Correlations Between Quality, Enzymatic Activities And Storage Stability Of Uht Milk In Tunisia

Destabilization of ultra-high temperature (UHT) milk (140 °C, 4s) in the form of gel or sediment may occur during storage. This phenomenon is a real industrial and scientific problem that could be caused by milk proteolysis and lipolysis. This study aimed to study the influence of storage conditions (time, temperature) on the quality of UHT milk produced under industrial conditions. The physicochemical characteristics of these different kinds of milk, including composition and stability as assessed by phosphate test, were determined. UHT heat treatment of milk resulted in a change in pH, acidity, and stability derived from phosphate test, as well as in a drop in psychrophilic bacteria counts. Proteolytic and lipolysis activities also decreased with UHT treatment. However, during UHT milk storage for 6 months, the milk proteolytic activity increased, and in consequence, an increase in acidity and non-casein nitrogen values was observed. Moreover, lipolytic activity was higher in milk stored at 30 °C, as compared to the milk kept at 4 °C. In addition, the stability values increased during UHT milk storage at 30 °C. Stability was inversely correlated with milk fat and acidity (p<0.05). Furthermore, it showed a strong relationship with proteolysis (r=0.765), but no significant relationship was found with lipolysis. The analysis of the sediment of UHT milk stored for 6 months showed levels of fat (15 %), proteins (40 %) and lactose (42 %). The composition of the sediment illustrated the complexity of the formation process.