Surface structure, casein micelles

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. 

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... 

Figure 7 shows the effect of protein adsorbed to the surface of the fat globule on resulting ice-cream structure. High levels of adsorbed protein, especially casein micelles, in the mix (Fig. 7A) impede fat adsorption at the air interface (Fig. 7C), impede fat partial coalescence and network formation (Fig. 7E), leading to rapid meltdown witii recovery of mostly intact fat globules (Fig. 7G). However, low levels of adsorbed protein in tiie mix (Fig. 7B) enhance fat adsorption at tiie air interface (Fig. 7D), promote fat partial coalescence and network formation (Fig. 7F), leading to slower meltdown as the fat network (Fig. 7H) must collapse after ice...

The casein micelle is an example of a naturally occurring nanoparticle formed when the different types of caseins (asl, 0 2, (5, and k) self-assem-ble around amorphous calcium phosphate. This allows it to be a natural carrier for calcium. The casein micelle also serves as a carrier for hydro-phobic bioactives (Livney and Dalgleish, 2007). Treatments such as ultra-high pressure have been reported to alter the structural characteristics of the casein micelle by partially removing parts of the surface of the casein (Sandra and Dalgleish, 2005). Altering the surface properties of these nanoparticles is expected to alter their functional properties. 

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). 

This brief review conprises three subject areas (i) the structure and properties of the K-casein surface layer in casein micelles (ii) the properties of the protein fraction in homogenized milks (i.e. basically intact casein micelles adsorbed at fat-water interfaces) (iii) the properties of caseinate and individual caseins adsorbed at the interfaces. In this, we are at present less... 

The gel structure can be controlled via changes in the hydrophobicity of the micelle surface. A decrease in hydrophobicity is possible, e. g., by heating milk (90 °C/10 min). Covalent bonding of denatured p-lactoglobulin to KT-casein (cf. 10.1.3.5) occurs, burying hydrophobic groups. 

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