Module 2. Skim milk and its by-products

Lesson 7

7.1 Introduction

The use of mineral acids has the advantage of completely continuous operation with no holding time for coagulation. Hydrochloric acid is most commonly used as a precipitant as it is usually available at a reasonable low price and is known to produce high quality casein. 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 quality of casein produced. In practice, hydrochloric acid is used in dilutions ranging from 1 part in 3 to 1 part in 9 and sulphuric acid is diluted 1 part in 20.

7.2 Continuous Manufacturing Process

Due to the advance in technology and automation, continuous casein manufacturing plants have taken over the batch processes for large production. These plants, not only, reduced manufacturing costs, but also elevated the status of casein for both industrial and food uses. A large casein plant with continuous hydrochloric acid precipitation having a capacity of 14000 l per h requires only one person to operate it. A typical continuous casein manufacturing plant is shown in Hydrochloric_acid_casein.swf .

7.2.1 Precipitation

Thorough and rapid mixing of milk and acid is achieved by spraying dilute (1.3-1.4 N HCl) acid under pressure into skim milk flowing in the opposite direction and the rate of coagulum formation is reduced by lowering the mixing temperature to as low as 25°C (mostly, 30-35°C) to ensure that equilibrium conditions are attained before coagulation begins. It has been reported that on mixing acid and milk at 43-46°C, coagulation occurs in less than 0.1 sec and that the curd is fibrous and sticky - equilibrium state of pH cannot be attained before coagulation of curd, resulting in high ash content in casein.

The pH is stabilized by using accurate measuring pumps to ensure a constant flow of milk and acid. The mixer (acid & milk) should be designed in such a way that the acid injection into the flowing milk would induce turbulence and produce a rapid and complete mixing of both liquids. In this way a close contact between the milk and the acid prior to onset of coagulation is ensured.

Steam injection is then automatically regulated to give the desired coagulation temperature and a holding or acidulation tube is used to ensure complete coagulation and agglomeration of the curd prior to separation of the curd and whey. This method reduces fines losses to less than 1% of the casein and still allows regulation of precipitation pH and temperature to give curd with the desired physical properties for further processing.

7.2.2 Drainage of whey

After precipitation, casein curd is concentrated by passage over stationary, inclined and fine mesh screens, which remove between 70 to 90% of the whey. Several dairy companies have installed and are successfully operating, roller presses and lately decanters for dewheying. Hydroclones or automatic desludging separators may be used to recover casein fines from whey and wash water, which may then be dissolved in sodium hydroxide and recycled for coagulation or may be recycled directly to the hot wash water.

7.2.3 Washing

For continuous washing of casein curd, the most common procedure is to use multi-stage washing system involving as many as five washing stages (mostly 3 stages), in which curd and wash water travel counter-current to each other and are separated between stages using screens. These tanks are of sufficient capacity to permit an average holding time of 20 to 30 min. This system reduces both the volume of water needed and the loss of casein fines.

7.2.4 Pressing

Continuous curd pressing is done in mechanically driven roller presses, belt or by passing through decanters, where water is sufficiently expelled for subsequent economical drying. The properties of the casein curd following washing should be such as to allow for maximum dewatering under the conditions of operation of the dewatering machine while at the same time maintaining the curd in a suitable condition for subsequent drying. The physical properties of the curd are also influenced by dewatering temperature, which should be about 39°C in case of roller press and decanter centrifuge fig_7.2_decanter.swf .

7.2.5 Drying, tempering & grinding

Pneumatic ring driers, which consist essentially of a large, stainless steel, ring-shaped duct through which high-velocity, heated air and moist, disintegrated casein curd is circulated continuously, are now widely used to dry casein. The short residence time of the product in the drier avoids heat damage and individual curd particles are dried to relatively uniform moisture content before exiting with the spent air via a manifold into product separators. The drying air may be heated by direct combustion of propane or natural gas.

Following drying, tempering and blending, the casein is ground in roller or pin-disc mills to produce the small-size particles required by users of casein. Milled material is separated on screens to products of desired particle-sized range and oversized material which is re-cycled for further milling.

A drying technique referred to as 'attrition drying', based on the principle of grinding and drying in a single operation, is now widely used in casein plants since it allows the production of casein product closely resembling spray dried casein. The drier consists of a fast revolving, multi-chambered rotor and a stator with a serrated surface. Turbulence, vortices and cavitation effect in the drier result in highly efficient grinding, which pulverizes the curd into very small particles with a large total surface area. These particles are simultaneously dried in a hot air stream that passes through the drier concurrently with the curd. The dried casein is very fine: no particle is larger than 600 µm, about 80% are smaller than 150 µm and the overall average particle size is about 100 µm. The particles have good wettability and dispersibility in water because they are irregularly shaped and many contain cavities due to the rapid evaporative process.

Selected reference

Gupta, V.K. 1989. Technology of edible casein. Indian Dairyman, 41: 643-650.

Last modified: Wednesday, 3 October 2012, 6:27 AM