Metal bottle caps can be a significant source of microplastic contamination in beverages, a new peer-reviewed study by France’s food safety agency finds.
Researchers compared microplastic levels in beer, water, wine and soft drinks, and found the substance in all samples, but liquid in glass jars showed the highest levels. The surprising source of the contamination – a polyester-based paint on the glass bottles’ metal caps.
The findings were “very surprising”, said Alexandre Dehaut, a study co-author with the French agency for food, environmental and occupational health and safety.
“Caps were suspected to be the main source of contamination, as the majority of particles isolated in beverages were identical to the color of caps and shared the composition of the outer paint,” the authors wrote in the study.
Microplastics are tiny bits of plastic either intentionally added to consumer goods, or that are products of larger plastics breaking down. The particles contain any number of 16,000 plastic chemicals, of which thousands, such as BPA, phthalates and Pfas, present serious health risks.
The substance has been found throughout the human body, and is a neurotoxicant that can cross the placental and brain barriers. It is linked to an increased risk of heart attack and cancer.
Diet is thought to be a main exposure route – testing in recent years has consistently identified microplastics in a range of foods and beverages, and packaging is one source of contamination.
Researchers in the new study checked beverages in water, glass, metal, and brick bottles, and found microplastics in all.
The levels in the glass bottles were highest – about 50 times higher than the plastic. The glass bottles used metal caps, while the plastic bottles came with plastic caps. The plastic caps did not use the same kind of paint as the metal caps, researchers noted.
Dehaut said they were led to the paint because the microplastic fragments they found in the beverages seemed to match the paint. Closer scrutiny revealed the microplastics matched material, color and polymeric composition of the paint lining the outside of the caps.
It appears the bottle caps are stored post production with thousands of other caps in bags or boxes, and those scrape each other as they are jostled, Dehaut noted. Once the caps are sealed to the bottles, the bits of plastic from the scratches end up in the beverage. The authors were able to see the tiny scrapes and scratches when they placed the caps under a microscope.
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Researchers also found that the problem may be easy to solve – the microplastic can be removed from the caps by rinsing and blowing them dry at the end of the manufacturing process. However, Dehaut said the strategy worked in the lab, but it may be more difficult to do on an industrial scale.
They also found microplastics that did not come from the paint, meaning the contamination occurred somewhere in the production process, or was in the product’s water.
Though the dangers of microplastics are coming into sharper focus, the health impacts of those that researchers found in the bottles are unclear because there’s so much variation in the type of plastic, and they did not run risk assessments, Dehaut said.
Consumers could avoid metal bottle caps. Dehaut said there is little people can do at home about the contamination because the microplastics are already in the beverages. He said the findings point to the need to investigate and avoid contamination further upstream during the production process.
“We should investigate such things, but don’t be paranoid,” Dehaut said.