BY-PRODUCTS TECHNOLOGY

Module 2. Skim milk and its by-products

Lesson 17
CO-PRECIPITATES: INTRODUCTION AND BASIC PRINCIPLES FOR PRODUCTION

17.1 Introduction

A co-precipitate of milk proteins may be defined as the product which separates as the solid phase after the heat treatment and precipitation of dairy fluid, or mixtures of dairy fluids, which contain both casein and heat coagulable whey proteins. The resultant product contains a significant proportion of the whey proteins and almost all the casein present in the raw material. Previously this name was confined to milk proteins only, but recent work has widen the term "Co-precipitates" in the sense that it covers the combination of milk proteins as well as the proteins derived from other biological systems. In the process of the manufacture of co-precipitates, up to 97% proteins have been reported to be recovered as compared to only 80% in casein manufacture (Muller et al., 1967). The structure of co-precipitates and their characteristics are highly influenced by the production process. It has led to many patents covering the production of co-precipitates having specific characteristics for particular uses. The concept of controlling calcium content of co-precipitates to produce a series of products with different physical or functional properties was introduced by Muller et al., (1967). The level of calcium in co-precipitate has major effect on its functional properties. The structure of co-precipitates and their characteristics are highly influenced by the production process. Control of calcium level is achieved by varying the amount of CaCl₂ added, changing the length of time at which the milk is held at about 90°C and varying pH of precipitation by acid for low and high calcium co-precipitates. The flexibility inherent in the co-precipitate process enables products to be produced, which vary in casein to whey protein ratio, solubility, ease of whipping, water binding, equilibrium moisture, viscosity and compatibility with other foodstuffs. Consequently, the range of application and properties of co-precipitates is large. Co-precipitates may be used as an ingredient in the preparation of various food products such as in dairy, meat, baked, confectionary, snack and animal and pet foods, either to contribute the desired functional properties or to improve the nutritional qualities of the products.

Co-precipitates are rich in proteins and low in lactose. They can be utilized for the manufacture of foods and beverages for lactose intolerant people or for fortification of infant foods without increasing the lactose and mineral contents. The cost of production of co-precipitates, as reported in the literature, is lower than that for skim milk powder and only slightly higher than that for casein.

17.2 Classification

Depending on the production process parameters, especially CaCl₂ quantity, pH value at precipitation and washing conditions, some authors classify co-precipitates in three varieties, each with differing amounts of calcium.

17.2.1 Muller et al., (1967) classified co-precipitates in three varieties given below

17.2.1.1 Low-calcium co-precipitate

The calcium content in these co-precipitates is 0.5-0.8%.

17.2.1.2 Medium calcium co-precipitate

The calcium content in these co-precipitates is 1.5%.

17.2.1.3 High calcium co-precipitate

The calcium content in these co-precipitates is 2.5-3.0%.

17.2.2 Kozhev et al., (1970) classified co-precipitates as

17.2.2.1 Acid co-precipitates

The calcium content in these co-precipitates is 0.8-1.0%.

17.2.2.2 Low calcium co-precipitates

The calcium content in these co-precipitates is 1.2-1.5%.

17.2.2.3 Medium calcium co-precipitates

The calcium content in these co-precipitates is 2.0-2.5%.

17.2.2.4 High calcium co-precipitates

The calcium content in these co-precipitates is 3.5-4.5%.

17.3 Basic Principle

Following precipitation of caseins from skim milk by acidification or renneting, the whey proteins remain soluble (in the whey). However, these can be precipitated in combination with the casein by first heating milk to such an extent that denature majority of the whey proteins and induce complexation of the whey proteins with casein, followed by precipitation of the milk protein complex by acidification to pH 4.6 or by a combination of added CaCl₂ and acidification. Precipitate of casein and whey proteins together from heated skim milk by acidification is termed as "co-precipitate". Thus co-precipitates of milk proteins are in principle formed by a two-stage process: i) Heat treatment of milk or a mixture of products which provide casein and whey protein, and ii) Precipitation of the proteins from the heated milk. The precipitated proteins are then separated from the serum, washed and dried.

In order to obtain the highest possible protein yield, it is necessary to heat the milk before the precipitation. Preheating to a temperature above 65°C promotes the interaction between β-lactoglobulin and k- casein, thus leading to complex formation between whey protein and casein. The complexed whey protein will then co-precipitate along with casein after the addition of the precipitating agent. For maximum protein recovery the heating temperature had to be at least 85°C. In addition to temperature, the time of heating also affects the protein yield. The interaction between β-lactoglobulin and k-casein changes the functional properties of the caseins considerably. Co-precipitate is a total milk protein preparation with properties different from those of the native protein fractions.

The calcium concentration in co-precipitates can be maintained by changing basic parameters in the production process: a higher pH value at precipitation results in a higher calcium concentration in the product, while the longer retention time at high temperature decreases calcium concentration. It has been found that CaCl₂ concentration of about 0.2% of the skim milk quantity results in the highest recovery of milk proteins (> 95%) and in a low calcium content in the whey. The level of CaCl₂ required in milk for the precipitation of high calcium co-precipitate was found (Southward et al., 1973) to be inversely related to the precipitation temperature. At 90°C, a concentration of 0.2% CaCl₂ in the milk was required for complete precipitation, whereas at lower temperature, it was found necessary to use up to 0.3% CaCl₂.

Selected reference

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.