Colloidal calcium phosphate solubilization

Acidification of milk is accompanied by a progressive solubilization of colloidal calcium phosphate and other colloidal salts from casein. Solubilization is complete below about pH 4.9 (Figure 5.11). 

Solubilizes of colloidal calcium phosphate and thereby affects cheese texture rapid acid production leads to a low level of calcium in the cheese and a crumbly texture (e.g. Cheshire) and vice versa (e.g. Emmental). 

Lowering the pH of milk to 4.6 solubilizes colloidal calcium phosphate. This removes its neutralizing effect, allowing electrostatic interactions between micelles. Under these conditions, micelles coagulate and precipitate from solution. Kudo (1980C) showed that release of whey proteins and K-casein from casein micelle surfaces as the pH is increased from 6.2 to 7.2 allows micelles to stick together and precipitate from solution. 

Acid coagulation Solubilization of the colloidal calcium phosphate and aggregation when the i.p. is approached (charge neutralization)... 

Solubilization of colloidal calcium phosphate, which, among other factors, affects curd (cheese) texture, stretchability, and meltability. 

The texture of Cheddar cheese is considered to be more dependent on pH than on any other factor (see Lawrence and Gilles, 1982 Fox et al, 1990, for references) for the same calcium content, the texture of Cheddar varies from curdy (pH 5.3), to waxy (pH 5.3 >5.1), to mealy (pH <5.1). Suggested explanations for this pH dependence include micelle hydration, especially in the presence of NaCl, and the extent to which colloidal calcium phosphate (CCP) is solubilized. 

Calcium and magnesium influence the titration curves of milk because as the pH is raised they precipitate as colloidal phosphates, and as the pH is lowered, colloidal calcium and magnesium phosphates are solubilized. Since these changes in state are sluggish and the composition of the precipitates depends on the conditions (Boulet and Marier 1961), the slope of the titration curves and the position of the maximum buffering depend upon the speed of the titration. 

The caseins exist in milk as polydisperse aggregates ranging in size from ca. 40 to 220nm (3), but the size distribution of micelles depends upon the method of measurement. These casein micelles scatter light and are responsible for the whitish, opaque nature of skim milk. The casein micelles are also associated with a colloidal apatite comprised of calcium-phosphate-citrate (CPC) which has a stabilizing influence on the micelle structure. The colloidal CPC is in equilibrium with soluble CPC in the milk serum phase and is solubilized as the pH is reduced. Thus, as the pH is reduced to the isoelectric point of the caseins (4.6), the colloidal CPC solubilizes, and the caseins precipitate (143). This phenomenon should be kept in mind during some of the following discussions. 

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