Instant infusion pasteurization for heat treatment of milk - Physical-chemical and functional properties

Summary

Instant infusion technology has been suggested as a gentle method for pasteurization of milk, and the overall aim of the research presented in the current PhD thesis was to investigate this suggestion. The effects of instant infusion pasteurization (IIP) was studied by characterization of the microbial inactivation and selected physical-chemical and functional properties of milk subjected to IIP at different temperatures in the range from 72°C to 120°C and holding times < 1s. Standard high temperature short time (HTST) pasteurization at 72°C for 15 s was used as reference treatment.
It was found that IIP provided high inactivation of microorganisms, even at 72°C the total aerobic counts were almost equal to those of milk subjected to HTST pasteurization. There was a further reduction of the bacterial counts when the temperature of IIP was increased, whereas changes in holding times between 0.1s and 0.7s had no effect on the total aerobic count.
The observed changes in the investigated physical-chemical properties were primarily due to disruption of the milk fat globules during IIP. The extent of the disruption increased with the temperature of IIP and was suggested to be due to cavitation occurring during the flash cooling step. This in turn affected the viscosity as well as the particle size distribution of the skim milk fraction of non-standardized milk. Furthermore, it was shown that front face fluorescence spectroscopy could be used to discern between milk subjected to different IIP treatments and also separate IIP treated milk from raw milk, HTST pasteurized milk and from milk heat treated by a more intensive pasteurization at 85°C for 30s. The composition of volatile compounds was analyzed by dynamic headspace analysis, and marked differences were seen comparing IIP treated milk to raw milk, HTST pasteurized milk and a more intensive pasteurization at 85°C for 30s.
The applicability of IIP for heat treatment of cheese milk was examined. The rennet coagulation was found to be impaired, when the temperature of IIP was increased, but the rennetability of IIP treated milk could be almost completely restored to the level of raw milk by addition of calcium. An open texture cheese was manufactured from IIP treated milk, and the resultant cheeses exhibited increased moisture content and lower pH at IIP temperatures 100°C and 120°C as compared to cheeses made using HTST pasteurization of the milk, and following the same manufacturing procedure. Moreover, a shorter texture, a whiter color, and a different profile of ripening were evident in the cheeses made from IIP treated milk.
Overall, the present work challenges the suggestion of IIP as being a gentle pasteurization method for milk due to the disruption of the milk fat globules. However, it seems possible to adjust the functional properties for cheese manufacture, thereby allowing heat treatment of cheese milk at higher temperature, but due attention to the observed color change and potential defects will be required.