RECOMBINED MILK PRODUCTS
Milk is a perishable commodity, and therefore scarce in many countries with little or no dairy production of their own. In such countries raw milk is replaced partly or fully by milk powder as raw material.
Recombination is an alternative method of supplying a product that closely resembles fresh dairy milk to markets where the genuine article is not available. The manufacture of recombined milk and milk products has been well established in many countries around the world, and a variety of processes and equipment have been developed for this purpose.
The principles of the processes are much the same. The initial applications were fluid milk, but this was followed by production of recombined evaporated milk and sweetened condensed milk. Today recombination also includes yoghurt, butter and cheese.
The processes have been developed over the years from simple batch operations to sophisticated systems with high capacities.
The main processes in the basic reconstitution and recombining operations are:
- Raw material handling
- Weighing and mixing
- Filtration, homogenization and heat treatment
The following definitions are given as a guide to clarify certain expressions used in the industry.
Reconstituted milk is the liquid milk obtained by adding water to skim milk powder (SMP), whole milk powder (WMP) or their mix.
Recombined milk is the liquid milk obtained by adding water to SMP and adding milk fat separately in such a quantity that the desired fat content is achieved.
Reconstituted milk products are the products resulting from addition of water to the dried or condensed form of product in the amounts necessary to re-establish the specified water/solids ratio.
Recombined milk products are manufactured by mixing milk fat and milk solids-non-fat (MSNF), with water. This combination must be made so as to re-establish the specified fat to MSNF ratio and dry matter (DM) to water ratio.
Recombined modified milk and milk products are products made from dairy-product ingredients with compositions other than normal dairy products, e.g. flavoured products, butter from fractionated fat, or dietary evaporated or condensed milk.
Filled milks and milk products are “semi-dairy” products in which the milk fat is replaced by vegetable oils, e.g. liquid milk, evaporated milk, condensed milk or cheese. Alternative terms could be called “imitation” or “substitute” milk products.
Fortified milk is made from fresh milk, reconstituted milk or recombined milk with the addition of one or more ingredients of dairy products.
Toned milk is fresh milk mixed with reconstituted or recombined skim milk in order to prepare normal composition milk or modified milk from high-fat milk, by adjusting the MSNF.
Anhydrous milk fat (AMF) is a pure dairy fat product obtained from fresh milk, cream or butter to which no neutralizing substances have been added.
Anhydrous butter oil is an all-fat product made from cream or butter of unspecified age.
Butter oil is a product made from cream or butter of unspecified age which may have a lower fat content.
Vegetable oils are refined, bleached, deodorized oils, preferably coconut, palm and soybean oils.
Non-fat solids for recombined milk are usually supplied in the form of skim milk powder. This is made by skimming the fat from the whole milk in centrifugal separators and then removing the water from the skim milk by evaporation and drying. The powder can be stored for months, or even years, without being spoiled, and dissolves easily in water to form reconstituted skim milk.
The most commonly used method of classifying skim milk powder (SMP) is to refer to the heat treatment, to which the skim milk has been exposed prior to evaporation and spray drying.
During the heat treatment of milk, the whey proteins are denatured to different degrees, depending on the temperature/time relationship. The degree of denaturation can be classified according to the Whey Protein Nitrogen Index (WPNI), which was discussed in Chapter 17.
The different recombined milk products usually require skim milk powder of various types of heat classification, (see Table 18.1).
Milk powder is typically supplied in 25 kg plastic-lined laminated bags.
In smaller plants, the powder is often emptied by hand, direct from the bags into the mixing system, but in the larger plants, the bags are emptied automatically. Even more sophisticated is the use of silo tanks to which the powder from the emptied bags is transferred pneumatically.
There are also rational methods for transporting milk powder to recombining plants in bulk bins containing 200 – 1,000 kg. The size of the containers is limited by the handling facilities in the locality receiving the powder.
Dissolving of milk powder
The dissolving properties of milk powders are very important for the recombined product quality and are affected by the following factors:
- Ability to sink
Analytical methods for these properties are given in:
- Standards for Grades of Dry Milk, Including Methods of Analysis, American Dairy Products Institute, Inc., USA
- ISO/IDF standard methods
The degree of wettability is very much a function of the particle volume, and especially of the capillarity.
Agglomerated powders have improved capillarity, resulting in increased wettability. Increased particle size (130 – 150 µm) also results in improved wettability. Good wettability is less than 30 seconds.
Ability to sink
The ability to sink is a function of specific volume and particle size. Agglomerated powders normally have the best ability to sink.
