PROTEOLYSIS AND STORAGE STABILITY OF UHT MILK AS INFLUENCED BY MILK PLASMIN ACTIVITY, PLASMIN BETA-LACTOGLOBULIN COMPLEXATION, PLASMINOGEN ACTIVATION AND SOMATIC-CELL COUNT/

Little residual plasmin activity was found in both low and high somati c cell count (SCC) milk which had been subjected to indirect ultra-hig h temperature (UHT) treatment (138 degrees Cx2.4s) and subsequently st ored at 20 degrees C. Milk with elevated SCCs showed definite sediment ation after 150 d storage but did not gel. Addition of potassium iodat e to the milk before UHT treatment led to considerably increased plasm in activity during subsequent storage, indicating the protective influ ence of potassium iodate on the thermal stability of plasmin. These mi lk samples underwent significant decrease in pH on storage, did not ge l, and urea-PAGE electrophoresis showed them to have almost complete b reakdown of beta-casein but incomplete hydrolysis of alpha(s1)-casein. When plasminogen was added to milk immediately after UHT treatment, i t was readily activated to plasmin. Activation occurred more rapidly i n the high SCC milk, suggesting the presence of cell-associated heat-s table plasminogen activators. UHT milk with added plasminogen showed d efinite gelation at 150 and 180 d storage and gelation was more pronou nced in the high SCC milk. Samples to which plasmin was added immediat ely after UHT treatment showed proteolysis patterns similar to those w ith added potassium iodate and showed slight gelation. Low levels of p roteolysis appeared to be linked to the onset of gelation. (C) 1997 El sevier Science Ltd. All rights reserved.