Major milk proteins

Milk protein system. The nomenclature and physico-chemical properties of the major milk proteins and their subunits have been provided by Whitney et al. (13) and Brunner (1 ). The conformation and related properties of the individual proteins and their subunits and aggregates have been reviewed by Morr (15) with special reference to their functional properties in food systems, and drawing heavily upon previous considerations by Bloomfield and Mead (16) and Slatterly (17). 

Milk protein products. As indicated in Table 1, the food industry is placing major emphasis on the production and utilization of milk protein products in a wide variety of formulated food products (20,21,22). Although nonfat dry milk (NFDM) and whey powder are major milk protein ingredients in formulated foods, casein and whey protein concentrates, which contain their proteins in a more highly concentrated and functional form, are essential for certain food product applications, such as those products that require the proteins as an emulsifier agent. Additional details on the processing methods and conditions used to produce the various milk protein products are available (23). 

Sasaki, M., Eigel, W. N. and Keenan, T. W. 1978. Lactose and major milk proteins are present in secretory vesicle-rich fractions from lactating mammary gland. Proc. Natl. Acad. Sci. USA 75, 5020-5024. 

Table 8 Molecular Structure and Properties of the Major Milk Proteins... 

Mather, I.H. 2000. A review and proposed nomenclature for major milk proteins of the milk-fat globule membrane. J. Dairy Sci. 83, 203-247. 

Among its effects on the epithelial cells of the mammary alveoli prolactin stimulates the production of the major milk proteins casein and lactalbumin [64,66-69,74-76]. Mouse mammary explants incubated in vitro with insulin and a corticosteroid produce very little of these proteins, but addition of prolactin to the culture medium induces their synthesis 10-20-fold within about 24 h. As has already been discussed, there is some uncertainty about the importance of glucocor-... 

This paper draws heavily upon the "Nomenclature Committee Report" ( 1) as well as several recent comprehensive reports that have considered the primary structure and conformation of the casein monomer subunits and how they are assembled into submicel-lar aggregates and casein micelles (2, 3). These basic relationships were utilized to develop additional projections relating to the conformation and functional properties of the major milk proteins, e.g., commercial caseinates and whey protein concentrates in food applications. 

The fractionation scheme and distribution data for the major milk proteins and their subfractions are given in Figure 1 (1). Although the caseins are easily separated from whey proteins by adjusting milk to pH 4.6-5.0, further separation and purification of the individual caseins is extremely difficult, due to their strong interaction, and requires the most sophisticated protein... 

Figure 1. Fractionation and distribution of the major milk proteins...

Synthesis of the major milk proteins occurs in the mammary gland the principal exceptions are serum albumin and some of the immunoglobulins, which are transferred from the blood. Polymerization of the amino acids occurs on ribosomes fixed on the rough endoplasmic reticulum of the secretory cells, apparently by a method common to all cells. 

Thus, all the major milk proteins contain sequences which, when liberated by gastrointestinal proteinases, possess opioid activity. These peptides are very resistant to proteolysis by gastrointestinal proteinases and, because of their high hydrophobicity, can be absorbed intact from the intestine. They possess physiological activity in vitro but their activity in vivo is as yet uncertain. 

Lactoferrin contents rise dramatically in the milks of mastitic cattle. This apparently represents a normal physiological defense mechanism similar to the nutritional immunity discussed in a previous section, except that here not only the UIBC but also TIBC-values of the milks increase drastically. In such milks, lactoferrin values may be as high as 6 mg/ml, lactoferrin thus becoming a major milk protein component. It is apparently synthesized by the epithelial cells rather than being released by the neutrophils that are also present in such milks (Harmon et al., 1976), The function of the transferrins in other types of infections in cattle have not yet received much attention, and should represent a fertile area for future research activity. 

Milk and colostrum glycoproteins contain sialic acids. The caseins are the major milk proteins, but only x-casein is a glycoprotein, with 5% carbohydrate. It has been isolated from human, bovine, ovine and goat sources (see Kobata 1977). 

Casein (CA), the major milk protein, is inexpensive, readily available, non-toxic and highly stable. CA comprises about 94% protein and 6% low-molecular-weight compounds collectively... 

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