The aim of the study is to determine heat-induced transformations of milk proteins under various pasteurization conditions. The object of the study was skim milk particles pasteurized at 70, 80, and 90 ºC for 30 s. Thermal skim milk processed at 45 ºC was used as a control sample. Protein profile changes were assessed using two-dimensional electrophoresis, followed by analysis of digital images of electropherograms obtained using an Epson Expression 1680 scanner and ImageMaster 2D Platinum 7 software. In the control image without thermal exposure, 26 protein spots were detected in the main molecular weights of 10–70 kDa and isoelectric points of 4.80–9.20 p.i. units, which corresponds to the native profile of milk. As the liquid system's processing temperature increases, the number and width of protein fragments decreases and/or their disappearance in the gel by up to 40 %. The most pronounced reduction was observed for a protein spot in the 70 kDa region, presumably related to bovine serum albumin, which virtually disappeared at 70 ºC. For spots in the molecular weight range of 15–25 kDa, associated in studies with casein fractions, disappearance was similarly established in the pasteurization mode of 90 ºC. The appearance of new colored fragments with a molecular weight of 10–20 kDa under a temperature load of 90°C can be explained by the partial degradation of the protein components of the milk matrix. Densitometric analysis revealed a decrease in the intensity of individual protein spots before they completely disappeared, while the total intensity of all detectable proteins in heat-treated samples decreased by 77 %. The results confirm that processing the milk system at temperatures above 65 °C causes changes in the secondary and tertiary structure of protein molecules, triggering the formation of protein aggregates and reducing their solubility under electrophoretic separation conditions.
Koroleva et al. (Fri,) studied this question.