Bangladesh Milk Producers Co-operative Union Ltd. (BMPCUL) is one of the largest national level cooperative organizations in Bangladesh. In the late 1960’s, two loss making dairy organizations were amalgamated by the Government to form the Eastern Milk Producers Co-operative Union Ltd (EMPCUL).
The federal union was called ‘Milk Union’. It used ‘Milk Vita’ as a brand name for its products. In the mid 1970’s, the Government of Bangladesh initiated a co-operative dairy endure with the financial and technical assistance from UNDP, FAO and DANIDA. Three chilling plants and one pasteurization /processing plant were commissioned in rural milk pocket areas. One processing and packaging plant was set up in Dhaka city for standardization of liquid milk and marketing of pasteurized milk and milk products to the city dwellers. In 1977, the name of the organization was changed to Bangladesh Milk Producers Co-operative Union Ltd (BMPCUL). Initially the co-operative started its activities in 110 village primary co-operatives having 4304 nos. of household members in four districts, procured 0.85 million liters of milk and paid Taka 1.85 million to the producers. In spite of gradual increased milk collection, extended support services for cattle development and marketing activities, the co-operative was a losing concern till1990-91 financial years. Development of management skills and commercial approach in business operation led the co-operative to emerge as a profit making organization since 1991-92 and its ever-increasing business success is continuing year after year. The developmental activities of 1998-99 financial year revealed that the co-operative procured 29.5 million liters of milk from 390 village milk co-operative societies spread in15 districts at a cost of Taka 467.42 million. The 1997/ 98 audited accounts of Milk Vita indicated a net profit of Taka 47.8 million (US $ 1.0 million) on a turnover of Taka 490.5 million (US$ 10.0 million)-much of which was distributed as a dividend to the milk producers. Four additional chilling centers were already set up in the milk pocket areas and one instant milk powder plant of 100,000 liters processing capacity per day commissioned at own fund and three more chilling centers are in pipeline for set up. Milk collection target for 1999-2000 financial years is 32.5 million liters. The current daily milk collection quantity is 115,000 liters and sale volume is around 90,000 liters. The direct beneficiaries of this co-operative organization are 80,000 landless, small and marginal household milk producers of 800 village primary milk co-operative societies which acronyms VMPCS. Other beneficiaries are- 300,000 family members, 800 employees of VMPCS, 300 rickshaw pullers of Dhaka city engaged in milk transportation to the retail shops and 700 employees of different dairy plants and Head Office. Having pasteurized liquid milk and other milk products at their doorsteps daily also benefits millions of city dwellers.
Milk Vita is the trade name for dairy products manufactured by the Bangladesh Milk Producers Cooperative Union Limited. It is established under the co-operative fold, the organization, popularly known as Milk Vita, makes necessary efforts to fulfill the demand for milk and milk products of city dwellers by collecting milk from remote places of the country. Presently, Bangladesh Milk Producers Co-operative Union Limited operates in 24 milk shed areas of the country viz Tangail, Manikganj, Tekerhat, Baghabarighat, Rangpur, and Sreenagar. It collects milk through networks established by its primary co-operative societies. BMPCUL is the central union of a total of 800 Primary Milk Producer’s Co-operative Societies and has a membership of about 3,00,000 milk-producing farmer-members. To become a member of a rural primary society, farmers have to own a milking cow and have to buy a share of Tk 10.00 and pay Tk 1.00 as admission fee. To maintain membership, a farmer has to supply at least 150 liters of milk in a year.
Members supply milk to societies twice a day on cash payment with a preferential system of weekly basis matched on the market day of each area. The rate of the farmer’s milk is decided on the basis of fat and solid non-fat (SNF) percentage. Milk collected from cooperative societies is transported to the nearest plant for preliminary processing and afterwards milk of Tangail, Manikganj, Tekerhat and Sreenagar areas is brought to Dhaka for the processing of liquid milk and production of cream, ice-cream, flavored milk and sweet yogurt. Milk collected from Rangpur and Baghabarighat areas is processed at Baghabarighat Dairy Plant for Powder Milk, Butter and Ghee (butter oil) production. All products of the organization are marketed under the trade name of ‘Milk Vita’.
Milk Consumptions & Constituents
World wide milk is a widely consummated natural product. It is said an ideal food because almost all of the natural minerals and vitamins is found in milk. In the following sections the consumptions and constituents will discussed.
World-wide Milk Consumption and Production
The total milk consumption (as fluid milk and processed products) per person varies widely from highs in Europe and North America to lows in Asia. However, as the various regions of the world become more integrated through travel and migration, these trends are changing, a factor which needs to be considered by product developers and marketers of milk and milk products in various countries of the world.
Even within regions such as Europe, the custom of milk consumption has varied greatly. Consider for example the high consumption of fluid milk in countries like Finland, Norway and Sweden compared to France and Italy where cheeses have tended to dominate milk consumption. When also consider the climates of these regions, it would appear that the culture of producing more stable products (cheese) in hotter climates as a means of preservation is evident. Table 1 illustrates milk per capita consumption information from various countries of the world. Table 2 shows the quantity of raw milk produced around the world.
|Country||Liquid Milk Drinks (Liters)||Cheeses (kg)||Butter (kg)|
|United Kingdom (2005)||111.2||12.2||3.7|
|European Union (25 countries)||92.6||18.4||4.2|
|New Zealand (2005)||90.0||7.1||6.3|
Table 2. Cow milk production (‘000 tones) in selected countries in the world (2006).
Composition and Structure
The role of milk in nature is to nourish and provide immunological protection for the mammalian young. Milk has been a food source for humans since prehistoric times; from human, goat, buffalo, sheep, yak, to the focus – domesticated cow milk. Milk and honey are the only articles of diet whose sole function in nature is food. It is not surprising, therefore, that the nutritional value of milk is high. Milk is also a very complex food with over 100,000 different molecular species found. There are many factors that can affect milk composition such as breed variations (cow to cow variations, herd to herd variations – including management and feed considerations, seasonal variations, and geographic variations. With all this in mind, only an approximate composition of milk can be given:
87.3% water (range of 85.5% – 88.7%)
3.9 % milk fat (range of 2.4% – 5.5%)
8.8% solids-not-fat (range of 7.9 – 10.0%):
Protein 3.25% (3/4 casein)
Minerals 0.65% – Ca, P, citrate, Mg, K, Na, Zn, Cl, Fe, Cu, sulfate, bicarbonate, many others
Acids 0.18% – citrate, formate, acetate, lactate, oxalate
Enzymes – peroxidase, catalase, phosphatase, lipase
Gases – oxygen, nitrogen
Vitamins – A, C, D, thiamine, riboflavin, others
Not only is the composition important in determining the properties of milk, but the physical structure must also be examined. Due to its role in nature, milk is in a liquid form. This may seem curious if one takes into consideration the fact that milk has less water than most fruits and vegetables. Milk can be described as:
- An oil-in-water emulsion with the fat globules dispersed in the continuous serum phase
- A colloid suspension of casein micelles, globular proteins and lipoprotein particles
- A solution of lactose, soluble proteins, minerals, vitamins other components.
Looking at milk under a microscope, at low magnification (5X) a uniform but turbid liquid is observed. At 500X magnification, spherical droplets of fat, known as fat globules, can be seen. At even higher magnification (50,000X), the casein micelles can be observed. The main structural components of milk, fat globules and casein micelles, will be examined in more detail later.
Figure: Structures of different molecules present in milk.
From above discussion it is clear that milk is a food with rich food contents than others which is found naturally. So there are various industries based on the milk treatment and processing all over the world. In Bangladesh there are several industries which process milk. Among them MILK VITA is the trailblazer of all. For the industrial training of Level 3, Term 2 we were assigned there. So we have the opportunity to have a closer look on that industry.
PRODUCTS BY MILK VITA
Milk Vita is a food processing industry which deals with treating milk and producing various milk products. Following milk products are produced in Milk Vita on daily basis:
1. Pasteurized Liquid Milk.
Ultra Heat Treated Milk.
3. Flavored Milk (Chocolate, strawberry and mango).
4. Ice cream.(Cup, Stick and container)
6. Yogurt.(Sweetened and non-sweetened)
7. Milk Powder.
8. Butter Oil (Ghee).
9. Sweet meat ( Rosho Malai ).
Among these products in the Mirpur plant only the first 4 section is covered during our training. The sweet meat is also produced in this plant. But this section is avoided due to shortage of time and as sufficient mechanical instruments and equipments are not used in this plant. Moreover the centrifuge machine used to produce butter was out of order during our training period. So unfortunately we didn’t have the chance to overview the butter making machine.
