Calcium caseinate phosphate complex

Commercial casein is usually manufactured from skim milk by precipitating the casein through acidification or rennet coagulation. Casein exists in milk as a calcium caseinate-calcium phosphate complex. When acid is added, the complex is dissociated, and at pH 4.6, the isoelectric point of casein, maximum precipitation occurs. Relatively little commercial casein is produced in the United States, but imports amounted to well over 150 million lb in 1981 (USDA 1981C). Casein is widely used in food products as a protein supplement. Industrial uses include paper coatings, glues, plastics and artificial fibers. Casein is typed according to the process used to precipitate it from milk, such as hydrochloric acid casein, sulfuric acid casein, lactic acid casein, coprecipitated casein, rennet casein, and low-viscosity casein. Differences... 

Manufacture of crystalline lactose from permeate derived by ultrafiltration of lactic casein whey presents special problems because of the low pH, high lactate concentration, and high calcium and phosphate concentrations (Hobman 1984). Research at the New Zealand Dairy Research Institute has led to a pilot-scale process whereby calcium phosphate complexes are partially removed before evaporation by an alkali and heat treatment to precipitate them, followed by centrifugation to clarify the treated permeate. Removal of about 50% of the calcium is sufficient to avoid problems during evaporation. 

Casein exists in milk as a calcium caseinate-calcium phosphate complex the ratio of these components is approximately 95.2 to 4.8. The dispersed casein particles appear to be spherical m shape and of various sizes. The size distribution of the casein micelles is nol constant, hut varies with aging, heating, concentration, and other processing treatments. Processing alters ihe water-binding of casein and this in turn affects the apparent viscosity of products that contain casein Changes in hydration have not been measured quantitatively although the casein panicles of raw milk... 

The casein phosphoproteins may be regarded collectively as calcium caseinate, but the number of Ca present can exceed the number of phosphoserine residues per casein molecule and the number required by stoichiometric calcium phosphate. This well-known binding capacity for excess calcium may be accommodated by attachment to ionised aspartyl or glutamyl residues present in the casein protein chains. Under some conditions, the presence of excess Ca leads to aggregation and precipitation of calcium caseinate complex (Chapter 10.2). 

Numerous effects of inorganic phosphates when added to milk have been reported. Many of these are explained in terms of precipitation of calcium phosphate and calcium caseinate, or the removal of these compounds by formation of soluble complexes. 

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). 

In principle, it would be logical to combine plots of the buffer index curves of each of the buffer components of milk and thus obtain a plot which could be compared with that actually found for milk. It is not difficult, of course, to conclude that the principal buffer components are phosphate, citrate, bicarbonate, and proteins, but quantitative assignment of the buffer capacity to these components proves to be rather difficult. This problem arises primarily from the presence of calcium and magnesium in the system. These alkaline earths are present as free ions as soluble, undissociated complexes with phosphates, citrate, and casein and as colloidal phosphates associated with casein. Thus precise definition of the ionic equilibria in milk becomes rather complicated. It is difficult to obtain ratios for the various physical states of some of the components, even in simple systems. Some concentrations must be calculated from the dissociation constants, whose... 

Stability of the complex protein system of milk or whey is decreased by concentration (Fox 1982 Muir and Sweetsur 1978 Sweetsur and Muir 1980B). In addition to closer packing of casein micelles and other proteins in concentrated milk, calcium phosphate is precipitated so that the pH decreases (Fox 1982). The pH effect causes protein which would be soluble at a normal solids concentration to precipitate. Casein in milk concentrated to three times its original solids level forms a flocculent after 1 to 3 weeks at -8°C (Lonergan 1978). 

Reynolds, E.C. 1998. Anticariogenic complexes of amorphous calcium phosphate stabilized by casein phosphopeptides a review. Spec. Care Dent. 18, 8—16. 

Protein Composition of Milk. Skim milk is a colloidal suspension of extreme complexity. The particulate phase, the casein micelles, consists primarily of a mixture of asi, as2, / , and x-caseins combined with calcium ions and an amorphous calcium-phosphate-citrate complex. The soluble phase contains lactose, a fraction of the caseins and calcium, and, in raw milk, the whey proteins, which are predominantly /3-lacto-globulin and a-lactalbumin. When milk is centrifuged at high speed (in our experiments, 30 min at 110,000 X gravity), the casein micelles sediment. This permits one to separate the two physical phases of skim milk and to measure changes in composition of the phases resulting from... 

