Submicelles

The presence of water-soluble macromolecules in solution at submicel-lar concentrations has been reported to enhance the water solubility of hydro-phobic organic chemicals in several instances [19, 106, 113]. The presence of macromolecules in solution can enhance the apparent solubility of solutes by sorptive interactions in the solution phase. The processes by which macromolecules enhance the solubility of pollutants are probably variable as a function of the particular physical and chemical properties of the system. A macromolecule possessing a substantial nonpolar region can sorb a hydrophobic molecule, thereby minimizing the interfacial tension between the solute and the water. 

K. Kozco, A.D. Nikolov, D.T. Wasan, R.P Borwankar, and A. Gonsalves Layering of Sodium Caseinate Submicelles in Thin Liquid Films-A New Stability Mechanism... 

Colloidal calcium phosphate (CCP) acts as a cement between the hundreds or even thousands of submicelles that form the casein micelle. Binding may be covalent or electrostatic. The casein micelles are not static there are three dynamic equilibria between the micelle and its surroundings ... 

K-casein aggregates in aqueous medium are more complicated than those of p-casein, being composed of star-like sub-micelles, where each submicelle contains nine K-casein chains and the total degree of association may reach about 140 (Thum el al., 1987a). Payens and Vreeman (1982) used sedimentation measurements to infer a critical micelle concentration for K-casein of 0.5 mg/ml. 

Figure 3. Proposed model for casein micelles and submicelles...

Figure 4.20 Submicelle model of the casein micelle (from Walstra and Jenness, 1984).

Data analysis methods depend upon the level of order in the sample. The degree of order, in turn, depends upon the scale of distance on which the sample is viewed. For example, casein micelles show great variation in size (20 to 300 nm diameter) and so must be treated as a polydisperse system. However, the density variations ( submicelles ) within the whole micelle are much more uniform in size. They can be treated as a quasi-monodisperse system (Stothart and Cebula, 1982) and analyzed in terms of inter-particle interference (Stothart, 1989). 

Figure 21. Yogurt prepared by the TA-F-O method and observed using a field emission SEM. In addition to clearly imaging casein micelles (CM) and submicelles (SM) the micrograph documents a resolution of 3 nm. (x 100,000). [From 86].

Indeed the images of casein micelles and submicelles in yogurt are impressive. As pointed out in the discussions with reviewers section of the paper the technique is exhaustive and utilizes a highly sophisticated SEM and therefore is not likely to find wide use. The capability to resolve particulates at the 3 nm size range in the SEM mode is truly noteworthy and this reference represents a step forward in defining new capabilities to address specific questions requiring these higher resolutions. 

Another controversial and evolving idea concerning casein micelle structure is the concept of the submicelle. That there is some substructure to the micelle can hardly be denied, because all of the appropriate techniques have revealed some inhomogeneities over distances of 5-20 nm. Proponents of submicellar models of casein micelle structure interpret this evidence in terms of spherical particles of casein, the submicelles, joined together, possibly, by the calcium... 

Notwithstanding this, claims have been made to have isolated the putative submicelles, on the basis that the particles have a size similar to the scale of the micellar substructure. Not surprisingly, the reported molecular weights differ between methods of preparation and depend on concentration (Schmidt, 1982). Their existence as discrete structures within the micelle is more problematic. In Fig. 13, substructure is depicted in a protein gel without requiring the existence of submicelles. 

This paper draws heavily upon the "Nomenclature Committee Report" ( 1) as well as several recent comprehensive reports that have considered the primary structure and conformation of the casein monomer subunits and how they are assembled into submicel-lar aggregates and casein micelles (2, 3). These basic relationships were utilized to develop additional projections relating to the conformation and functional properties of the major milk proteins, e.g., commercial caseinates and whey protein concentrates in food applications. 

Kakalis, L.T., Kumosinski, T.F., and Farrell, H.M. (1990). A multinuclear, high-resolution NMR study of bovine casein micelles and submicelles. Biophys. Chem. 38, 87-98. 

The submicellar model has undergone several refinements (see Schmidt, 1982 Walstra and Jenness, 1984 Ono and Obata, 1989). The current view is that the /c-casein content of the submicelles varies and that the ic-casein-deficient submicelles are located in the interior of the micelles with the K-casein-rich submicelles concentrated at the surface, giving the micelles a K-casein-rich layer but with some Xji-, j8-caseins also exposed on... 

The Golgi complex is also the locus of casein micelle formation. In association with calcium, which is actively accumulated by Golgi vesicles, the polypeptide chains associate to form submicelles, and then micelles, prior to secretion. 

---