Occurrence of microplastics in foodstuffs and the factors that affect their migration

This doctoral dissertation discusses the migration of MPs into food and the factors that influence their migration. It should be noted that all experimental procedures had the ultimate goal of studying real food under realistic conditions. Three types of food were investigated: cheese, cured meat, and honey. More specifically, the migration of microplastics on the surface of cheeses such as Edam, Kefalotyri, Parmesan, and Cretan Graviera was studied. In the case of the first three, small pieces of cheese were placed in LDPE and stored under refrigerated conditions for 28 days. The surface of the cheeses was examined using a Raman microscope with a 4-3-4-3-day pattern up to 28 days to detect MPs. In the case of Cretan Graviera, small pieces of different maturity cheese (4- and 8-month maturity) were placed in LDPE and PP for 21 days, under refrigerated conditions. Each sample was sealed under vacuum for complete contact with the plastic. The cheeses’ surface was examined using a Raman microscope every three days. MP signals were detected in the cheese samples using Raman and ATR/FTIR spectroscopy. More specifically, MP signals were detected on the surface of Edam, Kefalotyri, and Parmesan cheeses on their surface from the 14th day (Day 14) of storage. In contrast, MPs from both plastic packages were detected in Cretan Graviera cheese from the 3rd day (Day 3) of storage in both cheese maturity levels. Similarly, the migration of MPs on the surface of cured meat samples, such as salami, mortadella, and bacon, was studied. As in the cheese experiments, small pieces of cured meat were placed in LDPE for 28 days under refrigerated conditions. Each sample was sealed under vacuum to ensure complete contact with the plastic. The surface of the cured meat was examined using a Raman microscope every 3 days. MPs were detected on different days of storage in each type of meat sample. Bacon showed earlier signs of migration, unlike mortadella. Although mortadella has more fat, it is evenly distributed, unlike bacon, where the fat is concentrated in specific areas, which favors migration. ATR measurements were only possible up to day 12 due to technical limitations. The ATR technique did not detect all polyethylene peaks, possibly due to an uneven surface and a large measurement point.As for honey, it was thoroughly investigated in literature as a foodstuff in terms of its properties and characteristics, as well as its contamination with substances and MPs, and the results were published in the related review paper. In parallel, research was conducted on the plastic packaging used for storing honey, and it was found that PET is a very common plastic used for this purpose. PET migration simulations were carried out in honey by placing PET powder inside the honey sample to detect and isolate the PET particles. Several techniques were then employed, including density separation, to isolate the MPs. Different solvents were tested, and it was concluded that pentane is the most suitable solvent extractor for "collecting" MPs. The isolation of MPs was carried out by adding pentane to an aqueous solution of honey. After vigorous shaking, three phases were formed: the supernatant (organic pentane phase), the colloidal dispersion (mixture of pentane with honey and water, along with particles found in honey), and the aqueous phase (mainly honey sugars with water and some solids). Also, attempts were made to dye the MPs with DANS with success. The MPs were visible under a UV lamp due to fluorescence, and their identification was performed using micro-Raman spectroscopy. To detect NPs (< 200 nm), different extraction phases were examined after adding pentane as a density separator. These samples were examined by ESI-MS for the detection of NPs. Using the above techniques, MPs with diameters ranging from 1 μm to 74 μm were detected by Raman spectroscopy, optical microscopy, and fluorescence microscopy. Similarly, PET fragments were found in the colloidal dispersions and in the remaining samples after the density separation technique. Following the simulation study of microplastic migration in honey, and having a research protocol defined, a study was performed in commercially available honey packages. Single-use plastic honey containers, 8 g each, from a well-known brand, commonly available in cafes, were used. Two groups of these products, with different expiration dates, were examined for microplastic migration in three different storage conditions: 1) in a dark and dry place, 2) outdoors (on the balcony), and 3) in the refrigerator. After examining these real honey samples and conducting numerous tests, it was discovered that the simulation protocols that had been developed were quite helpful; however, new protocols were needed in order to detect and isolate MPs from actually packaged honey. This was probably due to the different nature of the migrating MPs. The specific packaging chosen for evaluation had PET as its outer material, while the inner material was PE. After testing many different protocols, H2O2 was used for the dissolution of sugars so that there is no fluorescence in the Raman spectroscopy signals, and NaBr as a density separator, to force the MPs to rise up to the surface of the sample solution. Each sample was divided into two separate samples (upper sample, down sample), each of which passed through an acetate cellulose filter with a pore size of 1.2 μm (upper filter, down filter). As a final step, these filters were observed under a Raman microscope to detect MPs, and finally, the filtrates were passed through 0.2 μm pore filters for analysis by GC-MS. Many MPs were found in all storage conditions, even in the control samples. Particularly notable was the exponential increase in MPs in the honey samples that were stored under refrigerated conditions. Their diameter ranged from 4 μm to aggregates larger than 80 μm. In conclusion, it was found that MPs migrated to the surfaces of cheese, cured meat, and honey from the plastic packaging in which they were stored. In 8 g single-use honey packaging, 1.28–12.8 mg of MPs were detected just one day after purchase, an amount that is released and consumed when diluted in a cup of tea.