Module 2. Skim milk and its by-products

Lesson 11


11.1 Introduction

Rennet casein is used exclusively for making casein plastics. The high mineral content of rennet casein results in its good adhesion and plastic characteristics. Casein plastic were first produced before the turn of century, after First World War, the manufacture of casein plastics increased all over World, since that time it is estimated that over 10,000 tonnes of casein plastic have been made throughout the World each year. Rennet casein plastic has retained a place in the market mainly because it can be dyed easily, giving colourful and lustrous products. Despite the introduction of newer plastic made chiefly from petro-chemical, rennet casein continuous to be used in this field is principally for the manufacture of buttons.

Due to low frequency of secondary structures (α-helix and β-sheets), caseins are mostly random coil polypeptides with a high degree of molecular flexibility able to form typical intermolecular interactions (hydrogen, electrostatic and hydrophobic bonds). This confers upon caseins good film-forming and coating abilities. The strongly amphipathic nature of caseins, arising from the balance of polar and non-polar amino acids residues, causes them to concentrate at interfaces to form a protein film. This confers upon casein good emulsify-ing and stabilizing properties. In this lecture, rennet casein plastic, casein based packaging films and biomaterials, application of casein as additive in paints, in rubber products and other miscellaneous products are discussed.

11.2 Rennet Casein Plastics

Rennet casein produces a plastic, which is far superior to that of acid casein. Rennet casein plastic was first available in France and Germany under the trade name of “Galalith®” in the early twentieth century but other casein plastics have been patented under the trade names of Erinoid® (UK), Aladdinite® (USA), Casolith® (Netherlands), Lactoloid® (Ja-pan) and Lactolithe® (France). The importance of casein plastics has now declined due to severe competition from synthetic plastics with better properties. In the production of casein plastic, the casein (if ungrounded) is milled, sieved through a screen with apertures of about 600 µm and mixed thoroughly with water to final moisture content between 20 and 35%. At this stage, filler such as titanium dioxide or zinc oxide may be added to produce either a white or opaque plastic, and dyes may also be included to produce coloured plastic. The wet casein is then stored for several hours or overnight to allow the water and casein to come to equilibrium and ensure uniform moisture content throughout the whole mixture. The extrusion mixture is then placed in a hopper feeding the extruder, which consists of a screw rotating in a water-cooled barrel. The casein mixture is delivered by the screw into a heated nozzle section, where it undergoes several compression and expansion stages and is consequently formed into casein plastic. Extrusion of the plastic mass generally occurs in the temperature range of 60-100°C to produce a smooth rod or strip sheet from the nozzle section. Extruders can be equipped with up to three screws, which may feed casein with different dyes into a single nozzle section. By manipulation of the feed rates of the casein in each barrel and alteration of the design of the mixing head where the various streams of casein plastic merge, it is possible to produce many beautiful and intricate designs in the plastic.

The warm plastic, which emerges from the nozzle of the extruder is initially soft and pliable. This is immediately immersed in cold water, which has the effect of hardening the plastic and preventing or reducing the development of internal stresses. Rods of casein plastic are subjected to 'dowelling' after they have cooled. This process trims the surface irregularities to produce smooth rods of uniform diameter. These are sliced into discs (or 'button blanks') which are subsequently placed into a dilute solution of formaldehyde for several weeks in order to 'cure' or harden the plastic. The cured blanks are later dried, machined into buttons and finally polished, usually by mechanical tumbling in the presence of wood chips and oil seeds.

If sheet plastic from casein is required, rods of casein plastic are placed side by side in a heated hydraulic press and then subjected to high pressure. The casein plastic sheet so produced must still be cured in formaldehyde for periods varying from 1 week to 6 months, depending on the thickness of the plastic. Once cured, the plastic sheets may be dried carefully to avoid rupturing the material or setting up stresses and strains within it during the expulsion of excess moisture and formaldehyde. Sheets of casein plastic still tend to warp during this process and must be straightened in low pressure hydraulic presses. Even when hardened, casein plastic can absorb moisture and its moisture content can vary according to changes in humidity. This limits its use in large panels or long rods which can warp badly.

Even though many different treatments of casein and casein plastic have been made in an effort to overcome the problems outlined above, they have not caused a significant increase in the commercial production of casein plastic. Although articles fashioned from casein plastic have included knife handles ('imitation ivory'), combs, imitation tortoise shell, pens, shoehorns and dominoes, the present range of casein plastic articles is somewhat more li-mited and includes buttons, buckles, novelties and knitting needles. In spite of this limitation, casein plastic articles do take up a great range of dyes to produce very attractive pat-terns which are somewhat more difficult to reproduce in the more common plastics made from petrochemicals.

11.3 Casein in Paints

Casein has a long history of use in paints. As paint technology evolved, synthetic resin emulsions were produced in which the ratio of casein to drying oil was much lower than in the oil-phase reinforced casein paints. The casein became the thickener and emulsion stabilizer and only a minor portion of the binder. After World War II, styrene-butadiene latex paints were developed in which casein was used mainly as a thickener and stabilizer, generally at a level of 1-2% by weight of the finished paint.

