Oligomers are one type of non-intentionally added substances (NIAS) that may be present in plastic food contact materials (FCMs). They are composed of two to ten monomers used in the synthesis of polymeric materials such as plastics. Oligomers can form as a side product during the polymerization or by the degradation of the (final) polymeric material. Scientific studies have demonstrated their presence in and migration from plastic FCMs into food (FPF reported, here, and here).
In a review article published on January 30, 2023, in the Journal of Agricultural and Food Chemistry, J. Alberto Lopes from European Innovation Council and SMEs Executive Agency, Brussels, Belgium, and E. D. Tsochatzis from Aarhus University, Denmark, and the European Food Safety Authority, Parma, Italy, provided an overview on polyesters (PES) and polystyrene (PS) oligomer analysis in FCMs, their occurrence, and migration into food, as well as associated methodological challenges. They focused on the polyesters polyethylene terephthalate (PET) since it is the most used PES as well as polybutylene terephthalate (PBT) since its use in FCMs is increasing. Kitchen utensils and disposable coffee capsules can be made of PBT, for instance (FPF reported).
The scientists summarized the different series of oligomers that have been identified in PES and PS FCMs. PET oligomers, for instance, are commonly divided into three groups: (i) the first series consisting of an equal number of terephthalic acid and ethylene glycol units, (ii) the second series, in which the ethylene glycol of half of the units are replaced by a diethylene glycol, and (iii) the third series, in which all the ethylene glycols are replaced by diethylene glycol units. Three series of PBT oligomers could exist but are “less likely to be found.” Also in case of PS several different oligomers can form but, compared to PES, cyclic versions are less likely due to a lack of an esteric bond in the compound.
Concerning toxicity, the authors pointed out that toxicological data is generally lacking for all PES and PS oligomers, making a risk assessment difficult. Thus, the Threshold of Toxicological Concern (TCC, in silico) approach has been applied to PET cyclic oligomers and PBT oligomers leading to their classification within the Cramer Class III (implying potential significant toxicity). While for PS oligomers toxicological data is absent, its monomer, styrene, was classified as a “probable carcinogenic compound” to humans (Group 2A) by the International Agency for Research on Cancer (IARC, FPF reported).
Currently, scientists in Switzerland, including from the Food Packaging Forum (FPF), are collecting exposure and hazard information for 34 oligomers that have been found to migrate from PET FCMs (FPF reported). And the FPF’s FCCmigex Database includes evidence from over 40 studies on all types of oligomers migrating from different plastic polymers, silicones, and can coatings. Also Alberto Lopes and Tsochatzis evaluated the data of PES and PS oligomer occurence in FCMs and food. Generally, they found that compared to other NIAS, relatively few studies have been performed and that more studies focused on the investigation of PET than PS monomers and oligomers in food and food simulants.
Only recently, M. A. Naziruddin and co-authors from Universiti Putra Malaysia, Selangor, Malaysia, analyzed styrene monomer quantities in three types of yogurt packaged in High Impact Polystyrene. The article was published on January 23, 2023, in the journal Food Control. The scientists measured styrene levels after purchase and then every four days until the expiry date using gas chromatography-mass spectrometry (GC-MS). They reported that styrene concentrations in the “three yogurts ranged from 5.740 ± 0.008 to 78.39 ± 0.42 ng/g throughout the 672 h migration test” and were always below the overall migration limit (OML) of Regulation (EU) No 10/2011.
To explain the lack of studies on PES and PS oligomer analysis in FCMs and food, Alberto Lopes and Tsochatzis posit three possible reasons: “(1) lack of analytical standards; (2) the physical-chemical characteristics of the oligomers per se, and (3) the complication arising from the nature of certain matrices for their qualitative or quantitative analysis.”
The authors also outlined current and future challenges in the field, especially with regard to analytical methods to assess oligomers. Challenges include a yet unknown but “presumably very high number of target substances” since oligomers are formed by many potential mechanisms, the absence of analytical standards of sufficient purity as well as of validated analytical methods, and the difficulty to perform a risk assessment given the oligomers’ unknown identity and quantity. Therefore, “new approaches are needed to steer the assessment of the presence of oligomers in FCM products.”
As one approach to tackle some of the challenges, the scientists proposed the development of multianalyte methods (MAMs). MAMs combine targeted screening of several substances in one go with nontargeted screening. According to the authors, the “EU network of FCM” would at least to a certain extent be ready to “monitor the existing EU legislation concerning this class of oligomers” since the European Union Reference Laboratory for Food Contact Materials (EURLFCM) has organized two Proficiency Tests on the determination of PET and PBT oligomers (FPF reported).
Another challenge mentioned in the review is the increase in recycled plastics which would “urge the FCM community to develop efficient methods for the analysis of oligomers, as this may impact the possibility of a new recycling technology to be used with PES and PS to even reach the market.”
Together with Oliver Kappenstein from the German Federal Institute for Risk Assessment (BfR), Alberto Lopes and Tsochatzis published another article on oligomers in the journal Food Packaging and Shelf Life on January 2, 2023. Here they focused on polyamide (PA) 6 and 66 oligomers and their migration from a multilayer packaging consisting of PA on the food contact side. After having developed a method based on a quadruple time-of-flight mass spectrometer (QTOF-MS) to quantify PA monomers and oligomers in food simulants, they applied it to investigate how ionic strength affects migration. Their analysis showed that “changes in the ionic strength may influence the migration of polar substances” and, therefore, they recommended that “migration testing should consider not only the fatty content of a food but also salt content.” Already in 2020 scientists came to the same conclusion by also pointing out that especially the migration of polar substances could be affected by the salinity content of the food (FPF reported).
Having used the TCC approach, the BfR assigned the polyamide (PA) oligomers to Cramer Class III (FPF reported).
References
Alberto Lopes, J. and Tsochatzis, E. D. (2023). “Poly(ethylene terephthalate), Poly(butylene terephthalate), and Polystyrene Oligomers: Occurrence and Analysis in Food Contact Materials and Food.” Journal of Agricultural and Food Chemistry. DOI: 10.1021/acs.jafc.2c08558
Tsochatzis, E. D. et al. (2023). “Study of the ionic strength effect on the migration of polyamide 6 and 66 oligomers into liquid simulants by a LC-qTOF-MS method.” Food Packaging and Shelf Life. DOI: 10.1016/j.fpsl.2022.101015
Naziruddin, M.A. et al. (2023). “Assessment of residual styrene monomer migration into yoghurt packed in High Impact Polystyrene pots using a modelling approach.” Food Control. DOI: 10.1016/j.foodcont.2023.109612
