In this article we will discuss about the methods used for manufacturing different types of milk. Learn about the manufacture of: 1. Soft-Curd Milk 2. Chocolate Milk/Drinks 3. Cultured Buttermilk Acidophilus Milk 4. Evaporated Milk.
Methods of Preparation of Soft-Curd Milk:
These are:
(i) Natural Soft-Curd Milk:
It is possible to assemble a herd of cows that will produce milk with a curd tension of 30 g. or less.
(ii) Dilution with Water:
This has the effect of lowering the casein and calcium contents.
(iii) Heat Treatment:
(iv) Homogenization:
A pressure of 2500-3000 psi markedly reduces the curd tension.
(v) Acidification:
A fine coagulation of casein is brought about by acidification with lactic acid, facilitating digestive action.
(vi) Enzyme Treatment:
When milk is acted upon under controlled conditions by certain proteolytic enzymes such as trypsin, the curd tension is reduced.
(vii) Addition of Salts:
The curd tension of milk may be lowered by the addition of the following salts: Sodium citrate, Sodium pyrophosphate, Sodium hexametaphosphate, etc.
(viii) Base-Exchange Treatment:
This process consists in passing milk acidified with citric acid through a zeolite filter-bed in which about 20 per cent of ionic calcium is exchanged for sodium.
Method of Manufacture of Chocolate Milk/Drinks:
Flow Diagram of Manufacture:
Details of Manufacture:
The following formula may be used:
(i) Cocoa powder 1 to 1.5%
(ii) Sugar 5 to 7.0%
(iii) Stabilizer 0.2% (Sodium Alginate)
(iv) Fat level in milk/drink—Minimum legal standard 2%.
The milk on receipt is standardized, pre-heated to 35-40°C and filtered; alternatively, after standardization it is pre-heated to 60°C, homogenized at 2500 psi and then clarified. To the warm milk, the desired amounts of cocoa-mix, sugar and stabilizer are slowly added and stirred so as to dissolve them properly.
(The cocoa powder may also be added in the form of syrup, and the stabilizer in the form of solution.) The mixture is then pasteurized at 71°C/30 minutes, cooled rapidly to 5°C, bottled and kept under refrigeration (5°C) until used. The bottles are invariably inverted up and down a few times before consumption.
Note:
Standardized milk is homogenized to prevent or delay the rising of cream. It may be homogenized after addition of cocoa and sugar, but this has the effect of increasing sedimentation. Stabilizer is usually added to delay or prevent settling of cocoa particles; it also aids in the prevention of cream rising.
Flow Diagram of Manufacture of Cultured Buttermilk:
Details of Manufacture:
(i) Preparation:
Fresh skim milk is preferable. The solids content should range from 9 to 10 per cent. The skim milk should be free from antibiotics and inhibitory substances. If milk containing an appreciable amount of fat is used, it should be homogenized. If cream or butterfat is desired in the product, it may be added to the cold buttermilk when the curd is broken up.
Manufacturing milk is heated in a holder pasteurizer vat to 82-88°C (180-190°F) for 30 minutes with frequent stirring, and then cooled to 22 C (72°F). One per cent bulk lactic starter is added and mixed thoroughly in the milk, and then allowed to incubate at 21-22°C (70-72°F) without further agitation until coagulation occurs and an acidity between 0.80 and 0.85 per cent is reached.
The curd is broken up by slow agitation to give a smooth consistency and cooled to 5-10°C. It is then packaged and stored at 5-10°C until distribution. Care is taken not to incorporate air in the product during agitation and filling. Acidity at packaging should be between 0.85 to 0.90 per cent.
(ii) Creaming:
Cream imparts a rich flavour to the buttermilk and has the effect of decreasing the viscosity. (Viscosity can also be decreased by adding 20 to 25 per cent of pasteurized sweet whole milk during break-up of the curd.)
(iii) Salting:
Salt at 0.1 per cent may be added to the finished product to increase its flavour.
