The basic principle in the production of condensed and evaporated milks is that high quality milk is filtered/clarified, standardized, forewarmed and condensed/evaporated to the desired level. The concentrated product is preserved by the addition of sugar for condensed milk and by heat-sterilization for evaporated milk.
1. Receiving Milk:
The quality of the incoming milk is one of the indispensable corner-stones upon which rest the quality and marketability of every form of concentrated milk. It is well known that the sanitary quality of the milk on the receiving platform depends on its background on the farm, viz., healthy cows, clean milk production, clean utensils, and freedom from colostrum, prompt cooling and refrigerated transport. However, all milk supplies must be systematically and thoroughly inspected each day by conscientious and experienced milk-graders.
When milk is received at the plant, its temperature should be at 10°C (50°F) or below. The milk should be clean, sweet, free from off-flavours and odours and reasonably free from extraneous material. Contamination by antibiotics, pesticides and other chemical residues and metals is highly undesirable. No abnormal milk should be accepted. Acid development is objectionable, for not only does this indicate an excessive bacterial count, but it also reduces the heat-stability of milk.
Among the various platform and laboratory tests usually performed on the intake milk to determine its acceptance/rejection, much reliance is placed on Alcohol (Ethanol) and Clot-On-Boiling (COB) tests to determine its acceptance for condensing.
These tests are discussed below:
(i) Alcohol (Ethanol) Test:
To make this test, 5 ml. of milk is placed in a test tube and an equal amount of a solution with 68 per cent alcohol added. The mixture is shaken and any formation of clots or flakes denotes a positive test, i.e., the milk is susceptible to heat-coagulation. Alcohol coagulation is affected by a disturbed salt-balance in the same manner as heat-coagulation.
The alcohol test is especially important for the manufacture of evaporated milk, as it detects the tendency of the milk to curdle during sterilization. This test detects- abnormal milk (including colostrum) which is high in mineral salts; developed acidity in milk; mastitis milk likely to result in sweet curdling; etc. It is more sensitive than the COB test.
(ii) Alcohol Index (AI):
Determined by placing absolute alcohol in the burette and 10 ml. milk in a beaker. The number of ml. of alcohol required for flake formation is known as the Alcohol Index (AI). An AI of 7 is indicative of good stable milk for acceptance, while 3 or less shows that the milk is fit for rejection.
(iii) Alcohol-Alizarin Test:
This test not only determines the heat-stability of milk but also the pH. (Milk is coagulated if the pH is 5.6 or below.)
Note:
Under Indian conditions today, where any developed acidity in milk is invariably neutralized by the addition of sodium bicarbonate by unscrupulous dealers resulting in a disturbed salt balance, acceptance of milk on the basis of Alcohol Tests alone may prove impractical since the bulk of the daily milk supply may then have to be rejected.
(iv) Clot-On-Boiling Test:
In this test, 5 ml. of milk is placed in a test tube and kept in a boiling water-bath for 5 minutes. Afterwards it is removed and examined for precipitation. If curd is observed, the milk is said to fail the COB test and should be rejected.
Note:
After the milk has been accepted on the basis of the above tests, it is weighed, sampled and tested for fat, SNF, etc.
2. Filtration/Clarification:
This is done in order to remove visible foreign matter, which is unsightly and may cause consumer complaints, from the milk. The milk is generally pre-heated (in warm- milk clarifiers) to 35-40°C to increase the efficiency of the operation. Afterwards, it is cooled so as to preserve its quality.
3. Standardization:
This is done so as to conform to legal standards in the finished product.
The standardization of raw milk is normally carried out in three stages:
(a) The first standardization, which establishes the desired ratio of Fat: SNF (usually 1: 2.44);
(b) The second, which establishes the desired ratio of added sugar to the total milk-solids;
(c) The third, which adjusts the concentration of the finished condensed milk to the desired percentage of total solids.
The fat/SNF ratio in raw milk is adjusted by adding a calculated amount of cream or skim milk to it, depending on whether there is a fat shortage or fat surplus, as shown in the examples below-
(I) Correcting Fat Shortage in the Batch by the Addition of Cream:
Problem:
Wanted 9.05% fat and 31 % total milk-solids in condensed milk. On hand, 10,000 kg. milk testing 3.60% fat and 12.50% TS, and cream (from the same milk) testing 40% fat. How much 40% cream must be added to provide the desired ratio of fat to SNF?
