Casein micelles phosphate—calcium fractions

Fractionation of milk and titration of the fractions have been of considerable value. Rice and Markley (1924) made an attempt to assign contributions of the various milk components to titratable acidity. One scheme utilizes oxalate to precipitate calcium and rennet to remove the calcium caseinate phosphate micelles (Horst 1947 Ling 1936 Pyne and Ryan 1950). As formulated by Ling, the scheme involves titrations of milk, oxalated milk, rennet whey, and oxalated rennet whey to the phenolphthalein endpoint. From such titrations, Ling calculated that the caseinate contributed about 0.8 mEq of the total titer of 2.2 mEq/100 ml (0.19% lactic acid) in certain milks that he analyzed. These data are consistent with calculations based on the concentrations of phosphate and proteins present (Walstra and Jenness 1984). The casein, serum proteins, colloidal inorganic phosphorus, and dissolved inorganic phosphorus were accounted for by van der Have et al (1979) in their equation relating the titratable acidity of individual cow s milks to the composition. The casein and phosphates account for the major part of the titratable acidity of fresh milk. 

When the rate of exchange of calcium between the micelle and milk serum is examined (Yamauchi et al., 1969 Yamauchi and Yoneda, 1977 Pierre et al., 1983 Wahlgren et al. 1990), a continuous distribution of rates appears to be present, so any division into fast- and slow-exchanging fractions that might be identified with the conceptual division into caseinate calcium and calcium phosphate calcium does not seem possible. 

Protein Composition of Milk. Skim milk is a colloidal suspension of extreme complexity. The particulate phase, the casein micelles, consists primarily of a mixture of asi, as2, / , and x-caseins combined with calcium ions and an amorphous calcium-phosphate-citrate complex. The soluble phase contains lactose, a fraction of the caseins and calcium, and, in raw milk, the whey proteins, which are predominantly /3-lacto-globulin and a-lactalbumin. When milk is centrifuged at high speed (in our experiments, 30 min at 110,000 X gravity), the casein micelles sediment. This permits one to separate the two physical phases of skim milk and to measure changes in composition of the phases resulting from... 

A better knowledge of casein micelle structure is a prerequiste for further characterization of casein derivatives. One way to study the structure of the micelle is by inducing controlled dissociation. The analysis of resultant Fractions provides information on the initial state of aggregation. Removal of calcium phosphate has been used to dissociate the micelle (6). The resultant complexes have been separated by chromatography, and their composition, and average size evaluated (7,8,9). However, the nature of interactions leading to their formation is still obscure. In the present work, we removed calcium to induce dissociation of the micelle, but afterwards, we paid attention to the interactive properties of the isolated complexes. ... 

Proteins that form micelles, namely caseins, are the major protein fraction in bovine milk (about 80% of the total milk protein). Several components may be identified, namely ag,i and 2-caseins, jS-casem and /cbreakdown product of yS-casein is y-casein. Similar to ovalbumin, caseins are phosphoproteins. Large spherical casein micelles are formed by association of s-, p and /c-casein in the presence of free phosphate and calcium ions. Molecules are held together by electrostatic and hydrophobic interactions. The as- and jS-caseins are surrounded by the flexible hydrophilic K-casein, which forms the surface layer of the micelle. The high negative charge of the /c-casein prevents collapse of the micelle by electrostatic repulsion. The micelle diameter varies between 50 and 300 run. 

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