Acid casein

Although acid caseins are employed for a number of purposes, rennet caseins in which the protein remains associated with calcium and phosphate are preferred for plastics applications. Rennet is the dried extract of rennin, obtained from the inner lining of the fourth stomach of calves, and is a very powerful coagulant. As little as 0.2 parts per million are said to be sufficient to coagulate slightly acidic milk. Its coagulating power is destroyed at 100°C. 

Sodium caseinate is produced by reacting the acid casein with sodium hydroxide. 

Milk from cows contains 3.2% protein, about 80% of which is casein. Casein is isolated by a precipitation process from milk, involving heating, rinsing to remove whey, and drying to a powder. The yield is about 3 kg/ 100 kg skim milk. Rennet casein is obtained when the casein is precipitated by chymosin enzyme, also known as rennet, and acid casein is produced when precipitation is accomplished by acidification. Acid casein is usually found in the form of sodium caseinate or calcium caseinate, which are water-soluble salts. Caseinates are made by reacting NaOH or CaOH with a slurry of casein curd or powder and then spray drying (Southward, 2010). 

Following research by Tossavainen et al. (1986), extrusion is often used to produce insoluble acid casein from skim milk powder and to convert acid casein into sodium caseinate. The procedure is faster than batch mixing (Akdogan, 1999). 

Tossavainen, O., Hakulin, S., Kervmen, R., Myllymaki, O., and Linko, P. (1986). Neutralisation of acid casein in a twin-screw cooking extruder. Lebensm. Wiss. Technol. 19,443 47. 

Dickinson, E., Matia-Merino, L. (2002). Effect of sugars on the rheological properties of acid caseinate-stabilized emulsion gels. Food Hydrocolloids, 16, 321-331. 

Initially, it was believed that milk contained only one type of protein but about 100 years ago it was shown that the proteins in milk could be fractionated into two well-defined groups. On acidification to pH 4.6 (the isoelectric pH) at around 30°C, about 80% of the total protein in bovine milk precipitates out of solution this fraction is now called casein. The protein which remains soluble under these conditions is referred to as whey or serum protein or non-casein nitrogen. The pioneering work in this area was done by the German scientist, Hammarsten, and consequently isoelectric (acid) casein is sometimes referred to as casein nach Hammarsten. 

Casein may be coagulated and recovered as rennet casein by treatment of milk with selected proteinases (rennets). However, one of the caseins, K-casein, is hydrolysed during renneting and therefore the properties of rennet casein differ fundamentally from those of acid casein. Rennet casein, which contains the colloidal calcium phosphate of milk, is insoluble in water at pH 7 but can be dissolved by adding calcium sequestering agents, usually citrates or polyphosphates. It has desirable functional properties for certain food applications, e.g. in the production of cheese analogues. 

Acid Casein B Casein A Casein B Casein A2 Casein A2 Casein A2 Casein A globulin A albumin B... 

Commercial casein is usually manufactured from skim milk by precipitating the casein through acidification or rennet coagulation. Casein exists in milk as a calcium caseinate-calcium phosphate complex. When acid is added, the complex is dissociated, and at pH 4.6, the isoelectric point of casein, maximum precipitation occurs. Relatively little commercial casein is produced in the United States, but imports amounted to well over 150 million lb in 1981 (USDA 1981C). Casein is widely used in food products as a protein supplement. Industrial uses include paper coatings, glues, plastics and artificial fibers. Casein is typed according to the process used to precipitate it from milk, such as hydrochloric acid casein, sulfuric acid casein, lactic acid casein, coprecipitated casein, rennet casein, and low-viscosity casein. Differences... 

Australian standards have been established for both acid and rennet caseins. The standards for acid casein are much the same as those for U.S. casein. Rennet casein usually has between 7.0 and 8.3% ash compared with 2.2% for acid casein. The fact that rennet casein is essentially a calcium caseinate accounts for this comparatively large ash value. 