Solution:
(ii) Correcting Fat Surplus in the Batch by the Addition of Skim Milk:
Problem:
Wanted 9.05% fat and 31% total milk solids in condensed milk. On hand 10,000 kg. milk testing 6.5% fat and 16.08 TS, and skim milk testing 0.1% fat. How much skim milk must be added to provide the desired ratio of fat to SNF?
Solution:
4. Forewarming/Pre-Heating:
This refers to the heating of milk before it is condensed, and serves the following purposes:
(i) To ensure that the finished product is free from microorganisms and enzymes;
(ii) To ensure uninterrupted boiling in the vacuum pan;
(iii) To provide an effective means of controlling objectionable age-thickening in the finished product.
The temperature-time of forewarming/pre-heating extends over a wide range, such as 82 to 93°C (180 to 200°F) for 5 to 15 minutes; or 116 to 149°C for 0.5 to 5 minutes. The modern trend is towards high-temperature short-time heating, such as 115 to 118°C (239 to 248°F), for No-Hold/Flash. The exact temperature-time of heating is so controlled as to provide optimum viscosity in the manufactured product without inducing excessive thickening or thinning during storage.
Tubular heat-exchangers are commonly used for forewarming; either double-tube or shell-and-tube heat-exchangers are preferred.
The several systems of forewarming may be grouped as below:
5. Addition of Sugar:
Purpose:
Sugar is added for the purpose of preserving the condensed milk without resorting to sterilization by heat.
Kind:
Generally sucrose is added as it has proved most suitable. It is either highly refined cane or beet sugar. Other sweetening agents, such as corn syrup solids, glucose and dextrose, have been used to replace sugar by 5 to 25 per cent. The disadvantages of these sweetening agents are their reduced sweetening capacity compared to sucrose and their adverse effects on colour and the rate of thickening in storage.
Amount:
This ranges from 40 to 45 per cent in the finished product, which requires 18 to 20 per cent sugar on milk basis. Hunziker advocated a Sugar Ratio (sugar-in-water concentration) of 62.5 to 64.5 per cent; this amount not only ensures proper protection against microbial growth, but also prevents sugar crystallization.
(i) Determination of Sugar Ratio (SR):
Either of the two formulae may be used:
Problem:
Condensed milk contains 31% total milk solids and 43.1% added sugar. What is the sugar ratio?
Solution:
(ii) Determination of Percentage of Sugar in Condensed Milk for Desired Sugar Ratio:
The following formula should be used:
Problem:
Using the same values as above, what should the percentage of sugar in condensed milk for a SR of 62.5 be?
Solution:
(iii) Determination of Sugar in Milk to Give 43.1% Sugar in Condensed Milk:
Determine ratio of concentration by dividing % total milk solids in condensed milk by % total milk solids in fresh milk. Then divide percentage of sugar in condensed milk by the ratio of concentration.
Problem:
Fresh milk contains 12.3% total solids and condensed milk contains 31% total solids. How much sugar should be added to milk to give 43.1% sugar in condensed milk?
Solution:
Quality:
Sucrose in granulated or syrup form must be of good quality. Liquid sugars (approximately 65 per cent sucrose) should be subjected to a high pasteurizing temperature to destroy the micro-organisms before they are added to condensed milk.
Method:
The temperature and time at which sugar is added to the milk in the batch have a definite effect on the keeping quality and physical stability (age-thickening) of the finished product. Sugar is added at the end of the condensing process. The dry sugar is dissolved in the least possible quantity of water. If added before condensing, an increase in viscosity and greater difficulty in the evaporation of moisture result.
Further, the presence of added sugar in the fresh milk during forewarming increases the heat- resistance and survival capacity of the micro-organisms, thereby adversely affecting keeping quality. In order to ensure freedom from extraneous material, the sugar syrup may be passed through a pressure filter or a centrifugal clarifier.
6. Condensing:
The basic principle consists in the removal of water from the standardized milk by boiling it under partial vacuum at a low temperature till the desired concentration is reached. This operation is carried out in an evaporator, which should preferably be of the single-effect type (also known as a vacuum pan).
The chief advantages of condensing milk in vacuum are- economy of operation, rapidity of evaporation and protection of milk against heat damage. Vacuum condensing achieves the object of obtaining a finished product which is free from any cooked flavours and can be readily reconstituted into the original milk.
