Peptides NAVPITPTLNR and EKVNELSK suppress urokinase-type plasminogen activator-mediated plasmin system activation associated with age gelation in direct ultra-high-temperature skim milk.

Age gelation remains a challenge limiting the shelf-life and stability of direct ultra-high-temperature (dUHT) milk. This study investigated the effects of dUHT treatments (142 °C/147 °C for 0.25/3/6 s) on protein stability and gelation behavior by evaluating particle size, casein hydrolysis, plasmin (PL) and plasminogen (PLG)-derived activities and gel microstructure. Gel-derived peptides were identified, and their interactions with urokinase-type plasminogen activator (u-PA) were assessed. Treatments at 142 °C for 3 s or 147 °C for 6 s induced gel formation after prolonged storage associated with moderate proteolysis and aggregation. Conversely, 0.25 s treatments caused rapid clarification due to high PL activity. Treatment at 142 °C for 6 s maintained colloidal stability with minimal protein degradation. NAVPITPTLNR and EKVNELSK were identified as u-PA inhibitors, suppressing PLG-to-PL conversion. These findings provide evidence that casein-derived peptides may participate in the regulation of PL system in dUHT milk, although their practical effects require further validation in real milk matrices.