Commercial Acid Casein

This commonest form of casein is obtained from cows milk (which contains 2-3% casein) by first removing the fat by centrifnging. The ranainder (skim milk) is then acidified with lactic or mineral acid to pH 4.5, which solubilises the calcium phosphate. On raising the temperature to about 46°C, the casein is precipitated as a curd which is then drained, washed, pressed, dried and ground to a powder. This acid casein chars at about 200 C. It has a density of p = 1.25 g/cc and is practically insoluble in water (0.1 gfl at 25 C). 

High-grade acid casein is creamy white, it contains around 10% moisture and has a pH 4.0-5.5. The specification and grading of industrial caseins vary somewhat in different countries, but their protein content is usually in the range of 90-95%. About 3 g casein can be obtained from 100 g bovine milk. It is subject to microbial attack when damp and biocides are added for some applications. 

Casein is soluble in alkali hydroxides and in this way the soluble Na, K and NH4 salts are obtained commercially. Calcium caseinate will form stable colloidal dispersions while sodium and potassium caseinates dissolve to form viscous solutions and eventually aqueous gels above about 17% concentrations. The alkali metal salts are very stable to heat over a wide range of conditions. They can be heated to 140°C for several hours at pH 7, without decomposition. 

The viscosities attained by water-soluble caseins depend not only on the concentration, but on the alkali used and the pH reached. At 14% concentration with added NaOH, a maximum viscosity is reached at pH 9. If the alkali employed is Na3P04, the viscosity at the same pH is about five times greater, while at pH 11, it is 10 times greater. 

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Commercial casein is usually manufactured from skim milk by precipitating the casein through acidification or rennet coagulation. Casein exists in milk as a calcium caseinate-calcium phosphate complex. When acid is added, the complex is dissociated, and at pH 4.6, the isoelectric point of casein, maximum precipitation occurs. Relatively little commercial casein is produced in the United States, but imports amounted to well over 150 million lb in 1981 (USDA 1981C). Casein is widely used in food products as a protein supplement. Industrial uses include paper coatings, glues, plastics and artificial fibers. Casein is typed according to the process used to precipitate it from milk, such as hydrochloric acid casein, sulfuric acid casein, lactic acid casein, coprecipitated casein, rennet casein, and low-viscosity casein. Differences... 

Casamino acids, commercial acid-hydrolyzed casein. Hydrolysis is carried out until all the nitrogen in the casein is converted to amino acids or other compounds of relative chemical simplicity. Prepn Mueller, Miller, J. Immunol 40, 21 (1941) Mueller, Johnson, ibid. 33. Typical analysis N 10%, NaCI 14%, ash 20%, phosphorus as P04 2%, Fe 15 mg/3 g. 

Table 3.15 shows representative amino acid profiles of some common foods and dietary protein supplements. The percentages are averages of several commercial products. Casein and whey are milk... 

Casein protein is recovered from skim milk by acid precipitation to pH 4.5. Mineral acids may be used or the milk can be cultured with bacteria that convert lactose (milk sugar) to lactic acid, which in turn precipitates the casein. The precipitated protein curd is washed free of acid with hot or cold water and is then dried and ground. The commercial designation for casein often includes its method of acid precipitation (e.g., lactic acid or sulfuric acid casein ). 

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. 

Acid casein finds use in a variety of applications, including adhesives (mainly for paper and wood) and paper coatings. In these latter uses, the casein is applied as a dispersion in aqueous alkali. Casein fibre may be produced by forcing an alkaline dispersion through a spinneret into an acid coagulating bath the fibre (commonly termed casein woof) was produced commercially during World War II in Italy and the U.S.A. but it is now of little importance. Acid casein is used rather than rennet casein in these various applications since it is more readily dispersed in aqueous alkali. 

Commercial casein is obtained from cow s milk, in which it is the main protein at about 3% concentration. Most of the casein for adhesives is obtained by acid precipitation however, some is precipitated from milk by rennet, a preparation made from the stomach of young calves. Only the use of acid casein is covered here, since a very limited amount of rennet casein is used for adhesives. 

Casein occurs in several animal and vegetable materials but the only source of commercial importance is cow s milk, in which the casein content is about 3%. In commercial practice, casein is isolated from skimmed milk by either acid coagulation or by rennet coagulation. In acid coagulation, dilute sulphuric acid is added to the milk at about 35°C, with stirring. Coagulation... 