Description of a Vacuum Pan:
The vacuum pan or evaporator is the heart of the milk condensary. It is used in the manufacture of every type of concentrated milk product.
It consists essentially of five major parts and numerous accessories, as given below:
(i) Heating Surface:
This determines the evaporative capacity of the vacuum pan. It is provided with either a steam jacket, or a series of steam coils, or both; or with product-tubes enclosed in a steam chest or calandria; or a series of plates with low pressure steam and product in alternate plates; etc.
(ii) Vapour Space:
This refers to that portion of the body of the pan which extends above the level of milk. It is here that the water contained in boiling milk is converted into vapour. The walls of the vapour space are equipped with a manhole, thermometer, vacuum gauge, vacuum break, observation glass and illumination glasses with lights. The milk intake pipe also enters through the wall of the condenser. This pipe connects with the forewarmer and discharges the hot fresh milk into the pan.
(iii) Entrainment Separator:
The purpose of this is to reclaim particles of milk entrained by the vapour currents that pass from the vacuum pan to the condenser at a high velocity—so as to prevent excessive entrainment losses of valuable milk solids and to minimize the danger of troublesome pollution of milk-factory wastes.
The latest designs of efficient entrainment separators are capable of reducing entrainment losses to a small fraction of one per cent. These may be of the centrifugal, deflector or reverse-flow types.
(iv) Condenser:
The purpose of this is to condense the milk vapour and to cool the entrained air and non-condensable gases. The condensing of milk vapours is essential for maintaining the desired vacuum in the pan; the cooling of the entrained air and non-condensable gases is necessary for smooth pan operation.
Condensers may be either surface or spray types. The spray type condenser is used exclusively in the milk condensery and may be jet or cataract, parallel or counter-current, with the condenser installed either inside or outside the vacuum pan. The condenser water is removed either by pumps or a barometric drain/leg (Usually 10.4 metre/34 ft. or more in length to remove water by gravity).
The counter-current condenser makes highly efficient use of the water supply, makes possible the advantageous use of a water-cooling tower or spray pond, and is therefore recommended in the tropics.
The efficiency of spray condensers depends on the volume and temperature of the available water, surface area of water spray and the duration of contact of hot vapours with the cooling water spray.
The amount of cooling water required in the condenser is determined by the temperature of the water supply, the pan operating temperature and the temperature of the condenser water discharge. On average, it requires 20 kg. of cooling water to remove the vapours of 1 kg. of water contained in the milk, in the tropics. It is obviously desirable to use potable water in the condenser because it may get into the milk in the vacuum pan at any time.
(v) Vacuum Pump:
The purpose of this pump is to produce and maintain a partial vacuum, so as to make possible the condensing of milk in the vacuum pan under reduced pressure and at a correspondingly reduced temperature. Instead of a vacuum pump, the partial vacuum may be produced and maintained by a steam ejector, thereby eliminating all moving mechanisms.
It also has the advantages of simplicity of construction, low cost of initial installation and absence of maintenance costs. There are two principal classes of vacuum pumps, viz., the wet and the dry vacuum pump. The former removes the condenser discharge water as well as the air and non- condensable gases while the dry vacuum pump disposes of the air and non-condensable gases only.
(vi) Accessories:
An important accessory is the condensed milk sampler. Its purpose is to draw samples representative of the boiling milk in the pan for determination of its density, without interrupting pan operation.
The sampler may be either a batch type, or capable of continuous pan operation; the latter is preferably installed near the pan platform so that the operator can continuously observe the density of the finished product by means of a hydrometer freely floating in the sampler.
Evaporator Classification:
Basis:
The evaporators may be classified on the following basis:
(i) Source of heat- Steam, direct-fire, etc.;
(ii) Position of heating tubes- Horizontal, vertical or inclined;
(iii) Method of circulation of product- Forced, natural;
(iv) Length of tubes- Long, short, medium;
(v) Direction of flow of film of product- Upward, downward, (rising film, falling film);
(vi) Number of passes of product- One, two or more;
(vii) Shape of tube assembly for heat-exchanger- Coil, basket, straight;
(viii) Location of steam- Inside or outside the tube, or both;
(ix) Location of tubes- Internal, external.