PLANTS AND PROCESSES
As the students of mechanical engineering we were interested in those processes which involve mechanical techniques and systems. Such sections in Milk vita are as follows:
Milk Collection & Transport.
Vehicle Refrigeration System
Liquid milk processing and packing.
Ice cream section.
If we take a closer look at these plants we would find that all plants are related to heating and refrigeration. We can talk about the Milk collection and transportation system. First the milk is collected from the village farmers and then stored to a chilled temperature and then it is conveyed to the processing plant. The transportation system used in this purpose should must equipped with refrigeration system or cooling system. This system should be reliable enough to ensure the required temperature for required time. Otherwise the milk collected can be wasted. The vehicle is subjected to several hours of traveling from the collection spot to the processing plant.
After reaching the plant the milk is then pumped to some vat for storing purpose from where milk is stored for further processing. The temperature on these vat‘s should also be in such a temperature that the milk should survive until the processing begins. So there is another usage of refrigeration plant.
At the next stage the milk is processed in two ways. One is Pasteurization and another is Ultra Heat Treatment process. Both are nothing but the heat exchanger where heat is exchanged in a heat exchanger to raise the temperature to the desired level to finish these processes. After finishing these process the milk is then send to another VAT to store. From there milk is passed through the packing machine. The packing machine packs certain amount of milk and then it is ready for sale in the market.
Another generous product of Milk Vita is its ice cream. In the Ice cream section the ice cream is produced from the heat treated milk by mixing several ingredients and then cooling them to freeze. So in the ice cream section there is another use of refrigeration system and for packing system distinctive type of machine is used here which are not similar to the packing machines used after the heat treating of milk.
The above named section has drawn our attention as mechanical perspective. So throughout the report they will be discussed in detail.
Milk Collection & Transport
Raw milk is collected from the regional plants in milk carrying vehicles. These vehicles are usually called “Milk Tankers”. Two different capacitated tankers of up to 5,000 and 10,000 liters are used to carry milk. These vehicles are internally insulated which enables them to keep the milk at a temperature of below 4 degree Celsius for up to more than 24 hours of disinfection period. Milk is collected in them after cooling in the chillers of local plants. The milk tankers bring the collections to the main pasteurizing plant of Mirpur. From there these milks are stored in a VAT. From the storing VAT milk is processed through several steps and the processed milk is then stored in a storing VAT. From there milk is supplied to the packing machine. After packaging the packets are distributed in selling zones by light vehicles. So the “Milk Tanker” plays an important role on the processing of milk.
Various types of tanker are used to transport milk from the remote area to the Dhaka plant. The details of the tankers are as follows
|Vehicle Name (Manufacturer)||Capacity(Liters)||Total number||Engine type
(No. of cylinder)
|SUZUKI MARUTI T-45||500||42||4||CNG|
In a food industry refrigeration is must. So there is no exception in the MILK VITA industry. The refrigeration system used in this industry is vapor compression refrigeration cycle. In the following section the vapor compression refrigeration system will be discussed.
Vapor Compression Refrigeration Cycle
Mechanical refrigerators working on vapor compression refrigeration cycle have four basic elements:
A condenser, and
A refrigerant flow control (expansion valve).
A refrigerant circulates among the four elements changing from liquid to gas and back to liquid. In the evaporator, the liquid refrigerant evaporates (boils) under reduced pressure and in doing so absorbs latent heat of vaporization and cools the surroundings. The evaporator is at the lowest temperature in the system and heat flows to it.
Figure Typical vapor compression refrigeration cycle.
This heat is used to vaporize the refrigerant. The temperature at which this occurs is a function of the pressure on the refrigerant: for example if ammonia is the refrigerant, at -18oC the ammonia pressure required is 1.1 kg/sq. cm. The part of the process described thus far is the useful part of the refrigeration cycle; the remainder of the process is necessary only so that the refrigerant may be returned to the evaporator to continue the cycle. The refrigerant vapor is sucked into a compressor, a pump that increases the pressure and then exhausts it at a higher pressure to the condenser. For ammonia, this is approx. 10 kg/sq. cm. To complete the cycle, the refrigerant must be condensed back to liquid and in doing this it gives up its latent heat of vaporization to some cooling medium such as water or air. The condensing temperature of ammonia is 29oC, so that cooling water at about 21oC could be used. In home refrigerators, the compressed gas (not ammonia) is sent through the pipes at the back, which are cooled by circulating air around them. Often fins are added to these tubes to increase the cooling area. The gas had to be compressed so that it could be condensed at these higher temperatures, using free cooling from water or air. The refrigerant is now ready to enter the evaporator to be used again. It passes through an expansion valve to enter into the region of lower pressure, which causes it to boil and absorb more heat from the load. By adjusting the high and low pressures, the condensing and evaporating temperatures can be adjusted as required. The following image is a schematic of a refrigeration cycle. It is described in detail below.
Figure Vapor compression refrigeration cycle used in Dairy plant
Refrigeration in Milk Vita
Vapor compression refrigeration cycle is involved in this plant where Ammonia is used as the refrigerant. Though ammonia is harmful for human body but it is widely used in the industry for is availability and low cost. The official name for ammonia is R-717. The several components of the refrigeration system used in the industry are as follows
The evaporator consists of a coil of copper, aluminum, or aluminum alloy tubing installed in the space to be refrigerated. Following figure shows some of this tubing. As mentioned before, the liquid ammonia enters the tubing at a reduced pressure and, therefore, with a lower boiling point.
Figure Shell and Tube type evaporator.
As the refrigerant passes through the evaporator, the heat flowing to the coil from the surrounding air causes the rest of the liquid refrigerant to boil and vaporize. After the refrigerant has absorbed its latent heat of vaporization (that is, after it is entirely vaporized), the refrigerant continues to absorb heat until it becomes superheated by approximately 10°F. The amount of superheat is determined by the amount of liquid refrigerant admitted to the evaporator. This, in turn, is controlled by the spring adjustment of the TXV. A temperature range of 4° to 12°F of superheat is considered desirable. It increases the efficiency of the plant and evaporates all of the liquid. This prevents liquid carry-over into the compressor.
The compressor in a refrigeration system is essentially a pump. It is used to pump refrigerant uphill from the cold side to the hot side of the system. The heat absorbed by the refrigerant in the evaporator must be removed before the refrigerant can again absorb latent heat. The only way the vaporized refrigerant can be made to give up the latent heat of vaporization that it absorbed in the evaporator is by cooling and condensing it. Because of the relatively high temperature of the available cooling medium, the only way to make the vapor condense is to compress it. When the pressure is raised, the temperature is also raised. Therefore, condensing temperature has to be raised, which allows use water as a cooling medium in the condenser. In addition to this primary function, the compressor also keeps the refrigerant circulating and maintains the required pressure difference between the high-pressure and low-pressure sides of the system.
Figure Motor-driven, single-acting, two-cylinder reciprocating compressor
(Courtesy SABROE, Finland)
Many different types of compressors are used in refrigeration systems. Compressors are of two general types
Reciprocating and Rotating.
In a reciprocating, or displacement, compressor which is used to produce high pressures, the air is compressed by the action of a piston in a cylinder. When the piston moves to the right, air flows into the cylinder through the intake valve; when the piston moves to the left, the air is compressed and forced through an output-control valve into a reservoir or storage tank.
Figure Typical reciprocating type compressor
A rotating air compressor, used for low and medium pressures, usually consists of a bladed wheel or impeller that spins inside a closed circular housing. Air is drawn in at the center of the wheel and accelerated by the centrifugal force of the spinning blades. The energy of the moving air is then converted into pressure in the diffuser, and the compressed air is forced out through a narrow passage to the storage tank.
Figure Typical Rotating Compressor
The designs of compressors vary depending on the application of the refrigerants used in the system. The figure shows a motor-driven, single-acting, two-cylinder, reciprocating compressor, such as those commonly used in refrigeration plants. Compressors used in ammonia systems may be lubricated either by splash lubrication or by pressure lubrication. Splash lubrication, which depends on maintaining a fairly high oil level in the compressor crankcase, is usually satisfactory for smaller compressors. High-speed or large-capacity compressors use pressure lubrications systems.
Generally reciprocating type compressor is used in the MILK VITA industry. The most common type compressor used looks similar as follows:
Figure Typical compressor used in the MILK VITA.
