Acid Casein

There are two types of caseins available in the market: edible casein and industrial casein. The above two types of caseins can be either acid casein or rennet casein.Edible casein is a long established dairy byproduct finding its use as an ingredient in many dairy and food products. The general development in technologies and the new uses in foods have increased the production and demand of this byproduct. Its manufacture differs from industrial casein (i.e. non edible) in that it is produced under sanitary conditions. Further, during its manufacture, food grade chemicals are to be used and it is sufficiently heat treated to make its safe for human consumption.

Appropriate national and international standards for this byproduct calls for rigorous control during its manufacture. The intensive investigation in manufacturing technologies over the years and the introduction of efficient plant designs have immensely improved the technology of edible casein.

i. Manufacturing Process

i.Raw material & its influence: In order to produce high quality casein, the raw material, skimmed milk, must be of good quality. A skim milk with a developed acidity of not greater than 0.15% lactic acid must be used. The fat content should not exceed 0.02-0.03%. If bacteria had time to act on the protein in the milk as a result of a change in acidity, this will affect the colour and consistency of the casein, which will acquire a grayish colour and smoother consistency.

Excessive heating of the milk before precipitation will not only cause assorted interactions among the lactose, casein and whey protein constituents but also give the casein a yellow or at worst a brownish colour.Some authorities hold that heat treatment of milk for casein manufacture causes slight insolubility.

In order to produce casein of a good bacteriological quality, without high heat treatment of the skim milk, the pasteurization plant may also contain a Microfiltration (MF) plant. To satisfy the high demands on the quality of casein intended for use in the food industry, not only must the production line be carefully planned right from the reception of milk, but the treatment and handling of the raw material prior to this stage must also be carefully controlled.

ii) Precipitation: Casein exists in milk as calcium caseinate-calcium phosphate complex. When an acid is added to milk, this complex is dissociated. As the pH of milk is lowered, the calcium is displaced from the casein molecules by hydronium ions, H3O+ and the calcium phosphate associated with the complex is converted into soluble Ca+2 ions and H3PO4 ions. At about 5.3 pH, the casein begins to precipitate out of solution and at the isoelectric point of casein (about pH 4.6), maximum precipitation occurs. At this pH all the calcium is solubilized. Not only is the calcium from the caseinate molecule removed but also the calcium phosphate is liberated in soluble form. This makes it possible to wash these soluble salts from the curd and achieve a low ash content in the final product.It might be expected that all the casein in a sample of milk would be precipitated simply by adding sufficient acid to bring the pH value to approximately 4.6.

However, the reaction of acid with caseinate complex is not instantaneous and the pH will tend to rise slowly with time. Therefore, ample time should be allowed for achieving equilibrium conditions. When casein is precipitated from skim milk by the direct addition of acid, the temperature and pH of precipitation and mechanical handling of the curd during its formation is very important in determining the subsequent properties of the curd.Any of the acid precipitation processes (hydrochloric acid casein, sulphuric acid casein or lactic casein process) can be used to produce edible quality casein. The choice of the method for reducing the pH of skim milk to precipitate casein is largely governed by economics. In terms of cost of acid, a lactic fermentation process is attractive especially when, with large scale processing by modern methods, the tendency for higher capital and operational costs are minimized.For lactic acid casein, the pasteurized skimmed milk is cooled to 22-26oC. A mesophilic, non-gas producing starter is then added at the rate of 0.5 to 1.0%.

The skimmed milk is then incubated at this temperature for 14-16 hours during which the pH reduces to 4.6 giving a coagulum. If the acidulation process is too rapid, it can result in problems such as uneven quality and reduced casein yield. The slow coagulating cultures exhibit less proteolysis and increased protein yield. Large tanks are usually used. This means that it can take such a long time to empty the tank that the degree of acidity may vary.

The precise rate of acid production by the starter is not important as coagulation usually takes place several hours before processing begins and at about 4.5 pH, the culture is in stationary phase of growth. The coagulum is cooked to 50-55oC to create a curd firm enough for subsequent processing. The acid and heat help in syneresis of whey.The use of mineral acids, on the other hand, has the advantage of a completely continuous operation with no holding time for coagulation. Hydrochloric acid has been found with experience, to be a superior coagulating agent. When sulphuric acid or hydrochloric acid is used to precipitate curd, it should be diluted before being added to the skim milk; otherwise local action of the acid may injure the curd, even though the agitation is rapid. Within reasonable limits, the more dilute the acid; the better will be the quality of casein produced. In practice, hydrochloric acid is used in dilutions ranging from 1 part in 3 to 1 part in 9 parts water and sulphuric acid is diluted 1 part in 20 parts water.

