Proteolysis and storage stability of UHT milk as influenced by milk plasmin activity, plasmin/β-lactoglobulin complexation, plasminogen activation and somatic cell count

Little residual plasmin activity was found in both low and high somatic cell count (SSC) milk which had been subjected to indirect ultra-high temperature (UHT) treatment (138°C x 2.4s) and subsequently stored at 20°C. Milk with elevated SCCs showed definite sedimentation after 150 d storage but did not gel. Addition of potassium iodate to the milk before UHT treatment led to considerably increased plasmin activity during subsequent storage, indicating the protective influence of potassium iodate on the thermal stability of plasmin. These milk samples underwent significant decrease in pH on storage, did not gel, and urea-PAGE electrophoresis showed them to have almost complete breakdown of β-casein but incomplete hydrolysis of α(s1)-casein. When plasminogen was added to milk immediately after UHT treatment, it was readily activated to plasmin. Activation occurred more rapidly in the high SCC milk, suggesting the presence of cell-associated heat-stable plasminogen activators. UHT milk with added plasminogen showed definite gelation at 150 and 180 d storage and gelation was more pronounced in the high SCC milk. Samples to which plasmin was added immediately after UHT treatment showed proteolysis patterns similar to those with added potassium iodate and showed slight gelation. Low levels of proteolysis appeared to be linked to the onset of gelation.