by absorption into LLDPE packaging than food products containing no or a small quantity of 8.4 The role of differing packaging materials An important requirement in selecting food-packaging systems is the barrier properties of the packaging material. Barrier properties include permeability of gases(such as O2, CO2, N2 and ethylene), water vapour, aroma compounds and light. These are vital factors for maintaining the quality of foods. a good barrier to moisture and oxygen keeps a product crisp and fresh, and reduces oxidation of food constituents. Plastics are widely used for food packaging due to their flexibility, variability in size and shape, thermal stability, and barrier properties PE and PP have been used for many years because of their good heat sealability low costs and low water vapour permeability. However, poor gas permeability makes laminating of PE with aluminium foil and paper necessary. During the last decades, PET and, to a lesser extent, PC have found increased use for food packaging. PET has good mechanical properties, excellent transparency and relatively low permeability to gases. PC is tough, stiff, hard and transparent, but has poor gas permeability properties and is still quite expensive &s Unlike glass, plastics are not inert allowing mass transport of compounds such water, gases, flavours, monomers and fatty acids between a food product, package and the environment due to permeation, migration and absorption. The quality and shelf-life of plastic-packaged food depend strongly on physical and chemical properties of the polymeric film and the interactions between food components and package during storage. Several investigations showed that considerable amounts of aroma compounds can be absorbed by plastic packaging materials, resulting in loss of aroma intensity or an unbalanced flavour profile(van Willige et al., 2000a, b, Arora et al., 1991; Lebosse et al, 1997, Linssen et al. 1991b, Nielsen et al., 1992; Paik, 1992) Absorption may also indirectly affect the food quality by causing delamination of multilayer packages (Olafsson and Hildingsson, 1995: Olafsson et al., 1995)or by altering the barrier and mechanical properties of plastic packaging materials (Tawfik et al., 1998). Oxygen permeability through the packaging is an important factor for the shelf-life of many packed foods. Little information is available in literature about the influence of absorbed compounds on the oxygen permeability of packaging materials. Hirose et al.(1988)reported that the oxygen permeability of LDPE and two types of ionomer increased due to the presence of absorbed d-limonene. Johansson and Leufven(1994) studied the effect of rapeseed oil on the oxygen barrier properties
influence on flavour absorption by plastic packaging materials in different food matrices has been unclear for a long time. Van Willige et al. (2000a,b) showed that food components can affect the quantity of absorbed flavour compounds by LLDPE in the following order: oil or fat >> polysaccharides and proteins > disaccharides. Because of the lipophilic character of many flavour compounds, food products with a high oil or fat content will lose less flavour by absorption into LLDPE packaging than food products containing no or a small quantity of oil. 8.4 The role of differing packaging materials An important requirement in selecting food-packaging systems is the barrier properties of the packaging material. Barrier properties include permeability of gases (such as O2, CO2, N2 and ethylene), water vapour, aroma compounds and light. These are vital factors for maintaining the quality of foods. A good barrier to moisture and oxygen keeps a product crisp and fresh, and reduces oxidation of food constituents. Plastics are widely used for food packaging due to their flexibility, variability in size and shape, thermal stability, and barrier properties. PE and PP have been used for many years because of their good heat sealability, low costs and low water vapour permeability. However, poor gas permeability makes laminating of PE with aluminium foil and paper necessary. During the last decades, PET and, to a lesser extent, PC have found increased use for food packaging. PET has good mechanical properties, excellent transparency and relatively low permeability to gases. PC is tough, stiff, hard and transparent, but has poor gas permeability properties and is still quite expensive. Unlike glass, plastics are not inert allowing mass transport of compounds such as water, gases, flavours, monomers and fatty acids between a food product, package and the environment due to permeation, migration and absorption. The quality and shelf-life of plastic-packaged food depend strongly on physical and chemical properties of the polymeric film and the interactions between food components and package during storage. Several investigations showed that considerable amounts of aroma compounds can be absorbed by plastic packaging materials, resulting in loss of aroma intensity or an unbalanced flavour profile (Van Willige et al., 2000a,b; Arora et al., 1991; Lebosse´ et al., 1997; Linssen et al., 1991b; Nielsen et al., 1992; Paik, 1992) Absorption may also indirectly affect the food quality by causing delamination of multilayer packages (Olafsson and Hildingsson, 1995; Olafsson et al., 1995) or by altering the barrier and mechanical properties of plastic packaging materials (Tawfik et al., 1998). Oxygen permeability through the packaging is an important factor for the shelf-life of many packed foods. Little information is available in literature about the influence of absorbed compounds on the oxygen permeability of packaging materials. Hirose et al. (1988) reported that the oxygen permeability of LDPE and two types of ionomer increased due to the presence of absorbed d-limonene. Johansson and Leufve´n (1994) studied the effect of rapeseed oil on the oxygen barrier properties Packaging-flavour interactions 153