Calcium sensitive casein

In the absence of calcium ions but in the presence of other casein components the normally insoluble K-casein is apparently stabilized by the calcium-sensitive caseins (157). Thus calcium ions are required to coagulate whole casein after treatment with rennin. Therefore in the native milk system the micelle-stabilizing power of K-casein is specifically destroyed by rennin, and in the presence of calcium ions in milk a coagulum is formed (2). This offers a dramatic example of how the functionality of an entire protein system can be altered by specific proteolytic action on a component of that system. 

K-Casein is the only eutherian casein that has conclusively been shown to contain carbohydrate moieties and it differs from the calcium-sensitive caseins in a number of respects. It is the only casein that is soluble in the presence of calcium ions and has a much smaller phosphate component than any of the other caseins. The phosphorylation sites are limited to the C-terminal region of the molecule and are present as single sites rather than the clusters found in the calcium-sensitive... 

Soybean Protein Isolates. Soybean protein isolates, having a protein content of >90 wt%, are the only vegetable proteins that are widely used in imitation dairy products (1). Most isolates are derived from isoelectric precipitation, so that the soybean protein isolates have properties that are similar to those of casein. They are insoluble at thek isoelectric point, have a relatively high proportion of hydrophobic amino acid residues, and are calcium-sensitive. They differ from casein in that they are heat-denaturable and thus heat-labile. The proteins have relatively good nutritional properties and have been increasingly used as a principal source of protein. A main deterrent to use has been the beany flavor associated with the product. Use is expected to increase in part because of lower cost as compared to caseinates. There has been much research to develop improved soybean protein isolates. 

Phosphates, which react with calcium to reduce the calcium ion activity, assist in stabilizing calcium-sensitive proteins, eg caseinate and soy proteinate, during processing. Phosphates also react with milk proteins. The extent of the reaction depends upon chain length. Casein precipitates upon addition of pyrophosphates, whereas whey proteins do not. Longer-chain polyphosphates cause the precipitation of both casein and whey proteins. These reactions are complex and not fully understood. Functions of phosphates in different types of dairy substitutes are summarized in Table 9 (see also Food additives). 

As discussed in Chapter 4, the casein micelles are stabilized by tc-casein, which represents 12-15% of the total casein and is located mainly on the surface of the micelles such that its hydrophobic N-terminal region reacts hydrophobically with the calcium-sensitive asl-, as2- and j8-caseins while its hydrophilic C-terminal region protrudes into the surrounding aqueous environment, stabilizing the micelles by a negative surface charge and steric stabilization. 

Schmidt, D. G. and Payens, T. A. J. 1963. The purification and some properties of a calcium-sensitive a-casein. Biochim. Biophys. Acta 78, 492-499. 

About 20% of milk protein is soluble in the aqueous phase of milk. These serum proteins are primarily a mixture of /3-lactoglobulin, a-lactalbumin, bovine serum albumin, and immunoglobulins. Each of these globular proteins has a unique set of characteristics as a result of its amino acid sequence (Swaisgood 1982). As a group, they are more heat sensitive and less calcium sensitive than caseins (Kinsella 1984). Some of these characteristics (Table 11.1) cause large differences in susceptibility to denaturation (de Wit and Klarenbeek 1984). 

The temperature-sensitive precipitation of unmodified and methylated /3-caseins in the presence of calcium was measured also (see Figure 3). Methylation caused an increase of up to 3°C in the precipitation temperature of calcium /3-caseinate. Results from rennet clotting of an asi-K casein micelle system indicated that replacing native asi-casein with the reductively methylated protein had little influence on clotting time, while replacing K-casein with its reductively methylated derivative re-... 

Calcium is capable of forming cross-links with a number of food proteins. Some proteins, such as casein and soy proteins, can be precipitated in the presence of appropriate amounts of calcium. Undenatured whey proteins are not calcium sensitive, but the concentration of calcium can greatly influence their gelation properties. Some prop-... 

The highest amount of caseins in milk belongs to asi-casein [2, 3]. Unlike other caseins, in Ksi-casein both N- and C-terminal segments of molecules are hydrophobic which leads to the formation of either intra- or inter-molecular interactions [5]. Amino acid sequence in asi-casein differs from that in as2-caseins and hence it is normal to possess different physical and chemical properties. Although both Ogi- and as2-caseins are calcium-sensitive, the same as ) -casein, Os2-casein is much more sensitive to Ca " [6, 7]. Indeed, 2 mM Ca " at pH 7 is sufficient for precipitation of s2-casien, while a concentration of 6 mM is needed to precipitate asi-casien [6]. However, si-casien is more sensitive to Ca in comparison with /S-casein [5]. 

Dickinson, E., Semenova, M.G., Belyakova, L.E., Antipova, A.S., Il in, M.M., Tsapkina, E.N., Ritzoulis, C. (2001). Analysis of light scattering data on the calcium ion sensitivity of caseinate solution thermodynamics relationship to emulsion flocculation. Journal of Colloid and Interface Science, 239, 87-97. 

We have seen earlier in this chapter how the self-assembly of casein systems is sensitively affected by temperature. Another thermodynamic variable that can affect protein-protein interactions in aqueous media is the hydrostatic pressure. Static high-pressure treatment causes the disintegration of casein micelles due to the dismption of internal hydro-phobic interactions and the dissociation of colloidal calcium phosphate. This phenomenon has been used to modify the gelation ability of casein without acidification as a consequence of exposure of hydrophobic parts of the casein molecules into the aqueous medium from the interior of the native casein micelles (Dickinson, 2006). High-pressure treatment leads to a reduction in the casein concentration required for gelation under neutral conditions, especially in the presence of cosolutes such as sucrose (Abbasi and Dickinson, 2001, 2002, 2004 Keenan et al., 2001). 

Casein micelle proteins are primarily a8i-, as2-, /3-, and -caseins in approximate proportions 3 .8 3 1. asi-Casein has eight or nine phosphate groups, depending on the genetic variant. aS2-Casein is the most hydrophilic of the caseins. It has two disulfide bonds which, by severe heat treatment, can be caused to interact with those of /3-lactoglobulin. It also has 10 to 13 phosphate groups and is very sensitive to the calcium ion concentration (Kinsella 1984 Swaisgood 1982). 

Casein is very hydrophobic and, therefore, temperature sensitive. Low temperature or removal of calcium causes dissociation of /3-casein from the micelle and destabilizes the remaining micelle (Carpenter and Brown 1985 Dalgleish 1982). Soluble /3-casein can form aggregates of up to 40 monomers when heated. The C-terminal (hydrophobic) portions of /3-casein monomers clump together, and the N-terminal (hydrophilic) portions extend outward into the surrounding aqueous medium (Kinsella 1984). 

Inorganic phosphate is released when casein is heated. Dephosphor-ylated casein is less able to bind calcium and is more heat labile (Howat and Wright 1934). a9-Caseins are especially sensitive to the calcium concentration because of their high phosphorylation levels and small amounts of secondary and tertiary structure (Kinsella 1984). 

Pyne, G. T. 1958. The heat coagulation of milk II. Variations in sensitivity of casein to calcium ions. J. Dairy Res. 25, 467-474. 

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