Not as simple as making a cup of tea

Figuring out how much plastic is possibly released from a teabag into a cup of tea is, as the AMI BioNanomaterials group has shown, far from simple, and requires advanced analytical methods. 

Research into the detection of plastic particles, especially in the micro-, sub-micro-, and nano-size ranges, in food and beverages has been gaining traction in the scientific community. The recent surge in interest has on the one hand been motivated by the fact that the growing use of plastic materials promotes their widespread accumulation in the environment, where they degrade, are taken up by plants and animals, and via the food chain transfer to humans. But since most drinks and food products are sold in plastic packaging, there is also the question if and how much of that plastic finds its way into our meals and potentially impacts our health.

While research has shown that larger microplastic particles can been detected in food and beverages such as bottled water, confirming, and quantifying the presence of small nanoplastic contaminants is more challenging. Studies of these particles in complex environments, which contain large amounts of other organic or inorganic matter, are plagued by a number of issues. These are related to nanoplastics’ mostly carbon structures, low densities, size, and generally low estimates of their concentrations in samples.

This has not prevented AMI researchers from attempting to tackle the problem, choosing teabags, and investigating just how many plastic particles leach out while steeping a cup. Researchers at other institutions had previously attempted to answer this question, but their results suggest that the application of known analytical methods and the interpretation of the data that they afford is not necessarily straightforward.

“For example, other researchers have used a method called Fourier transform infrared spectroscopy to detect plastic particles leached from teabags, but this method cannot be used to directly assess individual particles, but only yields an average composition of an up-concentrated sample” explains PhD student Jessica Caldwell. “Using electron microscopy to distinguish between nanoplastics and other organic materials can also be fraught with difficulty.” One key problem is to differentiate between plastic particles and other organic matter, for example the ingredients extracted from the tea itself, tea leaf-based particulate matter, or dust.   

To overcome these limitations, tests were carried out at the behest of Switzerland’s Federal Food Safety and Veterinary Office. The AMI researchers conducting these analyses say that greater care must be given to sample preparation and data interpretation to ensure accurate results. One approach that they investigated involved purifying samples by applying chemical digestion protocols that were designed to remove non-plastic compounds from the samples. Yet, despite using potassium hydroxide and hydrogen dioxide, both known for their capacity to eliminate organic matter, signs of the latter could still be observed in the samples assessed by the AMI researchers. Further testing on empty control teabags also indicated that such chemical purification steps may also degrade the plastic particles that should be observed.    

To circumvent this problem, the BioNanomaterials group researchers have proposed that future studies should investigate the possibility of using enzymes for the purification step. These bio-catalysts can be expected to digest exclusively bio-based organic matter and avoid plastic degradation. “We want to establish protocols for this type of analysis and establish our expertise in this field, as part of our wider research on plastic particles in complex environments”, explains Prof. Alke Fink. Alongside this, more sensitive analytical techniques such as surface-enhanced Raman spectroscopy could provide the finesse required to analyze samples at the single-particle or single- material basis.    

References:  

Caldwell, J.; Taladriz-Blanco, P.; Rothen-Rutishauser, B.; Petri-Fink, A. Additional Commentary on the Detection and Quantification of Plastic Micro- and Nanoparticles in Tea Samples. CHIMIA 2021, 75 (10), 882–882 

Caldwell, J.; Taladriz-Blanco, P.; Rothen-Rutishauser, B.; Petri-Fink, A. Detection of Sub-Micro- and Nanoplastic Particles on Gold Nanoparticle-Based Substrates through Surface-Enhanced Raman Scattering (SERS) Spectroscopy. Nanomaterials 2021, 11 (5), 1149.