In vitro intestinal digestibility of cyclic aromatic polyester oligomers from polyethylene terephthalate (PET) and polybutylene terephthalate (PBT)

Linear and cyclic oligomers are unavoidable non-intentionally added substances (NIAS) present in food contact materials made from common polyesters such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and polyester coatings. Although polyester oligomers can migrate into fats in significant amounts in high-temperature processes such as baking or frying, little is known about their toxicological properties and their behaviour in the human gastrointestinal tract. In the present study, first indications of a possible digestibility of polyester oligomers formed from the commonly used aromatic dicarboxylic acid terephthalic acid (TPA) are provided by in vitro experiments. Three cyclic polyester oligomers originated from PET (trimer) and PBT (dimer and trimer) were extracted from the raw materials, isolated and subjected to a simulated intestinal digestion. A fast cleavage (≥75% of the initial amount) of all three cyclic oligomers into their linear counterparts was detected already within the first hour of in vitro intestinal incubation. Subsequent hydrolysis to shorter chained linear oligomers was determined especially for the PET cyclic trimer. Degradation down to the monomer TPA was not observed. In terms of risk assessment and prioritisation for non-evaluated NIAS, the threshold of toxicological concern (TTC) concept is an appropriate tool. While cyclic polyester oligomers based on TPA are assigned to the TTC Cramer class III (high potential concern, exposure threshold 1.5 µg/kg body weight per day), the corresponding linear oligomers are expected to be of a lower probable toxicological concern (Cramer class I, 30 µg/kg body weight per day). A cleavage of cyclic polyester oligomers under human intestinal conditions, which was assessed to be likely by the provided in vitro experiments, could consequently affect the risk assessment on polyester oligomers.