Physicochemical Changes in Raw Milk Induced by High Hydrostatic Pressure and Thermal Processing: A Multi-Analytical Approach

Milk preservation requires technologies that extend shelf life while minimising undesirable changes in product quality. This study compared the effects of pasteurisation, sterilisation, and high hydrostatic pressure processing (HHP) on the physicochemical properties and protein profiles of raw whole bovine milk. The samples included untreated raw milk as the control, pasteurised milk (72 °C for 42 s), sterilised milk (121 °C for 15 min), and HHP-treated milk processed at 300, 400, 500, or 600 MPa for 5 min. Colour, pH, rheological behaviour, SDS-PAGE, and Native-PAGE were evaluated. Sterilisation caused the greatest total colour difference (ΔE* = 13.95), whereas pasteurisation resulted in a barely perceptible change (ΔE* = 0.64), and HHP-treated samples showed perceptible but lower colour differences (ΔE* = 3.42–3.97). Sterilisation also caused the most pronounced pH decrease and the highest shear stress values over most of the measured shear-rate range. SDS-PAGE profiles of raw and HHP-treated milk remained broadly similar, while Native-PAGE revealed pressure-dependent reductions in native β-lactoglobulin band intensity, particularly at 500 and 600 MPa. Pasteurisation and 600 MPa treatment reduced the κ-casein-associated band intensity by 54% and 58%, respectively. Overall, HHP induced less pronounced physicochemical and electrophoretic changes than sterilisation, although higher pressure levels caused measurable protein restructuring.