Effect of various heat treatments on plasminogen activation in bovine milk during refrigerated storage

Plasmin and plasminogen-derived plasmin activities were measured in heated milk with and without the addition of plasminogen activator, before and after storage at 4 degrees C for 96 h. The effect of a free sulfhydryl group donor, beta-lactoglobulin or cysteine, on plasminogen activation was investigated in a model system and milk. Heating milk to 75 degrees C enhanced plasminogen activation that was marked by a considerable increase in plasmin activity. Heating at 85 and 90 degrees C caused a significant decrease in plasmin and plasminogen-derived plasmin activities. However, after storage, significant plasmin levels were restored because of the activation of remaining unfolded plasminogen. Both beta-lactoglobulin and cysteine significantly decreased plasmin and plasminogen-derived plasmin activities in a model system. While endogenous beta-lactoglobulin was not sufficient to completely eliminate plasminogen activation in milk, cysteine addition prior to pasteurisation significantly decreased plasmin and plasminogen-derived plasmin activities. Results highlighted the importance of the remaining plasminogen in heated milk systems.