A typical compressor which are generally used for room cooling are usually rated as follows
Power 15 kW
Voltage 380 V
Current 32 Amps
Frame 160 Liter
Speed 1480 rpm
Suction pressure 1 Bar
Oil Pressure 4.8 Bar
Discharge Pressure 9.5 Bar
Intermediate Pressure 2 Bar
Powered by 3 phase induction motor
The compressor discharges the high-pressure, high-temperature refrigerant vapor to the condenser, where it flows around the tubes through which water is being pumped. As the vapor gives up its superheat (sensible heat) to the water, the temperature of the vapor drops to the condensing point. The refrigerant, now in liquid form, is sub cooled slightly below its condensing point. This is done at the existing pressure to ensure that it will not flash into vapor.
Figure Evaporative Condenser
A water-cooled condenser for an R-717 refrigeration system is shown in figure. Circulating water is obtained through a branch connection from the fire main or by means of an individual pump taking suction from the water stored under the condenser. The purge connection is on the refrigerant side. It is used to remove air and other non-condensable gases that are lighter than the ammonia vapor. Most condensers used in plants are of the water-cooled type. However, some small units have air-cooled condensers. These consist of tubing with external fins to increase the heat transfer surface. Most air-cooled condensers have fans to ensure positive circulation of air around the condenser tubes. In Milk Vita Evaporative condensers are used.
The receiver acts as a temporary storage space and surge tank for the liquid refrigerant. The receiver also serves as a vapor seal to keep vapor out of the liquid line to the expansion valve. Receivers are constructed for either horizontal or vertical installation. In Milk Vita Pasteurization system all receivers are manufactured by GRANDINA of Scotland.
Figure Receiver Tank.
The expansion valve is placed between the high-pressure side and the low-pressure side of the refrigeration cycle, and its purpose is to maintain the given pressures in these two regions in such a way that condensation and evaporation is carried out under the most convenient circumstances. The expansion valve controls the flow of fluid into the evaporator. The intention is to make the refrigerant evaporate while the pressure is as low as possible. On the other hand, in the condenser, one wishes the refrigerant to condense under as high pressure as possible. The valve is controlled manually by a programmable PLC, of by the embedded controllers of this PLC. The expansion valve has two advantages to the refrigeration cycle. They are to meter the liquid refrigerant from the liquid line into the evaporator at a rate commensurate with the rate at which vaporization of the liquid is occurring in the evaporator unit.
Figure Expansion Valve
To maintain a pressure differential between the high and low pressure sides of the system in order to permit the refrigerant to vaporize under the desired low pressure in the evaporator, while at the same time condensing at the high pressure in the condenser. The expansion valve used in the refrigeration cycle consists of a Badger Research Control Valve Model 73N-B.
Vehicle Refrigeration System
In almost all the milk processing industries milk is generally is collected from the remote areas. So it is quite difficult to get them at good condition for processing. So generally refrigeration unit in vehicles are necessary. But there is a limitation on these refrigeration units. Not like the normal refrigeration unit enough power can be used in theses vehicle refrigeration units. Generally three types of vehicles are observed on the MILK VITA industry. We focus on the large capable vehicle. All other vehicle refrigeration system is based on the same principle.
Generally in a vehicle refrigeration system we can divide the total system into two parts.
Electrical power system
Electrical Power system:
Generally vapor compression refrigeration system is equipped with a compressor. It is filthy to mention the importance of a compressor in a vapor compression refrigeration system.
Power Distribution Relay
Figure: Power Supply system of vehicle refrigeration system
This needs large amount of power to run. So electrical power supply is an important part of the vehicle refrigeration system
Generally the compressor can be run using two possible ways. These are
Using a generator
Using power supply from AC source
Generally when a vehicle is at running condition then power to the compressor is got from a generator. There is switch provided while anyone wants to use generator for refrigeration or not. When the vehicle is running then the engine of the vehicle is running. So we can use the power from vehicle engine to drive a DC generator. This is what is done on vehicle refrigeration system. A generator is coupled with the vehicle engine. When it is in generator mode the generator is connected with vehicle engine and thus necessary power is produced to drive the motor of the compressor. The generator produces 15 volt DC supply which is used to run the compressor
When the car is parked then power supply from AC source is available. So this AC power source is generally used on powering the compressor of the vehicle. But there is problem. The compressor is run by a motor which is designed to run on DC source but from AC source a DC motor can never run. So we need to convert the AC to DC supply.
Generally the input is 380 volt AC supply. But the motor to be run is designed for 15 volt. So a transformer is used to step down to 15 volt AC. A rectifier is used to convert the AC voltage to DC voltage. Generally a circuit breaker is used to provide securities from high current pass. This breaker is equipped with a magnetic conductor relay. The rating of this relay is ±12V with 15A current. There are total 3 relays. One is to provide securities to power supply and another two is for defrosting purpose.
So this is the overview of the power supply of a vehicle power supply.
The refrigeration system used in vehicles should be small enough in size so that it can easily be carried. So the several components of a refrigeration system should be of smaller than that of those used on industries.
Generally vapor compression refrigeration cycle is used on the vehicles also. The main components of refrigeration system are not different from other ones.
- Expansion valve
Figure: Refrigeration system of vehicle refrigeration system
Generally the capacity of a vehicle refrigeration system is not so large. So components are generally smaller in size in this type of system.
The compressor of a vehicle refrigeration system is generally set on the top of the driver’s space. The vehicle we observed was equipped with an open type compressor. The vehicle with higher refrigeration capability is generally equipped with open type compressor. Those who are having less refrigeration capability uses semi seal or seal` type compressor. Generally three type of compressor is widely used. They are
Semi seal type
At condenser the refrigerant give up heats to the ambient temperature and transforms into liquid at high pressure. Condenser is in most cases in the form of coil so that it is exposed to ambient with high surface areas. There are generally three types of coil used. They are
The whole truck space is used as the evaporator. The evaporator is made of heat resistant materials and insulator. The vehicles with smaller size are generally provided with eutectic plate. This help in stabilizing the temperature quickly. So it is easy for a low powered compressor to reach its equilibrium quickly. In most of the system a number of fans are provided to force air flow throughout the evaporator. This is because the refrigerant flow can’t be flowed around the entire evaporator wall uniformly.
Figure: Expansion Valves of refrigeration cycle used in vehicle refrigeration system
The next part of the system is the expansion valve. Expansion valves are used to drop the pressure of the refrigerant adiabatically. So the refrigerant then travels with lower pressure and in liquid state to the evaporator.
In Milk Vita plant the entire vehicles which have refrigeration system with it used R-22 as refrigerant. To control the flow of the refrigerant solenoid valves are used. For flow control solenoid valves are very suitable.
LIQUID MILK PROCESSING AND PACKING
Each and every country has its standard for their milk contents. So the desired amount of components should be maintained. Generally BSTI approved milk content is as follows:
Milk on an average generally contains
3.25 ~ 3.5% of Fat
8 ~ 9% of SnF (Solid non Fat)
87% of Water
But in general the milk collected in our country have 4 ~ 5% of fat. The excess fat is not good for human body. So the excess fat is removed from the milk and is used for making other useful products such as butter, cheese etc.
Milk’s fat content is generally 2 μm in average. If this is not shortened then the mixture of milk would not be equally distributed one. So homogenization is required to shorten the fat content of milk. After homogenization the fat content of milk is generally about 0.5µm in average. So an important step in milk processing industry is homogenization of milk. This is done by increasing the pressure of the milk. Generally a machine named homogenizer is used for this purpose. The detail of this machine will be discussed in the later sections.
In the most milk processing plant pasteurization process is followed to preserve the milk. Especially in developing country like Bangladesh pasteurization process is suitable. Without this process Ultra Heat Treatment is used in MILK VITA to process the milk. Generally UHT is used to process the chocolate milk in this plant. Chocolate milk process exigent UHT more than others because the storing time of chocolate milk generally greater than those of normal milks. So according to the need the UHT is involved in the processing of chocolate milk production.
Another important thing in the milk industry is packaging. If all processing is done perfectly and immaculately but if the product is packed in such a packet that it could easily be attacked by the external germs and destructive components then it is indubitable that the products will not survive. This is why packing is important in industries like MILK VITA. So they have to maintain some standard to pack their products. The packet they use for packing for the pasteurized milk ensure that it will survive up to 48 hours when stored at 4’C. But for the ultra heat treated milk they use completely different type of packet. Seven layers of food grade packet are used to store this kind of treated milk. If any liquid milk is ultra heat treated then it can sustain up to one year. But the packet used here provides 6 months of sustainability of the product packed using this packet. The product packed can sustain more than 6 months. But generally 6 month is the limit which it can sustain without any problem. At present in the MILK VITA industry the packing material is affected due to insufficient attention. So the UHT plant is currently out of production. So in a food industry packaging has an influential effect.
