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Journal of Dairy Science Vol. 43 No. 10 1407-1413
© 1960 by American Dairy Science Association ®
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The Solubility in Aqueous Urea Solutions of the Micellar Caseinates of Milk and Milk Products Subjected to Various Sterilizing Heat Treatments

Harrison S. Haller and Michael J. Pallansch

Dairy Products Laboratory, Eastern Utilization Research and Development Division, USDA, Washington, D. C.

ABSTRACT

A study was made of the changes that occur in the micellar caseinates when heated in their natural environment.

The caseinates were separated from heated milks by centrifugation at 27,000 times gravity in a Servall centrifuge, followed by washing and lyophilizing. The dry caseinates were equilibrated by shaking for 24 hr. at 25° C. in 6.6 M urea solution. After dilution to 3.3 M urea, and equilibration for 48 hr., the solutions were centrifuged to separate the insoluble casemates. The soluble caseinates of the solution were determined by micro-Kjeldahl analyses after dialysis to remove urea. Solubility curves, obtained by plotting the amount of the casein in 3.3 M urea solution against the amount of micellar caseinates present in the system, exhibit solubility phenomena characteristic of heterogeneous systems and show that:

  1. Increased heating decreases the solubility of the caseinates in aqueous urea.
  2. Fat present in milk during heating decreases the solubility in urea.
  3. Increased solids content of milk gives decreased protein solubility in aqueous urea after heating.
  4. High-temperature—short-time methods of sterilizing concentrated milk products generally produce less change in the protein solubility in urea than does the conventional sterilization method.
  5. The presence of citrate and phosphate ions in the urea system increases the protein solubility.
  6. The heat-produced protein component in the casein micelle concentrates in the 3.3 M urea insoluble fraction.






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Copyright © 1960 by the American Dairy Science Association ®.