Upon forming the micelle structure, colloidal calcium phosphate acts as a stabilizing agent and cross-links the network. Subsequently, αs1-caseins molecules are attached to the β-casein polymers, while k-caseins interact with αs1-caseins, forming aggregates of limited size. He assumed that β-casein monomers begin to self-associate into chain-like polymers. The first internal structure model was suggested by Rose in 1969. The last category of models is based on the properties of the isolated casein constituents, causing or directing the formation of the internal structure of the casein micelle. Separating the caseins from calcium and phosphate until this point in the assembly process is not really possible, since both calcium and phosphate are involved in the phosphorylation of the protein chain which occurs post-translation immediately and presumably before the association of the chains into submicelles. Another criticism of this model is the late entry of calcium phosphate into the assembly process by Slattery and Evard. This model does not explain what provokes the segregation of the k-casein or why k-casein molecules, having preferred to associate with their own kind to form aggregate patches in the k-rich submicelles, should then associate with the other caseins to complete the building of the submicelle. The k-casein congregate on the micelle surface, those submicelles poor (like αs and β-casein) or totally deficient in k-casein are located in the interior of the micelle ( Figure 2). The pattern of interaction is such that it brings about a variation in the k-casein content of these submicelles. In this model, the caseins first aggregate via hydrophobic interaction into subunits of 15–20 molecules each. The model, described by Slattery and Evard and Slattery, falls in the last category. The submicelles, estimated by sedimentation velocity studies, are stabilized by hydrophobic bonding and calcium caseinate bridges, and the submicelles are aggregated and held together by colloidal calcium phosphate linkages with a micelle structure covered by αs1- and k-casein. Morr considered that αs1-, β-, and k-casein monomers form small uniform submicelles. The first sub-micelle model was proposed by Morr in 1967. CSN3 stabilizes up to 10 times its weight of the Ca 2+-sensitive caseins via the formation of micelles ( Figure 1). Although k-casein is in a relatively low amount of the casein system (12–15% of whole casein), it is soluble in the presence of Ca 2+, whereas the remaining 85% of casein are precipitated by Ca 2+. The latter is important for the stability and properties of the casein micelle. The casein micelle in milk consists of four caseins: αs1- (CSN1S1), αs2- (CSNS2), β- (CSN2), and k-casein (CSN3). Casein micelle contains an average of 3.4 g H 2O per gram dry matter, which consists approximately of 93% protein and about 7% of inorganic component (Ca 2+), formed phosphoprotein complexes. The size, form, and structure of the casein micelle are of great importance for the milk industry especially for cheese making, yellow cheese, etc.
It is well known that the casein fraction of bovine milk exists as polydisperse, large, roughly spherical colloidal particles, 50–600 nm in diameter (average ~150 nm), called “casein micelles”.
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