iii) Temperature of Precipitation: Curd precipitated at temperature below 35oC is very soft and fine, and consequently, is slow to settle and difficult to wash without losses. Precipitated at temperatures between 35oC and 38oC, the curd is coarse provided stirring is not too fast. Stirring is necessary to distribute the acid uniformly, but rapid stirring at temperatures below 38oC produces a curd so fine that it settles very slowly during drainage and washing and may be lost to some extent in the whey and washings. Much more rapid equilibrium, more complete precipitation and, therefore, better yields are obtained by rapid and complete mixing before precipitation.

The curd can be made firm in either of two ways, by heating to a temperature above 38oC, or lowering further the pH to 4.1. Curd precipitated at above 43oC has a texture resembling chewing gum, being stringy, lumpy and coarse,containing practically no fine particles, and separating cleanly from the whey.However, this type of curd is difficult to wash.

iv) Draining of Whey: After precipitation has been completed and the curd has settled, the whey should be removed from contact with curd as soon as possible.The longer the curd stands in contact with whey, the more difficult it is to wash out acids, slats, whey protein and lactose, as the freshly broken curd tends to anneal itself, thereby enclosing these constituents within the protein film.

v) Washing: As soon as the whey is removed from the curd, wash water should be added equal in quantity to whey that has been drained off. The curd should be well stirred in the wash water, either by rakes or by mechanical agitators,but care should be taken not to break the curd into fine particles. Firm and friable curd particles are required to avoid creation of excessive fines. Rubbery and plastic curds cannot be washed efficiently. Three separate washings of casein curd are required with contact times of 15 to 20 min. each.Washing is required to improve the quality of casein. Large amounts of lactose, minerals and acids are trapped within the curd. It is necessary to allow sufficient holding time during each washing stage to permit diffusion of these whey components from the curd into the wash water. The diffusion rate depends on the size and permeability of curd particles, and the purity, amount and rate of movement of wash water. Smaller size and better permeability of curd particles are important for efficient washing.

pH of wash water: The pH of wash water should be about 4.6 for first two washings to avoid the formation of a gelatinous layer over the curd particles in excessively acid water and softening and redispersion of the curd in alkaline water. Formation of gelatinous layer over the curd particles inhibits drainage of salts and lactose from the particles. For pH adjustment, sulphuric acid is preferred, as casein is much less soluble in this acid, than in hydrochloric acid.The third wash should be given with neutral water.

Temperature of wash water: The temperature of the first wash should be the same as the precipitation temperature to give good curd shrinkage. With lactic acid casein, higher temperature (70oC or more) is necessary at some stage to reduce the bacteria, which multiply during incubation of milk with starter. The temperature of the last wash water is to be adjusted to 32 to 40oC for better expulsion of water during subsequent pressing.

vi) Pressing: Pressing is required for dewatering of washed casein curd. The curd after pressing should have moisture content of 55-60 percent. The pressing of curd should not be for less than 12 to 15 hrs with 3-4 kg/sq. cm pressures.

vii) Milling and Drying: The pressed curd is milled to produce particles of uniform size and surface for drying. Otherwise, uneven drying occurs. Large particles or lumps my dry on the outside forming a hard, impervious outer surface that prevents the diffusion of the remaining moisture from the interior of the particle.The ground curd is evenly spread on trays. Not more than 0.9 to 1.1 kg curd should be placed on each standard tray of 74 x 75 cm.Proper control of humidity of air coming in contact with the curd is essential for efficient drying. A temperature of 52 to 57oC for the air entering a tunnel dryer is suitable for any type of curd. During the early stages of drying it is desirable to circulate a portion of air so that the surface of the particles will not become fully dried while the interior is still moist. Drying once started should not be interrupted, but should continue until the percentage of moisture is about 8%.

viii) Grinding, Sifting and Bagging: Casein is generally ground by the use of roller mills. Prior to grinding the curd must be cooled and hardened. Cooling is achieved by storage in an agitated holding bin and/or by pneumatic conveyance or fluidized cooling. Attempts to mill curd, which is either too hot or too moist,will cause clogging of the rollers.

Following milling, the casein is passed through a sifter to yield the desired particle size, e.g. 30 mesh casein.The casein is then packed in sacks or bags of 100 to 200 lb capacity. Burlap sacks lined with closely woven cloth or with heavy papers of by three-ply paper bags may also be used.