In this article we will discuss about:- 1. Introduction to Industrial Casein 2. Types of Casein 3. Manufacture 4. Specifications 5. Defects 6. Uses.
Introduction to Industrial Casein:
An important utilization of skim milk is in the production of industrial casein which, however, is a non-food item. Because of its widespread demand, industrial casein is prepared extensively throughout the world.
There are two types of industrial casein, viz. acid casein and rennet casein. The casein precipitated by various acids is called acid casein, free casein or (simply) casein; while that precipitated by rennet is called rennet casein.
Types of Casein:
(a) On the Basis of the Coagulating Agent Used:
(i) Acid casein.
(ii) Rennet casein.
(b) On the Basis of the Particular Acid Used:
(i) Hydrochloric acid casein.
(ii) Sulfuric acid casein.
(iii) Nitric acid casein.
(Iv) Lactic Acid Casein:
1. Direct precipitation.
2. Self-soured.
(c) On The Basis of the Texture of the Curd:
(i) Grain curd casein.
(ii) Cooked curd casein.
(iii) Pressed curd casein.
Manufacture of Industrial Casein:
a. General Method of Manufacture of Acid Casein:
Flow Diagram of Manufacture:
Details:
(i) Receiving Skim Milk:
Fresh, clean, sweet skim milk which is as fat-free as possible should be used.
(ii) Precipitating:
Casein exists in milk as a calcium caseinate- calcium phosphate complex. When an acid is added to milk, this complex is dissociated. At about pH 5.3, the casein begins to precipitate from the solution; and at the iso-electric point of casein, about pH 4.7, maximum precipitation occurs.
The casein coagulates as a watery, gel-like curd. The gel is broken to allow for the expulsion of the water from it. The precipitation of casein involves three considerations: acidity and pH, temperature, and the speed at which it is stirred. Once out of solution, casein does not dissolve appreciably in either a slightly more acid or alkaline solution.
However, a little more acid is added after precipitation till a pH of 4.1 is reached in order to dissolve the calcium which flows out with the whey; the texture of the curd is thereby also improved (it becomes coarse). Washing, which is done so as to remove the impurities, viz., whey, proteins, lactose, salts and acid, also becomes easier.
The temperature should be carefully controlled during precipitation, since the casein curd is sensitive to heat. A jacketed vat is recommended for this purpose. The effect of temperature on the character of curd is shown in Table 12.5.
Stirring is essential to distribute the acid uniformly but it should always be done at slow speed. Rapid stirring at less than 38°C (100°F) produces so fine a curd that it settles very slowly when being drained and washed, and is likely to be partly lost in the whey and washings.
Note:
When hydrochloric or sulfuric acids are used for precipitation, they are diluted with water to prevent any local injury to the curd (through the action of concentrated acid, even though agitation may be rapid). The proportion of acid and water is given in Table 12.6.
(iii) Draining and Washing:
The longer the casein curd stands in the whey, the more difficult it becomes to remove the impurities (viz., whey proteins, lactose, salts and acid). Hence the whey is removed promptly. The first step in this regard is to allow the curd to settle and then push it back from the gate/outlet end of the vat. Next, the gate strainer is fixed in position and another cloth-lined strainer hung at the discharge end of the gate to recover small curd particles. Then the whey is drained off promptly.
As soon as the whey ceases to run freely, cold wash-water is added in quantities approximately equal to the amount of whey removed. The wash water is made very slightly acidic (pH 4.1), especially for the first washing. The curd is well-stirred in the wash- water, but care is taken not to break it into fine particles; it is then allowed to settle, and the wash water drained as above. It should be washed at least twice.
Note:
Inadequately washed casein is less soluble than thoroughly washed casein, has less adhesive strength and develops more colour when dry.
(iv) Pressing:
The wet curd is pressed to remove as much water as possible, so as to reduce the cost of drying. Normally it is not possible to reduce the moisture level in the pressed curd to less than 55 per cent.
(v) Milling and Spreading:
After being pressed, the curd is milled to produce particles of a uniform size and surface for drying. If it is not milled, it dries unevenly. Pressed curd should be promptly milled and dried, in order to prevent spoilage by mould and bacterial action.
Milling should be of the shredding variety. The milled curd should be evenly spread over standard perforated trays with a fixed amount of curd to ensure adequate drying. The lowest tray should be of a finer mesh than the others and may be covered with a cloth to catch fine particles falling from above.
(vi) Drying:
It is essential to control the temperature and humidity of the intake air correctly if the curd is to be efficiently dried. Once started, drying should not be interrupted till the moisture content has been reduced to less than 8 per cent.
The intake air temperature may range from 71 to 77°C (160 to 170°F), while the outlet air temperature should be between 52 and 57°C (125 to 135°F) in order to avoid the risk of discolouring the casein and impairing its solubility.
(vii) Grinding:
Before it is ground, casein should be properly cooled, since warm casein becomes plastic in a grinder and sticks to it. The fineness of grinding depends on the requirement of the user. Uniformity in size of ground casein is ensured by passing it through screens of a particular mesh size.
(viii) Packaging:
The dried casein, whether ground or unground, is normally packaged either in gunny bags lined inside with closely woven cloth, or in heavy 3-ply paper bags with polythene liners.
(ix) Storage:
The filled packages of casein should be closed air-tight and transferred to a clean, dry storage room maintained at a uniform temperature. Wide variations in storage temperature will cause the casein to ‘sweat’ and mould. If stored in a damp atmosphere, it will absorb water and deteriorate. For long storage, it is best to store casein in a basement, away from the sun.
b. Manufacturing Particulars of Some Acid-Caseins:
(a) Grain-Curd Casein:
A high-grade acid-casein, low in ash and readily soluble, is made by the grain-curd process, provided the pH and temperature are closely controlled. A jacketed vat is essential for accurately controlling the temperature. The best product is obtained by using hydrochloric acid, but lactic or sulfuric acids can also be successfully used.
