A model is proposed for the equilibria between the components of the salts in milk. The model includes complex formation between calcium ions and the various ionized forms of citric, lactic and phosphoric acids, and makes allowance for the effect of ionic strength. A computer program has been written to calculate the pH of the milk salt solutions and the concentration of each of the complexes formed. Calculated pH values agree with observed values for solutions of known composition.The principal components of the salts of cow's milk are the ions of calcium, magnesium, phosphate, citrate, sodium, potassium and chloride. To this list, it is desirable to add lactic acid because of its presence in some milk products. These ions interact to form soluble complexes with each other and with the milk proteins and especially with the caseins in the casein micelles, which also carry considerable amounts of inorganic Ca phosphate. These and other interactions with milk proteins are of considerable practical importance; they are known to influence the stability of milk to heat (Fox & Morrissey, 1977), coagulation by rennin (Ernstrom & Wong, 1974), feathering of creams in coffee (Anderson et al. 1977) and are implicated in variations of the quality of milk products, such as the whipping properties of cream.The nature and extent of the equilibrium interactions between Ca and its salts in milk were discussed by Pyne (1962). However, no detailed quantitative calculations have been published, based on physico-chemical measurements of the appropriate acid dissociation constants and the association constants of the various possible complexes. Such calculations are described in this paper for a model that consists of a milk salt solution in the absence of proteins, at pH values at or below 50; the method of calculation, which is general, is to calculate successive approximations in a series of nested iterative loops. The accuracy of our model depends only on the relevant physico-chemical assumptions involved and on the accuracy of the experimental values for the equilibrium constants used in the calculations. Our model, although limited by exclusion of the interactions with proteins, should be considered as a start to the construction of a more complete model involving proteins and all the other components of milk.
DESCRIPTION OF THE MODELFor the model we assume the presence of only the following components: citrate, phosphate, lactate, chloride, calcium, sodium, so that magnesium ions and proteins are ignored; potassium ions are counted as equivalent to sodium ions, because of the *
