Proteolysis of ultra-high pressure homogenised treated milk during refrigerated storage

Plasmin residual activity and its relation to proteolysis of milk subjected to ultra-high pressure homogenisation (UHPH; 200–300 MPa, inlet temperature = 30 °C and 40 °C) and to a high-pasteurisation treatment (90 °C, 15 s) were studied during refrigerated storage. Proteolysis was examined by capillary electrophoresis, HPLC peptide profiles, pH 4.6-soluble nitrogen and free amino acids. Inactivation of plasmin increased as homogenisation pressure did. Extensive proteolysis, was observed in 200 MPa 40 °C milk, due to its higher native and microbial enzyme contents, compared with the other samples. In general, hydrolysis of β-casein, hydrophobic peptide and pH 4.6-soluble nitrogen levels increased with higher residual plasmin activity, while hydrophilic peptides were not affected by the different treatments applied. β-Lactoglobulin was denatured to a greater extent by thermal treatment than by UHPH treatments. This study provides further insight into how UHPH treatments influence milk properties.