Samples of commercially processed ultra-high-temperature (UHT) milk were stored at 4, 20, 30 and 37°C for up to 2 years or until gelation occurred.
The stability of the proteins to ethanol, calcium ions and rennet decreased with time of storage. However, preliminary autoclaving of the UHT milk induced a high degree of stability.
The extent of protein decomposition that had occurred in the UHT milk samples depended on the time and temperature of storage. There was negligible decomposition when the samples had been autoclaved before storage at 4 and 20°C. Storage at 37°C led to significant decomposition.
UHT milk samples gelled after being stored for 13 months at 4, 20 and 30°C, but not at 37°C. The autoclaved milk was still fluid after being stored for 2 years at these temperatures.
The time of onset of gelation did not depend on the degree of protein breakdown, and it seemed therefore that proteolysis was not the primary cause of gelation on storage.
It was concluded that the proteins in UHT milk underwent several changes on storage that were apparently independent of each other and led ultimately to coagulation of the milk. These changes included proteolysis, and a progressive loss of stability under conditions that favoured aggregation of the casein micelles.