High pressure effects on the colloidal calcium phosphate and the structural integrity of micellar casein in milk - II. Kinetics of the casein micelle disintegration and protein interactions in milk

High pressure treatment results in a disintegration of micellar casein both in its native state and after UHT treatment when denatured whey proteins deposit on the casein micelles. Pressure treatments at lower temperatures lead to a stronger and quicker disintegration of micellar casein and to corresponding pH-value shifts. The pH-shift in UHT milk is higher than in pasteurized milk because of additional heat-induced insoluble calcium phosphate. At temperatures above 20 degrees C and pressures above 200 MPa the pressure-induced denaturation of whey protein superimposes this disintegration process which is, however, not the case in milk which already contains thermally denatured whey proteins. The degree of reversibility of pressure-induced shifts in pH and micellar size significantly differs between differently heat-treated milks. In pasteurized milk, the shift in pH is nearly reversible whereas a considerable part of the shift in turbidity remains after 24 hours storage at room temperature. In UHT milk the shift in turbidity is nearly reversible but increase of the pH-value partly remains.