Nomenclature:
The classification of a few well-known evaporators with their particulars is briefly given below:
(i) Vertical Short-Tube Evaporator:
Known popularly as a Calandria Evaporator. Commonly used throughout the world. The tubes, carrying steam internally, are placed vertically at the bottom of the cylindrical evaporating chamber. The calandria has a large central down-comer to allow vigorous natural circulation. Cleaning and inspection are easy.
(ii) Vertical Long-Tube Evaporator:
This uses natural circulation, the flow of the product being either upward or downward. With upward flow, it is known as a rising/climbing film evaporator; and with downward flow, it is called a falling film evaporator.
(iii) Forced Circulation Evaporator:
Used for viscous liquids, with the help of either a centrifugal or positive pump.
(iv) Plate Evaporator:
This uses an arrangement of gasketed plates, in place of calandria, and operates on a single pass climbing and falling principle. It is characterized by the short heat-contact time of the product.
(v) Multiple-Effect Evaporator:
Most commonly used. The vapour from the first vacuum-pan/effect, which contains considerable latent heat, may be used to heat the second, and so on. Thus two or more effects can be utilized in the evaporator to improve economy of operation, as shown in Table 8.6.
Note:
The temperature of the first effect is comparatively higher with an increase in the total number of effects. Hence, in order to prevent heat-damage, the use of the double-effect evaporator alone is preferred for milk processing.
(vi) Centrifugal Evaporator (Centri-Therm):
In this, centrifugal force is used not only in applying the product on steam-heated cones but also in removing the condensed product, steam condensate and vapour.
(vii) Expanding-Flow Evaporator:
Operates on the same principle as the Centri-therm, but use of inverted cones and counter-current flow of the product as well as heating medium is believed to increase efficiency of evaporation.
(viii) Vapour Recompression:
Heat-energy is added to the vapour removed from the first-effect by a mechanical compressor of steam jet (thermo-compressor) for heating the second-effect.
(ix) Low-Temperature Evaporator:
This is particularly useful for removing water from a heat-sensitive product.
Important Operating Points for Evaporators:
These have been listed below:
(i) The evaporator may be operated either as a batch or continuous system;
(ii) It should be sanitized before admitting the product to the pan;
(iii) The product should cover the heating tubes (coils) before steam intake, so as to prevent scorching;
(iv) The product should be maintained at a uniform level in the evaporator. This is made possible by controlling the rate of fresh product intake so that the volume of water removed is replaced;
(v) Excessively rapid boiling is avoided as it is likely to increase entrainment;
(vi) Air leaks in the system should be avoided;
(vii) Single-effect evaporator (vacuum pan) is normally operated for milk at 54-60°C (130-140°F), or 63.5 cm. of mercury vacuum;
(viii) In order to stop the evaporator, the following steps should be taken in the sequence given- turn off steam; turn off water to the condenser; stop the vacuum pump; and open the vacuum relief;
(ix) Dry saturated steam is more desirable for vacuum pan operation than wet or super-heated steam.
Striking the Batch:
When the boiling milk approaches the desired concentration, there are visual indications which show that the final stage has been reached, viz., the milk ‘settles down’ to a quiet boil, its-surface assumes a glossy and glistening lustre, there is a heavy roll from the periphery towards the centre, etc.
These signs should warn the operator as to the right time for ‘striking the batch’. (This term indicates that the correct concentration, as determined by specific gravity/density tests, has been reached.)
The sampling of the condensed milk should, however, begin sufficiently early to permit taking and testing for density several successive samples without the risk of objectionable over-condensing. The standard testing temperature is 49°C (120°F), close to which the pan temperature usually drops towards the end of the condensing period.
The most practical density tests usually applied are:
(i) Pycnometer test;
(ii) Hydrometer test;
(iii) Refractometer test:
(iv) Viscosimeter test.
The Baumé Hydrometer test is most commonly used for density- tests of pan samples of condensed milk. The hydrometer scale may record the density either directly or indirectly, or both. For condensed milk, the Baumé Hydrometer ranges from 30′ to 37 Bé.
The temperature correction factor is 0.03 for each °F of deviation from the standard (120°F); for temperatures above the standard, ‘add’ and for those below the standard, ‘subtract’. However, for the most dependable results, the sample should be brought to the standard temperature adopted for density tests.
