Functional physicochemical properties of caseins

Solubility. Solubility is an important functional property per se, i.e. in fluid products, and is essential for other functionalities since insoluble proteins can not perform useful functions in foods. The caseins are, by definition, insoluble at their isoelectric points, i.e. in the pH range c. 3.5-5.S the insolubility range becomes wider with increasing temperature. Insolubility in the region of the isoelectric point is clearly advantageous in the production of acid casein and is exploited in the production of two major families of dairy products, i.e. fermented milks and fresh cheeses. However, such insolubility precludes the use of casein in acid liquid foods, e.g. protein-enriched fruit juices or carbonated beverages. Acid-soluble casein can be prepared by limited proteolysis or by interaction with certain forms of pectin. 

Rheological properties. Viscosity, an important physicochemical property of many foods, can be modified by proteins or polysaccharides. The caseins form rather viscous solutions, a reflection of their rather open structure and relatively high water-binding capacity. While the high viscosity of caseinate may be of some importance in casein-stabilized emulsions, it causes production problems for example, due to very high viscosity, not more than about 20% protein can be dissolved even at a high temperature. The low protein content of caseinate solution increases the cost of drying and results in low-density powders which are difficult to handle. 

Hydration. The ability of proteins to bind and hold water without syner-esis is critical in many foods, e.g. comminuted meat products. Although the caseins are relatively hydrophobic, they bind c. 2g H20g protein, which is typical of proteins. Hydration increases with increasing pH and is relatively independent of NaCl concentration, which is especially important in the efficacy of casein in meat-based products. The water-holding capacity of sodium caseinate is higher than that of calcium caseinate or micellar casein. 

Caseins are among the most surface-active proteins available to food technologists, )S-casein being particularly effective. To exhibit good surface activity, a protein must possess three structural features  

It should be relatively small, since the rate of migration to the interface is inversely proportional to the molecular mass. In actual food processing operations, the rate of diffusion is not particularly important since the production of emulsions and foams involves a large imput of work with vigorous agitation which moves the protein rapidly to the interface. 

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