Good dispersability is obtained when powders added to the water are distributed as single particles, leaving no lumps. The structure of the powder particles, as well as the configuration of the protein molecules, is of importance. A powder with a high content of denatured proteins is very difficult to disperse. A dispersability of at least 90 % is normal for milk powders for recombination.
This property describes how well the powders dissolve and measures the amount of nondissolving protein. How good the solubility is depends very much on the technology used for production of the powder.
A good solubility index should be as low as 0.25 ml undissolved sediment in 50 ml recombined milk.
- Ability to sink
Fats and oils
Unsalted butter can be used in the manufacture of recombined milk products, but it must be kept under refrigerated storage.
The most common source of milk fat for recombination is anhydrous milk fat (AMF), which does not require such storage. It is typically packed in
19.5 kg cans or 196 kg drums. Provided that care is taken in the manufacture of the product, and that air is excluded by packing the product under inert gas (nitrogen), AMF will keep for 6 – 12 months even at elevated ambient temperatures of 30 – 40 °C.
Milk fat packed in cans can be melted by immersion in hot water at 80°C for 2 – 3 hours. Drums of AMF, however, require longer melting times. The normal method is to store the drums in a hot room at 45 – 50 °C for 24 – 28 hours before use, or to use a steam chest or tunnel which can melt the contents of the drums in about 2 hours. Once melted, the AMF should be transferred to a jacketed holding tank with facilities for maintaining the temperature.
Similar handling systems can also be employed when non-liquid vegetable oils are used in production of recombined “filled“ milk products.
Water is a raw materials in all types of reconstituted and recombined milk products. It must be of good drinking quality, free from harmful microorganisms and of acceptably low hardness, expressed as calcium carbonate (CaCO3) and should be below 100 mg/l, corresponding to about 5.5°dH. As only “distilled” water is removed in the production of milk powder, the water used for reconstitution or recombination must also be pure; an excessive mineral content will jeopardize the salt balance and heat stability of the reconstituted or recombined product, which in turn will cause problems in pasteurization, not to mention sterilization or UHT treatment.
Too much copper or iron in the water may cause off-flavours, due to oxidation of fat.
The maximum levels recommended are therefore:
- Cu (copper), mg/l 0.05
- Fe (iron), mg/l 0.1
See also Chapter 6.11, Table 6.11.1 regarding specifications for water.
Dry additives such as sugar, stabilizers and emulsifiers can be handled in the same way as the milk powder, i.e. they are dumped from the bags either directly into the mixing vessel or into the mixing system.
Recombination of milk products
Temperature and hydration time
The solubility of milk powder increases when the water temperature increases from 10 to 50 °C. There is no increase between 50 and 100 °C. Low-heat powder is easier to dissolve than high-heat powder. It is important that the proteins obtain their normal state of hydration, which takes less than 20 minutes at 40 – 50 °C.
As a rule fresh, high-quality powder requires the shortest hydration time. Insufficient hydration time may lead to a “chalky” defect in the final product. Recombined milk for cheese manufacture should be given two hours’ hydration time.
It is possible to reconstitute at 10 °C and then store the milk at this temperature overnight to obtain maximum hydration. However, more powder particles remain undissolved at a mixing temperature of 10 – 20 °C compared to 35 – 45 °C, even if the mix is kept for 24 hours. In milk with 8 % dry solids, the difference is very small. The proportion of undissolved powder in milk that is heated to at least 40 °C after reconstitution is very small, even in a mix with 26 % dry solids.
The air content of reconstituted milk increases at lower mixing temperatures.
The fat must be added at a temperature above its melting point. To assure this, AMF must be added at above 40 °C. The recombined milk should not be kept at this high mixing temperature for more than two hours because of risk for bacterial growth.
Fat addition and emulsification
Incorporation of fat into recombined products has always been a relatively
difficult operation. The fat has to be properly dispersed and emulsified, and this places certain requirements on the processing equipment and process parameters.
Traditionally, the melted fat is metered into the line during continuous operation, followed by thorough mixing in a static or mechanically operated mixer before entering a homogenizer.
In modern systems using high-shear mixing devices, fat can be
dosed directly into the mixing vessel of the unit. The dispersion of fat is
sufficient to create a stable emulsion, which allows eliminating homogenization at this stage of the process. Later on, the recombined products will be finally homogenized during pasteurization or UHT processing.
In small scale batch production, fat is sometimes added to the milk in a mixing tank. To ensure that the composition of the product is uniform when it is pumped to downstream processing, the milk has to be thoroughly agitated, often with a high-shear agitator. Even when a homogenizer is integrated into the system, it is important that the fat in the feed is uniformly distributed.
An emulsifier is sometimes added to facilitate and improve the
emulsification of milk fat.
