Chemical migration and the main factors that control it 121 The mechanistic basis of migration

Migration of chemical substances is a diffusion process subject to both kinetic and thermodynamic control and can be described by diffusion mathematics derived from Fick's Law. The mathematics describe the diffusion process as a function of time, temperature, thickness of the material, amount of chemical in the material, partition coefficient and distribution coefficient. The kinetic dimension of migration dictates how fast the process of migration occurs. The thermodynamic dimension dictates how extensively the transfer of substances will be when migration is finished - when the system is at equilibrium. The kinetic and thermodynamic aspects should not be confused. For example, migration may proceed at a slow rate but, if the chemical migrant has a higher affinity for the food than for the packaging material, then given enough time (e.g. a long shelf life) it may still migrate extensively into the food. On the other hand, if a different food or beverage is packed and the chemical is only poorly soluble in that food or beverage, then migration could be low no matter how long the shelf life is.

Being a molecular diffusion process, chemical migration is subject to the normal laws of physics and chemistry. There are several determinants of chemical migration and exactly what migration occurs depends first on the identities and concentrations of the chemicals present in the packaging material. Other important parameters are the nature of the food along with the conditions of contact. Lastly, the intrinsic properties of the packaging material itself are important considerations. If a material interacts strongly with the food it could give high migration by leaching. Conversely, an inert material with low diffusivity is likely to give low migration values. It is important to understand the factors that control chemical migration because from this understanding springs the ability to prevent or limit any undesirable migration into foods.

1.2.2 Composition of the packaging material

The packaging material is the source of any chemical migration. The extent of any migration depends first on the concentration of the chemical in the packaging. If a substance is not present in a packaging material then it cannot migrate. This is self-evident but easily forgotten, and is important especially when considering models to assess consumer exposure (Chapter 6). If a substance is present in the packaging then, other things remaining equal, migration levels will be higher if the concentration in the packaging is increased and vice versa.

1.2.3 The nature and extent of contact

The nature and extent of any contact between the packaging and the food is the next important parameter to consider. This depends on the physical properties of the food (solid foods make only limited contact whereas liquids make more extensive contact) and the size and shape of the pack. Consider an individual portion pack of margarine (say, 7 g in contact with 28 cm2, or 4000 cm2/kg) compared to a catering pack of the margarine (say, 2 kg in contact with 1050 cm2 or 525 cm2/kg). If the same plastic was used to make the two pack sizes (polystyrene or polypropylene would be candidates) then the same migration on a unit area basis would give rise to an eight-fold higher exposure per portion eaten for the individual portion pack, compared with the catering pack. The most extreme examples of this mass ratio of surface area to food are to be found outside the area of general packaging materials, for example, the relatively limited contact made by small gaskets used in a large food processing plant, gloves or conveyor belts used to handle tonnes of food in a packing plant, or tubing used to pipe tens or hundreds of thousands of litres of liquid during its service life.

Another factor that determines the nature and extent of any contact with the food is the presence of a barrier layer. If the chemical that may migrate is located in one layer of the packaging material but this is separated from the food by an intervening layer, then this barrier layer - between food and chemical migrant - may retard or prevent migration from occurring. This is quite a common situation with modern multi-laminate packaging materials where inks, adhesives, or one or more of the laminate plies do not touch the food directly. The packaging industry has long exploited barrier layers to protect food products from air, light and moisture, in controlling the inner atmosphere of the pack (MAP and CAP, modified and controlled atmosphere packs), and in retaining desirable aromas whilst protecting the packed food from undesirable odour pick-up. The same physico-chemical laws on barrier properties that endow these quality benefits can also be used to prevent or limit chemical migration.

1.2.4 The nature of the food

The nature of the food that touches the packaging is important for two reasons:


If the packaging is not compatible with a given type of food then there can be a strong interaction leading to an accelerated release of chemical substances. Examples are the interaction of fats and oils with certain plastics that leads to swelling of the plastic and leaching of substances from that plastic. Leaching, formally known as Class III migration, occurs because the diffusivity of the plastic increases with any swelling. This means that with swelling, the plastic starts to behave more like a fluid. An even more extreme example of an undesirable interaction between packaging and food is the corrosion of uncoated metal surfaces leading to high metal release into certain acidic foods, or the leaching of heavy metals from ceramic glazes. It is important to avoid such obvious mismatches and ensure that packaging materials are compatible with the food that it is intended to pack.


