Micelles calcium binding

Performance Indices Quality Factors Optimum E1LB Critical micelle concentration (CMC) Soil solubilization capacity Krafft point (ionic surfactants only) Cloud point (nonionic surfactants only) Viscosity Calcium binding capacity Surface tension reduction at CMC Dissolution time Material and/or structural attributes... 

Of the caseins, only K-casein is capable of exerting a stabilizing effect in systems vdiich contain calcium ions in appreciable quantities. This is the case in milk, where the other caseins are effectively rendered insoluble by their binding to calcium phosphate, and vdiere there are also appreciable pools of calcium ions. When the calcium is removed, all of the caseins act as surfactants, and can stabilize emulsions. Even in milk, the casein micelles can bind to and stabilize unprotected fat surfaces, as in homogenized milk. 

K-casein on the other (31). Moreover, these simple systems show appreciable differences from native casein micelles in their response to Ca ". In casein micelles, the binding sites for Ca appear to be some distance from the surface of the hairy layer (13) and the same argument can be presumably used for the individual caseins, and show that the calcium binding sites in the synthetic particles are within the surface of shear. On the other hand, the binding of Ca may cause conformational changes in the interfacial layer. 

Practical laundry detergent formulations usually include builders for control of polyvalent metal ion activities so that the relevant salts of the anionic surfactant, which is usually present, are not precipitated out of solution. The presence of builders also has implications for the effectiveness of antifoam action because they may prevent the precipitation of calcium soaps. We can illustrate the relevant issues if we consider, for example, the concentration of calcium harduess uecessary to just initiate calcium soap precipitation in the case of a solution built by sodium tripolyphosphate at a pH >10.5. If we iguore the effect of solubilizatiou of soaps and calcium binding in anionic surfactant micelles, then, according to Irani and Callis [4],... 

Calcium forms stable insoluble salt with oxalic acid (see Section 10.2.3.2). In plant cells with higher concentrations of oxalic add, caldum oxalate can be actually present in the form of crystals. Some plants have been shown to bind metals in mixed complexes. For example, chromium can be bound in an oxalate-malate complex, and nickel and zinc can form a dtrate malate complex. Citric add has been proven to be a low molecular weight zinc ligand in human milk, and in casein micelles it binds calcium. It is also used as a food additive (acidulant, synergist to antioxidants and sequestrant), so great attention has been paid to the formation of its complexes with metal ions. The addition to cereal products leads to increased solubihty of naturally present iron, due to its release from phytic acid salts (phytates). 

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 ... 

Figure 6.1 The dual binding model of the structure of the casein micelle as built up from the individual caseins (0 1, o 2, P and k) in combination with calcium phosphate (CaP). Reproduced from Home (1998) with permission.

Like as2-casein, /e-casein has two disulfide bonds which can form cross-links with /3-lactoglobulin. The N-terminal two-thirds of the molecule is hydrophobic and contains the two disulfide bonds. The C-termi-nal end is hydrophilic, polar, and charged. It varies in the number of attached carbohydrate moieties and has only one phosphate group. These characteristics make /c-casein ideal for the surface of casein micelles, where it is most often found. It is not susceptible to calcium ion binding, as the other caseins are, and when present on the surface of micelles, it protects the other caseins from calcium (McMahon and Brown 1984A Swaisgood 1982). 

Casein exists in milk as a calcium caseinate-calcium phosphate complex the ratio of these components is approximately 95.2 to 4.8. The dispersed casein particles appear to be spherical m shape and of various sizes. The size distribution of the casein micelles is nol constant, hut varies with aging, heating, concentration, and other processing treatments. Processing alters ihe water-binding of casein and this in turn affects the apparent viscosity of products that contain casein Changes in hydration have not been measured quantitatively although the casein panicles of raw milk... 

Micellar solutions of anionic amphiphiles are usually not stable with respect to the addition of di- or multivalent cations since a precipitation occurs (hard water). In exceptional cases, where precipitation does not occur, the question arises as to how the uni-, di- and multivalent ions compete for binding to the micelles. Due to the high value of I l close to the micellar surface a counterion of high charge will be strongly favored and there is a discrimination between the different types of ions299. For example for SDS close to the CMC, -e(r[)/kT — 7 and if the presence of small amounts of calcium ions does not affect [Pg.74]

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