In an article published on December 21, 2023, in the journal Food Packaging and Shelf Life, Gianluca Colombo and co-authors from Aarhus University, Denmark, compared the migration of cyclic oligomers and NIAS from industrially recycled polyethylene terephthalate (PET) trays with different ratios of virgin and recycled PET, and produced by two different extrusion processes.
The scientists acquired seven PET trays made of 0, 73, 90, and 100% recycled PET originating from single-screw or twin-screw extruded pellets. They applied the samples to MALDI time-of-flight mass spectrometry (MALDI-TOF-MS) to analyze molecular weights. In addition, they performed migration testing as recommended by the European plastic food contact material regulation (EC No 10/2011), i.e., at 40 °C for 10 days with food simulant D1 (50% v/v ethanol/water). Subsequently, they subjected the samples to ultra-high-performance liquid chromatography quadruple time-of-flight mass spectrometry (UHPLC-QTOF-MS) where they screened for linear and cyclic PET oligomers by targeted and for further (unknown) substances by untargeted analysis.
MALDI-TOF-MS showed a decrease in the molecular weight distribution of polymers in recycled compared to virgin PET trays, which indicates depolymerization due to mechanical recycling, according to the authors. From all seven trays, some of the 12 targeted cyclic PET oligomers were found to migrate into food simulant, but levels varied between the different trays. With increasing content of recycled PET, oligomers migrated in higher levels (especially 2nd and 3rd series dimers) which confirms previous findings (FPF reported). Moreover, “trays obtained with single-screw extrusion showed a significantly higher increase of the PET oligomers concentration.” The authors explained this phenomenon with the shear rate during the extrusion process, creating higher temperatures and friction within the polymer, which could have an impact on material degradation and subsequent oligomer release.
In the untargeted screening, Colombo and co-authors identified a total of 30 compounds, including antioxidants, photoinitiators, phthalates, and different amides. Based on multivariate analysis, they further identified marker compounds to discriminate recycled from virgin PET which were benzyl butyl phthalate (BBP), butyl lactate, dodecenamide, 2-methyl-1,3-dioxolane, PET 2nd series cyclic dimer as well as PET 1st series cyclic dimer.
The authors highlighted that “a better understanding of the choice of extruder type (i.e., single or twin screw) may be important to reduce the level of potentially harmful components migrating in the food.” During extrusion, strong forces are applied to the plastic, resulting in high temperatures and high pressure—both impacting material properties and migration.
The health risk of oligomers migrating from PET is essentially unknown. A systematic evidence map published earlier in 2023 summarized the presence of 34 types of oligomers in food contact materials and their migration into food or food simulant, while also looking into their toxicity. The study found 74% of the 34 oligomers migrated, with the majority not yet assessed for their safety (FPF reported). Further, the report found that several basic assumptions made in the risk assessment of PET oligomers are not based on scientific evidence, highlighting the need for more robust risk assessment of PET oligomers.
Oligomers are relevant for all types of plastics. Another review summarized the presence of oligomers in products made of polybutylene terephthalate (PBT), and polystyrene (PS) in addition to PET (FPF reported). Large knowledge gaps exist for health risks related to oligomer migration.
Reference
Colombo, G. et al. (2023). “Untargeted screening of NIAS and cyclic oligomers migrating from virgin and recycled polyethylene terephthalate (PET) food trays.” Food Packaging and Shelf Life. DOI: 10.1016/j.fpsl.2023.101227
