HIGH-PRESSURE EFFECTS ON THE COLLOIDAL CALCIUM-PHOSPHATE AND THE STRUCTURAL INTEGRITY OF MICELLAR CASEIN IN MILK .1. HIGH-PRESSURE DISSOLUTION OF COLLOIDAL CALCIUM-PHOSPHATE IN HEATED MILK SYSTEMS

Results of this study confirm that high temperature (118 degrees C/15 min) and high pressure (400 MPa/5 min) processing of skim milk, skim m ilk ultrafiltration and ultracentrifugation fractions, and model milk salt solutions cause dramatic shifts in their colloidal and soluble Ca phospate equilibrium that affect their pH, dissolved Ca content, turb idity, and casein micelle microstructure. The relations between high t emperature and high pressure processing-induced changes in the colloid al and soluble Ca phosphate equilibrium were evaluated in raw, pasteur ized, and high temperature treated skim milk, ultrafiltration retentat e and permeate of pasteurized skim milk, clear ultracentrifugation inf ranatant of pasteurized skim milk, and synthetic milk ultrafiltrates w ith and without lactose or Ca. The magnitude of the pH and dissolved C a shifts caused by high temperature and high pressure processing was a function of casein micelle concentration. Ultrafiltration permeate ex hibited the most drastic shifts in pH and dissolved Ca contents due to high temperature and high pressure processing. Although high temperat ure processing reduced the pH of ultrafiltration permeate from 6.59 to 6.03 and the dissolved Ca from 100% to 58%, high pressure processing reversed both of these changes. These changes in high temperature and high pressure processed milk, milk fractions, and model milk salt solu tions were related to microstructural changes in the casein micelles a s revealed by electron microscopy.