The specific gravity of condensed milk is obtained by the formula:
The specific gravity of liquids heavier than water is converted to the Baumé degree by the formula:
This is then corrected to the standard temperature used for the hydrometer test of the pan sample by the temperature correction factor.
Finishing the Batch:
On ‘striking the batch’ when the desired density has been reached, the condensing process is stopped. All steam to the pan is shut off, the valve in the water-line to the condenser is closed, the vacuum pump is stopped and the vacuum relief is opened.
The above operations should be carried out in the order stated to prevent milk from burning on to the heating surface and condenser water from flooding the pan. When the vacuum has been dissipated, the condensed milk is drawn from the pan. This should be done promptly.
Third Standardization:
Some manufacturers prefer to slightly over-condense the milk and then standardize it back to the exact concentration desired by the addition of the correctly calculated amount of water.
The following examples explain the method:
Problem I:
The batch sample tests 9.2% fat. The fat desired in condensed milk is 9.0%. The batch weighs 450 kg. How much water must be added to reduce the fat content to 9.0%?
Solution I:
The % fat of the batch is 9.2 or 1.022 times the amount desired. Hence the amount of water to be added is 450 × 0.022 or 9.9 kg. water.
Problem II:
(No means to weigh the batch.)
Weight of standardized fresh milk – 650 kg.
Fat in standardized fresh milk – 3.50 %
Fat desired in condensed milk – 9.00 %
Fat test of batch – 9.20 %
How much water must be added to reduce the fat in the batch to 9.00%?
Solution II:
7. Homogenization:
Hot condensed milk is invariably homogenized before it is cooled and crystallized by standard manufacturers of the product throughout the world. The object is to obtain a uniform fat emulsion and reduce fat separation to a minimum during storage. A special type of homogenizer suitable for handling a highly viscous product is used at a total pressure of 2500 psi (2000 psi in the first stage and 500 psi in the second stage).
8. Cooling and Crystallization:
Importance:
The cooling process occupies an important place in the manufacture of a marketable condensed milk. Prompt cooling is desirable to delay the tendency of age-thickening and discolouration, which is accelerated by prolonged exposure to heat. In addition, and even more importantly, on the method of cooling depends in a large measure the smoothness of texture of the finished product and its freedom from an objectionable sugar deposit.
Role of Lactose:
Lactose plays an important role in the successful control of the texture of condensed milk, which constitutes a highly concentrated solution of lactose. A considerable portion of the lactose content in the condensed milk held at ordinary temperature is present in crystal form.
The size of these crystals determines the relative smoothness of the product. Crystal size is also one of the factors upon which depends the presence or absence of sugar sediment in the container. The size of the lactose crystal is controlled by the procedure used for cooling the condensed milk.
Effect of Cooling Process on Texture of Condensed Milk:
The relative smoothness of condensed milk is controlled by the number and size of the lactose crystals it contains. It is the treatment which the hot sweetened condensed milk receives during the cooling process that determines very largely the number and permanent size of the lactose crystals. The correlation of the number and corresponding size of lactose crystals with degree of sandiness is given in Table 8.7.
Mechanism of Lactose Crystallization:
Under the temperature conditions that prevail in the manufacture of condensed milk, only α-lactose-hydrate will crystallize. The crystallization proceeds slowly because maintenance of the status of super-saturation of α-hydrate in solution requires the continued mutation from the highly soluble β-lactose anhydride to the less soluble α-hydrate form.
In condensed milk, the rate of lactose-crystallization is further impeded by the presence of milk colloids and the high viscosity which reduces the rate of diffusion. For normal condensed milk of average composition, the temperature of maximum rapidity of crystallization is approximately 30°C (86°F).
Importance of Mass Crystallization in Cooling of Condensed Milk:
The problem of ensuring a permanently smooth texture in the finished product is not to prevent the formation of lactose crystals during the cooling process, but to prevent the crystals that are present at the end of this process from subsequently growing larger. This is done by providing conditions in the cooling process that cause mass crystallization.
The Forced Crystallization Period:
The purpose of this is to produce mass crystallization of lactose. It is the period in batch cooling when condensed milk has reached, and is then held, at the temperature which helps mass crystallization. This temperature is optimum for seeding.
After seeding, the milk should be held at this temperature for at least an hour under vigorous agitation after which it should be cooled rapidly to the final temperature (for packaging). For the best results at the end of the condensing period, the hot condensed milk should be cooled from pan temperature as rapidly as possible to the seeding temperature.
