Development and validation of analytical methods for detecting and identifying microplastics in salts, soy sauce, and salted pollock roe

One of the main routes of human exposure to microplastics is food consumption; therefore, the quantitative analysis of microplastics in foods is important. Researchers have reported the occurrence of microplastics in different ways, necessitating the validation and standardization of analytical methods. In this study, we optimized pretreatment methods for three representative foods (i.e., salts, soy sauce, and salted pollock roe) and verified analytical methods, including material identification using Fourier-transform infrared spectroscopy, in four independent laboratories based on blind tests. We prepared custom-made cylindrical reference materials, visually identifiable under a microscope, using five plastic materials (polyethylene, polypropylene, polyethylene terephthalate, polystyrene, and polyamide-6). Each food sample was spiked with a randomized number of reference materials and sent to testing laboratories for analysis without notifying them the material type and number of reference materials. The average recovery rates of the reference materials for salt, soy sauce, and salted pollock roe were 73.2%, 76.9%, and 86.2%, respectively. Two-way analysis of variance of the experimental results demonstrated that the recoveries of the reference materials did not depend on any of the five plastic materials or the participating laboratories, indicating that the proposed methods are capable of reliably determining microplastics greater than 100 µm in selected foods. •Method validation on microplastics in three food matrices was conducted.•Optimized methods showed recoveries of reference materials (∼100 µm) over 70%.•Recoveries were independent of experimenters or five plastic reference materials.•First reports on the occurrence of microplastics in soy sauce and salted pollock roe.