casein

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casein

(kā`sēn), well-defined group of proteins found in milk, constituting about 80% of the proteins in cow's milk, but only 40% in human milk. Casein is a remarkably efficient nutrient, supplying not only essential amino acids, but also some carbohydrates and the inorganic elements calcium and phosphorus. The calcium caseinates form an insoluble white curd when acidified by hydrochloric acid or sulfuric acid, or when milk is soured by bacterial contaminants. Acid casein is used widely in cheese, adhesives, water paints, for coating paper, and in printing textiles and wallpaper. In neutral solutions the enzyme rennin converts one of the caseins to an insoluble curd; most of the protein in cheese is rennetrennet,
substance containing rennin, an enzyme having the property of clotting, or curdling, milk. It is used in the making of cheese and junket. Rennet is obtained from the stomachs of young mammals living on milk, especially from the inner lining of the fourth, or true,
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 casein curd. When treated with formaldehyde the curd forms casein plastic, used for manufacturing imitation tortoiseshell, jade, and lapis lazuli.

Casein

 

a complex phosphoprotein that is formed from its precursor caseinogen as a result of the splitting of peptide bonds in the process of milk curdling. Casein is readily soluble in saline solutions in neutral or alkaline media, and it precipitates when acidified. Its molecular weight is 75, 000–100, 000.

Casein is a heterogeneous protein; by physical and chemical methods it can be divided into three fractions that are similar to one another in amino-acid composition, (α-casein β-casein, and γ-casein). Dried casein is a tasteless and odorless white powder. Casein is the principal protein component of milk and milk products. Cow’s milk contains 2.8–3.5 percent casein; human milk, 0.3–0.9 percent. This most important dietary protein contains a complete complement of the essential amino acids; it contains especially high percentages of methionine (~3.5 percent), lysine (˜6.9 percent), tryptophan (~ 1.8 percent), leucine (˜12.1 percent), and valine (7.0 percent). The stomachs of mammals shortly after birth contain the enzyme chymosin, or rennin, which curdles milk (this can also be catalyzed by other proteolytic enzymes). In industry casein is used in the manufacture of paints, glues, synthetic fibers, and plastics.

I. B. ZBARSKII

casein

[′ka‚sēn]
(organic chemistry)
The protein of milk; a white solid soluble in acids.

casein

A protein; the chief nitrogenous ingredient of milk.

casein

a phosphoprotein, precipitated from milk by the action of rennin, forming the basis of cheese: used in the manufacture of plastics and adhesives