Determination of Optimum Temperature for Forced Crystallization:
The optimum temperature for mass crystallization varies chiefly with the ratio of lactose to water in the condensed milk. For milks of fairly normal concentration, the variations are usually limited to an approximate range of about 30-40°C (86-104°F). The optimum temperature for forced crystallization for any particular batch of condensed milk can be determined by reference to the standard forced-crystallization curves.
Seeding Condensed Milk:
‘Seeding’ refers to the introduction of lactose in very fine powder form during the cooling process to provide nuclei for crystallization. The purpose of seeding is to give the lactose present in the supersaturated state an added incentive to crystallize. Further seeding at an optimum temperature for mass crystallization, with properly prepared seed lactose in the presence of vigorous agitation, yields crystals of uniform size.
The seed material commonly used is powdered lactose of commerce (α-lactose-hydrate) or pulverized non-fatty dry milk, or sweetened condensed milk from a previous batch. For best ‘results, the seed lactose should be able to pass through a 200-mesh screen consistent with the preservation of sharp crystal edges.
Since the standard powdered lactose of commerce does not normally have particles of a sufficiently fine size, it has to be re-ground. This should preferably be carried out by an Impact-Mill or Hammer-Mill type grinder.
If sterilization of the resulting lactose dust is desired, the following procedure (suggested by Whittier) is recommended: heat the powdered lactose of commerce to 93°C (200°F), preferably under vacuum.
This converts the α-lactose hydrate to the α-anhydride form. Then grind the α-anhydride using an impact pulverizer mill. Fill the resulting lactose dust into cans, preferably with a friction top. Seal the cans and sterilize them at approximately 130°C (266°F) for one to two hours. The lactose dust is now ready to be used.
Amount of Seed Material to Use:
It is good practice not to use more seed material than is necessary for optimum mass crystallization. This ensures superior smoothness of texture in the finished product. The seed lactose usage rate is about 375-500 gm. per 1000 kg. of the original fluid milk, or 0.1 to 0.3 per cent of condensed milk. Twice as much non-fat dry milk and condensed milk is required when it is used as the ‘seed’.
Method of Adding Seed Lactose to the Batch:
The seed lactose should not be added to the batch in dry form. Such a practice causes it to lump together into large aggregates which do not disintegrate readily. In this condition the seed material is incapable of inciting mass crystallization. A means of ensuring uniform dispersion of the seed material is essential.
One procedure consists of blending the ‘seed’ into a small amount of condensed milk and then adding this to the batch during vigorous agitation. The agitation must continue while crystallization takes place, in order to stimulate the formation of numerous small crystals of lactose (rather than a few larger ones). The above operation must be carried out under strict hygienic conditions.
Lactose Crystal Formation:
Rapid crystallization leads to the formation of a large number of small crystals, giving a smooth texture to the condensed milk; on the other hand, slow crystallization creates a small number of large crystals which produce a sandy or gritty texture. After ‘seed’ lactose is uniformly dispersed, cooling of the product may be continued slowly to 24°C (75°F).
This should take approximately an hour. Then the cooling is completed to 13-18°C (55-65°F) with continued agitation. The rate of crystal formation is controlled by the amount of agitation, number of nuclei, total solids in the product, temperature and viscosity. Vigorous agitation (stirring) during cooling is highly important; it increases the rate of crystallization, besides ensuring uniform small- sized lactose crystals in the batch system.
Finishing the Cooling Process:
After seeding and forced crystallization, cooling is resumed under constant agitation, as rapidly as possible, until the final temperature is reached. Agitation is then generally continued for another hour or longer, when the product is ready for packaging.
A line diagram of cooling and crystallization of condensed milk is given as follows:
Systems of Cooling:
The systems of cooling are:
(a) Batch
(b) Continuous flow;
(c) Combined Batch and Continuous; and
(d) Vacuum.
(a) Batch:
The equipment for cooling and crystallization consists of an especially designed tank or vat, with water-jacketed sides and bottom, and a powerful rotary agitator. These coolers are provided with nylon/rubber scrapers of special design that press closely to the cooling surface.
The cooling is done by controlled circulation of refrigerated water through the jacket. The milk is seeded at the proper temperature for mass crystallization and the cooling resumed at the end of the forced crystallization period. Some of the batch coolers are operated under vacuum.
