Module 2. Skim milk and its by-products

Lesson 18


18.1 Introduction

Early developments in the manufacture of co-precipitates occurred mainly in the USA and USSR. Co-precipitate manufacture was first reported in the USSR in early 1950s, the commercial potential of co-precipitates was developed in the 1960s and 1970s by research workers at the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO). This, and subsequent work, has been extensively reviewed (Muller, 1971, 1982, South-ward and Goldman, 1975, Southward, 1985).

18.2 Manufacturing Process

The type of plant used for casein manufacture can be adopted for the manufacture of co-precipitates, by which it is possible to increase the protein recovery from 80-96% with a 28% reduction in B.O.D. of the whey. Continuous_manufaturing_plant_for_co-precipitates.swf shows how the acid casein plant is modified for the manufacture of co-precipitates. The modification made it possible to use the plant for either casein or co-precipitate manufacture.

The manufacture of a range of co-precipitates with different calcium contents from cow milk as described by Muller et al. (1967) has been outlined in (Fig. 18.2).

18.2.1 Heating of buffalo skim milk

Buffalo skim milk is heated in double jacketed stainless steel vat to 90.6°C by indirect heating or by direct steam injection.

18.2.2 Holding and precipitation

The skim milk is held at varied holding times, depending on the type of co-precipitate desired, at 90.6°C by maintaining the temperature of hot water in the outer jacket at 93-94°C. The precipitation of co-precipitates is done at different pH ranges as follows: Low calcium co-precipitates

For low-calcium co-precipitate production, 0.03% of CaCl2 is added to skim milk; the mix is then preheated to a temperature of 65-75°C by passage through a plate heat-exchanger. In the next stage, a final temperature of 90°C is obtained by steam injection into the mix and this temperature is maintained for 15-20 minutes in a holding vat. Two-stage heat treatment is employed for the following reasons: (a) the Plate-heat exchanger is to be used re-generatively in commercial operations and hence recovery of heat from the whey is possible. This process of ‘pre-heating’ the milk is less expensive than entirely heating is done with steam; and (b) If the plate heat exchanger is the only means of used for heating milk, there would be a tendency for ‘burn-on’ to occur in the plates from slight precipitation of the milk proteins, especially at temperatures above about 68°C and at a pH of 6.3-6.5. After heat treatment, the hot skim milk is cooled and then the pH value is adjusted (pH 4.6) by injecting dilute hydrochloric acid (1:6) through a spray counter-currently to the direction of milk flow to provide full mixing. The mixture is transformed into curd in 20-25 seconds in a holding tube. The precipitate is then separated from the whey on an inclined 90-mesh screen, washed with water, pressed and dried. The calcium concentration of low calcium co-precipitates produced in this way is 0.1-0.5%.

In the production of low calcium co-precipitates with a calcium content of 0.5%, coagulation temperature 65°C produced slightly sticky and soft curd. Losses of fines in the whey are fairly high (about 1ml/100ml, representing some 2% of co-precipitate), but are much lower in the first wash water (approx. 0.1 ml/100 ml) which is usually heated to a temperature near 40°C and maintained at a pH of about 4.6. Total fines losses amounts to approx. 2.5% of the low co-precipitate. Medium calcium co-precipitates

The same production process can result medium calcium co-precipitates (1.0-1.5% of calcium in the product) if 0.06% CaCl2 is injected in the skim milk, and the mix is retained at a temperature of 90°C for 10-12 min and then precipitated at a pH value of 5.3. In the production of co-precipitates with a calcium content of 1.0-1.5%, coagulation temperature of 65°C produced the firmest and most manageable curd. The most robust curd is precipitated at a pH of 5.2-5.3. When losses of fine curd particles are approximately 0.3 ml/100 ml in both the whey and wash water, and accounted for about 2.1% of the weight of co-precipitate. High calcium co-precipitates

For high-calcium co-precipitates production (2.5-3.0% of calcium in the final product), skim milk is heated at 90°C and retained at this temperature for 1-2 min, then 0.2% CaCl2 is added. It is not necessary to add acid to adjust the pH value of precipitation. The firmest high calcium co-precipitate curds and the clearest whey’s are produced from milk heated at pH 6.4 and 85°C for 6 min, when 0.2% CaCl2 is injected into the hot milk and the temperature of the mixture is 77°C or higher. Under these conditions, the fines in the whey are reduced to a level of 0.1 ml/100 ml or approximate 0.2% by weight of co-precipitate. When, however, the coagulation temperatures fell below 73°C, the whey become cloudy and the fine losses increased to 0.4 ml/100 ml or approximate 0.8% by weight of the co-precipitate.

18.2.3 Drainage of whey

After the precipitation of the co-precipitate curd, it is allowed to settle and the whey removed after filtering through the stainless steel strainer covered with muslin cloth. Filtration is done to avoid loss of fine particles.