(iv) Adding Butter Granules:
Butter granules or flakes are sometimes added to cultured buttermilk made from skim milk to simulate an old-fashioned churned buttermilk appearance. Butter granules can be made in several ways, e.g. by churning cultured cream, by passing hot butter-oil through a screen into cold buttermilk and/or spraying hot butter-oil into cold buttermilk.
One method is to prepare butter granules from 20 per cent cultured cream and add them to cultured skim milk when breaking up the curd. Churned buttermilk may be prepared by culturing milk containing 1-2 per cent butterfat, cooling the coagulated milk to 13°C (56°F), and then churning the milk until butter granules the size of rice grains appear. Butter colour may be added to increase the colour of the granule, if desired. The product is cooled and packaged.
Flow Diagram of Manufacture of Acidophilus Milk:
Details of Manufacture:
Skim, whole, or partly defatted milk may be used. Milk containing more than 1 per cent fat should be homogenized. Acidophilus milk can simply be made with added sugar or honey, or with tomato or carrot juice. Fresh milk is heated to 115°C (240°F) for 15 minutes (or heated intermittently) to obtain sterile milk.
The addition of 1 per cent of glucose or honey, or 5 per cent tomato juice helps to hasten the fermentation, but is not essential. The milk is cooled to 38-40°C (100-104°F) and inoculated with 3-5 per cent inoculum (mother starter).
The inoculated milk is mixed thoroughly and incubated at 38-40°C (100-104°F) until the milk coagulates. The coagulum is slowly broken up and cooled to 10°C (50°F). Five to ten per cent lactose or dextrin may be added if desired. The milk is stirred until smooth, packaged, and stored at 5°C (40°F) until used.
Note:
Strains of Lact. acidophilus are difficult to maintain and propagate because of the ease with which contaminating bacteria outnumber them.
Method of Manufacture and Storage of Evaporated Milk:
Flow Diagram of Manufacture:
Details of Manufacture:
1. Receiving milk.
2. Filtration/Clarification.
3. Standardization.
4. Forewarming/Pre-Heating:
The main purpose of this is to improve the heat-stability of milk and to ensure a medium viscosity in the finished product.
Note:
The forewarming of milk during the manufacture of evaporated milk is not relied upon for destruction of micro-organisms and inactivation of enzymes; this latter is the function of sterilization.
5. Evaporation:
Same as for condensed milk, except that no sugar is added and normally a continuous operation takes place. It is customary to slightly over-condense the milk to facilitate standardization later.
6. Homogenization:
After evaporation and before cooling, the evaporated milk is thoroughly homogenized to obtain a uniform fat emulsion and reduce separation of fat to a minimum during storage. A temperature and pressure combination that will reduce the fat globules to less than 2 microns in diameter is needed.
The temperature is usually about 49°C (120°F) as the product is removed from the last effect of the evaporator. Two-stage homogenization is generally recommended, 2000 psi in the first stage and 500 psi in the second.
Note:
In developed countries, evaporated milk is enriched with vitamin D. This is done either through irradiation i ultraviolet radiation from artificial sources) or fortification (addition of vitamin D-rich substances).
7. Cooling:
After homogenization, the evaporated milk is cooled to 7°C (45°F) if it is to be sterilized on the same day, as in small-scale handling; however, in large-scale handling it is cooled and held at 5°C (40°F). The cooled product is held in large storage tanks for re-standardization of fat or total solids. Stabilizing salts may also be added at this point if pilot tests indicate such a need.
8. Pilot Sterilization Test:
(a) Purpose:
To determine the amount of chemical stabilizer (trisodium citrate or disodium phosphate) to be added to any given batch of evaporated milk for the most satisfactory heat-stability.
(b) Apparatus:
The major items needed are:
(i) A pilot sterilizer with accessories;
(ii) A viscosimeter;
(iii) A sample can filler;
(iv) A weighing balance;
(v) Glassware for making and measuring the stabilizer.
