CASEIN, Ica'se-In (Lat. caseus, "cheese'") a colloidal substance resembling albumen in its general constitution, and obtained from milk. The older chemists gave the name °casein* both to the precipitated substance that is now known by that name, and to the corresponding substance as it exists in solution in the milk; but it is the scientific practice at present time to distinguish the latter as Caseinogen is the principal nitrogenous con stituent of milk, in which it occurs to the extent of about 3 per cent, forming 80 per cent of the proteid -content. It is precipitated by a curdy mass, when acetic acid or a mineral acid is added to milk that has been previously diluted by the addition of its own bulk of water. If the caseinogen so prepared is made into a paste and then treated with a small quantity of ren net, the mass sets at once into a solid clot, consisting of true casein; but Hammarsten has shown that if the caseinogen is first washed entirely free from calcium phosphate, rennet is without action upon it. The precise function of the calcium phosphate is obscure, and the same may be said of the chemical relations of the proteids in general. If rennet is added to fresh milk a bulky deposit of casein and butter fat comes down immediately; but to obtain the casein in pure form, the oily matters in the milk should be first removed by the action of a centrifugal separator. Caseinogen is not pre cipitated by heat, nor does it (like fibrinogen) coagulate spontaneously. The coagulation ob served when milk is boiled.is due to the albu men present, and not to the casein; and that, which occurs upon standing may be due either to the generation of lactic acid through the fermentation of the lactose present, or to the rennet-like action of the ptomaines liberated by micro-organisms that happen to fall into the milk from the air. According to the analyses of Chittenden and Painter, the elementary per centage composition of casein is as follows: Carbon, 53.30; hydrogen, 7.07; nitrogen, 15.91;
sulphur, 0.82; phosphorus, 0.87; oxygen, 22.03. Casein is insoluble in water, alcohol or ether, but dissolves easily in alkaline solutions. It also dissolves in very weak hydrochloric acid, from which it is again precipitated upon the addition of the same reagent in more con centrated form. Casein for commercial pur poses is prepared almost wholly from cow's milk The milk is first put through a centriflu gal separator to remove the fat, and then from 4 to 6 per cent of caustic alkali is added. The milk is then run into a vat, and dilute sulphuric acid is added, the whole being stirred con stantly. When the curd has settled it is washed with cold water and drained on cheesecloth filters. It is pressed in a cheese press as dry as possible, and then broken up into very small pieces in a curd mill; after which it is dried at a low temperature in a vacuum. The chief use of casein in the arts is as a glue. In this form it is widely utilized in the wall paper industry in the production of washable wall papers, and also in the production of enameled papers, in which casein glue is used as the binder for the materials of the enamel coating. It is also used in the making of erasable tablets, paper etc. As a binder for various °fillers" many solid articles are produced in imitation of ivory and tortoise-shell. A compound of casein and lime is made use of in the dyeing industry for uanimalizing" cotton fabrics so that the fibres will retain the colors applied to them. In medicine, casein is being more and more em ployed, its combination with certain drugs im mensely enhancing their efficacy. This is par ticularly marked in the administration of iodine, phosphorus, arsenic, mercury and iron, as well as many alkaloids and organic compounds. See PROTEIDS. Consult Hammarsten-Mandel, 'Phys iological Chemistry' (New York 1914); Sherer, R., 'Casein: Its Preparation and Technical Utilization) (London 1906).