Investigating microplastic release from plastic grinder heads during salt grinding

Microplastic contamination of table salt has been widely reported, but most studies focus on environmental sources rather than microplastic release from food-contact materials under routine household use. Here, we demonstrate that commonly available plastic grinder heads themselves can be a significant contributor of microplastics in salt during household use. We tested commercially available salt products from different local retailers (Retailer A, B, and C), each fitted with plastic grinder heads, using a ceramic mortar as the control group. We implemented strict QA/QC measures, performing procedural blanks and calculating the recovery rate for spiked samples. Using optical photothermal infrared (O-PTIR) spectroscopy, we analysed subsampled particles visually detected on the collection filters and matched their spectra against the Hummel polymer library for identification. The grinder heads from the products were made of either polyethylene terephthalate (PET; Retailers A and B) or polycarbonate (PC; Retailer C), and the majority of microplastic fragments released corresponded to these polymers. Plastic grinder heads released a significant number of microplastic particles per 50 g salt (Retailer A: 1091 ± 239, Retailer B: 2420 ± 1805, Retailer C: 15743 ± 1858 particles, each Retailer n = 3 grinder heads), significantly higher than the procedural blanks results (The number of particles found in the experimental environment was 0, 1, and 0 respectively.). Microplastic particles released during salt grinding were confirmed using O-PTIR spectroscopy, with characteristic particle sizes on the order of several tens of micrometres (mean particle sizes ± standard deviation of 49.6 ± 18.2 μm n = 3 for Retailer A and 55.3 ± 12.7 μm n = 3 for Retailer B). Spectra collected from all particles, along with the associated QA/QC data, are openly available on Zenodo, contributing to transparency and reproducibility in microplastic research. In summary, our findings provide direct evidence that everyday food-contact plastics can introduce microplastics into diets, particularly under conditions of high mechanical stress, such as grinding. This highlights the need to assess the wear resistance of food-contact plastics and consider adopting more durable materials or designs to reduce consumer exposure to microplastics.