Dairy Chemistry And Biochemistry

186 DAIRY CHEMISTRY AND BIOCHEMISTRY

molecules forming a gel-like structure in which microgranules of colloidal

calcium phosphate are an integral feature and from the surface of which the

C-terminal region of K-casein extends, forming a hairy layer (Figure 4.21).

These models retain two of the central features of the submicellar model, i.e.

the cementing role of CCP and the predominantly surface location of

K-casein.

Holt (1992, 1994) also proposed that, in addition to supplying amino

acids, caseins should be considered to have a biological function, i.e. to

enable a high concentration of calcium to be carried in stable form in milk;

without the stabilizing effect of casein, calcium phosphate would precipitate

in the mammary cells, resulting in ectopic mineralization, which might lead

to the death of the mammary gland or of the whole animal. A similar

situation occurs with kidney stones, gallstones and calcified synovial and

salivary fluid.

Since the micelles are closely packed, intermicellar collisions are frequent;

however, the micelles do not normally remain together after collisions. The

micelles are stabilized by two principal factors: (1) a surface (zeta) potential

of c. -20mV at pH 6.7, which, alone, is probably too small for colloidal

stability, and (2) steric stabilization due to the protruding K-casein hairs.

4.6 Whey proteins

About 20% of the total protein of bovine milk belongs to a group of

proteins generally referred to as whey or serum proteins or non-casein

nitrogen. Acid and rennet wheys also contain casein-derived peptides; both

contain proteose-peptones, produced by plasmin, mainly from p-casein,

and the latter also contains (g1yco)macropeptides produced by rennets from

K-casein. These peptides are excluded from the present discussion.

4.6.1 Preparation

The whey proteins, as a group, are readily prepared from milk by any of the
methods described in section 4.3, i.e.

1. the proteins remaining soluble at pH 4.6;


2. soluble in saturated NaCI;


3. soluble after rennet coagulation of the caseins;


4. by gel permeation chromatography;


5. by ultracentrifugation, with or without added Ca2+.

The whey prepared by any of the above methods, except 4, contains
lactose and soluble salts. Total whey proteins may be prepared from the