Casein

The only industrial products manufactured directly from skim milk are acid-precipitated and rennet-precipitated casein, which are relatively pure chemical products. Purified casein is suitable for food, but it is not necessary or even desirable that casein be purified for food use. Cottage cheese and cheeses of the harder varieties are essentially casein of the acid-precipitated and rennet-precipitated varieties, respectively, that have not been freed from other food substances present in milk. But, for industrial use, it is necessary that casein be refined by removing the non-protein organic nutrients to as great a degree as possible.

The chemical differences between acid-precipitated casein and rennet casein are not well understood, but a mixture of rennet casein with a small proportion of water is characterized by its formation of a plastic mass that is suitable for the manufacture of buttons and many other articles, such a umbrella handles, buckles, and costume jewelry. Plastic casein would have greater use were it not that no satisfactory means have been discovered to prevent the finished products from expanding in humid air and contracting in dry air. Efforts have been made to find plasticizers for casein so that it can be used as a molding powder rather than as an extruded plastic, but these efforts have been only partially successful.

The average quality of the acid-precipitated casein produced in the United States has improved markedly in the past 20 years. The devising of the grain-curd process in the Bureau of Dairy Industry and the development by several manufacturers of continuous processes of manufacture, all requiring careful control of acidities and effective washing, have been the major contributions to the wider production of uniform, high-grade casein. The quality of casein is highly important to the paper-coating industry, which used about three-fourths of the total supply in the prewar Years. Quality is of less importance to the casein adhesive and casein paint manufacturers, who used 10 and 5 percent, respectively, of the prewar supply. For the newer uses in fabrication of synthetic rubber and in making casein fiber, it is essential that the casein be of high grade.

The popular and relatively new water-emulsion paints usually contain casein to the extent of about 3 percent of the total solids. Casein is used in these paints principally for its emulsifying action, but it also functions as a binder. The use of the insecticide DDT in water-emulsion paints is expected to increase the market for casein.

Although the quantity of casein used in synthetic rubber is small, this use is important, since no adequate alternative substance has been found. The casein is said to act as an internal lubricant, and thus prevents excessive heating of synthetic rubber articles, such as tires, when they are flexed during use. The quantity of casein required in the future for this purpose depends, obviously, on the extent to which synthetic rubber is used, but there is a probability that casein will be used also in tires made from natural rubber.

A thin, transparent, wrapping material made from casein was in limited commercial use in the early 1930's. Its production was abandoned because it developed a cloudy appearance and brittleness. A recent patent on improvements in the manufacturing process indicates the possibility that it may again be on the market.

Casein has been used in small quantities in the textile industry for years for bonding, loading, finishing, and waterproofing fabrics, but only since 1940 has there been commercial development from casein of an artificial fiber that seems to be finding a definite place among our textile fibers. Application for the first practical patent for making fiber from casein was filed in 1935 in Italy and commercial production was begun there the following year. The first of a series of eight patents resulting from, research in the Bureau of Dairy Industry was issued in 1939 and, in the 6 years immediately following, about 40 United States patents were granted to other inventors on phases of this development. Currently, the production of casein fiber in this country, which began only a few years ago, probably approaches 10 million pounds a year.

The conversion of the casein of skim milk into textile fiber is not a process that can be carried out on the farm. The casein must be made by a controlled procedure possible only in a dairy plant or a plant making casein exclusively. The conversion of casein into fiber requires the knowledge and experience of textile engineers and equipment similar to that of plants producing viscose rayon. The casein is dissolved in alkali, various other substances are added, and the solution is extruded through the fine apertures of a spinneret into a bath containing acid and dehydrating and hardening agents. Next, the fiber is chemically treated. It is then cut into staple lengths, after which it it either felted in mixture with hair or wool or spun into yarn, which is woven into fabrics in mixture with cotton, rayon, or wool yarn. Most felt hats sold in the United States contain some casein fiber; garments and fabrics containing casein fiber may be purchased in stores.

Among the characteristics that have established casein fiber in the textile field are its property of felting in mixture with hair and wool whereby it acts as an extender of these more expensive fibers; the soft feel imparted to fabrics that contain it; and the fineness of fiber possible finer than the finest wool. The fact that casein fiber has somewhat less tensile strength than wool, especially when it is wet, has prevented its being woven into fabrics without admixture of other, stronger fibers. Fibers from soybean protein and from peanut protein resemble casein fiber but do not equal it, and have not yet been commercialized; the synthetic nylon is similar chemically to casein and offers serious competition since it is now being produced as a wool-like fiber.