The stability of the caseinate particles in milk can be measured by a test such as the heat stability test, rennet coagulation test, or alcohol stability test. Addition of various phosphates—especially polyphosphates, which are effective calcium complexing agents—can increase the caseinate stability of milk. Addition of calcium ions has the opposite effect and decreases the stability of milk. Calcium is bound by polyphosphates in the form of a chelate, as shown in Figure 5-3. 

It has been proven over the years that the effect of fouling can be lessened to some extent for the application of whey concentration by pretreating the feed streams for the ultrafilters. Whey contains many insoluble solids such as casein fines, lipoprotein complex, mineral precipitates, free fats and microorganisms. Clarification of these debris helps reduce fouling potential during ultrafiltration. In addition, it is quite evident that calcium phosphate minerals in whey are not stable and their precipitation in the membrane pores often results in flux decline. Demineralization of whey before ultrafiltration helps maintain high permeate flux considerably [Muir and Banks, 1985]. 

The fact that milk and milk products arc rich in calcium is known by most people. Casein is the main protein of milk, accounting for about Sd% of milk protein. Casein has a molecular weight of about 23,000 and, thu.s, is a relatively small protein however, the casein in milk occurs as a large complex of casein molecules having an overall molecular weight of about 100 sc 10, Casein is phosphorylated on residues of serine. These phosphate groups bind calcium ions. The calcium ions help maintain the stability of the large complex. 

It is well known that the addition of soluble calcium salts reduces the heat stability of milk, whereas the addition of calcium complexing agents with the appropriate control of pH improves heat stability. Phosphates and citrates have often been used to increase the heat stability of concentrated milks (Augustin and Clarke, 1990 Pouliot and Boulet, 1991 Sweetsur and Muir, 1982a). A reduction in activity by the addition of these salts contributes to the improved heat stability of concentrated milks, but the effects of salts on the equilibrium of caseins between the serum and micellar phases of milk also affect heat stability. 

Severe heat treatments, especially >80°C, adversely affect the rennet coagulation of milk. Although changes in calcium phosphate equilibria are contributory factors, complexation of /3-lactoglobulin and/or ot-lactalbumin with x-casein via intermolecular disulfide bond formation is the principal factor responsible (see Fox, 1988). Most authors agree that the primary and, especially, the seconday (nonenzymatic) phase of rennet coagulation are adversely affected by severe heat treatments, as is the strength of the... 

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. ... 

Milk contains about 40 gl of fat and 32 gl of protein. The milk fat occurs in droplets stabilizedby layers of phospholipid and protein [16]. Ofthe protein, about 25gl is casein and 7 gl is serum proteins (mostly lactoglobulins). The caseins form casein micelles , which involve both casein and calcium phosphate, may contain 500-10000 casein protein molecules, and are of the order of 200-300 nm in diameter [16, 18, 34, 36]. The casein micelles are quite complex. There are four types of casein molecules of which most (a j -, a 2 > P-caseins) complex with... 

Several different types of dental caries have been described by clinicians. Specifically these are smooth-surface caries, pit and fissure caries, enamel caries, dentinal caries, secondary caries, early childhood caries and root caries [12], All occur by the same essential mechanism, as described above, and all arise as a consequence of a disturbance to the demineralization-remineralization balance. Attack by organic acids produced by bacteria in the plaque favours demineralization, but the natural remineralization processes of the mouth can reverse this. Certain dietary and hygiene behaviours as well as clinical treatments can enhance this natural remineralization provided they occur early enough in the demineralization part of the process. For example, complexes of casein phosphopeptide with amorphous calcium phosphate have been shown in various studies to be capable of enhancing the remineralization step under certain conditions and in specific groups of individuals [16,17]. These are now available commercially as an anticaries treatment for patients. 

Most protein molecules, then, have many points where complexes may be formed with metals. Among these metals it is worthwhile to distinguish those which appear to be coordinated strongly and by many different polar side-chains such are mercury, silver, copper and zinc. The alkaline earth metals, like calcium, seem to be bound primarily by free carboxyl groups, or, in the phosphoproteins such as casein, by phosphate groups. 

---