Equilibrium casein micelle systems

Abstract

A casein micelle consists of a core of essentially insoluble «"-and/or /3-caseinates stabilized by a coat layer containing -casein. In a system, cores occur in a variety of sizes, thus producing a micelle distribution. Ultracentrifugation is used to characterize reconstituted micelle systems containing as-and -caseins with respect to the weight fractions of total protein in particles of different size. At final environmental conditions of 37°, pH 6.6, a monovalent ionic strength of 0.05 and 0.012 m calcium chloride, it is concluded that a system is in equilibrium when a particular micelle distribution is present and that the distribution is determined by the system weight ratio of 3-: -casein. (A) Micelle systems develop spontaneously as calcium chloride is increased progressively from zero. Using initial weight ratios, Ri, from 20 to 3, the micelle radius of the maximum weight fraction decreases as Ri is decreased, respectively, from ~425 to ~36 nm. Concomitantly, the concentration of nonmicellar protein (including coat) slowly increases; at 8 mg/ml of «¡,-casein, from 0.15 to 0.4 mg per ml. (B) Colloid particles of Cac V .ia spin micelles of bovine milk are nearly spherical and exist in a distribution of sizes. Average radii, by electron microscopy, range from about 10 to 400 nm, with over 95 % of the micelle mass in particles from 30 to 150 nm (Nitschmann,

Invited review: Caseins and the casein micelle: Their biological functions, structures, and behavior in foods

“…On the other hand, bovine κ-CN does not sequester ACP, it is not precipitated by calcium ions, even at high concentrations (Dickson and Perkins, 1971;Mekmene and Gaucheron, 2011, and it can stabilize the other calcium-sensitive caseins against precipitation by calcium ions (Waugh and Talbot, 1971). All milks contain an ortholog of κ-CN and, in the 6 examples where the composition of casein has been determined (Supplementary Table S1; http://dx.doi.org/10.3168/jds.2013-6831), the percentage of κ-CN in whole casein varies from less than 2% to more than 17% (Baranyi et al, 1995;Miranda et al, 2004;Boumahrou et al, 2009;Holt and Carver, 2012).…”

“…The remainder of κ-CN (para-κ-CN) is mostly composed of P,Q-rich sequences that can bind to similar sequences in other caseins. As a result, whereas κ-CN acts to limit the size of amorphous aggregates formed by mixtures with other caseins and calcium ions, para-κ-CN causes a similar mixture to precipitate or form a gel (Waugh and Talbot, 1971). With the appropriate number of sequestered nanoclusters of amorphous calcium phosphate (ACP), whole casein forms the amorphous aggregate known as the casein micelle.…”

Casein structures in the context of unfolded proteins

“…The physicochemical principles affecting the first growth mechanism are completely unknown but there is a natural limit to growth when all the phosphate centres have reacted to form the nanoclusters. The mechanism that limits the growth of the casein micelle by the second route has been recognised, although not fully understood, for many years (Talbot & Waugh, 1967;Waugh & Talbot, 1971), ever since κ-casein was first isolated and studied. Thus, the solution to the problem of indefinite association through the interaction of P,Q-rich sequences was to introduce another protein into the casein mixture, namely κ-casein, with the ability to bind promiscuously to the P,Q-rich sequences of the other caseins, and by so doing, terminate any further growth.…”

BEST USE FOR MILK ‐ A REVIEW I‐Effect of breed variations on the physicochemical properties of bovine milk

“…Table shows variations in casein micelle size for the different breeds (Waught and Talbot ; Holt and Baird ; Ng‐Kwai‐Hang et al . ; Glantz et al .…”