Casein micelle structure dissociation

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. ... 

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. 

To elucidate the surface morphology and size distribution of pressure treated casein micelles and their irreversible fragments, AFM experiments were performed. The samples were pressure-treated for 30 min in discrete steps (0.1-400 MPa) across the dissociation transition (Figure 19.22). Instead of a continuous evolution of the structure with pressure three characteristic morphologies can be observed The native micelles, existent up to 50 MPa, appear to be composed of sub-elements, suggesting... 

Citrate salts have long been used in the processed cheese industry as "emulsifying salts," and there is still interest in the mechanism of their action. Shirashoji et al. (2006) examined the effects of trisodium citrate on the properties of processed cheese. Increasing concentration of sodium citrate decreased the size droplets of the cheese. This effect is typical when emulsifying properties of a system are improved. This is expected as the complexation of calcium by citrate causes dissociation of the casein micelle, making the casein more available for emulsifying fat droplets. This possibly contributed to the reinforcement of the structure of the processed cheese. 

The dissociation of a quaternary structure or denaturation of proteins is required prior to emulsification. Therefore, casein micelles are adsorbed at an interface in a semi-intact form (Oortwijn et al., 1977). The thermal denaturation of globular proteins prior to emulsification was reported to improve the emulsifying properties. The high level of the thermally denatured whey protein fraction in mixed proteins (of denatured and undenatured proteins) increased the emulsion viscosity and coalescence stability compared with the low-level denatured fraction (Britten et al., 1994). 

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 in milk do not occur as monomers, but are aggregated into casein complexes and spherical particles called micelles (see Section 7.6.3.2.1). Caseins P and y can easily associate into polymeric structures at about 20 °C, while at temperatures <8 °C they dissociate back to the monomers. The aggregation of molecules of a -, P- and K-caseins into micelles occurs at temperatures >5°C. Molecules of a -, P- and K-caseins are first arranged into particles... 

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