Milk Composition and Purification of the Target Protein

Milk is a multiphasic fluid composed of a fat emulsion, a micellu-lar casein dispersion, a colloidal suspension of lipoproteins, and a solution of proteins, mineral salts, vitamins, organic acids, and minor components. When collected, milk is not sterile and contains a 5,000

a 5,000

Figure 1 (a) Milk and recombinant antibody production during the first natural lactation of a transgenic goat during the 1996 season, (b) Milk and Antithrombin III production during the second natural lactation of a transgenic goat during the 1995 season. Work from Cenzyme Transgenics Corp. (1996).

Lactation Day

Figure 1 (a) Milk and recombinant antibody production during the first natural lactation of a transgenic goat during the 1996 season, (b) Milk and Antithrombin III production during the second natural lactation of a transgenic goat during the 1995 season. Work from Cenzyme Transgenics Corp. (1996).

bacteria derived from the teat as well as somatic cells derived mostly from the lymphatic ducts of the udder. According to the Pasteurized Milk Ordinance (U.S. Department of Health and Human Services, 1993), the bacterial plate count should be less than 100,000 per milliliter.

Milk composition is species specific,- major differences are seen between human and ruminant milk. Within a species, the composition varies with breed, diet, and other factors. Volume and composition also vary during lactation. Table 3 gives the average percentage compositions of livestock milk. Milk is approximately 85-90% water; the pH is 6.5-6.7 and as high as pH 6.8 in ewe's milk.

It is important from a purification point of view that the fat is present in globular form in the size range of 0.1-10 (xm and with a density higher than the other constituents. The fat globule is enclosed in a membrane of polar lipids and proteins. Triglycerides make up 97% of the fat.

The three subgroups of casein, as-, (3-, and k-casein, display genetic polymorphism and consist of two to eight variants. Casein is present as 10 to 300-nm micelles formed of submicelles held together by phosphate and hydrophobic bonding. Each submicelle has a polar core and a heterogeneous distribution of as- and (3-casein with surface k-casein, as- and (3-caseins are almost insoluble, whereas the glycoprotein, (3-casein is highly soluble in water. Casein may be precipitated somewhat below the isoelectric point range (pH 5.1-5.3) at about pH 4.6-4.7 by acidification or by the addition of chymosin, which attacks the 105 (phenylalanine) and 106 (methionine) peptide bond of the k-casein. The hydrophilic amino

Table 3

Percentage Composition of Major Milk Constituents'1

Table 3

Percentage Composition of Major Milk Constituents'1

(%)

(%)

Goat

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