Casein is an organic compound allied to Albumen (q.v.), found in the milk of the Mammalia. It also exists in peas, beans, and other leguminous seeds, and is then known as Legumin. The proportion of casein in Milk (q.v.) varies, but averages about 3 per cent., and it may be coagulated and separated therefrom by the addition of a little Rennet (q.v.), as in the manufacture of Cheese (q.v.), or by the employment of a few drops of a mineral acid, such as dilute sulphuric acid. In either case the casein separates as curd, which still retains attached to it some oil and earthy salts, though the greater portion of these substances, along with the sugar, remain in the watery liquid or whey. The elementary bodies which enter into the composition of casein, and the proportion in which these are present in 100 parts, are—carbon 53.83; hydrogen, 7.15; nitrogen, 15.65; oxygen, 22.52; and sulphur, 0.85. The properties of casein are, that it is not coagulated by heat, as is well evidenced in the heating of milk, but is coagulated on the addition of rennet; sulphuric, hydrochloric, or nitric acids; alcohol, creosote, or infusion of galls, but not by acetic acid. It also forms insoluble precipitates with solutions of the poisonous salts, acetate of lead, nitrate of silver, and bicarbonate of mercury (corrosive sublimate), and hence the efficacy of taking large doses of milk in cases of poisoning by those deadly salts, as the casein in the milk, forming an insoluble compound with the poison, keeps it from exerting its deadly powers. When casein is heated with chalk, it forms a compound, insoluble in water, and hardening by exposure to air, which may be used as a cement. The compound with lime is used in destemper painting.
The form of casein obtained from plants, and termed legumin, is generally procured from leguminous seeds, like peas or beans, though it can also be extracted from the majority of vegetable substances, especially from sweet and bitter almonds, and even from tea and coffee. Dried peas contain a fourth of their weight of legumin, and this can be extracted by bruising the peas to powder, and digesting in warm water for two or three hours. The liquid is then strained through cloth, which retains the insoluble matters, and allows the water with the legumin dissolved therein, and with starch mechanically suspended, to pass through. On settling, the starch falls to the bottom of the vessel, and the clear liquid holding the legumin in solution, on the addition of a small amount of acetic acid, yields a precipitate of legumin or vegetable casein. So perfectly does the vegetable casein resemble the casein from milk, that the one can hardly be distinguished from the other by chemical tests or by taste. In various parts of China, especially near Canton, there is a form of cheese made from peas. Casein is a most important article of food. See DIET, DIGESTION, NUTRITION.