Plasmin activity as a possible cause for age gelation in UHT milk produced by direct steam infusion

The effect of enzymatic activity in direct steam infusion heat treated milk with ultra-short holding times (>150 °C for <0.2 s) on age gelation during storage was investigated. Preheating at either 72 or 95 °C for 180 s was performed. Milk pre-heated at 72 °C showed extensive proteolysis and exhibited bitter off flavour and contained <40% intact αS- and β-caseins after 6 weeks storage at 20 °C. No proteolysis of κ-casein was detected. Plasmin was identified as active protease and activation of plasminogen was observed as an increase in the rate of casein hydrolysis. Proteolysis in the stored samples correlated with a decrease in pH and with changes in colour. Gelation occurred after 10 weeks along with an increase in viscosity and extensive proteolysis of αS- and β-caseins. In conclusion, plasmin activity was involved in age gelation and bitterness caused by proteolysis was the shelf-life limiting factor.