Does bottling process or cap type (plastic vs metal) affect microplastic contamination?
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Executive summary
A recent multi-beverage analysis by France’s ANSES found higher microplastic counts in drinks sold in glass bottles than in plastic bottles or cans, and implicated painted metal caps as a major source [1][2]. The study also showed that simple cap-cleaning steps on the production line can sharply reduce particle counts, indicating the contamination pathway is plausibly preventable [3][4].
1. Glass bottles — the surprising hotspot
Multiple reports from the ANSES study and peer-reviewed publication show that beverages like cola, lemonade, iced tea and beer in glass bottles exhibited the highest microplastic concentrations—sometimes five to fifty times greater than the same drinks in plastic bottles or cans, and in some tests reaching over 100 particles per liter [2][4]. Water and wine were notable exceptions with relatively low microplastic levels across packaging types, a pattern ANSES says “remains to be explored” except where caps are involved [1][5].
2. The cap as a contamination vector — paint and friction
Laboratory analyses matched the color and chemical composition of many particles found in drinks to the polyester-type paints used on metal bottle caps, and microscopic scratches consistent with friction were observed on cap surfaces—evidence ANSES cites for paint-shedding into beverages [2][6]. The agency’s experiments point to cap handling and storage—caps rubbing against each other before sealing—as a likely source of abrasion that releases paint fragments into bottles during closure [1][5].
3. Opening, closing, and other mechanical sources
Separate studies emphasize that cap manipulation after bottling—repeated opening and closing—can also generate microplastics: controlled experiments using single-particle FTIR imaging showed cap opening can release measurable particles into bottled water, making consumer behaviors another plausible contributor [7]. Earlier work and reviews likewise flag screw caps and sealing gaskets as recurring origins of microplastics in bottled beverages [8][9].
4. Plastic vs metal caps — materials and measured outcomes
The ANSES dataset linked higher contamination specifically to painted metal caps used on many glass bottles, whereas plastic bottles in the study typically arrived with plastic caps that did not use the same paint and showed lower matching particle signatures [10][2]. That distinction—painted metal closures versus unpainted plastic closures—helps explain why “glass” containers in practice carried more microplastics: it is the cap composition and surface treatment, not the glass itself, that emerged as the dominant source in these tests [6][11].
5. Strength of evidence, limitations and alternative explanations
The ANSES team published laboratory-based matches of particle color and composition and tested remediation steps, supporting a causal chain from cap paint abrasion to beverage contamination, but broader generalization is limited by geography, product selection and the complexity of supply chains; the study sampled drinks sold in France and did not establish how universal the finding is across all cap formulations worldwide [6][12]. Other research flags additional vectors—internal bottle liners, gasket materials, recycling wear or consumer handling—that may contribute to observed microplastic loads and complicate single-factor attribution [9][7].
6. Practical implications: industry fixes and consumer options
ANSES documented that simple industrial mitigations—air-blowing and rinsing caps with water and alcohol—reduced microplastic counts by around 60% in trial fills, demonstrating a feasible manufacturing fix for painted metal caps [4][3]. For consumers the agency notes limited options to remove already-present particles in bottled drinks, though avoiding glass bottles with painted metal caps or preferring products sealed with corks or unpainted closures (as in many wines) could reduce exposure in settings matching the study sample [1][5].
7. Bottom line
The preponderance of evidence from the ANSES study and corroborating reports indicates that cap type and bottling processes materially affect microplastic contamination: painted metal caps (commonly used on many glass bottles) are a documented and reducible source, while plain plastic closures and unpainted seals generally corresponded with lower particle counts in the sampled products [2][6]. Broader surveillance, cross‑country replication and tracking of cap formulations are needed before declaring the pattern universal, but the mechanism identified points to a clear, actionable contamination pathway for manufacturers and regulators [1][11].