Casein micelles structure

Since the micelles are of colloidal dimensions, they are capable of scattering light and the white colour of milk is due largely to light scattering by the casein micelles the white colour is lost if the micelles are disrupted, e.g. by removing colloidal calcium phosphate (by citrate, ethylene 

They are stable to compaction, e.g. they can be sedimented by ultracentrifugation and redispersed readily by mild agitation. 

They are stable to commercial homogenization but are changed slightly at very high pressures (500 MPa). 

They aggregate and precipitate from solution when the pH is adjusted to the isoelectric point of caseins (c. pH4.6). Precipitation at this pH, which is temperature-dependent (i.e. does not occur at temperatures below 5-8°C and occurs over a wide pH range, perhaps 3.0-5.5, at higher temperatures, e.g. 70°C), occurs owing to the loss of net positive or negative charge as the pH approaches 4.6. 

As the pH of milk is reduced, the colloidal calcium phosphate (CCP) dissolves and is completely soluble at pH 4.9 (Chapter 5). pH adjustment, followed by dialysis against bulk milk, is a convenient and widely used technique for varying the CCP content of milk. As the concentration of CCP is reduced, the properties of the micelles are altered but they retain some of their structure even after removing 70% of the CCP. Removal of more than 70% of the CCP results in disintegration of the micelles into smaller particles (aggregates). 

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Farrell, H. M., Jr., Malin, E. L., Brown, E. M., and Qi, P. X. (2006). Casein micelle structure What can be learned from milk synthesis and structural biology Curr. Opin. Colloid Interface Sci. 11,135-147. 

Horne, D. S. (2009). Casein micelles structure and stability. In "Milk Proteins From Expression to Food", (A. Thompson, M. Boland, and H. Singh, Eds), pp. 133-179. Academic Press, San Diego. 

McMahon, D. J. and McManus, W. R. (1998). Rethinking casein micelle structure using electron microscopy. /. Dairy Sci. 81,2985-2993. 

Home, D.S. (2006). Casein micelle structure models and muddles. Current Opinion in Colloid and Interface Science, 11, 148-153. 

Nature itself gives us a spectacular example of a biopolymer-based delivery system in the form of the native casein micelle of mammalian milk (Lemay et al, 2007). This is primarily a colloidal delivery system for calcium, where the micronutrient is in the form of calcium phosphate, which does not give a bitter taste, and which provides good bioavailability owing to its colloidal size, amorphous state and quick dissolution in gastric conditions (pH 1-2). Nevertheless, the casein micelle structure is unique there are no other readily available natural delivery systems for most nutraceuticals. Therefore some new designs are clearly required (Velikov and Pelan, 2008 McClements et al, 2008, 2009). 

Marchin, S., Putaux, J.L., Pignon, F., Leonil, J. (2007). Effects of the environmental factors on the casein micelle structure studied by cryo-transmission electron microscopy and small-angle X-ray scattering/ultra-small-angle X-ray scattering. Journal of Chemical Physics, 126, 45-101. 

Schmidt, D.G. (1982). Association of caseins and casein micelle structure. In Fox, P.F. (Ed.). Developments in Dairy Chemistry. London Applied Science, pp. 61-86. 

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). 

Schmidt, D. G. 1982. Association of casein and casein micelle structure. In Developments in Dairy Chemistry, Vol. I. P. F. Fox (Editor). Applied Science Publishers, New York. 

Brown, R. J. 1984. Casein micelle structure. Symposium at the 79th American Dairy Science Association Meeting. College Station, Texas, June 24-27. 

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. 

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). 

Gamier, J. 1973. Models of casein micelle structure. Neth. Milk Dairy J. 27, 240-248. 

Slattery, C. W. 1976. Review Casein micelle structure an examination of models. J. Dairy Sci. 59, 1547-1556. 

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. 

This sequence of events has its parallel in the model of casein micelle structure proposed by Schmidt (1982) and Walstra (1990) in which small calcium phosphate particles link together discrete spherical protein subunits. 

Erdem, Y.K., Modification of casein micelle structure caused by ultrafiltration and heat treatment A spectrofluorimetric and kinetic... 

Needs, E., Stenning, R.A., Gill, A.L., Ferragut, V., and Rich, G.T. High-pressure treatment of milk effects on casein micelle structure and on enzymic coagulation, /. Dairy Res., 67, 31, 2000. 

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. ... 

Phadungath, C. Casein micelle structure a concise review. Songkianakarin J. Sd Technol 27(1), 201-212 (2005)... 

C.G. de Kruif, C. Holt, Casein micelle structure, functions and interactions, in Advanced Dairy Chemistry Proteins, vol. 1, ed. by P.F. Fox, P.L.H. McSweeney (Kluwer, Plenum New York, 2002) pp. 233-276... 

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