BY-PRODUCTS TECHNOLOGY

Module 2. Skim milk and its by-products

Lesson 19
PHYSICO-CHEMICAL PROPERTIES OF CO-PRECIPITATES

19.1 Introduction

The choice of manufacture of different calcium co-precipitate is influenced not only by consideration of yield and ease of manufacture, but also by the properties and intended use of the product. Co-precipitates with different calcium content have different physico-chemical properties.

19.2 Proximate Composition of Granular Co-precipitates

The proximate composition of granular (insoluble) co-precipitates, prepared from curd, which is washed twice in water, is shown in Table 19.1. The calcium content of a co-precipitate is determined mainly by its pH of precipitation (Muller et al. 1967). The composition of co-precipitates is also affected by the extent of washing of the curd.

Table 19.1 Proximate compositional analysis of granular co-precipitates

The fat content of the co-precipitates increases from 0.6% to 0.9% as the pH of precipitation is reduced from 5.9 to 4.9. The high ash content of high calcium co-precipitate leads to a consequential reduction of 3-5% in the protein content of the product relative to lactic and acid caseins and low calcium co-precipitate.

19.3 Proximate Composition of Soluble Co-precipitates

Proximate compositional analysis and physical properties of soluble and dispersible co-precipitates are presented in Table 19.2.

Table 19.2 Proximate compositional analysis and physical properties of spray-dried soluble co-precipitates

Soluble high calcium co-precipitate has very high ash content (13.5%) due in part to the presence of sodium tripolyphosphate and contains almost 2% sodium. Soluble low calcium co-precipitate has a composition similar to that of sodium caseinate. In order to render medium calcium co-precipitate substantially (> 90%) soluble at pH 7.5, it is necessary to use both, sodium hydroxide and sodium tripolyphosphate (or some other complex phosphate).

19.4 Physical Properties of Co-precipitates

The general physical properties of co-precipitates may be regarded as the physical properties of these products when examined alone or in water (dispersed or dissolved). These include pH solubility (in water, alkali or acid) and rate of solution, density, particle size, viscosity of solutions and colour of powder and solutions.

19.4.1 Solubility

Solubility of low-calcium co-precipitates in alkali over a range of pH values was similar to casein. Solubility is a characteristic of casein products such as co-precipitates which varies markedly under the influence of pH, agitation, mixing temperature, mixing time, the particle size and concentration of the casein product and the presence of other species such as dissolved salts. The solubility of co-precipitates has generally been considered in the pH range from 6 to 10. However, it has been demonstrated that co-precipitates can also be dissolved in several different acids such as phosphoric, hydrochloric and citric acid at pH 2-3.

19.4.2 pH

The pH of a water dispersion of an insoluble co-precipitates is determined particularly by the pH of precipitation, which also affects its calcium content. It is also affected to some extent by the number of washes, and by the alkalinity of the wash water used, particularly in the manufacture of low calcium co-precipitate. Where co-precipitates are rendered substantially or partly soluble or dispersible in water by the addition of alkali and/or complex phosphates, the pH of these solutions is usually near 7.

19.4.3 Bulk density

The packing density (bulk density) of casein products such as co-precipitates can vary markedly depending on the method of manufacture. Granular, insoluble co-precipitates have approximately density 0.6 g/ml, depending on particle size, whereas the density of spray dried soluble co-precipitates can be much lower. Buchanan et al. (1965) found the density of a spray dried high-calcium co-precipitates containing 2% sodium tripolyphosphate to be 0.34 g/ml compared with 0.25 g/ml for sodium caseinate.

19.4.4 Viscosity

The viscosity of solutions of soluble co-precipitate in water at different concentrations, temperature and shear rates was measured by Hays, Muller and Fraser (1969). They found that low calcium co-precipitate had a viscosity similar to that of acid casein. The viscosity of medium and high-calcium co-precipitate solutions was found to be relatively high when the pH was above 7. The viscosity of solutions of low-calcium co-precipitate was affected by the heat treatment of the milk from which it was made.

19.4.5 Colour

Hays, Dunkerely & Muller (1969) found that pH, calcium and polyphosphate content of co-precipitate solutions were the factors which particularly affected its whiteness.

19.5 Flavour

The development of “gluey” off-flavours in low calcium co-precipitates was inhibited by the addition of 0.01-0.05% sodium metabisulphite. The flavour of co-precipitates tends to follow a similar pattern to that of the caseins, high calcium co-precipitates being more stable than low calcium (acid) co-precipitates and fresh curd soluble co-precipitates being better than those reconstituted from dry granular insoluble co-precipitates. The co-precipitates also exhibit ‘cooked flavour’ due to high heat treatment given.

The successful and potential uses of co-precipitates in food products will depend upon the satisfactory functional properties of the proteins that are added in the food system so that in body, texture, flavour and nutritive characteristics etc. of the finished product are not adversely affected and remain acceptable to the consumers. The investigations conducted so far indicated that the co-precipitates are used mostly in a variety of food products in small proportions to improve their functional and nutritive properties.

Selected references

Buchanan, R.A., Snow, N.S. and Hays, J.F. 1965. The manufacture of “Calcium co-precipitates”. Aust. J. Dairy Technol., 20: 139.
Hays, J.F., Muller, L.L and Fraser, P. 1969. Studies on co-precipitates of milk proteins: Part 5-Investigation on viscosity of co-precipitates in dispersions of high concentrations. Aust. J. Dairy Technol., 24: 75.
Muller, L.L., Hays, J.F. and Snow, N. 1967. Studies on coprecipitates of milk proteins. Part I. Manufacture with varying calcium contents. Aust. J. Dairy Technol., 22: 12.