Structure of casein micelle

Muller-Buschbaum, P., Gebhardt, R., Roth, S.V., Metwalli, E., Doster, W. (2007). Effect of calcium concentration on the structure of casein micelles in thin films. Biophysical Journal, 93, 960-968. 

Thurn, A., Burchard, W., Niki, R. (1987a). Structure of casein micelles. I. Small-angle neutron scattering and light scattering from p- and K-casein. Colloid and Polymer Science, 265, 653-666. 

Waugh, D. F. 1971. Formation and structure of casein micelles. In Milk Proteins, Vol. 

Understanding milk-clotting is made more difficult by our rudimentary, and therefore often conflicting, views of casein micelles structure (Bloomfield and Morr 1973 Farrell 1973 Gamier 1973 McMahon and Brown 1984 Schmidt 1980 Slattery 1976 Swaisgood 1973). A complete explanation of milk-clotting will not be possible until more information, including the complete and correct structure of casein micelles, becomes available (Ekstrand et al 1980). 

Bloomfield, V. A. and Morr, C. V. 1973. Structure of casein micelles Physical methods. Neth. Milk Dairy J. 27, 103-120. 

McMahon, D.J., Yousif, B.H., and Kalab, M. (1993). Effect of whey protein denaturation on structure of casein micelles and their rennetability after ultra-high temperature processing of milk with or without ultrafiltration. Int. Dairy J. 3, 239-256. 

Recently, Slattery and Evard (171) proposed a model for the formation and structure of casein micelles from studies devoted to association products of the purified caseins. They proposed that the micelle is composed of polymer subunits, each 20 nm in diameter. In the micellar subunits the nonpolar portion of each monomer is oriented radially inward, whereas the charged acidic peptides of the Ca2+-sensitive caseins and the hydrophilic carbohydrate-containing portion of K-casein are near the surface. Asymmetric distribution of K-casein in a micelle subunit results in hydrophilic and hydrophobic areas on the subunit surface. In this situation, aggregation through hydrophobic interaction forms a porous micelle (Figure 10). Micelle growth is limited by the eventual concentration, at the micelle surface, of subunits rich in K-casein. 

The model of Slattery and Evard (171) explains many of the properties of micelles, including the events associated with clotting by rennin action. For example, it explains the crenated surface of micelles observed on electron micrographs (179), the subunit structure of casein micelles (180), the porous nature of micelles (177, 178) allowing syneresis by continued action of rennin on interior subunits, and the more or less random distribution of the caseins in the micelle (170,177, 178,183). 

In relation to the structure of casein micelles, our results indicate that the micelles are composed of two different fractions. Using the additivity law (23), we found that Fraction I and Fraction... 

Shukla, A., Narayanan, T., Zanchi, D. Structure of casein micelles and their complexation with tannins. Soft Matter 5, 2884-2888 (2009)... 

P NMR spectroscopy is a useful tool to discriminate between phosphorylated molecules in liquid or amorphous/solid-like sample with respect to their nature and dynamics. The major advantage of the NMR technique is that the sample can be analysed without pretreatment or extraction, and can be recovered since NMR is non-destructive. Phosphates in milk and in isolated casein micelles have been widely investigated using liquid-state P NMR spectroscopy As the restricted motion induced by the large colloidal structure of casein micelles does not permit the obtaining of hi ly resolved spectra, only the mobile phosphates (a part of easein phosphoprotein residues, the dissociated inorganic phosphate and the milk fat phospholipids) found in the soluble phase were detected by liquid-state NMR. 

Caseins are the major proteins in bovine milk and about 95% of the caseins exist as casein micelles. The structure and properties of casein micelles influence a wide range of technological uses of milk. Light microscopy, SEM, and TEM have been frequently used to study casein... 

Huppertz, T., de Kruif C.G. (2008). Structure and stability of nanogel particles prepared by internal cross-linking of casein micelles. International Dairy Journal, 18, 556-565. 

Pignon, F., Belina, G., Narayanan, T., Paubel, X., Magnin, A., Gezan-Guiziou, G. (2004). Structure and rheological behaviour of casein micelle suspensions during ultrafiltration process. Journal of Chemical Physics, 121, 8138-8146. 

Martin, A., Goff, H.D., Smith, A., Dalgleish, D.G. (2006). Immobilization of casein micelles for probing their structure and interactions with polysaccharides using scanning electron microscopy (SEM). Food Hydrocolloids, 20, 817-824. 

Micelle structure. Various models of casein micelle structure have been proposed and refined over the past 40 years. Progress has been reviewed regularly, including Schmidt (1982), McMahon and Brown (1984), Farrell (1988), Holt (1992, 1994), Rollema (1992) and Visser (1992). 

Not all of the salt constituents are found in the dissolved state in milk. Calcium, magnesium, phosphate, and citrate are partitioned between the solution phase and the colloidal casein micelles (see Chapter 9 for the composition and structure of these micelles). For analytical purposes, partition of the salt constituents can be achieved by equilibrium dialysis or by pressure ultrafiltration. In the latter technique, pressures must be limited to about 1 atmosphere to avoid the so-called sieving effect (pushing water through the filter faster than the dissolved components (Davies and White 1960). 

The association that occurs between the monomers of the same milk protein as influenced by their environment has been discussed in the section Structure and Conformation of Milk Proteins. However, in addition to this type of association, the milk proteins are known to form complexes with small ions and molecules, to bind water, and to form complexes with other macromolecules and with each other. The most important example of the last phenomenon is the formation of casein micelles, which is discussed in Chapter 9. 

Heth, A. A. and Swaisgood, H. E. 1982. Examination of casein micelle structure by a method for reversible covalent immobilization. J. Dairy Sci. 65, 2047-2054. 

McMahon, D. J. and Brown, R. J. 1984A. Composition, structure and integrity of casein micelles A review. J. Dairy Sci. 67, 499-512. 

Milk-clotting is a complex process, involving a primary enzymic phase in which K-casein is altered and loses its ability to stabilize the remainder of the caseinate complex, a secondary non-enzymic phase in which aggregation of the altered caseinate takes place, a third step where the aggregate of casein micelles forms a firm gel structure and a possibly separate fourth step where the curd structure tightens and syneresis occurs (McMahon and Brown 1984B). 

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... 

Historically, ideas of casein micelle structure and stability have evolved in tandem. In the earlier literature, discussions of micellar stability drew on the classical ideas of the stability of hydrophobic colloids. More recently, the hairy micelle model has focused attention more on the hydrophilic nature of the micelle and steric stabilization mechanisms. According to the hairy micelle model, the C-terminal macropeptides of some of the K-casein project from the surface of the micelle to form a hydrophilic and negatively charged diffuse outer layer, which causes the micelles to repel one another on close approach. Aggregation of micelles can only occur when the hairs are removed enzymatically, e.g., by chymosin (EC 3.4.23.4) in the renneting of milk, or when the micelle structure is so disrupted that the hairy layer is destroyed, e.g., by heating or acidification, or when the dispersion medium becomes a poor solvent for the hairs, e.g., by addition of ethanol. 

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