Module 2. Skim milk and its by-products

Lesson 6


6.1 Introduction

Casein exists in milk in complex groups of molecules (sometimes referred to as calcium phospho-caseinate) that are called micelles. The micelles consist of casein molecules, calcium, inorganic phosphate and citrate ions. As the initial pH value of the milk is decreased from 6.5, casein starts losing its colloidal dispersibility and stability and begins to precipitate at pH 5.3. Maximum precipitation takes place at pH 4.6, which is the isoelectric point of casein. Ion exchanges to replace cations in deproteinated whey with hydrogen ions has been applied in reducing the pH to about 1.8 prior to using the treated whey as the precipitant for acid casein. More recently, it has been proposed that ultrafiltration be used to increase the protein content of skim milk to 5-7% and to produce a permeate which is treated by cation exchange and then used to precipitate the casein. These techniques avoid incorporating the anions of hydrochloric or sulphuric acid in the whey resulting from casein manufacture and so aim to increase the value of the whey as a raw material for further processing. Casein may also be coagulated by other means, particularly proteolytic enzymes.

6.2 Processing Principle

Efficient separation of fat from milk is essential in production of edible casein. Fat needs to be reduced to the level less than 0.05%. Further, achievement of the microbiological standards for edible casein requires pasteurization of either or both the milk and the curd. The basic operations in the production of casein are the same irrespective of the type of casein produced. Such operations consist of the precipitation of the curd and its washing, pressing, and drying.

6.2.1 Precipitation

Casein exists in milk as a calcium caseinate-calcium phosphate complex. When an acid is added to milk, this complex is dissociated. As the pH of milk is lowered, the calcium is displaced from the casein molecules by hydronium ions, H3O+ and the calcium phosphate associated with the complex is converted into soluble Ca2+ ions and H2PO4- ions. At about pH 5.3, the casein begins to precipitate out of solution and at the isoelectric point of casein (about pH 4.6), maximum precipitation occurs. At this pH, all the calcium is solubilized. Not only is the calcium from the caseinate molecule removed, but also the calcium phosphate is liberated to the soluble form. This makes it possible to wash these soluble salts from the curd and achieve low ash content in the final product.

It might be expected that all the casein in milk would be precipitated simply by adding sufficient acid to bring the pH value to approximately 4.6. However, the reaction of acid with caseinate complex is not instantaneous and the pH will tends to rise slowly with time. Therefore, ample time should be allowed for achieving equilibrium conditions. When casein is precipitated from skim milk by the direct addition of acid, the temperature and pH of precipitation and the mechanical handling of the curd during its formation are very important in determining the subsequent properties of the curd.

Casein precipitated by acid usually includes the name of the acid in its description e.g. hydrochloric acid casein, lactic acid casein etc. but may simply be called acid casein. Any of the acid precipitation processes (hydrochloric acid casein, sulphuric acid casein or lactic casein process) can be used to produce edible quality casein. The choice of method for reducing the pH of skim milk to precipitate casein is largely governed by economics. Temperature of precipitation

The kind of curd formed is quite sensitive to heat. Curd precipitated at temperature below 35°C is very soft and fine, and consequently, is slow to settle and difficult to wash without loss. Precipitated at temperatures between 35 and 38°C, the curd is coarse provided stirring is not too fast. Stirring is necessary to distribute the acid uniformly, but rapid string at temperatures below 38°C produces a curd so fine that it settles very slowly during drainage and washing and may be lost to some extent in the whey and washings. Much more rapid equilibrium, more complete precipitation and, therefore, better yields are obtained by rapid and complete mixing before precipitation. The curd can be made firm in either of two ways; by heating to a temperature above 38°C; or the pH lowered to 4.1. Curd precipitated at about 43°C has a texture resembling chewing gum, being stringy, lumpy and coarse, containing practically no fine particles, and separating cleanly from the whey.

High-grade casein, low in ash and readily soluble, is made by the grain-curd process, provided pH value and temperature are closely controlled. The best product is made by the use of hydrochloric acid, but lactic and sulphuric acids may be used successfully. The temperature of precipitation of the skim milk should be held close to 37°C for hydrochloric acid curd. Stirring during precipitation

When acid is being added to skim milk, stirring is necessary to distribute the acid uniformly, but rapid string at temperatures below 38°C produces a curd so fine that it settles very slowly during drainage and washing and may be lost to some extent in the whey and washings. Much more rapid equilibrium, more complete precipitation and, therefore, better yields are obtained by rapid and complete mixing before precipitation.

6.2.2 Drainage of whey

After the precipitation has been completed and the curd has settled, whey should be removed from contact with the curd as soon as possible. The longer the curd stands in contact with the whey, the more difficult it is to wash out acids, salts, whey protein and lactose, as the freshly broken curd tends to anneal itself, thereby enclosing these constituents within a protein film.

