Comparison of the effects of high-pressure treatments and heat pasteurization on the whey proteins in goat's milk

Goat's milk was subjected to pressures up to 500 MPa at 25 or 50 degrees C, and the extent of denaturation of the individual whey proteins, determined from their loss of solubility at pH 4.6, was examined by gel permeation FPLC (fast protein liquid chromatography) and SDS-PAGE. On pressure treatment at 25 degrees C, beta-lactoglobulin readily aggregated, whereas the immunoglobulins and alpha-lactalbumin were more resistant. At 50 degrees C, the effect was greater, and the immunoglobulins and alpha-lactalbumin were also partially denatured. SDS-PAGE showed that after pressure treatment the proteins in the acid precipitate were disulfide linked. Some small soluble aggregates of beta-lactoglobulin remained in the acid whey from pressurized milk, and these were not present in acid whey from heat-treated milk. Thermal and pressure treatments of milk could be distinguished by their different effects on denaturation of the individual whey proteins. Also, the activity of alkaline phosphatase in goat's milk was reduced by pasteurization but not by high-pressure treatment up to 500 MPa for 10 min. The possible use of these differences for monitoring thermal and pressure treatments in milk, and the importance of high pressure treatment in denaturation of the whey proteins, are discussed.