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