Recombined cream can be made from skim milk powder or buttermilk powder and anhydrous milk fat to a fat content of about 40 %. The stability is improved by addition of emulsifiers and stabilizers.
Skim milk powder normally contains a total of about 40 % air by volume, consisting of occluded and interstitial air. The mixing equipment may also cause air addition if not properly operated and maintained.
Tests indicate that the air content in reconstituted skim milk, dissolved at 50 °C and with 14 to 18 % dry solids, is the same as in normal skim milk. At a mixing temperature of 30 °C, the air content is 50 – 60 % higher even after a holding time of one hour. With 41 % dry solids, the air content of the mix was 10 times higher than in normal skim milk.
Too much air in the recombined milk has the following disadvantages:
- Burning-on in the heat exchanger
- Cavitation in the homogenizer
- Whey formation in cultured-milk products
- Increased risk of oxidation of fat
As recombination is accompanied by foaming, the volume of the mixing tank(s) should be about 20 % larger than the volume of the batch, to avoid foam forcing its way out of the manhole.
Some mixing equipment make it possible to recombine products under vacuum. By constantly extracting air from the mixing vessel, a certain vacuum is maintained, which reduces foaming to a minimum.
Proportioning of skim milk powder is based on the simple rule that the weight of the powder is one tenth of the weight of milk produced. For small plants, manual emptying of a calculated number of sacks of a given weight into the mixing tank is the easiest and most practical solution, but production can be mechanized for higher capacities.
Milk powder handling creates a lot of dust. Large-scale sack-emptying therefore requires special equipment, including a dust collecting unit, as seen in Figure 18.1.
Powder can also be supplied in containers. In this case, suitable equipment comprises a variable-speed screw feeder, which takes powder from the bottom of the container and discharges it into the mixer. The container can be lifted into position by a tilting rack, (Figure 18.2), or by a hoist.
In highly mechanized plants, the powder is supplied in bulk. It is stored in bulk silos and transferred pneumatically to a day bin, from which it is batched into the process via a weighing hopper and a screw feeder. The system in Figure 18.3 also includes a unit for central dust collection.
When a vacuum mixing system is used, the powders are sucked into the mixing vessel from specially designed powder silo tanks. It is also possible
to prepare mixtures of different dry ingredients in these silos before
dissolving takes place.
Design of recombination plants
Recombination plants are built for capacities of up to 20,000 l/h. In larger plants, parallel lines are installed to meet higher capacity requirements.
The sequence in a large plant is essentially the same as in a small one, except that it requires more tanks for storage and melting of fat, mixing, and buffer storage of the finished product. The degree of mechanization may also differ as described above.
In large plants, it is necessary to use weighing tanks for fat dosage, in order to achieve the necessary accuracy. In a smaller plant, the weighing tank can often be replaced by a proportioning pump.
In small plants, where mixing of material in a processing tank is just enough, the product will be naturally and satisfactorily deaerated if a reconstitution temperature of approx. 40 °C has been maintained and, when all powder has been dissolved, the resultant solution is allowed to stand for 20 minutes with the agitator switched off.
The same procedure should also be applied in large-scale production. To maintain uninterrupted production, however, it is advisable to deaerate the product by vacuum treatment in connection with heat treatment.
The design of the plant is influenced not only by its capacity, but also by the method of heat treatment of the recombined milk. Three alternative methods are used:
• Pasteurization at a temperature of at least 72 °C for 15 seconds
followed immediately by cooling to 4 °C.
• UHT treatment by direct or indirect heating to 132 – 149 °C
for a few seconds, followed by cooling to approximately
20 °C prior to aseptic packing.
In very small-scale production, 1,000 – 2,000 l, batch processing is most common. Mixing and processing are carried out in a jacketed mixing tank with a two-speed shearing agitator and with heating and cooling facilities. The plant is shown in Figure 18.4. However, such low production capacities are more and more unusual.
After a suitable volume of water has been measured into the tank and heated to 43 – 49 °C, powder is added steadily and agitation applied until all the powder has dissolved. The resulting solution should be allowed to stand for 20 minutes with the agitator switched off. At the end of this time, the agitator is restarted and the temperature is raised to 54 – 65 °C, before milk fat is added. This has previously been liquefied by being held in a warm room at 38 – 45 °C. If processing is to be continued in the tank, the agitator is switched to high speed for several minutes to disperse the fat. The agitator is then switched to the stirring position, and the process concludes with pasteurization and homogenization followed by cooling to packing temperature.
Figure 18.5 shows a large recombination plant for continuous operation, where the fat is dosed into the vessel of the mixer.
Water of food quality is metered into one of the mixing tanks (4). On the way, it is heated in a plate heat exchanger, as the skim milk powder dissolves more easily in warm water than in cold.
