BY-PRODUCTS TECHNOLOGY

Module 2. Skim milk and its by-products

Lesson 10
INDUSTRIAL USES OF CASEINS-I

10.1 Introduction

Casein is unique amongst dairy products in that it has a long history of use in foods and in non-food industrial or technical applications. The major uses of casein until the 1960s were in technical, non-food applications. Considerable efforts are being made to prepare more and more food and pharmaceutical grade casein during the past 35 years, not only because it gives better return, but also because it is an excellent protein nutritionally and functionally. Still, a sizable amount of casein for non-food uses is being prepared for industrial and technical applications. Due to their high amount of polar groups, caseinate also shows good adhesion to different substrates such as wood, glass or paper and this hydrophilicity makes caseinate films excellent barriers to non-polar substances such as oxygen, carbon dioxide and aromas. Considering different properties, caseinate-based polymers can be used in several technical applications such as protective coating and foams, paper coating, adhesives or injection moulding disposables. The specific properties of casein relevant for technical applications are listed in Table 10.1. An excess of fat present in casein may make the casein unsuitable for certain uses. In paper coating it may result in faulty imprints, in plastics it would prevent binding of the individual components, and in glues it would decrease their adhesive strength.

10.2 Application of Casein in Glues

Casein glues came to be used extensively in Europe probably in the 1890s, but they did not become widely known in the United States until 1917. When that country became involved in the First World War, a need arose for water-resistant glue for the construction of military aircraft which were made mostly of wood. Interest was aroused in casein glue, especially for plywood, and this led to a thriving industry in its manufacture. Although animal glues had been commercially important for wood gluing long before casein was used, it was difficult to make them water-resistant and their durability was consequently often poorer than that of casein. By the time aircraft construction had shifted largely to metal, casein glue was firmly established in other woodworking industries.

For marketing purposes, casein glues have been classified as (a) prepared glues and (b) wet-mix glues. Prepared glues were sold in the form of dry powders which contained all the necessary ingredients except water. The proportion of powder to water was usually about 1:2 by weight and even large batches could be prepared with a mechanical mixer in less than 30 min. Once mixed, the glues generally had to be used within a working day. Wet-mix glues were prepared by mixing together ground casein, water and additional chemicals according to the formula.

Table 10.1 Principle technical applications of casein

Besides casein and water, an alkali must be used to dissolve the casein. This is often sodium hydroxide and provides the third ingredient in simple glue. Lime may be added if the glue is to be water resistant. The lime promotes cross-linking of the casein and, over a period of several hours, will cause a casein glue to form an irreversible jelly which is insoluble in water. Various additives may be employed to change the properties of casein glues, e.g., sodium silicate prolongs the working life of the glue while addition of a soluble copper salt such as copper chloride increases the water resistance of the glue after it has dried. Preservation of solutions of casein glue against putrefaction and mould growth may be accomplished by using chlorophenol derivatives, for instance, and the viscosity of solutions of casein glues may be reduced by addition of a viscosity-modifying agent such as urea or ammonium thiocyanate.

Casein glue is used for gluing timber in internal woodwork, such as laminated beams and arches and in interior doors, plywood, wood particle board and in bonding of Formica laminate to timber. Casein adhesives may also be used for bonding paper, in packaging and in foil laminating, in holding the seam of a cigarette together, in the seaming of paper bags, in the assembly of milk cartons and in securing the abrasive strip on the covers of match boxes.

10.3 Casein as an Adhesive in Coating Paper and Cardboard

Coated paper was developed to satisfy the needs of printers for a paper upon which illustrations, especially fine halftones, could be reproduced satisfactorily. The coating is prepared by mixing mineral material with a solution of an adhesive and applying this mixture in a thin, even layer to the surface of a sheet of paper. The function of the adhesive is to bind the coating material so firmly that it will not be removed or 'picked' off during printing. The coating material, which covers the individual fibres on the paper surface and also fills any hollows between them, forms a surface which is receptive to printing ink. After calendering (polishing), the surface of the coated paper is smooth, even and continuous which is needed for high quality reproduction of illustrations, in particular. Casein is still a preferred binder for cast-coating and in enamel grades of coating. It is also used in coating stock, label stock and bleached kraft board for food cartons.

10.4 Casein in Sizing

In such utilizations, casein acts as a binder for the coating material, generally a mixture of mineral materials, which is applied as a thin layer on the surface of the material. Casein glue has long been used as a sizing material, sometimes for the sealing of absorbent surfaces prior to subsequent treatment. The film-forming ability of casein is retained even when deposited from a very dilute solution, and casein sizings have been used on such diverse products as shot-gun shells, heels of ladies' shoes, in varnishes, in paper making, leather finishing and textile manufacture.

Casein has also been applied to wool to reduce its felting properties and to artificial textile materials such as Nylon in order to ensure successful wearing or knitting.

The casein film may be given a high degree of water resistance by the inclusion of 'hardeners', either in the solution or by post-application to the film. Casein then becomes a permanent finish which can be applied to paper to enhance its snap, lustre or stiffness.

