Effect of Storage Temperature on Age Gelation of Ultra-High Temperature Milk Processed by Direct and Indirect Heating Systems

Age gelation in ultra-high temperature milk processed by direct (Dasi) and indirect (Sordi) systems was investigated at three temperatures (4C, 22 to 25C, and 37C) over 182 d. Sordi processed milk showed no residual plasmin activity but 19% of original plasminogen activity after processing, whereas Dasi milk contained 19% and 37% of original plasmin and plasminogen activities. During storage, plasminogen activity gradually decreased and plasmin activity increased in all samples. Age gelation was observed only in Dasi processed milk stored at 22 to 25C after 84 to 98 d. In general, age gelation was retarded at low (4C) and high (37C) temperatures, and rate of increase in proteolysis was directly proportional to temperature. No relationship was found between extent of proteolysis and gelation time.