The demand for ultra-high-temperature (UHT) processed and aseptically packaged milk is increasing worldwide. UHT milk has advantages as it is bacteriologically stable at ambient temperature for extended shelf life, and it has lower energy costs due to the elimination of required refrigeration during storage. Besides these positive attributes of UHT processed milk, undesirable changes may also occur during storage, of which the irreversible gelation of UHT milk (age gelation) is a major factor that limits shelf life and commercial exploitation. Although this problem has been known for a long time, mechanistic insights into this age-gelation process of UHT milk are lacking.
The onset of age-gelation is specifically promoted by the proteolytic activity due to residual enzymes in UHT milk, which can survive the UHT treatment and remain active during storage, so further research on both of them can be used to better understand age gelation. Detailed determination of the action of plasmin and psychrotrophic bacterial enzymes in proteolysis, followed by kinetic modelling of age-gelation, may help in explaining the mechanisms and kinetics of age-gelation. Based upon the obtained data, age gelation in UHT milk will be traced back to the milk quality on farm level, and this study will thus providing guidance for setting quality requirements for raw milk.
The objective of this project is thus to improve the stability of UHT milk by studying the gelation mechanisms and the effects of endogenous and exogenous proteinases using enzymology, reaction kinetics, chromatography, mass-spectrometry, as well as rheological approaches.