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.

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

Calculation of the ion equilibria in milk diffusate and comparison with experiment.

Changes in Physical Properties of Bovine Milk from the Colostrum Period to Early Lactation

Milk is secreted by all species of mammals to supply nutrition and immunological protection to the young. It performs these functions with a large array of distinctive compounds. Interspecies differences in the quantitative composition of milk (Jenness and Sloan 1970) probably reflect differences in the metabolic processes of the lactating mother and in the nutritive requirements of the suckling young.

Composition of milk.

1. The volume of the udder and the composition of the secretion have been followed in five goats through pregnancy to the onset of lactation. 2. During the middle of pregnancy udder volume was minimal and there was little or no fluid in the teats. 3. Two stages of commencing secretory activity (lactogenesis) were defined. In the first, starting up to ten weeks pre-partum, udder volume increased and the fluid in the teats changed from an extracellular-fluid-like to a milk-like composition and acquired a high concentration of immunoglobulins. Four goats accumulated several litres of a pre-colostral fluid with a high [lactose] 6-7 weeks pre-partum. 4. Comparison of the rate of increase in udder volume with previous data for the rate of increase in empty udder volume in pregnant goats showed that the rate of secretion, even in the last few days of pregnancy, was only a few per cent of the rate immediately after parturition. 5. In the last 2-3 days of pregnancy there was a three to elevenfold increase in [citrate] in the secretion; this heralded the onset of copious secretion at about the time of parturition. 6. The changes in mammary gland activities are discussed in relation to changes in plasma hormone concentrations during pregnancy.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1975.sp011120

Secretory activity of goat mammary glands during pregnancy and the onset of lactation.

The accuracy of methods that utilize the colour-forming reaction between pyridine, acetic anhydride and citric acid for the determination of citric acid in milk and milk sera has been examined. A method based on that of Saffran & Denstedt (1948), in which a trichloroacetic acid (TCA) filtrate is used, gave values 3–5% low for the citric acid content of both milk and milk ultrafiltrate. The method of Merier & Boulet (1958), in which aqueous samples are analysed, gave an accurate value for the citric acid content of milk. The same technique but applied to aqueous-NaOH samples (Marier & Boulet, 1960) gave values about 7% high for the citric acid content of milk and about 3% high for the citric acid content of ultrafiltrate. However, a method using the technique of Marier & Boulet and TCA filtrate gave accurate values for the citric acid content of both milk and milk ultrafiltrate.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022029900011353

D. T. Davies

Papers

The determination of citric acid in milk and milk sera.