A study from France’s food safety agency, ANSES, has revealed a counterintuitive finding that challenges assumptions about beverage packaging safety. Published in the Journal of Food Composition and Analysis in 2025, the research demonstrates that glass bottles contain significantly higher levels of microplastic contamination compared to plastic bottles, with some beverages showing contamination levels up to 50 times greater in glass containers.
The study examined water, soft drinks, beer, and wine purchased from French retailers in June 2023. Researchers analyzed six samples from each product category, testing various container types including plastic, glass, brick, cans, and cubitainers.
The findings reveal average contamination levels of 2.9 microplastics per liter in waters, 31.4 in colas, 28.5 in teas, 45.2 in lemonades, 82.9 in beers, and 8.2 in wines.
The research identified glass bottles as the most contaminated containers across nearly all beverage categories. For colas, glass bottles contained an average of 103.4 microplastics per liter, compared to just 2.1 in plastic bottles and 3.4 in cans.
The pattern held for other beverages: iced tea in glass bottles averaged 86.3 microplastics per liter versus 2.2 in plastic, while lemonade in glass reached 111.6 compared to 1.5 in plastic containers.
Beer showed particularly high contamination in small glass bottles at 133.7 microplastics per liter, though larger glass bottles and cans contained significantly less at approximately 32 microplastics per liter each. Wine represented the only exception to the glass contamination pattern, with brick containers showing the highest levels at 30.0 microplastics per liter.
The study traced the source of contamination to an unexpected culprit: the painted metal caps sealing glass bottles. Researchers observed that the majority of microplastic particles isolated from beverages matched the color of bottle caps and shared the chemical composition of the outer paint coating. Microscopic examination of new caps revealed scratches on their outer surfaces and pieces of paint adhered to their inner surfaces.
To confirm this hypothesis, the research team conducted encapsulation experiments using clean glass bottles filled with filtered water and sealed with new caps. Bottles capped without any pre-treatment contained an average of 287.3 microplastics per liter. When caps were blown clean with compressed air before use, contamination decreased to 105.8 microplastics per liter. Caps that were both blown clean and rinsed with water and ethanol reduced contamination further to 86.7 microplastics per liter.
Analysis of the rinsing solution revealed an average of 47.8 yellow paint particles per cap, providing direct evidence that cap paint represents a significant contamination source. The researchers theorize that caps stored in bulk packaging experience abrasion and surface friction when they collide, causing paint to flake off and contaminate beverages upon opening.
Chemical analysis identified the polyester cluster, including PET and alkyd lacquers, as particularly prevalent in glass containers. This polymer group reached concentrations of 26.3 microplastics per liter in teas, 28.5 in lemonades, 33.2 in colas, and 95.9 in beers packaged in glass. These polyester compounds match the composition of paints used on metal caps.
The study found that 96.9% of microplastics in water, 93% in colas, 93.4% in teas, 96.1% in lemonades, 92.7% in beers, and 72.9% in wines appeared as fragments rather than fibers. Particle sizes ranged from 30 to 500 micrometers, with distribution varying by beverage type and container.
While glass has traditionally been considered a safer alternative to plastic for beverage storage due to concerns about chemical leaching, these findings demonstrate that the total packaging system, including closures, must be evaluated for microplastic contamination.
The study authors note that implementing a cleaning step for caps before bottling could substantially reduce beverage contamination, though it would not eliminate microplastics entirely. Beyond microplastic levels, the contamination from cap paint raises additional concerns about potential additives present in these coatings.
