Heterogeneity of milk proteins

Initially, it was believed that milk contained only one type of protein but about 100 years ago it was shown that the proteins in milk could be fractionated into two well-defined groups. On acidification to pH 4.6 (the isoelectric pH) at around 30°C, about 80% of the total protein in bovine milk precipitates out of solution this fraction is now called casein. The protein which remains soluble under these conditions is referred to as whey or serum protein or non-casein nitrogen. The pioneering work in this area was done by the German scientist, Hammarsten, and consequently isoelectric (acid) casein is sometimes referred to as casein nach Hammarsten. 

The ratio of casein whey proteins shows large interspecies differences in human milk, the ratio is c. 40 60, in equine (mare s) milk it is 50 50 while in the milks of the cow, goat, sheep and buffalo it is c. 80 20. Presumably, these differences reflect the nutritional and physiological requirements of the young of these species. 

There are several major differences between the caseins and whey proteins, of which the following are probably the most significant, especially from an industrial or technological viewpoint  

In contrast to the caseins, the whey proteins do not precipitate from solution when the pH of milk is adjusted to 4.6. This characteristic is used as the usual operational definition of casein. This difference in the properties of the two milk protein groups is exploited in the preparation of industrial casein and certain varieties of cheese (e.g. cottage, quarg and cream cheese). Only the casein fraction of milk protein is normally incorporated into these products, the whey proteins being lost in the whey. 

Casein is very stable to high temperatures milk may be heated at its natural pH (c. 6.7) at 100°C for 24 h without coagulation and it withstands heating at 140°C for up to 20 min. Such severe heat treatments cause many changes in milk, e.g. production of acids from lactose resulting in a decrease in pH and changes in the salt balance, which eventually cause the precipitation of casein. The whey proteins, on the 

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It is also noteworthy that milk is rich in the amino acid tryptophan, a precursor of niacin hence, milk is an excellent source of niacin equivalents. (One niacin equivalent is defined as 1 mg of niacin or 60 mg of tryptophan.) It is noteworthy, too, that the immunoglobulins in milk are a heterogeneous group of antibody proteins that serve as a source of passive immunity for the newborn, and that colostrum contains large quantities of globulins. A discussion of milk proteins follows ... 

Ice cream serves as a wonderful (and tasty) example of a complex, dynamically heterogeneous food system. A typical ice cream mix contains milk or cream (water, lactose, casein and whey proteins, lipids, vitamins, and minerals), sucrose, stabilizers and emulsifiers, and some type of flavor (e.g., vanilla). After the ingredients are combined, the mix is pasteurized and homogenized. Homogenization creates an oil-in-water emulsion, consisting of millions of tiny droplets of milk fat dispersed in the water phase, each surrounded by a layer of proteins and emulsifiers. The sucrose is dissolved in... 

The configuration of bovine serum albumin isolated from milk has not been investigated, but extensive investigations of this protein isolated from bovine blood serum have been made. The protein exhibits at least three different kinds of heterogeneity (1) due to polymer formation, (2) related to the sulfur linkages in the molecule, and (3) microheterogeneity. Fractionation of bovine serum albumin on DEAE-Sephadex A-50 resulted in a monomer and two dimer fractions (Janatova et al. 1968),... 

Mather, I. H., Weber, K. and Keenan, T. W. 1977. Membranes of mammary gland. XII. Loosely associated proteins and compositional heterogeneity of bovine milk fat globule membrane. J. Dairy Sci. 60, 394-402. 

The first soybean protein ingredients made commercially available for food use included full-fat and defatted soy flours and grits (3, 7, 8). These products contain ca. 46-59% protein (NX 6.25) on a moisture-free basis and are available with various heat treatments for specific end-use. Soy protein concentrates and soy protein isolates were introduced into the market about 15 years ago (3, 9, 10, II). By definition soy protein concentrates must contain no less than 70% protein (N X 6.25) and isolates no less than 90% protein (N X 6.25), all on a moisure-free basis. In the past several years there has been much activity in the commercialization of textured soy protein products intended for the extension and replacement of meat. These textured products may be obtained through fiber spinning, shred formation, extrusion, or compaction (12, 13, 14, 15). In addition, soybean milk solids and the heterogeneous proteins in soybean whey might serve as useful substrates in chemical modifications for food use. This short recitation of commercial products illustrates the type of crude protein fractions available for practical modification. Many useful functional properties have been ascribed to these new food proteins. 

Table X (142) lists the composition of the major proteins existing in milk. Even those proteins listed in Table X are heterogeneous in nature. For example, the a8 group of proteins is comprised of a80, aBv a82, a8v aH, 8g. Furthermore, all of the proteins listed in Table X exist as polymorphic genetic variants differing in primary structure and separable by polyacrylamide or starch gel electrophoresis. However, the existence of these variants will not have a marked effect on the ensuing discussion.

Casein is present in several animal and vegetable sources. Commercially, however, casein is primarily obtained from milk that contains about 3% of this protein. The polymer is isolated either by acid coagulation or with the help of enzymes obtained from animal stomachs. It is very heterogeneous. The molecular weight of a large portion of bovine casein is between 75,000 and 100,000. It consists of two components, a and p. Casein belongs to groups of proteins that are identified as phosphoproteins because the hydroxyl residues of the hydroxy amino acids are esterified with phosphoric acid. 

Numerous and conflicting reports on the protein requirements of the adult are to be found in the literature (23, 64, 65, 66). The use of heterogeneous natural food stuffs in the majority of these studies does not permit a ready estimation of the human amino acid requirements from the nitrogen data. In balance studies in which milk proteins constitute the principal nitrogen component of the diet, it is possible to approximate... 

Additional whey proteins, including P-lactoglobulin and bovine serum albumins, are also allergenic. Bovine serum albumin (67 kDa) comprises approximately 1 % of total milk protein and is quite heterogeneous in nature, with several different isoforms. 

Animal glue is a complex colloidal mixture of proteins. The related gelatins are also complex heterogeneous mixtures of proteins. They are strongly hydrophilic and rich in the amino acids glycine, proline, lysine, hydroxyproline and hydroxylysine. Casein is a phosphoprotein obtained from the milk of mammals. 

Bovine Serum Albumin. Since Polis et al. (1950) crystallized bovine serum albumin from whey and demonstrated that it was identical in all properties investigated to blood serum albumin, except in its electrophoretic behavior at pH 4.0, very little work has been done on this protein as isolated from milk. However, much work has been done on the protein isolated from bovine blood plasma. There is considerable evidence that serum albumin is heterogeneous. For example, Spencer and King (1971) have demonstrated several protein bands by electrophoretic focusing, with two major isoelectric components differing by one unit of charge. The chemical nature of this difference is not known. 

Nitrogen soluble in 12% TCA is referred to as non-protein nitrogen (NPN), of which milk contains 250-300mgl i.e. 5-6% of total milk nitrogen. The NPN is a very heterogeneous fraction (Table 4.7). 

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