European Biophysics Journal

, Volume 33, Issue 5, pp 421–434 | Cite as

An equilibrium thermodynamic model of the sequestration of calcium phosphate by casein micelles and its application to the calculation of the partition of salts in milk

  • Carl HoltEmail author


An equilibrium thermodynamic model of the interaction of calcium, phosphate and casein in milk is described in which the micellar calcium phosphate is assumed to be in the form of calcium phosphate nanoclusters. A generalized empirical formula for the nanocluster is used to define the molar ratios of small ions (Ca, Mg, Pi and citrate) to a casein phosphorylated sequence (phosphate centre, PC). From this model, a method of calculating the partition of milk salts into diffusible and non-diffusible fractions is obtained. No arbitrary assumptions are made, no fitting of adjustable parameters is done and the PCs in the caseins are defined by inspection of their primary structures. In addition to the salt partition, the mole fractions of the individual caseins not complexed to the calcium phosphate through one or more of their PCs are computed and a generic stability rule for milks is derived. The use of the model is illustrated by calculations of the partition of salts in a standard milk and by comparison with experimental data on the partition of salts in the milk of individual cows. The generic stability rule is applied to the individual milks to determine whether the micellar calcium phosphate is thermodynamically stable. According to the calculations, compositions that might lead to pathological calcification in the lumen of the mammary gland were seldom found in primiparous healthy cows in early or mid lactation but occurred more often in multiparous animals, in late lactation and during mastitic infection.


Calcification Calcium phosphate Casein Milk Salt partition 



amorphous calcium phosphate






calcium phosphate nanocluster


dicalcium phosphate dihydrate




ion activity product


micellar calcium phosphate


molecular weight cut-off


octacalcium phosphate


phosphate centre


tricalcium citrate



The author is grateful to Drs. D.T. Davies, A.J.R. Law and J.C.D. White for allowing use of their original compositional data on cows’ milk.


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Copyright information

© EBSA 2004

Authors and Affiliations

  1. 1.Hannah Research InstituteAyrUK

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