CASEIN. The milk of all mammals contains casein. Cheese is a modified form of casein. It is a complex product akin to white of egg, belonging to the class of chemical compounds known as albumins. In milk, casein occurs in combination with lime (calcium oxide) as a calcium salt, and in order to isolate it the milk must be broken up by treatment with acid. A white curd is precipitated which is washed and dried, the product thus obtained being sufficiently pure for most technical purposes. From skim milk of average composition, between 3 and 3.25% of casein can be obtained.
Various acids are used commer cially for precipitation, the three main processes are :
the rennet process; (2) the process in which an acid, e.g., dilute sul phuric or hydrochloric acid, is added to the milk; and (3) the self-curdling process, in which the milk is allowed to rest until it is curdled by the action of lactic acid formed from lactose or milk sugar of the milk by the activities of a bacillus (B. acidi lactici). Chemically the substance precipitated by the action of rennet is termed paracasein, as distinct from that precipitated by acids. In all these processes the precipitated curd is separated from the whey, washed with cold water, passed through a press or centrif ugal separator, broken up, and dried by currents of dry air or other means. The dry casein is ground and packed for sale.
Pure casein is a white powder without taste or smell. Chemically it is composed of carbon, hydrogen, nitrogen, sulphur and phosphorus. The food value of casein is comparable with that of the proteins in meat; its content of amino-acids, some 15 of which have been isolated, contributes to its nutri tive qualities, these latter being enhanced also by the presence of vitamins. It is a weak acid, readily soluble in solutions of caustic alkalis and of the carbonates, borates and other salts of the alkali metals. Water dissolves casein only to a very small extent, but causes it to swell. Casein is converted by the action of formaldehyde, or formalin, into a hard material quite insoluble in water. Casein is non-inflammable. Its specific gravity is about 1.26. Commercial casein of good quality is a dry, friable white, or slightly brown powder; casein of inferior quality has an un pleasant smell and is usually moist and dark coloured.
In many countries casein is produced on a commercial scale, and a large amount is used in the making of confectionery and special foods. Preparations are also used as media for the medicinal administration of quinine, lithium salts, iodides and other drugs. Casein forms the basis of many cosmetics, creams and ointments. It is used to some extent for printing on calico, sizing yarns, and sizing, waterproofing and coating in the paper industry. It is also used largely in the production of cold-water paints and distempers, of cements for porcelain and glass, and of glues for use in bookbinding, wood-working and aircraft con struction.
The production of casein moulded prod ucts or plastics intended to imitate ivory, tortoise-shell, amber, horn, ebony, agate, malachite and other decorative materials is an important industry all over the world. Some thousands of tons are made annually in many of the leading countries.
Two methods of manufacture are in use, known as the wet and dry processes. In the wet process, casein prepared by acid pre cipitation is dissolved in an alkaline solution and the necessary dyes and filling ingredients are added ; it is then re-precipitated by acid, washed, and the curd roughly pressed to remove water. The product is then ready for pressing in moulds of any desired shape. The moulded objects thus prepared must be "hardened" and rendered as waterproof as possible by long soaking in a solu tion of formaldehyde. After prolonged drying and seasoning the article is ready for use.
Rennet casein is used in the dry process ; it is first ground to a fine powder, and about one-third of its weight of water added and efficiently mixed. The apparently still dry powder is then well kneaded or pressed through a heated screw-press extruding machine from which it emerges as a soft plastic dough. This dough is then ready for moulding under pressure to the various shapes required. Moulded objects require "hardening" and sea soning, as described for the wet process.
Casein plastics are readily machined and polished and can be produced in a variety of colours, either opaque or transparent; they are difficult to ignite, of good mechanical strength, and when dry can be used for electrical insulation. A disadvantage, par ticularly for electrical purposes, is the readiness with which the material absorbs water. Combs, beads, buttons, manicure sets, cigarette holders and umbrella handles are among the objects which are made extensively of this material.
The production of dried casein (skim-milk product) in the United States in 1924 was 20,683,000 lb., according to the Sta tistical Abstract of the United States, 1925.
Sutermeister, Casein and its Industrial ApplicaBibliography.-E. Sutermeister, Casein and its Industrial Applica- tions, Chemical Catalogue Co., New York (1927), a comprehensive symposium by various specialists; E. L. Taque, Casein, Its Prepara tion, Chemistry and Technical Utilization (New York, 1926) .
(A. A. D.)
