Casein micelles surface area

The effects of homogenization on milk components have been summarized by Walstra and Jenness (1984) and Harper (1976). Homogenization disrupts fat globules and results in an increase in fat surface area (about 4-10 times). Casein micelles adsorb on the fat surface and constitute part of the fat globule membrane. The curd tension of milk is thus lowered. Walstra and Jenness (1984) have described the effect of homogenization on rennet coagulation. 

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. 

Colloidal dispersions have a very large surface area for their volume. Therefore the surface properties of the phases have a large influence on the properties as a whole. Ice cream is simultaneously an emulsion (fat droplets), a sol (ice crystals) and a foam (air bubbles), and also contains other colloids in the form of casein micelles, other proteins and polysaccharides in the matrix. 

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... 

From this model it is evident that K-casein on the surface would be readily available to rennin. The release of the macropeptide by rennin would result in the subsequent reduction in charge and hydrophilicity on the surface. That rennin-treated micelles do not aggregate in the cold is evidence of the fact that hydrophobic interaction is mainly responsible for clotting. The K-casein areas exposed to rennin action would result in hydrophobic patches separated by one subunit diameter (20 nm), whereby the micelles could cohere to form threads and fibrils characteristic of coagulation. 

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