The method of preserving milk by sterilizing evaporated milk in sealed containers was developed at the beginning of the 1880s. Earlier, in about 1850, the method of preserving evaporated milk by the addition of sugar had been perfected by an American. The manufacture of condensed milk, using these two methods, has developed into a large-scale industry. Later production by using continuous heating systems followed by aseptic packaging technology has been introduced.
A distinction is made between two different types; unsweetened and sweetened condensed milk. Both products can be made from fresh milk or recombined milk (milk powder, fat and water).
Unsweetened condensed milk (also called double concentrated milk) is a sterilized product, light in colour and with the appearance of cream. The product has a large market, for example in tropical countries, at sea and for the armed forces. It is also used where fresh milk is not available as a fresh milk substitute. In many countries where normal milk is available, it is used as a coffee whitener.
Unsweetened condensed milk is also used as a substitute for breast milk. In this case, vitamin D is added.
The condensed milk is made from whole milk, skim milk or recombined milk with skim milk powder, anhydrous milk fat (AMF) and water (see also Chapter 18, Recombined milk products). The increased dry matter is reached by either evaporation of fresh milk or by recombination by milk powder.
Outline of condensed milk
The evaporated product, the unsweetened condensed milk, is normally packed in cans, which are then sterilized in autoclaves or horizontal sterilizers, or UHT-treated and aseptically packed in paperboard packages.
Sweetened condensed milk is basically concentrated milk, to which sugar has been added. The product is yellowish in colour and high viscous. The high sugar concentration in sweetened condensed milk increases the osmotic pressure to such a level that most of the microorganisms are destroyed. This product is not heat treated after packaging as its high sugar content preserves it for a long shelf-life. The sugar concentration in the water phase must not be less than 62.5 % or more than 64.5 %. At the latter level, the sugar solution reaches its saturation point and some sugar may then crystallize, forming a sediment.
The manufacturing processes for these two products are shown as block diagrams in Figures 16.1 and 16.2.
The first stage, in both cases, comprises precision standardization of the milk fat content and the dry matter content. This is followed by heat treatment, which serves partly to destroy the microorganisms in the milk, and partly to stabilize the milk, so that it will not coagulate in the subsequent sterilization process. Raw material requirements and the initial treatment are identical for both products. After that, the processes differ slightly.
Unsweetened condensed milk
In the manufacture of unsweetened condensed milk (also called evaporated milk), the heat-treated milk is pumped to an evaporator, where it is concentrated. The concentrate is then homogenized and cooled. Checks are carried out on the coagulation stability, and a stabilizing salt, usually disodium or trisodium phosphate, is added if necessary. In the case of canned products, the concentrate is packed and sterilized in an autoclave. The cans are cooled before being placed in storage.
In the case of UHT-treated product, the stabilized concentrate is first sterilized and then packed aseptically.
Figure 16.3 shows the process stages in the manufacture of unsweetened condensed milk from fresh milk as raw material. Similar technology is utilized also when production of concentrated milk is based on recombination. In this case, standardization is taking place during recombination or reconstitution.
The quality of the raw material for condensed milk is basically the same as that used in the manufacture of ordinary long-life milk products.
There are two important considerations for the manufacture of condensed milk:
- The number of spores and heat-resistant bacteria in milk
- The ability of the milk to tolerate intensive heat treatment without coagulating (thermal stability).
Bacteriological quality of the raw material
Evaporation takes place under vacuum at a temperature which should not exceed 65 – 70 °C. At temperatures below 65 °C, spores and heat-resistant bacteria will have ideal growth conditions, which could result in the entire process being spoiled. Precise control of the bacteria in the process is thus an essential requirement in the manufacture of condensed milk.
Thermal stability of the raw material
The ability of milk to withstand intensive heat treatment depends to a great extent on its acidity, which should be low, and on the salt balance in the milk. The latter is affected by seasonal variations, the nature of the fodder and the stage of lactation. It is possible to improve the ability of the milk to withstand the required level of heat treatment by additives or pre-treatment.
Pre-treatment is essentially for the final quality and includes standardization of fat content, solids-non-fat, as well as heat treatment.
Condensed milk is marketed with a stipulated content of fat and dry solids. The figures vary with the applicable standard, but are normally 7.5 % fat and 17.5 % solids-non-fat. This product is often called “double concentrated milk” (25 % total solids). Another common standard is “triple concentrated milk “with 33 % total solids (often 4 – 10% fat content).
