Food and packaging interactions: Use of methyl red as a probe for PVC swelling by fatty acid esters

A study of the interaction of fatty foods with rigid PVC packages was undertaken, using fatty acid esters CH 3 (CH 2 ) n-2 COOR (8 ≤ n ≤ 18 ; R = Me, Et) as food simulants. Determination at 50°C of weight uptaken by PVC and of depth of penetration of the simulant into the material allowed us to define the time required to reach a thermodynamic equilibrium and the amount of liquid sorbed. Sorption at equilibrium decreased when chain length increased. When the simulant was tinted by methyl red, the dye adsorbed by the material was located exclusively in the diffusion layer. In contrast to sorption of the simulant, the absorbance of the dye in PVC went through a maximum for n = 14. Its spectrum depended on the extent of swelling of the polymer. In materials swollen by short-chain esters, the dye preferred a nonplanar conformation, stabilized through intermolecular solvatation by fatty acid esters. When solvatation and swelling were weak (n = 16, 18), the dye adopted a planar conformation, driven by intramolecular chelation, with a higher λ max and a large extinction coefficient. Methyl red thus appeared as a probe for the study of interactions with slight swelling.