Module 2. Skim milk and its by-products

Lesson 9

9.1 Introduction

Rennet casein is obtained by the enzymatic coagulation of skim milk. Rennet casein has characteristic bland, stable flavor and high nutritional value. Industrially, rennet casein is used for the production of plastic. There is growing interest in the use of rennet casein as a food ingredient partly because of its suitability for use in making cheese analogues and partly because of its flavour stability. For the manufacture of rennet casein, several different enzyme preparations are now available. These include chymosin (previously known as rennet or rennet extract), the milk-clotting enzyme extracted from the stomachs of young calves, and a number of so-called microbial rennet, which are enzymes that have been produced by means of microbial fermentation techniques. The caseins produced using any of these enzyme preparations are all known as rennet casein and all have similar properties. Rennet obtained from calf stomach has been used traditionally in the production of rennet casein, but high costs, shortages of calf rennet and being considered non-vegetarian, have encouraged dairy products manufactures to seek acceptable alternatives. Rennet substitutes are now being widely used. Porcine pepsin, bovine gastric extracts or microbial rennet from Mucor miehei and Mucor pulsillus compares very favourably with calf rennet for the manufacture of rennet casein.

9.2 Rennet Action

The coagulation of the casein by rennin is a two-stage process. In the first stage, enzyme hydrolyses the Phe-Met (residue 105-106) bond of k-casein macromolecule splitting it into para k-casein and glycomacropeptide. The hydrophilic glycomacropeptide is released into the whey. This action destabilizes the casein micelles. In the second stage of the reaction, the remaining para k-casein forms a three dimensional clot in the presence of calcium ions.

9.3 Traditional Batch Manufacturing Process

High quality milk is used for the preparation of rennet casein, since milk of poor quality with developed acidity will have some of its calcium solubilized, which will result in poor curd. Poor microbiological quality of milk also results in a plastic of dark brown colour. It is important that the milk is separated efficiently because even small amount of fat in skim milk is detrimental to the quality of rennet casein prepared. In the traditional batch method, skim milk is pasteurized at 72°C for 15 s, cooled to 31°C and added with sufficient quantity of calf rennet (1: 4500) to give setting time of 20-30 min (Fig. 9.1: Flow diagram for manufacture of rennet casein). The mixture is stirred vigorously for 10 s and then left to stand unstirred at 31°C for setting.

9.3.1 Cutting of curd, Cooking and Dewheying

The set coagulum is cut with cheese knives and the agitation of the curd is started after about 2 minutes of cutting. Steam is then injected into the vat jacket at controlled rate (sufficient to raise the temperature by 0.5°C/min) to cook the casein curd to about 50-60°C with continuous gentle agitation. Cooking of curd is further continued for 30 min at 50-60°C. Cooking process promotes whey synersis, facilitates separation of rennet casein curd and whey, improve curd strength for further processing and minimize potential curd losses as fines during rennet casein manufacture. After the cooking process, the casein curd is allowed to settle and the whey is drained out.

9.3.2 Washing and dewatering

After dewheying, the curd is washed with water to remove lactose, whey protein and soluble minerals, the same way as lactic acid casein curd. Washing is influenced by particle size distribution, wash water temperature, number of washings, holding time at each washing stage and degree of agitation. A typical wash water temperature profile for a four stage washing system is 30, 60, 75 and 30°C. After washing, rennet curd is dewatered to 60-70% moisture by pressing it for 10-12 h, usually overnight. Mechanical presses are usually employed for this purpose, and then the casein dried and packed in similar manner as hydrochloric acid casein.

9.4 Production of Rennet Casein by Continuous Process

In continuous rennet casein production process (Fig. 9.2), the rennet (1:7000 to 1: 8175) is mixed with skim milk stored in large silos (each with a capacity of up to 250 000 L) for setting of curd. When the coagulum has progressed to the desired stage, the gel is broken by agitation to give optimally sized curd particles for further processing and then pumped from the coagulation silo to a cooking pipe, where direct steam is injected to raise the temperature to 50-55°C. The pipe length is such as to hold the curd/whey mixture for about 45 sec for whey synersis. After cooking, the processing steps of the rennet curd are same as for the continuous manufacture of lactic acid casein.

In continuous method, less rennet is required, less steam is required for cooking, higher processing rates is feasible and labour requirements are lower. Also, rennet casein, produced by the continuous cooking process, retains less than half the fat retained by batch-cooked rennet casein. When separated from whey, the curd is firmer than that from the batch process. When plastic is made from the rennet casein by continuous cooking, it has better clarity due to its lower fat content, but generally has gas pin-holes than did plastic prepared from traditional rennet casein.

9.5 Effect of Manufacturing Variables on Colour of Rennet Casein Plastics

The manufacturing variables that has the greatest influence on the colour of both rennet casein and rennet casein plastics are heat treatment of the milk and number of washes. Pasteurization or similar heat treatment of the milk used for casein manufacture leads to rennet casein plastics with a darker brown colour than plastics produced from unpasteurized milk. So, the rennet casein intended for use in plastics should preferably be made from unpasteurized skim milk. High lactose content in poorly washed casein also leads to brownish colour in plastics. Iron in skim milk or wash water can also discolor (reddish brown) the plastic, especially if the iron content of the casein exceeds 10mg/kg.

Selected references

Michele, M., Sullivan, O., Singh, H., Peter A Munro and Daniel M Mulvihill 2002. Effect of cooking and washing temperature during pilot-scale rennet casein manufacture on casein hydration characteristics in disodium orthophosphate solution. International J. Dairy Technol. 55: 18-19.
Varghese, S and Sachdeva, S 2002. Development of Rennet casein based processed cheese preparation. International J. Dairy Technol. 54 (1): 1-7.
Last modified: Wednesday, 3 October 2012, 6:35 AM