Polyethylene terephthalate (PET) bottles have become an intrinsic part of everyday life whether from drinking bottled water and carbonated beverages to collecting empty PET bottles for recycling.
Around six million metric tonnes of PET are used annually to make bottles for mineral water with continued future growth being forecast by the plastics industry. The dominance of PET for bottled beverages is understandable as it has many attractions as a food contact material, not least its strength, impermeability, clarity and relative inertness compared to other polymers like polyolefins and PVC.
Bottled Water Testing – PET Degradation and Acetaldehyde Production
Despite having few plastics additives that might leach into beverages, unfortunately PET can degrade at high temperatures during bottle blowing, leading to formation of acetaldehyde. Trapped in the walls of the PET bottle, but not chemically bound, acetaldehyde can slowly diffuse (migrate) leading to trace levels of contamination in beverages. With a characteristic, slightly fruity smell, acetaldehyde can be detected at levels as low as 20 ppb as a discernible taint in bottled water. This is not a concern for stronger tasting beverages like fruit juices where acetaldehyde is a natural component anyway, or for other strong-tasting carbonated beverages such as cola. However, acetaldehyde is not acceptable in bottled water and any tainting is also an infringement of EU and US plastics regulations.
Clever Chemistry to ‘Mop-up’ Acetaldehyde
There are various reactive chemicals known as scavengers that can be used to ‘mop-up’ acetaldehyde during pre-form PET bottle production. 2-Aminobenzamide is one of these scavenging agents which is widely used to reduce the concentration of acetaldehyde in the PET bottle wall. However, having solved the taint problem by using a scavenger, it is important not to introduce another safety issue related to migration of 2-aminobenzamide and its resulting reaction product.
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Modelling and Predicting Migration Behavior
In a 2016 publication in the journal Food Additives & Contaminants Part A, Roland Franz and co-workers from the Fraunhofer Institute in Germany have studied the migration of 2-aminobenzamide from PET bottles into food simulants and into natural mineral water.
Using a Thermo Scientific TSQ Quantum™ Max triple stage LC-MS, 2-aminobenzamide was monitored in aqueous simulants and in carbonated mineral water using the transition m/z 137→m/z 120. LC-MS/MS provided a fast, sensitive and specific measurement tool to generate a body of migration data for 2-aminobenzamide.
From migration kinetics the authors determined the diffusion coefficient of 2-aminobenzamide from PET and from modelling experiments were thus able to predict migration at different residue levels in PET and at different time/temperature storage conditions. The authors concluded that for the PET bottle manufacturers the most economical way of ensuring compliance with migration limits is to routinely measure the free 2-aminobenzamide concentration in the PET bottle wall and thus calculate migration for room temperature (or similar) for long term storage.
In this way, the bottle wall concentration of 2-aminobenzamide can be taken as an easily measurable compliance parameter to check against the EU specific migration limit of 50 μg L–1.
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