Casein emulsifying effect

We now look more closely at the structure of casein. It is a long molecule with different ends one end is polar and the other is nonpolar. In milk, the polar group (ending with a phosphate group) is positioned to face the polar water, and the non-polar end faces the oil. In effect, each particle of oil has a double coating the inner layer is the non-polar end of the casein emulsifier, and the outer layer is a sheath of polar phosphate groups. 

Spinelli et al. (14) determined the emulsion stability and emulsion capacity of polypeptides recovered from hydrolysates of fish myofibrillar protein using hexametaphosphate. In general the emulsion capacity and emulsion stability increased through 30 min of proteolysis but then declined. Even the unhydrolyzed protein-phosphate complex yielded better emulsion stability and capacity values compared with sodium caseinate. The effect of the residual hexametaphosphate in the hydrolysate conceivably could have a beneficial effect on emulsifying characteristics of the modified fish myofibrillar proteins. 

Whey protein concentrates (WPC), which are relatively new forms of milk protein products available for emulsification uses, have also been studied (4,28,29). WPC products prepared by gel filtration, ultrafiltration, metaphosphate precipitation and carboxymethyl cellulose precipitation all exhibited inferior emulsification properties compared to caseinate, both in model systems and in a simulated whipped topping formulation (2. However, additional work is proceeding on this topic and it is expected that WPC will be found to be capable of providing reasonable functionality in the emulsification area, especially if proper processing conditions are followed to minimize protein denaturation during their production. Such adverse effects on the functionality of WPC are undoubtedly due to their Irreversible interaction during heating processes which impair their ability to dissociate and unfold at the emulsion interface in order to function as an emulsifier (22). 

Figure 14 Effect of low temperature on hydration of bovine casein micelles and of interfacially bound protein in ice cream mix with (+E) and without (-E) emulsifier (saturated mono-diglyceride).

Although critical concentrations of a polysaccharide may enhance the emulsifying and stabilizing ability of a protein, small additions may have the opposite effect (Cao et al., 1990). The discovery of destabilization by 0.05% xanthan on caseinate led Dickinson and Euston (1991a) to remark that small changes in polymer structure or solvent conditions can easily tip the balance in favor of depletion or bridging flocculation. 

Vakaleris, D.G., Sabharwal, K. 1972. Stability of fluid food emulsions. II. Interacting effects of electrolytes, sodium caseinate and emulsifiers. J. Dairy Sci. 55, 283-288. van den Berg, G. 1982. Developments in buttermaking. Proc. XXI Int. Dairy Congr. (Moscow) 2, 153-159. 

Dickinson, E. 1989 Surface and emulsifying properties of caseins. J. Dairy Res. 56, 471-477. Goff, H.D., Liboff, M., Jordan, W.K., Kinsella, J.E. 1987. The effects of polysorbate 80 on the fat emulsion in ice cream mix evidence from transmission electron microscopy studies. Food Microstruct. 6, 193—198. 

The cubic phases can also be dispersed by amphiphilic proteins. Caseins, for example, which also are very effective as emulsifiers, can disperse the cubic phase just as do simple surfactants, such as bile salts. The mechanism is exactly the same as the solubilisation of bilayers by detergents at the cmc - the mixed system has a different surfactant parameter, i.e. local curvature, and the system takes on a different structure. 

Citrate salts have long been used in the processed cheese industry as "emulsifying salts," and there is still interest in the mechanism of their action. Shirashoji et al. (2006) examined the effects of trisodium citrate on the properties of processed cheese. Increasing concentration of sodium citrate decreased the size droplets of the cheese. This effect is typical when emulsifying properties of a system are improved. This is expected as the complexation of calcium by citrate causes dissociation of the casein micelle, making the casein more available for emulsifying fat droplets. This possibly contributed to the reinforcement of the structure of the processed cheese. 

Darewicz, M., and Dziuba, J. (2001). The effect of glycosylation on emulsifying and structural properties of bovine beta-casein. Nahrung 45, 15-20. 

Mizuno, R., and Lucey, J.A. (2005). Effects of emulsifying salts on the turbidity and calcium phosphate-protein interactions in casein micelles. /. Dairy Sci. 88,3070-3078. 

There have been a limited number of studies on the effects of enzymic modification of protein concentrates on functional properties other than solubility. Studies on functional properties, as modified by enzymic treatments, emphasize foam formation and emulsifying characteristics of the hydrolysates. Treatment of chicken egg albumen alters the functional properties of the egg proteins in terms of foam volume and stability and the behavior of the proteins in angel food cakes (25). Various proteolytic enzymes were used to degrade the egg albumen partially. However, proteolytic enzyme inhibitors indigenous to the egg proteins repressed hydrolysis of the egg proteins compared with casein. 

The high content of nucleic acid in yeast is a potential problem associated with the consumption of large amounts of yeast nucleopro-tein in foods. Phosphorylation is one of the methods used for reducing nucleic acid of yeast proteins (Table 2) [67]. Huang and Kinsella [68] reported the effective removal of nucleic acid from yeast proteins by POCl3-phosphorylation. The phosphorylated protein showed improvement in emulsifying activity and emulsion stability, and produced stable but weak foams at neutral pH. The authors also noted that the in vitro digestibility of yeast protein was not affected by phosphorylation as reported for casein [60] and soy protein [65]. 

Williams and Janssen (20) studied the behavior of droplets in a simple shear flow in the presence of a protein emulsifier. The effect of two structurally diverse protein emulsifiers, P-lactoglobulin and P-casein, upon the breakup behavior of a single aqueous droplet in a Couette flow field has been studied over a wide range of protein concentrations. It was found that P-casein and low concentrations of P-lactoglobulin cause the droplets to be at least as stable as expected from conventional theories based on the equilibrium interfacial tension. In such cases the presence of the emulsifier at the deforming interface is thought to enhance the interfacial elasticity. This effect can be characterized by... 

For example, succinylated wheat gluten is quite soluble at pH 5 (cf. Fig. 1.40). This effect is related to disaggregation of high molecular weight gluten fractions (cf. Fig. 1.41). In the case of succinylated casein it is obvious that the modification shifts the isoelectric point of the protein (and thereby the solubility minimum) to a lower pH (cf. Fig. 1.42). Succinylation of leaf proteins improves the solubility as well as the flavor and emulsifying properties. 

Liu M, Damodaran S. Effect of transglutaminase-catalyzed polymerization of beta-casein on its emulsifying properties. 7 A ric Food Chem. 47 (4) 1514-1519,1999. 

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