(c) Preparation of Stabilizer Solution:
An approximate 10 per cent solution of stabilizer is prepared for use, so that 0.1 ml. of the solution is equivalent to the addition of 1 g. of dry salt for every 16 kg. of evaporated milk.
(d) Method of Adding Stabilizer Solution:
The amount of stabilizer to be added to any batch is determined by trial. About 8 to 10 sample cans are used. They are then individually filled first with different accurately measured amounts of the stabilizer solution and then with 170 g. (6 oz.) of evaporated milk to be tested, as shown in Table 8.8.
Note:
In Table 8.8, Can X represents the evaporated milk without any addition, Can 1 contains no stabilizer but represents the evaporated milk in diluted form, the rate of dilution being the same as that of all other samples. The remaining samples represent an addition of stabilizer in increasing amounts, but with the dilution in all cases equal to Can 1.
(e) Sterilizing the Samples:
The sample cans, containing measured amounts of stabilizer solution and a fixed quantity of evaporated milk, are sterilized at 11 7°C (242°F) for 15 minutes and then cooled rapidly to 24°C (75°F). As soon as they have been cooled, the cans are opened, examined for smoothness and colour, and tested for viscosity.
According to Mojonnier and Troy, a viscosity of 150°R on the Mojonnier-Doolittle viscosimeter represents the correct viscosity of evaporated milk as it comes out of the sterilizer. Changes in viscosity during the different stages in the manufacture of evaporated milk are given in Table 8.9.
(f) Determining the Amount of Stabilizer to be Added:
If the viscosity tests of pilot-sterilized cans show that no stability correction is necessary, the batch is ready to be filled into the cans. However, if the tests show otherwise, as in Table 8.10, stability corrections become necessary.
In Table 8.10, it will be seen that Can 2 has come out of the sterilizer with a smooth texture and to this can 0.2 ml. of the 10 per cent solution of stabilizer was added. This amount is equivalent to 2 g. of dry disodium phosphate crystals per 32 kg. evaporated milk.
(g) Adding Stabilizer to the Batch:
The calculated amount of stabilizer should be added to the evaporated milk in the form of a solution, using just enough water to dissolve it. The stabilizer solution should be added slowly, thoroughly agitating the evaporated milk both during addition and for several hours thereafter.
In practice, a portion of the required amount of stabilizer is added to the milk in the forewarmer and then correction completed by adding the’ remainder to the evaporated milk in the storage tank. (The previous day’s figures may be used as guidelines.)
9. Packaging:
The cans are now mechanically filled with volumetric fillers. The types of can used for this purpose are- the sanitary can; the can with a solder seal; and the vent hole can. The cans should be able to withstand pressure changes during sterilization and the seal should be absolutely air-tight and strong.
The temperature of evaporated milk when filling the cans should be neither high nor low, but around 5°C; a higher temperature causes foaming, while a lower one increases the tendency towards ‘flipping’. (Flipping refers to the sudden snapping of can ends from their slightly concave contour to a convex outward bulge, which is usually accompanied by an audible sound.)
Note:
The cans should be filled as nearly full as possible. Filling the cans too much also causes flipping. They should be properly sealed.
10. Sterilization:
Immediately after sealing and before sterilization, each can is tested by means of a dependable leak detector, where the cans pass submerged in a hot-water bath. In case of leaky cans, air bubbles rise to the top; these cans are removed for repair. The filled and sealed cans, which have passed the test for leaks, are now ready for sterilization.
Purpose:
The primary purpose of sterilization is to destroy all germ life and enzymes present, thereby preserving the product permanently. In addition, the sterilizing process is utilized to increase the viscosity and improve the body so as to give a creamy consistency to the finished product.
During sterilization, the temperature of evaporated milk is raised rapidly to 116-118°C (240-245°F) and held at this temperature for 15 minutes. If the cans cannot be sterilized within an hour or two, they should be held under refrigeration (5-7°C/40-45°F) especially in hot weather.