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Journal of Dairy Science Vol. 40 No. 11 1395-1404
© 1957 by American Dairy Science Association ®
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Nitrogen Factor for the Calcium Caseinate-Calcium Phosphate Complex in Milk

T. P. Ford, G. A. Ramsdell, Shirley G. Landsman1 and T. G. Alexander2

Eastern Utilization Research and Development Division, Agricultural Research Service, USDA, Washington, D. C.

ABSTRACT

  1. Skimmilks were progressively depleted of casein colloids by centrifuging, and the supernatant liquids and deposited fractions analyzed. The dry weights of calcium caseinate-calcium phosphate complex contained in the separated colloids were calculated from the analyses. For many milks and many particle-size fractions the average ratio of dry weight of complex to casein nitrogen found is approximately 6.89 ± 1.3%. This nitrogen factor is applicable to any milk sample of unknown composition.
  2. The complex: nitrogen factor is shown to be correlated with the calcium : nitrogen ratio for the separated colloid, i.e., with the ratio of complex bound calcium removed to casein nitrogen removed with the colloid. When this ratio is known, a more accurate nitrogen factor can be calculated by means of the statistically determined equation: The probable error when this equation is used is about 0.6%.
  3. Since it has already been shown that the calcium : casein nitrogen ratio tends to increase with particle size, it follows that the nitrogen factor does also. The range of calcium : nitrogen ratios found experimentally is from 0.181 to 0.232; the corresponding range of nitrogen factors given by the equation is 6.82 to 6.96.
  4. The experimental minimum nitrogen factor, 6.82, is in close agreement with a theoretical factor, 6.8005, calculated for a casein complex unit considered as composed of one mol of calcium caseinate and one-half mol of tricalcium phosphate. Adjusting this by adding trimagnesium phosphate in the proportion in which magnesium is known to be present, the theoretical minimum factor becomes 6.814.
  5. It is emphasized that the factors calculated are for the casein complex and not for the total colloidal particle, which also contains those serum solids associated with the solvate liquid. Colloid factors can be calculated from the complex factors by adding these additional solids. They range from approximately 7.7 to 7.8, in agreement with experimental results.
  6. The nitrogen factors given do not apply to the very largest casein colloid particles in milk, and may not apply to the very smallest. The largest particles are known to differ in composition from the rest and were, therefore, in most cases purposely removed before the fractionations were made. The smallest particles were not all removed in the centrifuging times used, and no close fractionations of small particles only were attempted.

FOOTNOTES

1 Present address, Stanford Research Institute, Menlo Park, California.

2 Present address, Food and Drug Administration, Washington, D. C.






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Copyright © 1957 by the American Dairy Science Association ®.