The circulation pump is started when the tank is half full and water flows through a bypass line from the mixing tank to a high-shear mixer (3).
In the high-shear mixing unit shown in Figure 18.6, the dry ingredients and fat are added to a mixing vessel filled with water at a rate determined by the size of the selected module. Anhydrous milk fat is added from the fat storage tank (1).
The heart of the mixing system is the mixing head (Figure 18.6), which is located at the bottom of the mixing vessel. It consists of a rotor and perforated stator, and is designed for three functions; mixing, pumping and dispersing. The ingredients and the liquid are sucked down into the mixing unit by a pump wheel and pressed out through the holes in the perforated ring by impeller wings underneath the pump wheel. When passing the perforated stator, high shear forces will effectively dissolve and disperse the added ingredients. As the outlet of the mixer is positioned after the mixing unit, all added ingredients have to pass through the mixing unit at least once.
The agitator in the mixing tank (4) is started at the same time as the circulation pump. Water continues to flow into the tank while mixing is in progress until the specified quantity has been supplied.
When all the powder has been added, the agitator and the circulation loop are stopped and the contents of the tank are given a certain resting interval for hydration of the proteins. This will take about 20 minutes at a temperature of 35 – 45 ºC. At the end of this period, the agitator is restarted.
When all the ingredients have been added to the first batch, the process is repeated in the next tank.
The skim milk/fat mixture is drawn from the full mixing tank by the pump, which forwards the mixture through duplex filters (6) where any foreign objects such as pieces of string or sacking are trapped. After being pre-heated in the heat exchanger (7), the product is pumped to the homogenizer (9), where the dispersion of fat globules is completed.
During the powder-mixing operation, the product may pick up large volumes of air, which can cause burning-on in the pasteurizer as well as homogenization problems. A vacuum deaerator vessel (8) can be installed in the line before the homogenizer to eliminate this. The product is pre-heated to 3 °C above homogenization temperature before being flashed in the deaerator, where the vacuum is adjusted so that the outgoing product has the correct homogenization temperature, typically 65°C.
The homogenised milk is pasteurized and chilled in a plate heat exchanger (7) before being pumped to storage tanks (10) and further to packaging or to UHT treatment.
When the fat can not be added in the mixing step, the fat has to be added through an in-line injector just before homogenization. All steps are the same as in Figure 18.5, except that the liquid fat is continuously metered into the flow by a positive displacement proportioning pump. A static mixer completes the blending downstream the injector and before entering the homogenizer.
In some mixing equipment, mixing under vacuum is possible. In Figure 18.7 a recombination plant with a vacuum mixer is shown. Since vacuum is applied in the mixing vessel, very little air will be incorporated in the product during the mixing process, and foaming will also be minimized. When using vacuum during mixing, the ingredients are sucked into a vessel filled with water (2) at a point below the liquid surface. The wetting of the powders is thus improved and the risk of floating powder lumps on the surface is eliminated. Another benefit of vacuum mixing is that it facilitates automatic powder addition, as the powders are sucked directly from silo tanks (5) into the mixing vessel (2). This also makes it possible to organize the handling of the dry ingredients in a separate room, and to have the mixer and mixing plant in the processing hall.
It is necessary to consider the handling of the recombined milk in
conjunction with the planning of the plant. This ensures that the product reaches the consumer in good condition.
Recombined milk normally flows directly from the production line to packing. A buffer tank may be needed to compensate for temporary stoppages in the production or packing lines. In the case of sterilized milk, this tank must be of aseptic design (Figure 18.8), to avoid the risk of reinfection.
Once sterile milk has been packed, it can be stored in any conditions provided that the packages are intact. Pasteurized milk must be kept in cold storage rooms. UHT-treated and sterilized milk are to be preferred in markets where the refrigeration chain is absent or incomplete.
The milk should be packed as soon as possible after production. UHT-treated milk must flow in a closed aseptic system to the aseptic carton filling machine.
Pasteurized milk can be packed in paper or plastic packs or glass bottles. If bottles are used, they should be of dark glass, which prevents the flavour of the milk from being spoiled by exposure to light.
The package must always be airtight to protect the milk from oxidation. It should also be strong enough for stacking in crates or boxes.
Since UHT treated and sterilized milk can be stored in ambient temperature the whole distribution chain is more simple than for pasteurized milk, which need to be refrigerated all the time. UHT-treated milk can, for example, be transported on an ordinary lorry for long distances and exposed for sale in a shop with no refrigeration facilities where people come to buy perhaps once a week. Pasteurized milk, on the other hand, requires a refrigeration chain of insulated distribution vans, chilled counters in the shops, daily shopping and, preferably, home refrigerators.