## 4 Discussion
Additionally, as found for the sheet with the lower residual styrene levels (PS-S-11), migration from the pots seemed not to considerably increase over time. After 30 and 40 days, respectively, the migration of styrene from pots PS-P-32 and PS-P-22 into yoghurt seemed slightly to decrease, but considering the low levels measured, this could be due to the uncertainty of the analytical method. In Figure 2, the relative migration of styrene in yoghurt was depicted after storage at 8 \({}^{\circ}\)C for 10, 20, 30, 40, and 50 days. In this figure, to allow for comparisons, the percentage values have been recalculated with an area weight normalized to 1 g/dm\({}^{2}\) for all the sample materials. The styrene levels measured in yoghurt within this study are in general agreement (only slightly higher) with previously published data on refrig  
Figure 2: Relative migration of styrene in yoghurt at 8 \({}^{\circ}\)C (area weight normalized to 1 g/dm\({}^{2}\)). in PS pots taken from the supermarket [19; 20]. In these studies, using natural stirred yoghurt products with 3.5% fat content analyzed on their best-before date (BBD), the styrene levels ranged from 5.2 ug/6 dm\({}^{2}\) to 11 ug/6 dm\({}^{2}\). In different milk desserts with a fat content of 0.2-15%, on their BBD, migration values ranged from 2.8 ug/6 dm\({}^{2}\) to 10 ug/6 dm\({}^{2}\). It should be noted that in the present study, for experimental reasons, strips cut from the pots were used for the migration contact. These strips were totally immersed in yoghurt within a glass vial with minimal headspace. In this experimental design, the edges of the PS material were in contact with the yoghurt, and the losses of styrene were minimized. This does not reflect the real-use contact conditions of pots filled with yoghurt but can be considered the worst case. Polystyrene is a low-diffusive polymer. Therefore, the migration of styrene from sheets of approximately 1000 um in thickness and pots of approximately 150 um-300 um in thickness should be similar from a theoretical migration point of view. From both types, only a minor percentage of the initial residual styrene content was transferred to the simulants or yoghurt (Table 3). Therefore, both thicknesses can be regarded as infinite for migration. In Figure 2, the migration of styrene is normalized to equal material thickness (area weight) and equal styrene concentration (expressed as a percentage of the initial amount) for better comparison. The reasons for the higher styrene migration from the extruded sheets compared to the thermoformed pots are not clear. The minimally different residual contents of styrene that were determined in the sheets and in the pots (manufactured from the corresponding sheets) do not explain these findings. The sheets with greater thickness have more cut edges relative to the surface area, compared to the pots. More precisely, the area of the sheet-edges was approx. 2.5 cm\({}^{2}\) (with simulants) and 1.1 cm\({}^{2}\) (with yoghurt); the area of the pot-edges was approx. 0.5 cm\({}^{2}\) (with simulants) and 0.2 cm\({}^{2}\) (with yoghurt). In all the cases, the area of the cut edges was less than 10% of the surface area. The \"edge effect\" is known, especially for some polymers like PS, to influence the migration extent when testing by total immersion [22]. Cut edges have different migration properties compared to the smooth surfaces of the sheets or the pots. Another explanation might be differences in the depletion of styrene in the near-surface region of the sheets and the pots during production, which were not equalized during storage before the experiment. The pots were manufactured by thermoforming. This process involves heating the extruded sheet to the point of thermal deformability and then forming it using a mold by means of vacuum or air pressure to obtain the desired pot shape.