In the next few sections the details of treatment process and packing system will be described. They will be focused mostly in the mechanical viewpoint.
Centrifugation is a process that involves the use of the centripetal force for the separation of mixtures, used in industry and in laboratory settings. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components of the mixture migrate towards the axis. In chemistry and biology, increasing the effective gravitational force on a test tube so as to more rapidly and completely cause the precipitate (“pellet”) to gather on the bottom of the tube. The remaining solution is properly called the “supernate” or “supernatant liquid”. The supernatant liquid is then either quickly decanted from the tube without disturbing the precipitate, or withdrawn with a Pasteur pipette. The rate of centrifugation is specified by the acceleration applied to the sample, typically measured in revolutions per minute (RPM) or g. The particles’ settling velocity in centrifugation is a function of their size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particle and the liquid, and the viscosity.
Centrifugal separation is a process used quite often in the dairy industry. Some uses include:
- Clarification (removal of solid impurities from milk prior to pasteurization)
- skimming (separation of cream from skim milk)
- whey separation (separation of whey cream (fat) from whey)
- Bactofuge treatment (separation of bacteria from milk)
- quark separation (separation of quark curd from whey)
- butter oil purification (separation of serum phase from anhydrous milk fat)
Centrifugation is based on Stokes Law. The particle sedimentation velocity increases with:
- increasing diameter
- increasing difference in density between the two phases
- decreasing viscosity of the continuous phase
If raw milk were allowed to stand, the fat globules would begin to rise to the surface in a phenomenon called creaming. Raw milk in a rotating container also has centrifugal forces acting on it. This allows rapid separation of milk fat from the skim milk portion and removal of solid impurities from the milk.
In our country BSTI allowed fat percentage on milk is 3.5%~4%. So the extra amount of fat is separated from the milk. This extra amount of fat then can be used in making butter, cheese etc. Centrifuges are used to separate the cream from the skim milk. The centrifuge consists of up to 120 discs stacked together at a 45 to 60 degree angle and separated by a 0.4 to 2.0 mm gap or separation channel. Milk is introduced at the outer edge of the disc stack. The stack of discs has vertically aligned distribution holes into which the milk is introduced. Under the influence of centrifugal force the fat globules (cream), which are less dense than the skim milk, move inwards through the separation channels toward the axis of rotation. The skim milk will move outwards and leaves through a separate outlet.
Separation and clarification can be done at the same time in one centrifuge. Particles, which are denser than the continuous milk phase, are thrown back to the perimeter. The solids that collect in the centrifuge consist of dirt, epithelial cells, leucocytes, corpuscles, bacteria sediment and sludge. The amount of solids that collect will vary; however, it must be removed from the centrifuge. More modern centrifuges are self-cleaning allowing a continuous separation/clarification process.
This type of centrifuge consists of a specially constructed bowl with peripheral discharge slots. These slots are kept closed under pressure. With a momentary release of pressure, for about 0.15 s, the contents of sediment space are evacuated. This can mean anywhere from 8 to 25 L are ejected at intervals of 60 min. For one dairy, self-cleaning translated to a loss of 50 L/hr of milk. The following image is a schematic of a clarifier.
In the plant we were assigned has got only one separator. But unfortunately that was out of order. So we were dispossessed from having sufficient knowledge from that. Generally the fat particle is removed in the Baghabarighat and the milk is then sent to the Mirpur plant using the tankers.
While the fat content is removed from the milk then the substance looses the desired ratio. To maintain the standard ratio the streams of skim and cream after separation must be recombined to a specified fat content. This can be done by adjusting the throttling valve of the cream outlet; if the valve is completely closed, all milk will be discharged through the skim milk outlet. As the valve is progressively opened, larger amounts of cream with diminishing fat contents are discharged from the cream outlet. With direct standardization the cream and skim are automatically remixed at the separator to provide the desired fat content.
The following steps can be observed generally in a standardization process:
Milk is pumped out from the tankers.(Pumping capacity 25,000 lit/hr)
2) Filtration applied.
3) Chilled water is passed through it with the help of a heat exchanging device to cool it down again to 8ºC.
4) Chilled milk is stored in a storage tank where it is continuously churned by motorized churner. (Tank capacity is 10,000 lit)
5) Sample milk is taken to the laboratory of quality control division of the industry and the amount of present nutrition facts and fat are measured through biochemical testing.
6) If the measurements assure BSTI standard of fat and SNF constituents it is sent to pasteurization unit.
7) If fat% is below 3.5% then full cream powder milk is added to it to level the lacing. If fat% is higher than 3.5% low fat powder milk is added to it with water and then sent to pasteurization unit.
8) Amount of SNF is kept as 8% as prescribed by BSTI.
9) Standardized milk is sent to pasteurizer.
The process of pasteurization was named after Louis Pasteur who discovered that spoilage organisms could be inactivated in wine by applying heat at temperatures below its boiling point. The process was later applied to milk and remains the most important operation in the processing of milk.
Pasteurization is used to kill harmful microorganisms by heating the milk for a short time and then cooling it for storage and transportation. Pasteurized milk is still perishable and must be stored cold by both suppliers and consumers. Dairies print expiration dates on each container, after which stores will remove any unsold milk from their shelves. In many countries it is illegal to sell milk that is not pasteurized.
Generally milk is pasteurized to increase the sustainability. Pasteurized milk can survive up to 48 hours without causing any problem. So it is suitable to apply where milk supply can be delayed. Pasteurization is a scientific technique to lower the amount of present active bacteria in milk and hence to increase the longevity of nutrient facts present in it. Famous French microbiologist Louis Pasteur devised this technique which keeps the milk beyond decomposing infection of micro-organisms and hence keeps the quality intact for longer period.
The heating of every particle of milk or milk product to a specific temperature for a specified period of time without allowing recontamination of that milk or milk product during the heat treatment process is known as pasteurization.
There are two distinct purposes for the process of milk pasteurization:
Public Health Aspect – to make milk and milk products safe for human consumption by destroying all bacteria that may be harmful to health (pathogens)
Keeping Quality Aspect – to improve the keeping quality of milk and milk products. Pasteurization can destroy some undesirable enzymes and many spoilage bacteria. Shelf life can be 7, 10, 14 or up to 16 days.
The extent of microorganism inactivation depends on the combination of temperature and holding time. Minimum temperature and time requirements for milk pasteurization are based on thermal death time studies for the most heat resistant pathogen found in milk. Thermal lethality determinations require the applications of microbiology to appropriate processing determinations.
To ensure destruction of all pathogenic microorganisms, time and temperature combinations of the pasteurization process are highly regulated
To pasteurize milk must be heated, with agitation, in such a way that every particle of the milk, including the foam, receives a minimum heat treatment of 150°F (66°C) continuously for 30 minutes or 161°F (72°C) for 15 seconds. The temperature should be monitored with an accurate metal or protected glass thermometer.
Commercial operations commonly use a high temperature, short-time process in which the milk is heated to 170°F (77°C) for 15 seconds and then cooled immediately to below 40°F (4°C) to increase storage life without any noticeable flavor change in the milk.
Mainly two types of pasteurization techniques are available in the whole world. They are:
1) High Temperature Short Time method. (HTST)
2) Low Temperature Long Time method. (LTLT) also known as VAT pasteurization.
The temperature range of these two types of pasteurization is as follows
|161°F (72°C) 15 seconds
(high temperature, short time
If a closer look at the pasteurization is given it will be found that a heat exchanger is needed to do this job.
A closer look at the pasteurizer would reveal that it’s a heat exchanging device where milk is heated, held for few seconds and then cooled again. The heat exchanger used in pasteurization is plate type in MILK VITA industry.
In pasteurization process cold raw milk in a constant level tank is drawn into the regenerator section of pasteurizer. Here it is warmed to approximately 57° C – 68° C by heat given up by hot pasteurized milk flowing in a counter current direction on the opposite side of thin, stainless steel plates. The raw milk, still under suction, passes through a positive displacement timing pump which delivers it under positive pressure through the rest of the HTST system.
The raw milk is forced through the heater section where hot water on opposite sides of the plates heat milk to a temperature of at least 72° C. The milk, at pasteurization temperature and under pressure, flows through the holding tube where it is held for at least 16 sec. The maximum velocity is governed by the speed of the timing pump, diameter and length of the holding tube, and surface friction. After passing temperature sensors of an indicating thermometer and a recorder-controller at the end of the holding tube, milk passes into the flow diversion device (FDD). The FDD assumes a forward-flow position if the milk passes the recorder-controller at the preset cut-in temperature (>72° C). The FDD remains in normal position which is in diverted-flow if milk has not achieved preset cut-in temperature. The improperly heated milk flows through the diverted flow line of the FDD back to the raw milk constant level tank. Properly heated milk flows through the forward flow part of the FDD to the pasteurized milk regenerator section where it gives up heat to the raw product and in turn is cooled to approximately 32° C – 9° C.
