Polypropylene micro- and nanoplastics affect the digestion of cow's milk proteins in infant model of gastric digestion.

Infants may ingest large quantities of micro- and nanoplastics (MNP) due to the heating and sterilization of the feeding bottles, which promotes the generation and migration of MNP into the milk or milk formula. The effect of MNP on the digestion of proteins, which are crucial for the growth and development of infants remain unknown. The current study investigated the in vitro digestion of cow's milk proteins (1 mg/mL) with or without polypropylene (PP) microplastics (MP, pristine or oxidized) (20 mg/mL and 63-180 μm) in simulated gastric fluids (SGF) using an infant (pH 5.0; pepsin activity; 268 U/mL) or adult (pH 3.0; pepsin activity; 2000 U/mL) digestion model at 37 °C for 5, 30, and 120 min. Secondly, the effect of the presence of agglomerated PP nanoplastics (NP, 10, 50, and 100 μg/mL) in milk on the in vitro digestion process using the infant model of gastric digestion was investigated. The profiles of protein digestion products, soft corona and hard corona, were analyzed using SDS-PAGE, and nLC-MS/MS, respectively. Cow's milk protein digestion with or without PP-MP was significantly slower in the infant compared to the adult model, and oxidation of the PP-MP enriched some proteins in the soft and hard corona. In the presence of agglomerated PP-NP, an additional decrease in the rate of milk protein digestion was observed, especially proteins of Mw between 18 and 20 kDa, presumably allergenic β-lactoglobulin. Irrespective of the type of PP, six different types of proteins, including casein α-S1, α-S2, β and κ, β-lactoglobulin, and α-lactalbumin, were present both in the soft and hard corona. Our results indicate a direct impact of PP-MNP on the rate of milk protein digestion in the infant model of gastric digestion. Aging of PP MP through oxidation and smaller size nanoplastics exert more pronounced effects on the digestion of cow's milk proteins in vitro. This suggests that PP-MNP could affect the biological functions of milk proteins and promote chronic health problems.