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Relation Between Micellar and Serum Casein in Bovine Milk¹

Division of Biology, National Research Council of Canada, Ottawa, Ontario

ABSTRACT

Little or no additional casein dissolved when milk was diluted with large volumes of milk dialysate. Some dissolution of micellar to serum casein occurred when sedimented micelles were redispersed in ultra-filtrate mechanically, but the concentration of serum casein remained far below that of the original milk. It is concluded that micellar and serum casein do not form an equilibrium system controlled by the solubilities of the various caseins.

_s-, ß-, and -Casein dissolved in the serum when milk was cooled, with ß-casein accounting for about 55% of the total increase in serum casein. Serum casein content decreased by addition of calcium, but was increased by addition of phosphate, by addition of acid in the range from pH 6.7 to 5.3, and by removal of colloidal calcium phosphate. The results suggest that, at a fixed level of calcium caseinate, the calcium phosphate content of the micelles and the degree of polymerization of temperature-sensitive casein components (mostly ß-casein) are the major factors controlling the proportion of casein present in micellar form. Loss of either calcium phosphate or of temperature-sensitive casein from the micelle releases additional casein to the serum; loss of both has a more than additive effect.

¹ Issued as N.R.C.C. no. 10483.

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