(i) Direct Precipitation:
Skim milk is held close to 38°C (100°F) for lactic, 37°C (98°F) for sulfuric and 35°C (95°F) for hydrochloric acids. The pH of 4.1 is obtained by slowly adding dilute acid while also gently stirring the milk. The curd is settled, drained, washed twice or thrice with water at 32°C (90°F) and pH 4.1, and finally, once with cold neutral water. The washed curd is pressed, dried, ground and packaged.
(ii) Self-Soured (Lactic):
Skim milk is held at 38°C (100°F). An active, high-acid producing, lactic starter is added at 10 per cent of the skim milk and thoroughly mixed into it. Acidity is allowed to develop up to 0.55 per cent. Then the temperature of the curd is raised to 40-41°C (104-I06°F), and the curd stirred until clear whey separates. The curd is now settled and the whey drained off as quickly as possible.
The speed at which it is drained is more important than the thoroughness of the operation. Then lactic whey, containing about 2 per cent acid and previously heated to 38°C (100°F), is added and stirred until the pH is 4.1. The granular curd is settled, drained and washed twice or thrice with acidulated water of pH 4.1 and temperature 32°C (90°F), and finally once with cold neutral water. The washed curd is pressed, dried, ground and packaged.
(b) Continuous Process Casein:
There are two processes which are used commercially, viz. the Sheffield process and the Universal process. Their principle of manufacture may be outlined as follows- the skim milk and precipitating acids are mixed and the mixture dropped into a riffle-trough. The slope of the trough may be altered to vary the size of the curd particles the steeper the slope, the finer the particles obtained.
From the riffle-trough, the curd and whey mixture drops to an inclined vibrating screen, which allows the whey to pass through but advances the curd to a 2-roll press where it enters a beater.
Here it is broken up sufficiently to allow it to pass through the perforated bottom to a second inclined vibrating screen, where it is washed with sprays of water. The washed curd enters another 2-roll press and passes through another beater, after which it is ready for drying.
c. Method of Manufacture of Rennet-Casein:
High quality fresh, sweet skim milk is essential. This is heated in a jacketed vat up to 36-38°C (96-100°F). Sufficient rennet and calcium chloride are added to the skim milk to give a setting time of 20-30 minutes. The rennet should previously have been diluted with about 15-20 times its weight in water (by volume); calcium chloride is also used @ 1 per cent solution.
The coagulum is agitated for 2-5 minutes after coagulation begins but before the coagulum reaches a solid clot, so as to produce curd particles of an optimum size for further processing. Simultaneously with agitation, the temperature of the curd is raised to 54-66°C (130-150°F). The curd is cooked at this temperature for about 30 minutes. After cooking, the curd is processed in the same manner as acid caseins (wash-water being neutral).
d. Method of Manufacture of Buttermilk-Casein:
The method for the isolation of casein from buttermilk/lassi may be briefly outlined as follows- the casein in the buttermilk is first dissolved by adding enough sodium hydroxide solution at 32°C (90°F) to raise the pH to 7.5. It is then precipitated by adding dilute hydrochloric acid to a pH of 4.6. The curd is settled, whey drained, curd washed and pressed, and then the wet curd dissolved once more, this time at 38°C (100°F) in dilute alkali.
The solution is passed through a cream separator to remove fat. The casein from the separated portion is re-precipitated by dilute acid, the curd settled and whey drained; the curd now obtained is finally washed, pressed, dried, ground and packaged. Although a fairly acceptable casein is obtained by this ‘Double alkali treatment-cum-mechanical separation process’, it has been found to be uneconomical.
Note:
The buttermilk casein tends to precipitate in a soft, finely divided condition, which makes it difficult to wash the curd ‘ without excessive loss. Moreover, it contains a greater quantity of fat and becomes highly coloured during drying.
Yield of Acid-Casein:
The yield is roughly equal to the percentage of casein in the skim milk and may vary from 2.8 to 3.2 per cent, depending on whether it is cow or buffalo skim milk.
Specifications for Casein:
(a) Acid Casein:
This is normally yellowish white in colour for cow and chalky white for buffalo milk, has a faint pleasant odour, a slightly acid taste; a total acidity equivalent to not more than 10.5 ml and N/10 alkali per g., not more than 10 per cent moisture, not more than 1 per cent fat (on moisture-free basis), not more than 4.0 per cent ash (on moisture-free basis) and not less than 14.25 per cent nitrogen (on a moisture-, fat- and ash-free basis).
(b) Rennet Casein:
This is normally light-yellowish in colour for cow and whitish for buffalo milk, has a faint agreeable odour, is tasteless, has a pH of about 7.0, not more than 10 percent moisture and approximately 7.5 per cent ash.
Defects in Casein, Their Causes and Prevention:
Defects in casein arise from the use of low-grade raw material (skim milk), faulty methods of production and drying, etc. The important defects, their causes and prevention are given in Table 12.7.
Uses of Casein:
The two types of casein, viz. acid and rennet, have separate uses and one type cannot be used as a substitute for the other.
The various uses of industrial casein are given below:
(a) Rennet Casein:
Plastics (such as buckles, brush-backs, buttons, costume jewellery, umbrella handles, etc.).
(b) Acid Casein:
(i) Adhesive (glue);
(ii) Paper coating;
(iii) Paint;
(iv) Fibre;
(v) Bristles;
(vi) Films;
(vii) Leather finish;
(viii) Textile finish;
(ix) Spreader for insecticide;
(x) Medicine (silver and mercury caseinate).
Note:
The food uses of casein are given under ‘edible casein’.