6.2.3 Washing

The most positive quality improvement in casein is achieved through efficient washing. Improperly and insufficiently washed casein has a high acidity, is less soluble, has a lower adhesive strength, and develops a yellowish colour on drying when compared to properly washed casein. Significant amounts of lactose, minerals and acids are trapped within the curd, which prevents their ready removal during washing of the curd. It is important to keep the mineral content, particularly that of calcium, as low as possible in acid casein to maintain high solubility and low viscosity of the casein. It is necessary to allow sufficient holding time during each washing stage to permit diffusion of these whey components from the curd into wash water. The diffusion rate depends on the size and permeability of the curd particles, and the purity, amount and rate of movement of the wash water. Smaller size and better permeability of the curd particles are important for efficient washing. Three separate washes of casein curd are required with contact times of 15-20 min each. As soon as the whey is removed from the curd, wash water should be added equal in quantity to whey that has drained off. The curd should be well stirred in the wash water either by rakes or by mechanical agitators, but care should be taken not to break the curd into fine particles. Firm and friable curd particles are required to avoid creation of excessive fines. Rubbery and plastic curds cannot be washed effectively. A marked increase in the efficiency of washing can be achieved by removal of as much whey as possible at the whey off stage. Even small amounts of whey contamination in wash water can cause a sharp decrease in washing efficiency.

An important consideration in the design and operation of a casein washing system is minimization of fine casein particles in the wash water. Processing conditions must be chosen to give firm casein curd particles at precipitation and to avoid shattering them during washing. Counter-current washing with a low wash water ratio assists in minimizing losses by restricting the opportunity for loss to one effluent wash-water flow and by minimizing the volume of such water that has to be processed to effect fines recovery. pH of wash water

The pH of wash water should be about 4.6 for first two washings to avoid the formation of a gelatinous layer over the curd particles in excessively acid water and softening and redispersion of the curd in alkaline waters. Gelatinous layer if formed over the curd particles inhibits drainage of salts and lactose from the particles. The adjustment of same pH of wash water as that of casein facilitates in maintaining the equilibrium. For pH adjustment, sulphuric acid is preferred, as casein is much less soluble in this acid, than in hydrochloric acid. The third wash should be given with neutral water. Temperature of wash water

Casein curd has the usual property of acting somewhat like a sponge in water, contracting to expel water when heat is applied (synersis) and relaxing when the water temperature is lowered. On the application of heat, the curd also becomes hard and rubbery, while cold water softens it and causes the curd to be quite fragile and readily broken. The temperature of the first wash should be about the same as the precipitation temperature to give good curd shrinkage. In practice, it is usual to adjust the temperature of last wash water to 32-40°C for better expulsion of water during subsequent pressing.

6.2.4 Pressing

The objective of pressing the curd is to reduce its moisture content and thus minimising the energy required for removal of remaining water by drying. If the pressing has not been adequate, the subsequent grinding will give lumps of curd that will dry on the outside to give hard, impervious surface that prevents the escape of moisture from the inside, a condition known as case hardening. The pressing of the curd should not be for less than 12 to 15 h with 34 kg/cm2 pressure.

The proportion of water in washed curd and its ease of removal depend upon the type of curd made. Precipitation of the curd at pH of 4.1-4.3, and the curd well-washed in waters, also of the proper pH and at temperatures of 41°C would give a firm, friable curd which would drain well and press well. The final moisture content is usually 55-60%.

6.2.5 Milling and drying

Pressed curd is liable to deterioration by action of moulds and bacteria and therefore, should be shredded and dried as promptly as practicable. The pressed curd is milled to produce particles of uniform size and surface for drying. Otherwise, uneven drying occurs. Large particles or lumps may dry on the outside forming a hard, impervious outer surface that prevents the diffusion of the remaining moisture from the interior of the particle. The kind of grinder used is dictated by the kind of drier used. To produce a stable, storable product that meets the internationally recognized compositional standards for edible grade product, the casein curd is dried to <12% moisture in any one of variety of drier types. Dried casein is relatively hot as it emerges from the drier and the moisture content of individual particles varies. Therefore, it is necessary to temper and blend the dried product to achieve a cooled final product of uniform moisture content. Proper control of temperature and humidity of air coming in contact with the curd are the essentials of efficient drying of casein. A temperature of 52-57°C for the air entering a tunnel drier is suitable for any type of curd. Higher temperatures may be used for well-washed curd, but they are not recommended because of the risk of discolouration and defects in the sensory attributes of the dried casein and impairing its solubility. Especially during the early stages of drying, it is desirable to circulate a portion of the air so that the surface of the particle will not become fully dried while the interior is still moist. Care should be taken that the temperature in the drier does not rise above 57°C towards the end of the drying when comparatively small amounts of water are being evaporated. Drying once started should not be interrupted, but should continue until the percentage of moisture is approximately 8%. Properly dried casein has mush the same fine, granular characteristics as the properly ground curd from which it is made.

6.2.6 Tempering, grinding, sieving and bagging

Tempering means the holding of the casein for a period (24 h) to allow efficient cooling, hardening of the casein and evenness of moisture throughout the batch. Casein shows variation in moisture content during a day's run as it comes from the drier. Agitation is necessary for efficient tempering. The most efficient tempering consists of recirculating the dried casein by pneumatic conveyance. It has the advantage that air used for transport of the casein assists in cooling the curd. (Fig. 6.1: Basic principle of casein manufacture)

The cool, tempered casein is ground. The casein must be cooled before grinding because warm casein is plastic and causes "burn on" of the rollers. An object of the grinding and sieving operation is to produce the highest proportion of the product in the size range desired by the buyer.

6.2.7 Yield

The yield of edible acid casein is about 3 kg/100 kg skim milk.

Selected reference

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

Last modified: Wednesday, 3 October 2012, 6:25 AM