The warm milk passes through the cooling section where it is cooled to 4° C or below by coolant on the opposite sides of the thin, stainless steel plates. The cold, pasteurized milk passes through a vacuum breaker at least 12 inches above the highest raw milk in the HTST system then on to storage tank filler for packaging.
Figure: Basic Flow of Milk in Pasteurizer.
The whole process in the pasteurizer can be viewed as follows:
Milk is taken through two regenerators in a heat exchanger where it takes heat from steam which comes from a boiler.
From regenerators the milk can be taken out for fat separation and homogenization and then again sent to heater.
After heating it is held in a holder for 15 seconds.
Then it is sent to the chilling section of pasteurizer where it is chilled down to 4ºC by heat exchanging with cooling water from a chiller.
Chilled milk is now pasteurized and sent to storage tank.
Figure: Flow through Pasteurizer plate heat exchanger
Figure HTST continuous plate pasteurizer
Figure Residence time profile of plate heat pasteurization.
Heat exchanger Plate
Figure: Typical Plate Type Pasteurizer
The holding time is an important factor in pasteurization process. The holding time is same for the flowing milk. An important factor which effect the holding time is the flow condition. Types of flow when fluids move through a pipe, either of two distinct types of flow can be observed. The first is known as turbulent flow which occurs at high velocity and in which eddies are present moving in all directions and at all angles to the normal line of flow. The second type is streamline, or laminar flow which occurs at low velocities and shows no eddy currents. The Reynolds number is used to predict whether laminar or turbulent flow will exist in a pipe. For Reynolds number:
Re < 2300 for laminar
Re > 4000 for fully developed turbulent flow
There is an impact of these flow patterns on holding time calculations and the assessment of proper holding tube lengths.
The holding time is determined by timing the interval for an added trace substance (salt) to pass through the holder. The time interval of the fastest particle of milk is desired. Thus the results found with water are converted to the milk flow time by formulation since a pump may not deliver the same amount of milk as it does water.
For continuous pasteurizing, it is important to maintain a minimum amount of pressure on the pasteurized side of the heat exchanger. By keeping the pasteurized milk at least 1 psi higher than raw milk in regenerator, it prevents contamination of pasteurized milk with raw milk in event that a pin-hole leak develops in thin stainless steel plates. This pressure differential is maintained using a timing pump in simple systems, and differential pressure controllers and back pressure flow regulators at the chilled pasteurization outlet in more complex systems. The position of the timing pump is crucial so that there is suction on the raw regenerator side and pushes milk under pressure through pasteurized regenerator. There are several other factors involved in maintaining the pressure differential:
- The balance tank overflow level must be less than the level of lowest milk passage in the regenerator
- Properly installed booster pump is all that is permitted between balance tank and raw regenerator
- No pump after pasteurized milk outlet to vacuum breaker
- There must be greater than a 12 inch vertical rise to the vacuum breaker
- The raw regenerator drains freely to balance tank at shut-down
- Basic Component Equipment of HTST Pasteurizer
- The several components that a HTST Pasteurizer uses are as follows.
The balance or constant level tank provides a constant supply of milk. It is equipped with a float valve assembly which controls the liquid level nearly constant ensuring uniform head pressure on the product leaving the tank. The overflow level must always be below the level of lowest milk passage in regenerator. It, therefore, helps to maintain a higher pressure on the pasteurized side of the heat exchanger. The balance tank also prevents entering air in the pasteurizer by placing the top of the outlet pipe lower than the lowest point in the tank and creating downward slopes of at least 2%. The balance tank provides a means for recirculation of diverted or pasteurized milk.
Heating and cooling energy can be saved by using a regenerator which utilizes the heat content of the pasteurized milk to warm the incoming cold milk. Its efficiency may be calculated as follows:
% regeneration = temperature increase due to regenerator/total temperature increase
For example: Cold milk entering system at 4° C, after regeneration at 65° C, and final temperature of 72° C would have an 89.7% regeneration:
The indicating thermometer is considered the most accurate temperature measurement. It is the official temperature to which the safety thermal limit recorder (STLR) is adjusted. The probe should sit as close as possible to STLR probe and be located not greater than 18 inches upstream of the flow diversion device.
The STLR records the temperature of the milk and the time of day. It monitors, controls and records the position of the flow diversion device (FDD) and supplies power to the FDD during forward flow. There are both pneumatic and electronic types of controllers. The operator is responsible for recording the date, shift, equipment, ID, product and amount, indicating thermometer temperature, cleansing cycles, cut in and cut out temperatures, any connects for unusual circumstances, and his/her signature.
The homogenizer may be used as timing pump. It is a positive pressure pump; if not, then it cannot supplement flow. Free circulation from outlet to inlet is required and the speed of the homogenizer must be greater than the rate of flow of the timing pump.
Milk Pasteurization Plant in Milk Vita
The pasteurization plant used in the MILK VITA industry is capable of producing 10,000l/h of pasteurization product. The plant used is of “Monoblock” type. This means that all the components like as constant level balance tank, plant heat exchanger, pumps, valves, probes and control panel board are assembled on an only one frame provided of adjustable feet. The plant used is designed and manufactured to work the milk through a succession of thermal exchanges.
The most important part of the plant is the Gea Ecoflex plate heat exchanger consisting of 5 sections.
1st section of heat regeneration:
In this section product is heated to + 30 °C to + 65 °C by means of product already pasteurized from + 69 °C to + 33 °C
2nd section of heat regeneration:
In this section product is heated from + 65 °C to + 78 °C by means of product already pasteurized from + 85 °C to + 69 °C.
In this section product is heated from + 78 °C to + 85 °C by means of hot water at + 90 °C.
Natural water cooling section:
In this section product is heated from + 33° C to + 20 °C by means of 15.000 l/h of natural water at I6°C.
Chilled water cooling section:
Here product is cooled from + 20° C to + 2 °C by means of 20.000 l/h of chilled water at 1 °C.
On plate heat exchanger there is a derivation at +65 °C after the first heat regeneration section. With controlled temperature, for milk homogenization, and another derivation at + 78°C, after the second regeneration section, for milk degassing. The separation of the incondensable gas from the milk is carried out by a chamber maintained to milk boiling temperature with vacuum made by a proper pump. The gas are sucked and condensed by means of proper system and eliminated with the sucked water by the vacuum pump, while the milk is sucked by a centrifugal pump with fluxed mechanical seal and reintroduced into the plate heat exchanger for the following phase of pasteurization. An electronic level that modulates the capacity of the milk extraction pump by means of a sanitary valve maintains the level of the milk into the chamber of the degasser constant. A series of temperature probes have the function to take the values of milk and water temperature, with recording of the same on the recorder installed on the panel board. The regulators, elaborating the inlet data, check the modulating valves to maintain the temperatures and the levels on the required values for the milk treatment. The most important data are measured and recorded on paper by a recorder, that origins proceeding lines of pasteurization temperature and product outlet temperature.
Packing Machine for Pasteurized Milk
|PREPAC IS.6 MC||Number present in
|High output, twin head automatic filler.
Standard conformity: AFNOR, CSA, European.
|OUTPUT||5 000 pouches/hour in 1L,
6 000 pouches/hour (1/10 to 1/2L.),
3 960 pouches/hour in 1/2 gallon and 2L
|CAPACITY||1/10L, 1/8L, 1/4L, 1/2L, 1L,
Also in 1/2 Gallon and 2L
|OPTIONS||Programmable PC version
|DIMENSIONS||H x L x D : 2 950 x 1 000 x 1 160mm|
|Elecstar B-34||Number present in
|High output, three head automatic filler.
Standard conformity: AFNOR, CSA, European..
|OUTPUT||7 200 pouches/hour from 0.25L to 1L,
6 400 pouches/hour in 1/2 L,
4 800 pouches/hour in 2L
|CAPACITY||1/4L, 1/2L, 1L,
Also in 2L
|OPTIONS||Code marker, print registration|
|DIMENSIONS||H x L x D: 2 950 x 1 000 x 1 160mm|
These are the two types of filling machine used in the packaging of pasteurized milk in the MILK VITA industry. During our training the Elecstar B-34 machine was out of order. So only the Prepack machine was available for observation.