10.5 Paper Industry

The hydrophobic/hydrophilic balance of casein increases its affinity for pigments, its ink-binding properties and its adhesion to various substrates. In the paper industry, casein is used as size for high quality glazed papers or for fine halftone illustrations. Water resistance can be achieved by exposure to formaldehyde vapours or by dipping in concentrated solutions of formaldehyde. Formaldehyde or dialdehyde are crosslinking agents which bind free amino groups to protein to give a three dimensional network. For example, wallpaper becomes water washable by adding a coating of casein solution brushed over with a solution of formaldehyde. With high solid contents (china clay, kaolin, chalk…) coating solutions become too viscous, so in some applications, flow modifiers are added to enhance the casting process. Viscosity can be lowered by reducing the molecular weight of caseins and by denaturing the protein. This can be achieved by addition of urea, by alkaline or enzymatic hydrolysis, or by disulphide bond reducing agents such as mercaptoacetic acid or 2-mercaptoethanol. Pigments can be directly mixed with the formula to obtain coloured size and coatings. In the middle of the twentieth century, paper and board coating accounted for the major uses of casein. Nowadays casein is still used in high quality paper finishing and for enamel grades of paper, but a decline in the use of casein in paper coating can be mentioned due to casein’s high costs.

10.6 Textile Industry

For producing casein fibres for textiles, acid casein is dissolved in an alkali, such as sodium hydroxide, at a concentration of about 200 g/litre, and the solution is then forced through a spinneret into a coagulating bath. The bath usually contains acid, inorganic salts and often heavy metal salts. The fibres thus formed resemble wool, except that they have a lower tensile strength and do not 'felt' (i.e. shrink on washing) like wool. The dissolved salt produces a large osmotic pressure and causes a considerable shrinkage in the diameter of the freshly extruded filaments. It also reduces the tendency of the filaments to stick together. Aluminium sulphate is used in such coagulating baths and can also be employed with formaldehyde for stretching and hardening the fibres. The hardening of the fibres is very important since the strength of the wet casein fibres is generally less than half that of the dry fibres. Amongst the more successful hardening process developed is acetylation.

The principal proprietary casein fibres which were developed throughout the world in the decade from 1936 to 1945 included: Aralac (National Dairy Products Corp., USA), Casolana (Co-op Condensfabriek Friesland, Netherlands), Fibrolane (Courtaulds Ltd., UK), Lanital and Merinova (Snia Viscosa, Italy). Of these, only Fibrolane and Merinova were still in production by 1971, though two other casein fibre products were being produced elsewhere (Wipolan, in Poland and Chinon, in Japan).

Casein fibres were used during and after the war years, usually in combination with wool and other fibres, such as cotton, viscose, rayon, etc., in a variety of products, such as flannel, woolen spun cloth (overcoats, blankets), felt hats (up to 25% casein fibre with wool), filling materials such as artificial horsehair, and in carpets and rugs. Bristles were also produced from casein fibres for use in brushes of various types. Caseins also combined with acrylates become a protective coating against greying of cotton. Chemically-modified caseins, either grafted with acrylate esters or crosslinked, are used as anti-static finishing of natural (wool, cotton and silk) and synthetic (polyester) textile fibres.

The importance of casein fibre for textiles has now declined in the face of competition from other fibres. However, co-polymer fibres containing casein have been prepared in Japan as a substitute for silk, which was still being undertaken during the preparation of this article.

10.7 Leather Industry

The use of casein in the leather industry is confined almost entirely to the last of the finishing operations, which consist in coating leather with certain preparations and then subjecting it to mechanical operations such as glazing, plating, brushing and ironing. After finishing in this way, leather is said to have been seasoned. Polyol plasticised casein is used in the finishing operations in the leather industry combined with additional components such as acrylates, phenol derivatives, pigments for coloured products or binders such as gelatine or sulfonated castor oil. For application to leather, casein is first dissolved in alkalis such as ammonia, borax, sodium hydroxide or trisodium phosphate or acids. An acid solution of casein (concentration, 1-6% by weight) produces a clear, bright finish for naturally-finished vegetable tanned leather, commonly known as russet leather, to which up to five coats may be applied. The particular properties of casein which make it desirable for these applications are: its film-forming properties, adhesive strength and viscosity-enhancing characteristics, which prevent the film from running before it has set. Casein is hard, but tends to be brittle, and it is for this reason that oils and glycerol may be added to the casein solution to increase its plasticity and reduce the brittleness of the film. A casein product, treated with a chlorocarbonate plasticizer, has been proposed to overcome the natural brittleness of casein on its own. Casein may also be incorporated with other binders such as shellac, carnauba wax, blood albumin, gelatin and sulphonated castor oil. More recently, various acrylates have been used with casein (as graft co-polymers) in leather finishing.

Selected reference

Audic, J.-L., Chaufer, B. and Daufin, G. 2003. Non food applications of milk components and dairy co-products: A review. www.edpsciences.org, Lait 83: 417-438.