In general, casein is used in paints for its ability to disperse both white and coloured pigments and its power to thicken the binder. It may also be used as a protective colloid, as a film former and to improve flow and levelling properties of the paint. The paints are marketed in both powder and paste form.

11.4 Casein in Rubber Products

One of the less well-known applications of casein is its use as a reinforcing agent and stabilizer for rubber used in motor vehicle tyres. Casein hardened by formaldehyde was used to replace part of the carbon black used in the vulcanizing of rubber. Measurement of such properties as resistance to breaking, extensibility, resistance to tearing, hardness and abrasion of rubber which contained, for instance, 18% carbon black and 10% casein were either similar to or better than those for rubber containing 28% carbon black and no casein.

The Dunlop Rubber Company Ltd. used casein with paraformaldehyde as a protective colloid to improve the stability of a dispersion of a resin-latex composition, which was used to treat textile fibres intended for reinforcing rubber products. Casein gave significantly greater adhesion of textile fibres to rubber compared with fibres without any added casein.

11.5 Casein-Based Packaging Films and Biomaterials

Transparency, biodegradability and good technical properties (barrier properties for a polar gas such as O2 and CO2) make casein films innovative materials for packaging. Nevertheless, casein-based materials have two major drawbacks in common with other protein-based biomaterials: limited mechanical properties and water sensitivity.

To overcome weakness and brittleness, plasticizers are added to enhance workability, elasticity and flexibility. Plasticizers reduce intermolecular hydrogen bonding while increasing intermolecular spacing. By decreasing intermolecular forces, plasticizers cause an increase in material flexibility but also a decrease in barrier properties due to increasing free volume. To summarise, an initially hard and brittle material becomes soft and flexible when plasticized enough.

For casein-based materials, the most common plasticizers are polyols, sugars or starches owing to their miscibility with the protein and their ability to enhance elasticity and flexibility. The incorporation of polyol-type plasticizers (glycerol and sorbitol) in protein-based films causes a decrease in tensile strength and an increase in ultimate elongation. The major difference between protein- (casein or whey protein) based films and synthetic films (LDPE, HDPE and PVC wrap film) concerns elongation at break. The maximum elongation is rather low in plasticized protein - based samples (less than 85%) compared with synthetic films (from 150% for plasticized PVC to 500% for LDPE), which may limit application domains for protein based films. Compared with starch-based materials, the most commonly used substitute for synthetic polymers, milk protein-based films exhibit better mechanical properties. The second drawback of caseinate films deals with their water sensitivity and water vapour permeability. Mixing the protein with oils, waxes or acetylated monoglycerides is an easy route to drastically reduce water sensitivity. Casein can also be hydrophobised by attachment of hydrophobic ligands, generally alkyl groups incorporated by esterification or by using monofunctional aldehydes. Water sensitivity of caseinate films can also be reduced by crosslinking with (i) calcium ions (ii) transglutaminase (iii) γ -irradiation and (iv) formaldehyde or dialdehydes. As for other protein materials, casein based films are hydrophilic, making them excellent gas barriers to non-polar substances such as oxygen, carbon dioxide and aromas. Casein based films and biomaterials obtained from caseinates can find many applications in packaging in edible films and coatings for fruits and vegetables or in mulching films.

11.6 Miscellaneous Technical Applications of Casein

Casein presents good metal and ion binding properties, making it suitable for absorbing and recovering chromate in wastes from manufacturing processes such as electroplating and water purification. Amongst the large number of other technical (non-food) applications where casein has been used (or at least claimed to be used) are in cleaners and dish washing liquids, hair setting products and cosmetics and cheese marks. In cosmetics, caseinate is used as surface active agent in soaps and various cosmetics such as cold wave lotions, hair sprays and hand cream. Casein hydrolysates could also be active substances for skin hydration. However, little information is available about these applications and their markets, which seem rather limited.

In building and civil engineering, uses for casein are claimed in the preparation of bitumen emulsions, in light weight concrete, in gypsum wallboards, in the preservation and restoration of old stone buildings and as a foaming agent for de-icing equipment, roads and run ways. In printing, casein is claimed to be used as a film-forming transfer regulator in a thermal printing adhesive ink and as a binder for a printable coating on a foamed polystyrene sheet. Casein has been used in photo etching for the production of shadow masks for colour television sets, computer circuitry and electronic ignition components for motor vehicles. Cross-linked casein has been used for water purification and for recover of chromium from waste electroplating liquors. In agriculture and horticulture, casein has also been used in insecticide sprays (as a spreader), in fungicides (as an adhesive), as a fertilizer and in coated seeds (as an adhesive).

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.

Last modified: Tuesday, 4 September 2012, 5:49 AM