The nature of a food has a pronounced influence on chemical migration because it determines the solubility of any packaging chemical in that food. This influences the amount of migration that may occur. Foods are conventionally classified into five categories: aqueous, acidic, alcoholic, fatty, and dry. The three main drivers of migration with respect to these different food categories can be characterised by the type of substances that have a high affinity for them and so tend to migrate more readily (Table 1.1).

1.2.5 The temperature of contact

The migration of chemicals is like virtually all chemical and physical processes in that it is accelerated by heat. So migration will occur faster if the temperature is raised. Packaging materials are increasingly used under a very wide range of temperature conditions, ranging from storage deep frozen, refrigerated and at ambient temperature, to boiling, sterilisation, microwaving and even baking in the pack. Clearly, a material suitable for one particular application may not necessarily be suitable for another.

1.2.6 The duration of contact

Materials suitable for short duration contact may not be suitable for longer service times. The kinetics of migration are, to a first approximation, firstorder in that the extent of migration increases according to the square-root of the time of contact: M ^ t1/2. The time (duration) of contact for common packaging can vary enormously:

• days (e.g. fresh milk, meat, fruit and vegetables)

• months and years (e.g. frozen foods, dry goods, canned foods, drinks). The performance requirements of the material must be specified accordingly.

Table 1.1 Classification of foods and the types of chemical migration that they are likely to elicit

Nature of the food in contact

Nature of chemicals most likely to migrate

Acidic foods, aqueous foods

Polar organic chemicals, salts, metals

and low alcohol beverages

Fatty foods, distilled spirits

Non-polar, lipophilic ('fat-loving') organic


Dry foods

Low molecular weight, volatile substances

1.2.7 Mobility of the chemicals in the packaging

The mobility of a chemical in the packaging material depends on the size and shape of the molecule, any interaction it experiences with the material, and the intrinsic resistance to mass transfer that the material presents. It is assumed that the chemical is compatible with the material. If the chemical is not compatible with the material then it could 'bloom' to the surface and give enhanced migration. The exact mechanisms of migration and its mathematical modelling are considered in depth in Chapter 8. To provide a general understanding, it is helpful to consider three general cases: impermeable materials, permeable materials and porous materials. These are depicted in Figs 1.1-1.3 and described below.



Impermeable materials:

- glass and ceramics

- metals and alloys






Fig. 1.1 Depiction of chemical migration from an impermeable material.

Fig. 1.1 Depiction of chemical migration from an impermeable material.



Fig. 1.2 Depiction of chemical migration from a permeable material.


Fig. 1.3 Depiction of chemical migration from and through a porous material.

Food y

Fig. 1.3 Depiction of chemical migration from and through a porous material.

Impermeable materials

These are exemplified by 'hard' materials such as metals, glass and ceramics. The material is an absolute barrier and there is no migration from the interior. Migration is confined to a surface phenomenon only.

Permeable materials

These are exemplified by 'plastic' materials such as plastics, rubbers and elastomers. The material offers some limited resistance to migration but this can occur not only from the surface but also from the interior of the material. The resistance to mass transfer depends on the structure, density, crystallinity, etc., of the material.

Porous materials

Exemplified by paper and board materials with a heterogeneous, open network of fibres with large air spaces or channels. Low molecular weight substances in particular can migrate rather rapidly with little hindrance offered.

1.2.8 Summary of factors that control the migration process

Migration from packaging materials is a diffusion process that is subject to the normal laws of chemistry and physics. Migration increases with

• increased duration of contact

• increased temperature of contact

• higher levels of the chemical in the packaging material

• surface area of the contact

• aggressive foodstuffs.

Migration decreases with

• higher molecular weight substances in the packaging material

• only dry or indirect contact

• low diffusivity ('inert') packaging materials

• presence of a barrier layer.

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