Figure: Prepack filling machine schematic
Figure: Basic components of a Prepack filling machine
The several important parts of this filling machine are as follows:
Cam vertical press
Pre unrolling pinion
Cam horizontal press
Milk is an oil-in-water emulsion, with the fat globules dispersed in a continuous skimmed milk phase. If raw milk were left to stand, however, the fat would rise and form a cream layer. Homogenization is a mechanical treatment of the fat globules in milk brought about by passing milk under high pressure through a tiny orifice, which results in a decrease in the average diameter and an increase in number and surface area, of the fat globules. The net result, from a
Figure Fat particles in milk.
practical view, is a much reduced tendency for creaming of fat globules. Three factors contribute to this enhanced stability of homogenized milk: a decrease in the mean diameter of the fat globules (a factor in Stokes Law), a decrease in the size distribution of the fat globules (causing the speed of rise to be similar for the majority of globules such that they don’t tend to cluster during creaming), and an increase in density of the globules (bringing them closer to the continuous phase) owing to the adsorption of a protein membrane. In addition, heat pasteurization breaks down the cryo-globulin complex, which tends to cluster fat globules causing them to rise.
Mechanism of Homogenization
Auguste Gaulin’s patent in 1899 consisted of a 3 piston pump in which product was forced through one or more hair like tubes under pressure. It was discovered that the size of fat globules produced were 500 to 600 times smaller than tubes. There have been over 100 patents since, all designed to produce smaller average particle size with expenditure of as little energy as possible. The homogenizer consists of a 3 cylinder positive piston pump (operates similar to car engine) and the valve seat (the land) and exits in about 50 microsecond. The homogenization phenomena is completed before the fluid leaves the area between the valve and the seat, and therefore emulsification is initiated and completed in less than 50 microsecond. The whole process occurs between 2 pieces of steel in a steel valve assembly. The product may then pass through a second stage valve similar to the first stage. While most of the fat globule reduction takes place in the first stage, there is a tendency for clumping or clustering of the reduced fat globules. The second stage valve permits the separation of those clusters into individual fat globules. It is most likely that a combination of two theories, turbulence and cavitations, explains the reduction in size of the fat globules during the homogenization process.
Figure: Typical Homogenizer
Energy, dissipating in the liquid going through the homogenizer valve, generates intense turbulent eddies of the same size as the average globule diameter. Globules are thus torn apart by these eddy currents reducing their average size.
The pump is turned by electric motor through connecting rods and crankshaft. To understand the mechanism, consider a conventional homogenizing valve processing an emulsion such as milk at a flow rate of 20,000 l/hr. at 14 MPa (2100 psig). As it first enters the valve, liquid velocity is about 4 to 6 m/s. It then moves into the gap between the valve and the valve seat and its velocity is increased to 120 meter/sec in about 0.2 millisecond. The liquid then moves across the face of
It is considerable pressure drop with charge of velocity of fluid. Liquid capitates because its vapor pressure is attained. Cavitations generates further eddies that would produce disruption of the fat globules. The high velocity gives liquid a high kinetic energy which is disrupted in a very short period of time. Increased pressure increases velocity. Dissipation of this energy leads to a high energy density (energy per volume and time). Resulting diameter is a function of energy density. In summary, the homogenization variables are:
Type of valve
- Single or two-stage
- Fat content
- Surfactant type and content
Also to be considered are the droplet diameter (the smaller, the more difficult to disrupt), and the log diameter which decreases linearly with log P and levels off at high pressures.
Figure: Double stage homogenizer.
Effects of Homogenization:
After homogenization several changes that are observed are as follows:
|No homogenization||15 MPA (2500 psig)|
|Av. diam. (μ m)||3.3||4|
|Max. diam. (μ m)||10||2|
|Surf. area (m2/ml of milk)||0.08||0.75|
|Number of globules (μ m-3)||0.02||0.12|
The milk fat globule has a native membrane, picked up at the time of secretion, made of amphiphilic molecules with both hydrophilic and hydrophobic sections. This membrane lowers the interfacial tension resulting in a more stable emulsion. During homogenization, there is a tremendous increase in surface area and the native milk fat globule membrane (MFGM) is lost. However, there are many amphiphilic molecules present from the milk plasma that readily adsorb: casein micelles (partly spread) and whey proteins. The interfacial tension of raw milk is
FIG: Effects of Homogenization
1-2 mN/m, immediately after homogenization 22 it is unstable at 15 mN/m, and shortly becomes stable (3-4 mN/m) as a result of the adsorption of protein. The transport of proteins is not by diffusion but mainly by convection. Rapid coverage is achieved in less than 10 sec but is subject to some rearrangement.
It is a measure of how much protein is adsorbed; for example 10 mg/m2 translate to a thickness of adsorbed layer of approximately 15 nm.
ULTRA HEAT TREATMENT (UHT) OF MILK
Ultra-High Temperature (UHT) pasteurization, a relatively new technique, is used to sterilize foods for aseptic packaging. Milk preserved by the UHT process is sold in cartons often called a brick that lack the peak of the traditional milk carton. Milk preserved in this fashion does not need to be refrigerated before opening and has a longer shelf life than milk in ordinary packaging. It is more typically sold unrefrigerated on the shelves in Europe than in America.
While pasteurization conditions effectively eliminate potential pathogenic microorganisms, it is not sufficient to inactivate the thermo resistant spores in milk. The term sterilization refers to the complete elimination of all microorganisms. The food industry uses the more realistic term “commercial sterilization”; a product is not necessarily free of all microorganisms, but those that survive the sterilization process are unlikely to grow during storage and cause product spoilage.
In canning we need to ensure the “cold spot” has reached the desired temperature for the desired time. With most canned products, there is a low rate of heat penetration to the thermal centre. This leads to over processing of some portions, and damage to nutritional and sensory characteristics, especially near the walls of the container. This implies long processing times at lower temperatures.
Milk can be made commercially sterile by subjecting it to temperatures in excess of 100° C, and packaging it in air-tight containers. The milk may be packaged either before or after sterilization. The basis of UHT, or ultra-high temperature, is the sterilization of food before packaging, then filling into pre-sterilized containers in a sterile atmosphere. Milk that is processed in this way using temperatures exceeding 135° C, permits a decrease in the necessary holding time (to 2-5 s) enabling a continuous flow operation.
Some examples of food products processed with UHT are:
liquid products – milk, juices, cream, yoghurt, wine, salad dressings
foods with discrete particles – baby foods; tomato products; fruits and vegetables juices; soups
larger particles – stews
Advantages of UHT
Though UHT method is costly than the pasteurization but it is used widely in the developed countries. There are several reasons why this method is used. In the next few lines the advantages of UHT method will be described.
The D and Z valves are higher for quality factors than microorganisms. The reduction in process time due to higher temperature (UHTST) and the minimal come-up and cool-down time leads to a higher quality product.
Long shelf life:
Greater than 6 months, without refrigeration, can be expected if milk is processed using UHT method. Generally the UHT treated milk sustains of about 1 year depending on the packet used.
Processing conditions are independent of container size, thus allowing for the filling of large containers for food-service or sale to food manufacturers (aseptic fruit purees in stainless steel totes).
The preserving cost of UHT treated milk is drastically reduced. Because refrigerated storage is reduced.
Difficulties with UHT
As UHT method is providing several advantages over the normal milk treatment process it will consists of some complex process and requires some complex and costly machines to perform this process.
Complexity of equipment and plant are needed to maintain sterile atmosphere between processing and packaging (packaging materials, pipe work, tanks, and pumps); higher skilled operators; sterility must be maintained through aseptic packaging.
With larger particulates there is a danger of overcooking of surfaces and need to transport material – both limits particle size.
There is a lack of equipment for particulate sterilization, due especially to settling of solids and thus over processing.
Heat stable lipases or proteases can lead to flavor deterioration, age gelation of the milk over time – nothing lasts forever! There is also a more pronounced cooked flavor to UHT milk.
All milk treatment process used is generally consists of heating the milk at a certain temperature and then cooling it to a desired temperature with minimum amount of time required. There is no difference in this process also. UHT process generally follows heating the milk up to 140’C then cooling it to 2~4’C in only 4 seconds. So we can say that the only thing used here is another heat exchanger. The shell and tube type heat exchanger is used in the MILK VITA industry. Another matter of concern is that the milk is to be cold in a short time of only 2 seconds. So we can also say that fully developed turbulent flow is occurred in the heat exchanger. Generally there are two types of system used to UHT treat the milk. They are
High temperature short time (HT)
Low temperature high time (LT)
The method that is followed on the MILK VITA industry is the first one that is High temperature low time (HT) method.
