Migrants, Migration potential

An important quantity which can be calculated at equilibrium conditions is the amount of substance migrated into the food or food simulant at equilibrium, mF,e. Provided that the migration potential in the polymer, i.e. the initial amount of migrant dissolved in the polymer, mP0, is known then from mass balance calculations the following equation can be derived ... 

Typical polymers Potential migrants Migration risk Hazard assessment... 

When applying the conventional food contact ratio the worst-case migration potential did not exceed the SML for any of the substances derived from plastics nor did the calculated worst-case exposure exceed the ADI/TDI or other exposure restriction value in any products. However, when the actual food contact ratio was applied the ADI/TDI or other exposure restriction value of several substances could theoretically be exceeded. The worst-case calculations assume that intimate contact is made with the entire surface of the packaging. This is not the case for the majority of snack foods that are solids or semi-solids and so the actual area of contact made will be less than the total area available for contact (e.g. crisps). The levels of those migrants (diisobutyl phthalate, dicyclohexyl phthalate, dibutyl sebacate, diphenyl 2-ethylhexyl phosphate and 2-ethyl-1-hexanol) that had the potential to exceed the assigned restrictions, assuming 100% migration, were determined in foods. Of the five substances tested for, only one, dicyclohexyl phthalate, was detected in one of the foodstuffs (tortilla corn chips), at a concentration of 0.60 ppm. 

Soto-Valdez, H., Gramshaw, J.W. and Vandenburg, H.J. (1997). Determination of potential migrants present in Nylon microwave and roasting bags and migration into olive oil, Food Addit. Contam., 14, 3, 309-318. 

For some food-package combinations the concentration of chemical migrants in the food can approach that of substances used as direct food additives, at levels of tens of parts-per-million (mg substance per kg of food, mg/kg). Therefore, all parties involved in the production, transport, selling and consumption of foods need to be aware of the potential for chemical migration and ways of minimising it. Everyone involved needs to ensure that packaging materials are correctly specified and used for the intended application so that there is no excessive chemical migration. The chain of care involves ... 

A more recent Dutch retail survey looked at the migration of A-nitrosamines, A-nitrosatable substances and 2-mercaptobenzothiazole (MET) from 19 samples of teats and soothers. In addition to these species, screening work was also carried out for any other potential migrants. The majority of the teats and soothers were found to be based on sihcone mbber, and the extractable substances were found to be siloxanes. The remaining samples were natural rubber and only one was found to be above the permissible limits, and that was for nitrosatable substances at 0.23 mg/kg. MET was found in only one of the natural rubber products and this was below the migration limit of 0.3 mg/teat. 

The concepts discussed in Section 14.4 describe the situation that will eventuate when a multiphase packaging system reaches thermodynamic equilibrium. However, the rate of mass transfer of permeant, sorbate, and migrant molecules in the polymer is not addressed by these equilibrium considerations. For example, if we consider a potential migration process, we know that eventually the migrant will be transferred to the food and it will finally reach equilibrium, but based on the equations presented in Section 14.4 we cannot predict how long the process will take. Similarly, these relationships will not allow us to estimate the shelf life of a product in a particular package system. For this, we need to look at diffusion. 

Migration is the result of diffusion and equilibrium processes involving the transfer of low molecular mass compounds from a plastic package into a food or food simulant. The migrants diffuse through the amorphous portion of the polymer matrix toward the interface where they are partitioned between the two media until their chemical potential values in both the polymer and the food reach equilibrium. Migration can be mathematically described by Pick s second law (Equation 13.1) ... 

Example Consider a similar system as in the previous example, but now the contacting phase has a limited volume Aa, with a = 2.5 cm. Estimate the time to transfer 25 percent of the total migrant that would potentially migrate at equilibrium (infinite time) if = 1 x 10 and Dp = 1 X 10 cm /s. 

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