(b) Continuous-Flow:
This system is represented by the internal tube counter-current. This type of cooler is used in large-scale operations, particularly when condensed milk is the main product of the plant.
(c) Combined Batch and Continuous:
A common combination is to use the continuous internal tube cooler from the pan to seeding temperature, and finish the operation in a crystallizer tank. Alternatively, the batch is run from the pan into the crystallizer tank, then cooled to seeding temperature, seeded, and the cooling finished by means of the continuous-flow internal-tube cooler.
(d) Vacuum Cooling:
Principle:
This system utilizes a high vacuum as the cooling medium; in other words, the temperature of milk in this cooler is reduced by evaporation under vacuum.
Equipment:
This consists mainly of a vacuum pan without a heating surface. The vapours arising from the milk are condensed in a counter-current condenser. The non-condensable gases and air are compressed and eliminated by multi-stage ejectors with intermediate condensers.
The water from the main condenser is discharged over a barometric leg, or by a centrifugal pump. To accelerate the circulation of the increasingly viscous milk, the cooler is equipped with a powerful rotary agitator, which assists proper thermo- circulation, promotes uniform cooling throughout the batch and expedites the rate of evaporation and cooling.
Operation:
The milk is condensed in the regular vacuum pan or evaporated to a predetermined point, after making due allowance for the additional evaporation that will occur in the cooler. The batch is dropped from the pan directly into the vacuum cooler. When the temperature in the cooler has been lowered to about 32°C (90°F), the batch is seeded (by blowing lactose ‘dust’ through the side of the cooler, using pressure difference as the motivating force).
This causes mass crystallization. At the end of the forced crystallization period, the milk is further cooled to 10°C (50°F). Agitation is continued throughout until the product is packaged.
Advantages over Other Systems:
(i) The time required for cooling is relatively short;
(ii) The cooling is uniform throughout the batch:
(iii) This cooling system yields mass crystallization; the size of most of the lactose crystals ranges from 5 to 8 microns;
(iv) The finished product has an exceptionally smooth and velvety texture;
(v) It enhances the keeping quality of the finished product.
9. Packaging:
The condensed milk is now ready for packaging. Bulk packaging may be done in barrels of various sizes, drums with polythene liners, or tin-containers. For the retail market, fillers are used to package condensed milk in cans. After filling, the cans are sealed, labelled and packed in cases for storage and distribution.
The retail cans are filled with automatic filling machines. In general, the filling machine consists of multiple-piston pumps. The cylinder charge can be adjusted to the size of the cans to be filled. It is important to fill the cans fully in order to exclude as much of the air from the container as possible.
Since cans filled with condensed milk do not undergo any subsequent sterilization, strict sanitary conditions should be observed during the filling process so as to prevent contamination, which will adversely affect the keeping quality of the finished product.
These sanitary measures may be listed as follows:
(i) The filling machine, connecting pipe-lines, etc., should be thoroughly cleaned and sanitized before use.
(ii) The filling and sealing room should be kept in strict sanitary condition, and preferably be closed to the outside and also to visitors during operation. Its air supply should consist of efficiently filtered pure air.
(iii) It is good practice to reject the first few cans when starting the machine (returning their contents to the forewarmer).
(iv) The filling machine should be hooded when not in use to protect it from dust, stray insects and other agencies of contamination.
(v) Working personnel should wear masks, so as not to breathe contaminated air into the product during filling. The personnel should also observe sanitary habits.
(vi) The cans and lids, on their passage to the filler, should be sterilized. (This is usually done by passing them under or over a battery of suitable gas jets.)
(vii) It is important to fill the cans fully (so as to exclude as much air from the container as possible) and seal them promptly after filling.
10. Storage:
The main consideration in the storage of condensed milk is the temperature of storage, which should be such as to prevent such defects as sandiness, sugar separation and viscosity changes. During storage, a wide temperature variation may increase the tendency to sandiness.
A very low storage temperature such as 0°C (32°F) or below may not only cause sandiness but also sugar (sucrose) separation. Cool storage is important to prevent changes in viscosity. The trend in recent years has been to store condensed milk at 10°C (50°F) or slightly below. The humidity of the surrounding air should be low (below 50 per cent) to check spoilage of cans and labels.