1.1. GENERAL OVERVIEW

The program SML is a joint development of the Swiss Federal Office of Public Health (BAG) and the company Advanced Kinetics and Technology Solutions AG (AKTS AG).

The verification of compliance of food packaging by the application of recogniz>ed diffusion models was introduced in recent the European legislation [1].

SML is a simulation program. It can be used to predict the amount of a substance (additive, contaminant or residual monomer) that migrates from a plastic packaging material into the contained food during a given time. This program is designed as a scientific tool for people active in food packaging: the material producer could check the compliance of a material with the SML values (comparison of Qm with SML), the laboratory specialist could use the calculated concentrations to design the experimental conditions of a migration test. It also includes a non-exhaustive list of starting substances (monomers and additives) used in the fabrication of plastic material coming into contact with food.

SML focuses on the simulation of release of additives from multilayer packaging both in extended temperature ranges and under temperature conditions at which ordinary investigation would be very difficult. These difficulties are prevalent when temperature fluctuates during the observation time. Complex surrounding temperature profiles can be considered such as stepwise, modulated, shock and additionally for temperature profiles reflecting real atmospheric temperature changes (yearly temperature profiles of different climates with daily minimal and maximal fluctuations). Employing Finite Element Analysis (FEA), the modeling is extended to predict the amount of a substance (decomposition products, additive, contaminant or residual monomer) that migrates from a plastic packaging material into the wrapped food. The technique allows the simulation of complex packaging (different geometries and up to 10 multilayer films). Calculation of the diffusive process is based on Fick’s law. It considers the Arrhenius equation and the last version of the Piringer model with a refined Ap constant for the approximation of the diffusion coefficients [2,3]. Diffusion and concentration distribution inside all package layers can be computed for both migrant leaving and food components entering packaging. The layers are divided into a series of N mesh planes each having IxJ elements. Applying FEA, the position of the mesh planes is moved along the time-axis allowing the calculation of the concentrations of both species (migrant and stimulant) at each location for every x, t grid points of each layer. The boundary conditions are derived from comparison of fluxes at the interface between the different layers and between the layer and the food. The functions of the mass balance are singular at the interface of the different layers and at the beginning of the diffusion process (times around 0). Therefore the grid-point distribution is chosen with variable step lengths. The mathematical approach used is unconditionally stable with the accuracy of a differencing scheme that is second-order in both space and time. Grid points are added in regions of high gradients to generate a denser mesh in that region, and subtracted from regions where the solution is decaying or flattening out. The generation of adaptive meshes allow to achieve a desired resolution in localized regions and decreases by orders of magnitude the calculation time.

[1] Commission directive 2002/72/EC, OJEC L220of 15.08.2002
[2] Piringer, O., Food Additives and Contaminants, 11 (1994) 221.
[3] Materials and articles in contact with foodstuffs – Plastics: Estimation of migration by generally recognized diffusion models in support of EU Directive 90/128/EEC (Migration modelling), CEN/TC194/SC1/WG4 N106, version 1, August 2001.