Testing limonene diffusion through food contact polyethylene by FT-IR spectroscopy: Film thickness, permeant concentration and outer medium effects

A number of relevant factors not usually considered in the analysis of the transport properties of aroma components through food packaging films have been the subject of this study, using infrared spectroscopy as the main probing tool. More in particular, the diffusion behavior of limonene in low density polyethylene (LDPE) has been analyzed by transmission FT-IR spectroscopy as a function of sample thickness, permeant concentration and the outer medium. From the results, a surprising reduction in diffusion (D) and permeability (P) coefficients with reducing film thickness was found, which is most likely attributed to morphological differences arising during cast film extrusion. Moreover, the limonene desorption kinetics were found to slow down considerably when desorption of the volatile was carried out in water as compared to desorption in air. Finally, the sorption kinetics of limonene in LDPE were found to be much slower when the polymer was put in contact with pressed orange juice (similar to a real juice packaging case), than when it was put in contact with the pure volatile. A remarkable observation arising from this work is that the diffusion coefficient of limonene in LDPE can vary by up to two orders of magnitude depending on the testing conditions, mainly limonene concentration but also polymer morphology and, consequently, these observations may well account for some of the extensive variability reported for this permeant in the existing literature.