## 4 Discussion
Previous studies [22; 23; 28] have already postulated that some physical treatments such as injection molding, extrusion, and thermoforming could lead to a deviation from the expected diffusion-controlled (Fickian) migration process. In thermoforming, the polymer chains become more orientated compared to the sheet, which improves the barrier property of the PS and might partly explain the lower migration of styrene from the pots. Scanning electron microscopy (SEM) images of the surfaces of the pots and sheets are shown in the Supplementary Materials. The pictures of the sheets show some surface damage (probably barrel scratches) parallel to the extrusion direction. Such scratches are less evident in the microstructures of the pots. An elongation of the polymer matrix around spherical or elliptical holes takes place, following the direction of thermoforming. However, the SEM pictures did not allow for a clear interpretation of differences in polymer orientation or material porosity. However, other authors [29] investigated HIPS yoghurt pots using transmission electron microscopy (TEM) and could clearly see the stretching of both the polymer matrix and the embedded rubber particles when using different processing conditions. Higher temperatures and production rates during thermoforming lead to higher degrees of stretching, which result in an increase in the mechanical quality (i.e., strength and stiffness) of the products. The relative migration of styrene from the tested PS materials (calculated by dividing the mass of styrene found in the food simulant by the initial mass determined in the sample material) ranged from 1% to 6% in 50% ethanol samples tested at 40 \({}^{\circ}\)C. That recorded in yoghurt stored for 50 days at 8 \({}^{\circ}\)C ranged from 0.1% to 0.4%, which agrees with the previously reported values [19,20]. These values demonstrate that actual styrene migration in food (and also in simulants) is far below the total transfer of the residual monomer level in the PS materials. The styrene migration levels measured in yoghurt from the PS pots were lower than those from the same pots in the food simulants. Styrene migration from PS pots into yoghurt after storage at 8 \({}^{\circ}\)C for 50 days reached up to 15 \(\upmu\)g/kg (when applying the EU cube model), which would be conservatively covered by testing the PS pots with 20% ethanol at 10 days/20 \({}^{\circ}\)C. A significantly higher migration was found with the 20% ethanol and 50% ethanol food simulants when tested for 10 days at 40 \({}^{\circ}\)C. These values ranged from 65 to 98 \(\upmu\)g/kg (20% ethanol) and from 155 to 212 \(\upmu\)g/kg (50% ethanol), respectively, at the surface-to-volume ratio of the EU cube model. In previous studies [16, 17, 18], the swelling effect caused by ethanol/water mixtures at elevated temperatures on the different styrene polymers was related to the major overestimation of styrene migration. Other authors [15] have already shown that the level of styrene migration after 24 h at 40 \({}^{\circ}\)C in 3.6% fat milk correlates approximately with that seen in 15% ethanol. In a previous study [26], the styrene concentration found in 3.5% fat milk was found to be comparable to that in 50% ethanol. However, in that study, the experiments were carried out by article-filling and storage at 30 \({}^{\circ}\)C for 10 days in pots sealed with aluminum lids. Eventually, the losses of styrene from the 50% ethanol solution were higher during migration contact with this experimental design, although 50% ethanol was less critical in terms of losses than 10% or 20% ethanol. However, a major difference is that the samples were stored under gentle movement conditions, which might increase the migration of styrene into the liquid milk compared to semi-solid yoghurt in static contact. The styrene migration levels measured from the extruded sheets in simulants and after 50 days of storage in yoghurt would exceed the foreseen precautionary SML for styrene of 40 \(\upmu\)g/kg [5].