Protein casein micelles

Co-preclpltate is an insoluble milk protein product that is produced by heating skinimllk to high temperatures ( > 90 C) to denature the whey proteins and complex them with the casein micelles. The heated system is subsequently adjusted to isoelectric point conditions of pH 4.5-5 to precipitate the complexed whey protein-casein micelles, centrifuged or filtered to recover the precipitate, washed and dryed. The resulting product, which is virtually insoluble, exhibits only minor functionality in most typical emulsification applications. 

Cl%Ca++ Lactose Whey proteins Casein micelles Fat globules Bactcrias ... 

Most dairy products, including yogurts and cheeses, are based on casein micelles (protein globules 100nm in diameter) and whey proteins. Casein micelles and coagulated whey proteins must be examined by EM because LM techniques do not provide sufficient resolution. 

A useful property of the red seaweed extracts is their abiUty to form gels with water and milk. Kappa-carrageenan reacts with milk protein micelles, particularly kappa-casein micelles. The thickening effect of kappa-carrageenan in milk is 5—10 times greater than it is in water at a concentration of 0.025% in milk, a weak thixotropic gel is formed. 

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... 

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. 

The elastic modulus (G ) of MP, BCAS, and BLG5 rapidly rose to plateaus that corresponded to different G saturations (Gjat) (Table 2). MP and BCAS coagula showed the more important Gsat value (142 N/m ), meaning that the emulsions stabilized by skim milk proteins (mainly casein micelles) and 6-casein formed the coagula with the strongest protein network. 

Similarly, the protein in milk is very rich in colloidal chemistry. Most of the protein is bound within aggregates called casein micelles (see p. 512). The colloids in milk are essentially stable even at elevated temperatures, so a cup of milky tea, for... 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

A colloid suspension of casein micelles, globular proteins, and lipoprotein particles... 

The following factors must be considered when assessing the stability of the casein micelle The role of Ca++ is very significant in milk. More than 90% of the calcium content of skim milk is associated in some way or another with the casein micelle. The removal of Ca++ leads to reversible dissociation of P-casein without micellular disintegration. The addition of Ca++ leads to aggregation. The same reaction occurs between the individual caseins in the micelle, but not as much because there is no secondary structure in casein proteins. 

The viscosity of milk and milk products is reported to be important in the rate of creaming. The viscosity of milk increases with decrease in temperature because the increased voluminosity of casein micelles temperatures above 65°C increases viscosity due to the denaturation of whey proteins pH an increase or decrease in the pH of milk also causes an increase in casein micelle voluminosity. Fat globules that have undergone cold agglutination may be dispersed due to agitation, causing a decrease in viscosity. 

The weak physical forces that hold together self-assembled nanoparticles are, of course, susceptible to disruption under the influence of thermodynamic and/or mechanical stresses. Hence some workers have investigated ways to reinforce nanoscale structures via covalent bonding. For instance, improved stability of protein nanoparticles, in particular, casein micelles, can be achieved by enzymatic cross-linking with the enzyme transglutaminase, which forms bonds between protein-bound glutamine and lysine residues. By this means native casein micelles can be converted from semi-reversible association colloids into permanent nanogel particles (Huppertz and de Kruif, 2008). 

These different casein monomers combine with calcium phosphate to form discrete particles on the nano-size scale. The phosphoserines of the caseins are seemingly clustered for the purpose of linking within the micelle to putative calcium phosphate microcrystallites, also known as nanoclusters (Holt, 1992 Home, 1998, 2002, 2003, 2006 Holt et al., 2003 Home et al., 2007). Structural evidence for the existence of such nanoclusters has come from neutron and X-ray scattering (de Kruif and Holt, 2003 Holt et al., 2003 Pignon et al., 2004 Marchin et al., 2007). The presence of nanoclusters allows native casein micelles to be effective natural suppliers of essential calcium salts in the human diet in a readily assimilated functional form. Protein-nanocluster interactions are the central concept of the cross-linking mechanism in Holt s model of casein micellar assembly (Holt et al., 2003 de Kruif and Holt, 2003). Any analogy with conventional soap-like micelles is considered to be... 

The self-assembly of caseins may be readily manipulated by processing methods that affect the integrity of native casein micelles and the character of the casein interactions in aqueous media. Examples of such procedures are (Dickinson, 2006) (i) acidification toward the isoelectric point (p/) (pH 4.6-4.8), leading to a neutralization of the net protein charge (ii) enzyme action, as exploited in the production of cheeses and fermented milks (iii) addition of divalent ions, especially, Ca2+ ions (iv) addition of sucrose or ethanol (v) temperature treatment and (vi) high-pressure treatment. 

These calculations also demonstrate the general theoretical principle, which has been confirmed in practice for various dairy-type emulsions, that the depletion interaction is of insufficient magnitude to induce flocculation when the non-adsorbed protein species are too small (e.g., individual protein molecules) or too large (e.g., native casein micelles). 

We have seen earlier in this chapter how the self-assembly of casein systems is sensitively affected by temperature. Another thermodynamic variable that can affect protein-protein interactions in aqueous media is the hydrostatic pressure. Static high-pressure treatment causes the disintegration of casein micelles due to the dismption of internal hydro-phobic interactions and the dissociation of colloidal calcium phosphate. This phenomenon has been used to modify the gelation ability of casein without acidification as a consequence of exposure of hydrophobic parts of the casein molecules into the aqueous medium from the interior of the native casein micelles (Dickinson, 2006). High-pressure treatment leads to a reduction in the casein concentration required for gelation under neutral conditions, especially in the presence of cosolutes such as sucrose (Abbasi and Dickinson, 2001, 2002, 2004 Keenan et al., 2001). 

NFDM, which retains casein micelles similar to those in fresh milk, is produced by pasteurization of sklmmllk, vacuum concentration and spray drying under processing conditions that result in either "low heat" or "high heat" product. Low heat NFDM is required for most applications that depend upon a highly soluble protein, as the case for most emulsification applications, since it is manufactured under mild temperature conditions to minimize whey protein denaturation and complexation with casein micelles. 

Principal micelle characteristics. The structure of the casein micelles has attracted the attention of scientists for a considerable time. Knowledge of micelle structure is important because the stability and behaviour of the micelles are central to many dairy processing operations, e.g. cheese manufacture, stability of sterilized, sweetened-condensed and reconstituted milks and frozen products. Without knowledge of the structure and properties of the casein micelle, attempts to solve many technological problems faced by the dairy industry will be empirical and not generally applicable. From the academic viewpoint, the casein micelle presents an interesting and complex problem in protein quaternary structure. 

Schmidt, D.G. (1982) Association of caseins and casein micelle structure, in Developments in Dairy Chemistry, Vol. 1 Proteins (ed. P.F. Fox), Applied Science, London, pp. 61-86. 

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