Studies On The Causes And Control Of Age Gelation In UHT Milk During Storage

Proteolysis, viscosity, pH change and age gelation were studied in three batches of skim UHT milk and three batches of whole UHT milk, respectively.The results showed that age gelation formed in one batch of the whole milk group and all the three batches of skim milk group after 7 months of storage at room temperature, viscosity increased sharply in gelled batches in the 30 days before age gelation, pH in all batches decreased gentally during storage, while free amino groups, non-casein nitrogen and non-protein nitrogen increased in all batches .It was found that the level of proteolysis caused by protease activity was related with gelation time (R2=0.728 for whole milk, R2=0.996 for skim milk). The influence of storage temperature on the level of proteolysis in UHT milk was also examined, and 83% and 96% of proteolysis was inhibited in the UHT milk stored at 4oC when compared with that stored at room temperature. The influence of low-temperature-inactivation treatment (LTI) after UHT on proteolysis and gelation was also investigated. Compared with the control, LTI treatment at 55oC for 10, 30 or 60 min inhibited proteolysis between 34.4-82.8%, 64.6-91.8% and 73.8-93.4%, respectively; LTI treatment at 60oC for 10, 30 or 60 min inhibited proteolysis between 45.8-68.1%, 62.4-91.8% and 75.9-95.5%, respectively, and the occurrence rate of age gelation in the LTI treated group was much lower than that in the control group when stored for 8 months, implying that LTI treatment did result in retardation of the onset of age gelation.