Electrophoretic Methods. Several electrophoretic procedures have been developed to fractionate or purify the various caseins (McKenzie 1971C Thompson 1971 Whitney 1977). Wake and Baldwin (1961) fractionated whole casein by zone electrophoresis on cellulose powder in 7 M urea and 0.02 ionic strength sodium phosphate buffer at pH 7 and 5°C. Payens and co-workers employed several somewhat different electrophoretic conditions for the fractionation and purification of the caseins on cellulose columns (Payens 1961 Schmidt and Payens 1963 Schmidt 1967). Three fractions, as-, k-, and /3-caseins, were separated at pH 7.5 and 30°C with 4.6 M urea-barbiturate buffer. The purification of asi-casein and the separation of the genetic variants of K-casein were accomplished by altering the electrophoretic conditions. Manson (1965) fractionated acid casein on a starch gel column stabilized by a density gradient at 25 °C. 

Figure 3.18. Elution pattern for a 250-mg sample of reduced and alkylated acid casein from DEAE-cellulose with NaCl gradient in buffer containing 6.6 M urea. Fraction (1), minor /3-caseins and para-K-casein-like material (2) K-casein and some /3-casein-1 P (f 29-209) (3) major /3-casein (4) as-caseins. (From Rose et al. 1969 Yaguchi and Rose 1971. Reprinted with permission of the American Dairy Science Association.)...

Whey proteins are slightly superior to casein because of the limiting quantity of the total sulfur-containing amino acids (methionine plus cystine) in casein. However, because whey proteins have a relative surplus of these amino acids, casein and whey proteins, as found in milk,... 

Further information on such a novel process, as well as the preparation of acid caseinates, electrochemical coagulation of milk proteins or electroreduction of disulfide bonds of milk proteins, using conventional or bipolar ED stacks, is given by Bazinet (2004). It is however worthy underlining that all these processes do not seem to have been used on an industrial scale yet. 

Savello, P. A., Ernstrom, C.A., Kalab, M. 1989. Microstructure and meltability of model process cheese made with rennet and acid casein. J. Dairy Sci. 72, 1-11. 

Assay Acid Casein Not less than 90.0% protein Rennet Casein Not less than 86.0% protein Caseinate Salts Not less than 84.0% protein, calculated on the dried basis. 

Free Acid (Casein only) Passes test. 

Sample Solution Transfer approximately 1 g of sample, accurately weighed, into a 150-mL beaker. If the sample is Acid Casein, add 0.10 g of sodium hydrogen carbonate. If the sample is Rennet Casein, add 0.10 g of sodium tripolyphosphate. Add 25 mL of water, and dissolve the sample by gently swirling while warming to 60° to 70° on a hot plate. Cool the solution to ambient temperature, and add 15 mL of water, 8 mL of 0.1 IV hydrochloric acid, and 1 mL of a 10% solution of acetic acid. Mix well by swirling, and after 5 min, add 1 mL of 1 M sodium acetate, and mix well. 

Casein is the material that curdles when acid is added to skim milk. Most of us think of it as cottage cheese. It normally comprises about 3% of the weight of whole milk. Since the type of acid used for curdling has a noticeable effect on the quality and purity of casein, it is normally classified as lactic, hydrochloric, or sulfuric acid material. Lactic acid casein is the product of natural self-souring, although this is normally promoted by inoculating skim milk with lactobacillus. A small portion of casein is produced by the rennet process. This material is... 

However, the procedures used in the dairy industry, rennet and acid casein, have the disadvantages to produce large volume of chemical effluents due to the addition of bases and acids during treatments, and to generate inherent risk linked to handling, stocking, and transportation of concentrated bases and acids. In acid casein production, milk pH is decreased to the isoelectric point of the casein by addition of strong acid (hydrochloric, sulfuric, nitric, lactic, etc.). H" " concentration of milk is increased... 

Bolzer R, Installation and process for the preparation of acid caseinates. 1985. United States Patent no 4 559 119. 

---