Destruction of the casein micelles in the milk with subsequent precipitation of the casein can be accomplished in a number of ways. The action of heat or the action of alcohols, acids, salts and the enzyme rennet all bring about precipitation. In commercial practise the two techniques used employ either acid coagulation or rennet coagulation mechanisms. 

Nitrogen sources include proteins, such as casein, zein, lactalbumin protein hydrolyzates such proteoses, peptones, peptides, and commercially available materials, such as N-Z Amine which is understood to be a casein hydrolyzate also corn steep liquor, soybean meal, gluten, cottonseed meal, fish meal, meat extracts, stick liquor, liver cake, yeast extracts and distillers solubles amino acids, urea, ammonium and nitrate salts. Such inorganic elements as sodium, potassium, calcium and magnesium and chlorides, sulfates, phosphates and combinations of these anions and cations in the form of mineral salts may be advantageously used in the fermentation. 

In addition to more rapid absorption of lipids in animals fed casein, another mechanism that may be operative is decreased clearance of circulating lipids. Rabbits fed a casein-based semipurified diet excreted significantly less cholesterol but more bile acids in their feces than animals fed a commercial diet (18). The total sterol excretion in feces of the animals fed the casein diet was half that of the rabbits fed the stock diet. Huff and Carroll (19) found that rabbits fed soy protein had a much faster turnover rate of cholesterol and a significantly reduced rapidly exchangeable cholesterol pool compared with rabbits fed casein. Similar studies performed in our laboratory revealed that the mean transit time for cholesterol was 18.4 days in rabbits fed soy protein, 36.8 days in rabbits fed casein, 33.7 days in rabbits fed soy plus lysine, and 36.3 days in rabbits fed casein plus arginine. These data suggest that addition of lysine to soy protein... 

Enzymatic hydrolysates of various proteins have a bitter taste, which may be one of the main drawbacks to their use in food. Arai el al. [90] showed that the bitterness of peptides from soybean protein hydrolysates was reduced by treatment of Aspergillus acid carboxypeptidase from A. saitoi. Significant amounts of free leucine and phenylalanine were liberated by Aspergillus carboxypeptidase from the tetracosapeptide of the peptic hydrolysate of soybean as a compound having a bitter taste. Furthermore, the bitter peptide fractions obtained from peptic hydrolysates of casein, fish protein, and soybean protein were treated with wheat carboxypeptidase W [91], The bitterness of the peptides lessened with an increase in free amino acids. Carboxypeptidase W can eliminate bitter tastes in enzymatic proteins and is commercially available for food processing. 

Four genetic variants have been identified, with variant A the most common in Western commercial breeds. The complete amino acid sequence of as2-CN A-l IP is shown in Fig. 3. Although the sites of phosphorylation are not all identified, it is thought that aS2-casein exists, at least predominantly, in four phosphorylated forms containing 10-13 mol P/mol. 

Casein has been well studied and is well characterized in the literature. Salzberg et al (4) characterize it as a globular polypeptide of molecular weight 33,600 to 375,000, osmotic pressure measurements leading to lower values and sedimentation giving higher ones. Macy et al. (10), in their compilation of 1,500 reports, break casein down into the amino acids and elements shown in Table I. Since casein is variable, these are representative compositions. Commercial caseins for adhesives use are generally characterized as shown in Table II. 

The major casein monomer subunits have random coil conformation that facilitates strong protein-protein interaction via hydrophobic and ionic bonding. The unique amphiphilic structure, which arises from separately clustered hydrophobic and negatively charged (acidic and ester phosphate) amino acid residues along the polypeptide chain, makes them susceptible to pH and Ca ion concentration effects. This amphiphilic nature is probably responsible for the excellent surfactant properties of commercial caseinate in a variety of food applications. 

Effect of pH on amino acid composition of casein. Conditions 1% commercial casein 65°C 3 hours. Numbers are mole per cent for each amino acid. 

Butter-like products with reduced-fat content are manufactured in several countries. Stabilizers, milk and soy proteins, sodium albumin or caseinate, fatty acids, and other additives are used. A product is now available on a commercial scale in the former U.S.S.R. that has the following composition 45% milkfat, 10% nonfat solids, and 45% moisture. It has a shelf life of 10 days at 5°C (91). Each country has established its own standards for butter and butter fat products. Many are still developing standards for a reduced-fat butter product to meet the growing consumer demand. 

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