Calculation by computer of individual concentrations in a simulated milk salt solution. I

Theoretical model for calculating ionic equilibria in milk as a function of pH: comparison to experiment

Acidification is fundamental for the processing of milk into cheeses, caseins, and fermented dairy products. It is established that a pH decrease changes the ionic equilibria of milk, inducing the solubilization of micellar calcium phosphate. This study aimed to present a theoretical model calculating ionic equilibria in milk as a function of pH. From the pH and total concentrations of minerals and caseins, the model calculated the concentrations of all ionic species and their partition between micellar and aqueous phases of milk. As the pH decreased, the minerals present in the micellar phase were gradually displaced into the aqueous phase. The calculated concentrations of minerals were in a good agreement with the experimental ones determined from acidified milk. A very satisfactory accuracy of the calculations, estimated by a root-mean-square error (RMSE) value of 5% for Ca and P(i) and a slope of the plot close to a unit, was obtained. The model is proposed for the simulation of ionic equilibria and the partition of salts between the aqueous and micellar phases of milk and dairy formulations during acidification.

Calculation by computer of individual concentrations in a simulated milk salt solution: II. An extension to the previous model

The computer program described by Wood, Reid & Elvin (1981) has been extended to include the complexes formed in alkaline solutions of Ca citrate and phosphate. Mg complexes are also included. Supersaturation with respect to various solid phases is tested for and the composition of the aqueous phase adjusted accordingly. Additional tests of the program have confirmed its accuracy and shown how the results depend on the values chosen for the relevant equilibrium constants.

Relationship between ethanol stability of bovine milk and natural variations in milk composition

Several ionic components of ultrafiltrate were measured in bulk and individual cow milks and an assessment was made of their relationship with the parameters of the corresponding ethanol (EtOH) stability/pH profiles. From linear regression analysis the strongest relationships (P < 0·001) were between soluble salt balance [expressed as (Ca+Mg) minus (Pi+Cit) or as the ratio to (Pi+Cit)] and pK (correlation coefficient, γ ∼ 0·82) or Smax, the maximum stability at high pH (γ ∼–0·72), and between Pi and pK(γ = –0·84)or Smax (γ = –0·61). These relationships agree with the view that the parameters of the EtOH stability/pH profile are determined by pH-induced changes in concentration of divalent cations. Natural variations in these parameters may be attributed to variations in relative concentrations of divalent cations and their chelators. EtOH stabilities at the natural pH of bulk milks from winter/spring- and autumn-calving animals were lowest in early and late lactation. The most important contributory factors appeared to be a high salt balance ratio in late lactation and a low natural milk pH in early lactation. The main component responsible for variable salt balance ratio was usually soluble Pi. Decrease in EtOH stability at the natural pH of late lactation milks reflected a more general change in the characteristics of the EtOH stability/pH profile, seen as an increase in pK and, in extreme cases, a decrease in Smax and profile gradient.

Interrelationships of the concentrations of some ionic constituents of human milk and comparison with cow and goat milks

1. Milk and milk ultrafiltrate samples from four women (Homo sapiens) during the first 16 weeks of lactation were analysed for the main salt constituents, protein and lactose and the interrelationships of these constituents were determined. 2. A good positive correlation was observed between ultrafiltrate calcium and citrate concentrations, as previously observed for cow (Bos taurus) and goat (Capra hircus) milk but the expected negative correlation of lactose with ultrafiltrate salts was poor. 3. An invariant ion activity product for milk calcium phosphate was found for a dicalcium phosphate stoichiometry but the milk serum was less supersaturated with respect to calcium phosphate and citrate salts than ruminant milks. 4. The observed interrelationships are discussed in relation to the constraints on human milk composition arising from a fixed osmotic pressure, overall charge neutrality and saturation with respect to calcium phosphate.