Dairy Chemistry And Biochemistry

218 DAIRY CHEMISTRY AND BIOCHEMISTRY

4.15.4 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. 3.5-5.5; the

insolubility range becomes wider with increasing temperature. Insolubility

in the region of the isoelectric point is clearly advantageous in the produc-

tion 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 proper ties. 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 produc-

tion 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. 2 g H,O g-' 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.

Gelation. One of the principal functional applications of proteins is the
formation of gels. In milk, caseins undergo gelation when the environment
is changed in one of several ways, but the most important are rennet-
induced coagulation for cheese or rennet casein manufacture (which is
discussed in Chapter 10) or on acidification to the isoelectric point (pH 4.6),
which is exploited in the preparation of fermented milk products and
isoelectric casein. In addition, casein may be gelled or coagulated by organic
solvents, prolonged heat treatments or during storage of heat-sterilized
products; these changes are usually negative. Heat-induced gelation is used
in the preparation of many food products but, as discussed in Chapter 9,