The ultrasonic properties of skim milk related to the release of calcium from casein micelles during acidification

Milk samples were acidified with different amounts of glucono-δ-lactone (GDL) and samples of permeates were collected rapidly by ultrafiltration at intervals during acidification. Calcium contents of these sera were measured to define the extent and rate of dissociation of micellar calcium phosphate. In parallel samples, measurements were made of the ultrasonic velocity and the ultrasonic attenuation of milks and permeates. This allowed the establishment of a complete profile of the changing mineral distribution of milk and changes in ultrasonic properties, as functions of pH. The results showed that changes in both of ultrasonic parameters depended almost completely on changes in composition of milk serum, and that states of casein micelles made little contribution. This makes ultrasound an excellent method for following the release of calcium and the progress of acidification in milks.