Modern automatic standardization systems permit continuous and extremely accurate standardization of both fat content and the relation between fat content and solids-non-fat of the basic milk. More information on standardization will be found in Chapter 6.2, Centrifugal machines and milk fat standardization systems.
Before being sterilized, the standardized milk undergoes intensive heat treatment to destroy microorganisms and to improve its thermal stability. The heat treatment, often integrated in the evaporation plant, takes place in a tubular or plate heat exchanger at a temperature of 100 –120 °C for 1 – 3 minutes, followed by chilling to about 70 °C before the milk enters the evaporator.
During heat treatment a great part of the whey proteins is denatured, while calcium salts are precipitated. In this way, the protein complex of the milk is stabilized so that it can withstand subsequent sterilization, without coagulation during the process or subsequent storage.
The nature of the heat treatment will largely determine the colour and viscosity of the end product, and is thus extremely important for the product quality.
The evaporator is usually of the multistage falling-film type. The milk passes through steam-heated tubes under vacuum. Boiling takes place at between 65 and 70 °C. The dry matter content of the milk increases as the water is boiled off. The density is checked continuously.
The concentrated milk is pumped from the evaporator to a homogenizer, which operates at a pressure of 5 – 25 MPa (50 – 250 bar). Homogenization disperses the fat and prevents the fat globules from coalescing during subsequent sterilization.
Homogenization should not be too intensive, because that might impair the stability of the protein, with the consequent risk of the milk coagulating during sterilization. It is therefore necessary to find the exact homogenization pressure that is high enough to produce the required fat dispersion, yet low enough to eliminate the risk of coagulation.
For in-can sterilization, the pressure is generally between 125 and 250 bar (two-stage). In UHT processing, homogenization during pre-treatment is normally low, in order to avoid separation during storage of the concentrate prior the final heat treatment. The main homogenization then takes place in the UHT treatment normally in downstream (aseptic) position.
Final standardization and intermediate storage
After homogenization the pre-treated milk is cooled to about 14 °C before packaging, or to 5 – 8 °C, if it will be stored to await sample sterilization. A final check of the fat content and the solids-non-fat is usually made at this stage.
As mentioned previously, the heat stability of the condensed milk can be improved by the addition of stabilizing salts, usually sodium phosphates. The quantity of phosphate to be added is determined by sample sterilization, to which varying amounts of stabilizing salts are added. Tests are needed because variations occur between batches of milk. It is a time-consuming test-procedure and further processing must wait until the results are available. In the meantime, the concentrate must be stored. However, long-term storage should be avoided, not only to prevent bacterial growth, but long cold storage may increase the tendency of age gelation of the final product.
Any addition of vitamins is also done at this stage.
Canning machines for condensed milk automatically fill and seal the cans before sterilization. The canning temperature is selected to give the lowest possible froth formation.
The filled and sealed cans pass from the filling machine to the autoclave, which operates either continuously or on the batch principle. In the batch autoclave, the cans are first stacked in special crates, which are then stacked inside the autoclave. In the continuous autoclave, the cans pass through on a conveyor belt at a precisely controlled speed (see also Figures in Chapter 9, Long-life milk).
In both types, the cans are kept in motion during sterilization to distribute the heat more quickly and more evenly through the cans. Any protein precipitated during heat treatment is uniformly distributed throughout the milk. After a certain period of heating, the milk reaches the sterilization temperature of 110 – 120 °C. This temperature is maintained for 15 – 20 minutes, after which the milk is cooled to storage temperature.
The heat treatment is intense. This results in a light brown colouration, because of chemical reactions between the protein and the lactose (Maillard reaction or browning reaction).
UHT treatment, mainly in tubular heat exchanger plants (described in Chapter 9, Long-life milk), can also be used for heat treatment of condensed milk. In this case, following sample sterilization and addition of a stabilizer, if required, the milk is pumped to the UHT plant, where it is heated to 122 –140 °C for a period ranging from 4 seconds to 8 minutes.
The time/temperature combination of the UHT treatment as well as the homogenizing conditions (mainly conducted aseptically, but sometimes in both aseptic and non-aseptic modes) are to main extent determining the final product colour, viscosity and storage stability. After cooling, the milk is packed aseptically in paperboard packages and stored.
