Casein acid coagulation

At the pH = Jt there is a balance of charge and there is no migration in an electric field. This is referred to as the isoelectric point and is determined by the relative dissociation constants of the acidic and basic side groups and does not necessarily correspond to neutrality on the pH scale. The isoelectric point for casein is about pH = 4.6 and at this point colloidal stability is at a minimum. This fact is utilised in the acid coagulation techniques for separating casein from skimmed milk. 

Destruction of the casein micelles in the milk with subsequent precipitation of the casein can be accomplished in a number of ways. The action of heat or the action of alcohols, acids, salts and the enzyme rennet all bring about precipitation. In commercial practise the two techniques used employ either acid coagulation or rennet coagulation mechanisms. 

Minerals found in milk which are insoluble remain in water in the curd and are more concentrated in the cheese than in milk. About two-thirds of the calcium and one-half of the phosphorus of milk remains in cheese. A major portion of the milk calcium is retained in the curd of cheese made with coagulating enzymes. Acid coagulation alone results in the loss of portions of both calcium and phosphorus salts in the acid whey, since these minerals are more soluble in the acidic medium. Most milk fat and fat-soluble vitamins are retained in the curd, but a considerable amount of water-soluble vitamins is lost during cheese manufacture. Retention of part of some B-complex vitamins in curd is due to their extended association with casein in the original 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. 

Bovine casein is listed by the International Union of Immunological Societies as a single allergen, Bos d 8. However, it contains four main protein components, a-sl-, a-s2-, 3-, and K-Casein, in approximate proportions of 40% 10% 40% 10%, respectively (Bernard et al. 1998). The fractions obtained after acid coagulation of individual caseins are cross-linked to aggregates called nanoclusters, which combine into micelles. Their central part is hydrophobic and peripheral hydrophilic parts contain sites of phosphorylation. 

Hinrichs, R., Bulca, S., and Kulozik, U. (2007). Water-mobility during lenneting and acid coagulation of casein solutions a differentiated low-resolution nuclear magnetic resonance (NMR) analysis. Int J. Dairy Technol 60, 37-43. 

The expression transition refers to a change in physical state and, in a food, the transition of concern is often either from liquid to solid, solid to liquid, or solid to solid. It is caused primarily by a change in temperature (heating and/or cooling) or pressure (Roos, 1998). However, auxiliary conditions, such as pH and presence of divalent ions, as well as enzymatic action aid liquid to solid transitions. For example, gels can be created from Casein either by enzymatic action followed by precipitation with Ca + or by acid coagulation. 

Bringe NA and Kinsella JE. Eorces involved in the enzymatic and acidic coagulation of casein micelles. In Hudson BJF, Ed. Developments in Food Proteins, Vol. 5, Elsevier Applied Science Pubhshers, London, 1987, pp. 159-194. 

When an acid-coagulated milk gel is broken, the pieces synerese, expressing whey. Acid-coagulated casein is the starting material for a family of cheeses that are usually consumed fresh, i.e., are not matured (ripened) major examples are Cottage and cream cheeses, Quarg, and fromage frais. 

The conversion of milk to cheese curd essentially involves coagulating the casein, either isoelectrically or enzymatically if present, the milk fat is occluded in the curd. The mechanisms of rennet and acid coagulation of casein and the subsequent manipulation of the coagula to produce cheese curd are described in the following sections. 

Bringe, N. A., and Kinsella, J. E. (1990). Acidic coagulation of casein micelles Mechanisms inferred from spectrophotometric studies. J. Dairy Res. SI, 365-375. 

Casein is present in several animal and vegetable sources. Commercially, however, casein is primarily obtained from milk that contains about 3% of this protein. The polymer is isolated either by acid coagulation or with the help of enzymes obtained from animal stomachs. It is very heterogeneous. The molecular weight of a large portion of bovine casein is between 75,000 and 100,000. It consists of two components, a and p. Casein belongs to groups of proteins that are identified as phosphoproteins because the hydroxyl residues of the hydroxy amino acids are esterified with phosphoric acid. 

Casein fibres can be made by dissolving casein in an alkaline solntion and extrnding throngh fine nozzles into an acid-coagulating bath, followed by treatment with formaldehyde to reduce water penetration. 

Acid casein finds use in a variety of applications, including adhesives (mainly for paper and wood) and paper coatings. In these latter uses, the casein is applied as a dispersion in aqueous alkali. Casein fibre may be produced by forcing an alkaline dispersion through a spinneret into an acid coagulating bath the fibre (commonly termed casein woof) was produced commercially during World War II in Italy and the U.S.A. but it is now of little importance. Acid casein is used rather than rennet casein in these various applications since it is more readily dispersed in aqueous alkali. 

Fig. 10.15. Heating of milk. 1-3 Pasteurization 1 high temperature treatment, 2 short time and 3 long time heat treatment 4 and 5 UHT treatment 4 indirect and 5 direct 6 sterilization, a Killing pathogenic microorganisms Tubercle bacilli as labelling organism), b c inactivation of alkaline/acid phosphatase, d, d2, d denat-uration (5, 40, 100%) of whey proteins, e casein heat coagulation,/ start of milk browning...

Bottazzi, V Other fermented dairy products. In Biotechnology (Eds. Rehm, H.-J., Reed, G.), Vol. 5, p. 315, Verlag Chemie Weinheim. 1983 Bringe, N.A., Kinsella, J.E. The effects of pH, calcium chloride and temperature on the rate of acid coagulation of casein micelles. Am. Dairy Sci., Annual Meeting, 23-26 June, Davis, CA. (1986)... 

To produce caseinates, enough alkali, commonly called calcium hydroxide or sodium hydroxide, is added to acid coagulated casein to reach a pH of 6.7. The resulting suspension is pasteurized and spray dried. 

Casein occurs in several animal and vegetable materials but the only source of commercial importance is cow s milk, in which the casein content is about 3%. In commercial practice, casein is isolated from skimmed milk by either acid coagulation or by rennet coagulation. In acid coagulation, dilute sulphuric acid is added to the milk at about 35°C, with stirring. Coagulation... 

Definition Obtained from fresh and pasteurized skimmed milk by acid coagulation of casein... 

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