Plate Type Heat Exchangers:
Similar to that used in HTST but operating pressures are limited by gaskets. Liquid velocities are low which could lead to uneven heating and burn-on. This method is economical in floor space, easily inspected, and allows for potential regeneration.
Tubular Heat Exchangers:
There are several types:
shell and tube
shell and coil
Heat exchanger (Shell)
Heat Exchanger (Tube)
Figure: Typical Shell and Tube Type Heat Exchanger
All of these tubular heat exchangers have fewer seals involved than with plates. This allows for higher pressures, thus higher flow rates and higher temperatures. The heating is more uniform but difficult to inspect. The shell and tube type heat exchanger is used in the MILK VITA industry for UHT method. The figure shows the typical shell and tube type heat exchanger for the UHT treatment of milk.
Packaging for Aseptic Processing
The UHT treated milk can sustain up to the desired time if it is packed in a sterilized packet. So the contact from any kind of human touch is not expected here. All handling of product post-process must be within the sterile environment. And the packet used to pack UHT treated milk is washed with tergitol. Tergitol is the concentrated solution of hydrogen peroxide.
There are 5 basic types of aseptic packaging lines:
Fill and seal: preformed containers made of thermoformed plastic, glass or metal are sterilized, filled in aseptic environment, and sealed
Form, fill and seal: roll of material is sterilized, formed in sterile environment, filled, sealed e.g. tetrapak
Erect, fill and seal: using knocked-down blanks, erected, sterilized, filled, sealed. e.g. gable-top cartons, cambri-bloc
Thermoform, fill, sealed roll stock sterilized, thermoformed, filled, sealed aseptically. e.g. creamers, plastic soup cans
Blow mold, fill, seal:
There are several different package forms that are used in aseptic UHT processing:
thermoformed plastic containers
flow molded containers
It is also worth mentioning that many products that are UHT heat treated are not aseptically packaged. This gives them the advantage of a longer shelf life at refrigeration temperatures compared to pasteurization, but it does not produce a shelf-stable product at ambient temperatures, due to the possibility of recontamination post-processing.
UHT Process in MILK VITA
In the MILK VITA industry the HT UHT process is generally followed. In the UHT plant Milk is pumped to the product storage tank using a pump. The pump rating is as follows:
Power 4 kW
Power factor 0.87
Then the product is then passed through the homogenizer using another pump. The pump used in the homogenizer is rated as follows:
Power 3 kW
Voltage 22/400 Volt
The homogenizer used in this plant is the same as those used in the pasteurization plant. This is of 3 pistons and is capable of producing pressure of 200 Bar. Then the product is pumped to the degasser. Degasser is used to remove the bad taste of unwanted flavor of the product. The pump used to pump product to the degasser is rated as follows:
Power 1.5~1.7 kW
Voltage 220-420 when connected to Y
380-420 when connected to Δ
Current 6.1-5.9 A when connected to Δ
3.5-3.4 A when connected to Y
Product Storage Tank
Heat exchanger (L2,L6,L4,L5)
4 Way Valve
Aseptic tank (Storage)
7 Layer Tetra pack
Packed with Dozen of pack
Figure Flow diagram of UHT process followed in MILK VITA industry
After degasification the product is again passed through another homogenizer then it is passed to the heat exchanger for UHT treatment. After finishing the UHT treatment the product is then passed through a 4 way valve. From this valve product can be passed to an aseptic tank or sterilizer or filling machine. Generally product is send to the filling machine. If generally there is any problem with the filling machine then it is send to the aseptic storage to store the product for packing at later time. The product can also be send to the sterilizer for further sterilization.
This is the process of UHT that is followed in the milk vita industry. All the process performed is controlled by a set of PLC devices. So handling the processes becomes easier. In this industry only chocolate milk is UHT treated.
The packing system used in the UHT treated product should be aseptic and must be clean before starting the machine. This is because the UHT treated milk sustains the desirable time if and only if it is packed in an aseptic packet. At present the product of MILK VITA is not sustaining the desirable time period as because there are some problem with the packet. The manufacturer of the tetra packet suggested storing the packet not above 20’C. But due to some problem the packets was not stored at that temperature. So the product was not sustaining up to the desired time period.
In the filing machine of UHT section the following process happens sequentially.
First the product is pumped from the heat exchanger to the filling machine or from the aseptic machine. In the filling machine there is a roller of the tetra pack. The tetra pack is drawn to a certain position and then stamped on the open side with heated sealer. This sealed the packet. By this time the packed moves over the filling nozzle. The nozzle then allows certain amount of product specified by the operator at the beginning of the process. The amount to be filled is set by the instructor according to the capacity of the packet. After filling the packet then proceeds further and the edge and the sides of the packet is then folded to give it the shape of a box. This is then generally called the brick. After finishing all these process the product brick then ride on a belt pulley then travel to a certain distance to reach the wrapping machine. After reaching the wrapping machine the product is then moved to one side and waits for the desired number packet to proceed. When there are 10 or 12 no of packets then these packets are pushed through the wrapper inside. The no of packet to be wrapped can be set by the operator. So when the product is wrapped then they are passed through a heating chamber to seal the wrapping material. On exit of this heating chamber they wait for a while under a fan or blower to cool the heating place of the wrapper. Then it passes through the exit and moves over some roller to the rest. From there product is collected and stored for marketing.
Figure Typical wrapping machine used in MILK VITA
Figure Typical filling machine used in the MILK VITA industry
Figure: Filling machine description
The aseptic filling machine used to pack product in the MILK VITA industry contains the following sections as shown in the figure. They are described below:
Web in feed and longitudinal scoring:
The role of package material is feed directly into the machine and unwinds through longitudinal scoring rolls which outlines the basic packaging shape.
The Inner face of the web goes through a hydrogen peroxide application system where the food contact surface is wetted with the sterilizing solution. The web then travels to stainless steel drum heated at 85° C, where the web is held In contact with the heated drum for 7 to 9 seconds- Here a vigorous reaction takes place, rendering the food contact surface of the paper web commercially sterile- After passing through the heated drum, the paper web continues to travel to the folding tower.
Web Folding Tower
After sterilization, the paper web is folded into a tent-shaped configuration, while web sterility I maintained by pre-sterilized air and the residual hydrogen peroxide is removed. The pre-sterilized air is produced by heating air to a temperature in excess of 300° C. Then it is cooled by water to 100° C for distribution. The pre-sterilized air blown inside the folding tower, crown and the filling and sealing areas of the machine, thus providing positive pressure of sterile air to prevent contamination of the sterile web and product during the packaging process and virtually eliminating the residue of hydrogen peroxide.
When the web reaches the top of the machine, it is folded (n half long the central score made by the longitudinal scoring. The open edge of the web travels inside the sterile air manifold where a constant pressure of sterile air is maintained
The folded web leaves the crown section and then it is transversely scored- The scoring facilitates the formation of the long vertical sides of the package and the triangular sections of the tabs on the carton.
After transverse scoring, the sterile web Is longitudinally sealed through inductive heating. It is sealed along the vertical open edge to form a tube for product filling.
The formed tube travels to a filling area where a pre-sterilized product is delivered into the package material- The filling section consists of two stainless steel pipes- The shorter one provides positive pressure of sterilized air in the chamber, thus maintaining sterility during the filling operation. The longer pipe is used to deliver the product into the tube.
Transverse Sealing & Cutting
Once the product is placed in the tube, the bottom edge of the tube is transversely sealed through inductive heating. The web is pulled down one package length and the top edge of the tube is then transversely sealed by the same transverse sealer. The length of the package is properly maintained by a registration system which senses a registration mark on each package. A pouch-shaped package is formed and then cut off the web.
Once leaving the web, the individual package is dropped into the pocket conveyor. The conveyor indexes the package forward through the forming arch- The forming arch presses down the tabs on the bottom edge of the pouch to form one tong side of the package. The formers, then, form the top and bottom of the pouch into the final brick shape with all four tabs extending from it. The tabs are then heat-sealed and folded until they are bounded- The final brick package is ready.