Storage and inspection
The canned condensed milk can be stored for practically any length of time at a temperature of 0 – 15 °C. The milk goes brownish if the storage temperature is too high, and protein may precipitate if the storage temperature is too low.
UHT treated condensed milk has normally a shelf-life of 6 – 9 months.
Sweetened condensed milk (SCM)
Sweetened condensed milk can be made from whole milk or skim milk, or recombined condensed milk – based on skim milk powder, anhydrous milk fat (AMF) and water.
In the manufacture of sweetened condensed milk the heat-treated milk is pumped to the evaporator, where it is concentrated. Sugar in solution is usually added to the concentrate during evaporation, but the sugar can also be added dry, in the correct proportion calculated on dry substance, before evaporation. After concentration, the product is cooled in such a way that the lactose forms very small crystals in the supersaturated solution. These crystals must be so small, less than 10 µm, that they cannot be detected by the tongue. After cooling and crystallization, the sweetened condensed milk is packed.
Figure 16.4 shows a process line for sweetened condensed milk manufactured from fresh milk. Before evaporation, the fat and solids-non-fat values of the milk have been standardized to predetermined levels in the same way as for unsweetened condensed milk. The milk has also been heat-treated to destroy microorganisms and enzymes which could cause problems and to stabilize the protein complex. Heat treatment is important to the development of product viscosity during storage, and is particularly important in the case of sweetened condensed milk. A gel can form if the heat treatment is too severe. The milk is usually heat-treated at 82 °C for 10 minutes if a product with a relatively high viscosity is required. If a low-viscosity product is required, the temperature/time combination should be at a higher temperature for a shorter time, for example 116 °C/30 sec.
The addition of sugar is a key step in the manufacture of sweetened condensed milk. It is important that the correct proportion is added, as the shelf life of the milk depends on its osmotic pressure being sufficiently high. A sugar content of at least 62.5 % in the aqueous phase is required to produce an osmotic pressure high enough to inhibit the growth of bacteria.
Two methods are used for addition of sugar:
- Addition of dry sugar before heat treatment
- Addition of sugar syrup in the evaporator.
The stage at which the sugar is added affects the viscosity of the end product. One theory maintains that early addition of sugar can cause the product to become too viscous during storage.
- Addition of sugar and evaporation
- Cooling to about 30 °C
- Seeding and subsequent cooling to 15 – 18 °C (sugar crystallization)
- Canning (or packing) and inspection
Evaporation of sweetened condensed milk is carried out in essentially the same way as for unsweetened. When sugar is added in the evaporator, the syrup is drawn into the evaporator and mixed with the milk at the half-way stage of the process. Evaporation then continues until the required dry matter content has been reached. The dry matter content is checked indirectly by determining the density of the concentrate.
Some manufacturers homogenize the concentrate at 5 – 7.5 mPa (50 – 75 bar) immediately after evaporation, as a measure to regulate the viscosity of the end product.
Cooling and crystallization
Sweetened condensed milk must be cooled after evaporation. This is the most critical and important stage in the whole process. The water in the condensed milk can only hold half the quantity of lactose in solution. The remaining half will therefore be precipitated in the form of crystals. If the surplus lactose is allowed to precipitate freely, the sugar crystals will slowly become larger and the product will be gritty and unsuitable for many applications. It is consequently preferable to control the crystallization of lactose, so that very small crystals are obtained. The largest crystal size permitted in first-grade milk is 10 µm. These crystals will remain dispersed in the milk under normal storage temperatures, 15 – 25 °C, and are not felt on the tongue.
The required crystallization is accomplished by cooling the mixture rapidly under vigorous agitation, without air being entrapped, and very often by flash cooling. Seed crystals, in the form of finely ground lactose crystals, are added at a rate of about 0.05% of the total mix, either as powder or as a slurry, when the milk has cooled to crystallization temperature (about 30 °C). This is the temperature at which the sugar solution is supersaturated, so that the seed lactose does not dissolve. However, the temperature must not be so low that spontaneous nucleation can occur before the seed crystals are mixed in.
The viscosity of sweetened condensed milk is high, which means that a very robust agitator in storage tanks is needed.
Packing and inspection
Sweetened condensed milk should be yellowish in colour and has a high viscosity. Traditionally, it is packed in cans, which in this case must be cleaned and sterilized before filling, as no sterilization takes place after canning.
Nowadays, it is also possible to pack sweetened condensed milk in aseptic paperboard packages.