18.2.4 Washing

The co-precipitate curd is given 2, 3 or 4 washings with the acidulated water with 15 min holding time for each washing. In low calcium co-precipitate, the curd is washed in water (up to four batch washes, each of 400-500 litre) at a pH (usually pH 4.6) and at temperatures (usually 45 to 55°C).

In medium calcium co-precipitate, the curd is washed, dewatered and dried in a manner similar to that described for low co-precipitate. Temperature of the first wash water is generally varied between 45°C and 55°C and its pH is usually adjusted with dilute sulphuric acid to between 4.5 and 5.5 in order to assist in maintaining the firm structure of the curd. During this type of washing, the properties of fine curd loses is reduced in each succeeding wash.

In high calcium co-precipitate, the temperature of the wash water is very important. When the wash water temperature is reduced to less than 60°C, the curd becomes very soft and mushy with a consistency similar to that of toothpaste. Wash water temperatures are therefore, adjusted to 60-75°C in the manufacturing of high calcium co-precipitates. Under these conditions, the losses of fine co-precipitate particles in the wash water, therefore any fines recovery amounted to approx. 0.5ml/100ml or about 2.5% by weight of the co-precipitate. No measurable fines losses are observed in the wash water effluent which is discharged for the centrifuge used to dewater the co-precipitate curd before it is dried. Combined fines losses from whey and wash water consequently represents about 2.7% by weight of the high calcium co-precipitate. Generally the wash water is acidified to pH 6.0 which is near the pH of the co-precipitate whey (pH 5.8-5.9).

18.2.5 Pressing

Pressing of curd is done with same principle as in casein. The curd can be subsequently de-watered using a decanter.

18.2.6 Drying

The co-precipitates is finally dried in a granular form, to a moisture content near 10% in a pneumatic conveying ring drier. A typical drier (F.W. Berk & Co. Ltd., London, England) has a water evaporation capacity of approx. 3.5 kg/h using inlet and outlet temperatures of 190°C and 90°C, respectively. After drying, the co-precipitates goes for tempering, milling, blending, bagging and storage in same manner as described for casein.

18.3 Manufacturing Process For Soluble Co-Precipitates

The solubility of co-precipitates depends on the calcium concentration in the product. Up to 90% of low calcium co-precipitates are soluble in water at a pH of 7.0, while high calcium co-precipitate is practically insoluble in water under the same conditions. In order to adequately dissolve medium-calcium co-precipitate at neutral pH, Smith and Snow (1968) added sodium tripolyphosphate, a calcium sequestering agent, to the mixture of co-precipitate and alkali. High calcium co-precipitate was dissolved at neutral pH by addition of sodium tripolyphosphate alone (6% on co-precipitate). The reason why complex phosphates such as sodium tripolyphosphate are so effective in dissolving medium and high calcium co-precipitate in water was ascribed to their calcium sequestering ability. When 2% sodium tripolyphosphate is included, high calcium co-precipitate solubility is approximately 10%, but a 6% sodium tripolyphosphate addition increases the co-precipitate solubility up to 80% at a pH of 7.0.

Smith and Snow (1968) found that solution of low, medium and high-calcium co-precipitate (5% w/w) were stable to a heat treatment of 120°C for 20 min. The heat stability of similar co-precipitate solutions containing lactose, however, was generally lower than that of the co-precipitate solutions alone. Neutralization with alkali of low and medium calcium co-precipitates yielded products which were substantially water-soluble (e.g. C.S.I.R.O., 1968).

For spray drying, the wet curd of low, medium and high calcium co-precipitates are dispersed in water containing 2, 4 and 6 percent sodium tripolyphosphate (STPP) respectively, calculated on the dry weight basis of the curd. The mixture is heated in a water bath to about 85°C and continuous stirring is done for about 30 min. This heat treatment gives a pasteurization treatment to the co-precipitates. Slowly and slowly 2.5 N sodium hydroxide solution is added to the curd with constant stirring in such a way that pH did not rise to more than 7.0. The mixture is then passed through the mini-pulveriser in place of colloid mill. The redispersed material is then filtered through muslin cloth. The pH of the dispersion is adjusted to 7.0 at this stage. Then the material obtained is spray dried. The manufacturing processing for soluble co-precipitates is given in (Fig. 18.3).

Selected references

Muller, L.L., Hays, J.F. and Snow, N. 1967. Studies on co-precipitates of milk proteins. Part I. Manufacture with varying calcium contents. Aust. J. Dairy Technol. 22: 12.
Muller, L.L. 1971. Manufacture and uses of casein and co-precipitates. Dairy Sci. Abst., 33: 659-674.
Smith, D.R. and Snow, N.S. 1968. Aust. J. Dairy Technol., 23: 8.

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