The specification of the machine used in this industry is as follows
|Filler Weight||2.820 Kg (200-250 ml)|
|Electrical connections||220V 60 Hz 3-Ph
380V 50 Hz 3-Ph
|Average electrical consumption||15 kW (200-250 ml)|
|Cooling water consumption||11 l/min|
|Cooling water temperature||10-20° C|
|Cooling water pressure||2,7-3,3 bar|
|Stream pressure||0,5-0,8 bar|
|Stream consumption||9-11 Kg/hour/sterilization cycle|
|Compressed air pressure||6-7 bar|
|Compressed air consumption||40 m3/hour|
|4.000 (*) (200-250 ml)
3.650 (*) (375-750 ml)
3.300 (*) (1.000 ml)
Ice Cream Section
The basic steps in the manufacturing of ice cream are generally as follows:]
• Blending of the mix ingredients
• Aging the mix
The flow diagram may be figured as follows:
Figure: Flow diagram of Ice cream production
The red section represents the operations involving raw, unpasteurized mix, the pale blue section represents the operations involving pasteurized mix, and the dark blue section represents the operations involving frozen ice cream.
First the ingredients are selected based on the desired formulation and the calculation of the recipe from the formulation and the ingredients chosen, and then the ingredients are weighed and blended together to produce what is known as the “ice cream mix”. Blending requires rapid agitation to incorporate powders, and often high speed blenders are used.
Figure: Ice cream mixing
The mix is then pasteurized. Pasteurization is the biological control point in the system, designed for the destruction of pathogenic bacteria. In addition to this very important function, pasteurization also reduces the number of spoilage organisms such as psychrotrophs, and helps to hydrate some of the components (proteins, stabilizers).
Pasteurization (Ontario regulations): 69° C/30 min. 80° C/25s
Both batch pasteurizers and continuous (HTST) methods are used. Batch pasteurizers lead to more whey protein denaturizing which some people feel gives a better body to the ice cream. In a batch pasteurization system, blending of the proper ingredient amounts is done in large jacketed vats equipped with some means of heating, usually steam or hot water. The product is then heated in the vat to at least 69 C (155 F) and held for 30 minutes to satisfy legal requirements for pasteurization, necessary for the destruction of pathogenic bacteria. Various time temperature combinations can be used. The heat treatment must be severe enough to ensure destruction of pathogens and to reduce the bacterial count to a maximum of 100,000 per gram. Following pasteurization, the mix is homogenized by means of high pressures and then is passed across some type of heat exchanger (plate or double or triple tube) for the purpose of cooling the mix to refrigerated temperatures (4 C). Batch tanks are usually operated in tandem so that one is holding while the other is being prepared. Automatic timers and valves ensure the proper holding time has been met. Continuous pasteurization is usually performed in a high temperature short time (HTST) heat exchanger following blending of ingredients in a large, insulated feed tank. Some preheating, to 30 to 40 C, is 40 necessary for volatilization of the components. The HTST system is equipped with a heating section, a cooling section, and a regeneration section. Cooling sections of ice cream mix HTST presses are usually larger than milk HTST presses. Due to the preheating of the mix, regeneration is lost and mix entering the cooling section is still quite warm.
The mix is also homogenized which forms the fat emulsion by breaking down or reducing the size of the fat globules found in milk or cream to less than 1 μ m. Two stage homogenization is usually preferred for ice cream mix. Clumping or clustering of the fat is reduced thereby producing a thinner, more rapidly whipped mix. Melt-down is also improved. Homogenization provides the following functions in ice cream manufacture:
• Reduces size of fat globules
• Increases surface area
• Forms membrane
• Makes possible the use of butter, frozen cream, etc.
By helping to form the fat structure, it also has the following indirect effects:
• Makes a smoother ice cream
• Gives a greater apparent richness and palatability
• Better air stability
• Increases resistance to melting
Homogenization of the mix should take place at the pasteurizing temperature. The high temperature produces more efficient breaking up of the fat globules at any given pressure and also reduces fat clumping and the tendency to thick, heavy bodied mixes. No one pressure can be recommended that will give satisfactory results under all conditions. The higher the fat and total solids in the mix, the lower the pressure should be. If a two stage homogenizer is used, a pressure of 2000 – 2500 psi on the first stage and 500 – 1000 psi on the second stage should be satisfactory under most conditions. Two stage homogenization is usually preferred for ice cream mix. Clumping or clustering of the fat is reduced thereby producing a thinner, more rapidly whipped mix. Melt-down is also improved.
The mix is then aged for at least four hours and usually overnight. This allows time for the fat to cool down and crystallize, and for the proteins and polysaccharides to fully hydrate. Aging provides the following functions:
• Improves whipping qualities of mix and body and texture of ice cream.
It does so by:
• Providing time for fat crystallization, so the fat can partially coalesce;
• Allowing time for full protein and stabilizer hydration and a resulting slight viscosity increase;
• Allowing time for membrane rearrangement and protein/emulsifier interaction, as emulsifiers displace proteins from the fat globule surface, this allows for a reduction in stabilization of the fat globules and enhanced partial coalescence.
Aging is performed in insulated or refrigerated storage tanks, silos, etc. Mix temperature should be maintained as low as possible without freezing, at or below 5 C. An aging time of overnight is likely to give best results under average plant conditions. A “green” or un-aged mix is usually quickly detected at the freezer.
Freezing and Hardening
Following mix processing, the mix is drawn into a flavor tank where any liquid flavors, fruit purees or colors are added. The mix then enters the dynamic freezing process which both freezes a portion of the water and whips air into the frozen mix. The “barrel” freezer is a scraped-surface, tubular heat exchanger, which is jacketed with a boiling refrigerant such as ammonia or Freon. Mix is pumped through this freezer and is drawn off the other end in a matter of 30 seconds, (or 10 to 15 minutes in the case of batch freezers) with about 50% of its water frozen. There are rotating blades inside the barrel that keep the ice scraped off the surface of the freezer and also dashers inside the machine which help to whip the mix and incorporate air.
Figure: Continuous barrel freezer
Ice cream contains a considerable quantity of air, up to half of its volume. This gives the product its characteristic lightness. Without air, ice cream would be similar to a frozen ice cube. The air content is termed its overrun, which can be calculated mathematically. As the ice cream is drawn with about half of its water frozen, particulate matter such as fruits, nuts, candy, cookies, or whatever you like, is added to the semi-frozen slurry which has a consistency similar to soft-serve ice cream. In fact, almost the only thing which differentiates hard frozen ice cream from soft-serve, is the fact that soft serve is drawn into cones at this point in the process rather than into packages for subsequent hardening.
After the particulates have been added, the ice cream is packaged and is placed into a blast freezer at 30° to -40° C where most of the remainder of the water is frozen. Below about -25° C, ice cream is stable for indefinite periods without danger of ice crystal growth; however, above this temperature, ice crystal growth is possible and the rate of crystal growth is dependent upon the temperature of storage. This limits the shelf life of the ice cream
Hardening chamber inside
Tray for holding the Ice cream package
Figure: Typical Hardening Chamber used in MILK VITA
A primer on the theoretical aspects of freezing will help you to fully understand the freezing and re-crystallization process.
Hardening involves static (still, quiescent) freezing of the packaged products in blast freezers. Freezing rate must still be rapid, so freezing techniques involve low temperature (-40oC) with either enhanced convection (freezing tunnels with forced air fans) or enhanced conduction (plate freezers).
The rate of heat transfer in a freezing process is affected by the temperature difference, the surface area exposed and the heat transfer coefficient (Q=U A dT). Thus, the factors affecting hardening are those affecting this rate of heat transfer:
• Temperature of blast freezer – the colder the temperature, the faster the hardening, the smoother the product.
• Rapid circulation of air – increases convective heat transfer.
• Temperature of ice cream when placed in the hardening freezer – the colder the ice cream at draws, the faster the hardening; – must get through packaging operations first.
• Size of container – exposure of maximum surface area to cold air, especially important to consider shrink wrapped bundles – they become a much larger mass to freeze. Bundling should be done after hardening.
• Composition of ice cream – related to freezing point depression and the temperature required ensuring a significantly high ice phase volume.
• Method of stacking containers or bundles to allow air circulation. Circulation should not be impeded – there should be no ‘dead air’ spaces (e.g., round vs. square packages).
• Care of evaporator – freedom from frost – acts as insulator.
• Package type, should not impede heat transfer – e.g., Styrofoam liner or corrugated cardboard may protect against heat shock after hardening, but reduces heat transfer during freezing so not feasible.
UTILITIES AT MILK VITA
The boiler is a steam producing unit. This widely used in the MILK VITA industry. Steam is an essential part of the processes used such as pasteurization and UHT process. The following section will describe steam and boiler.
Production and Utilization of Steam
This section will describe the production and utilization of both steam utilities absolutely essential to the operation of a dairy processing plant.
Figure: Simultaneous use of refrigeration and steam in Pasteurization.
Steam Production and Utilization
The steam produced is used in heat exchanging with the milk while pasteurizing and ultra heat treating. So the characteristics of this steam should be clearly understood.
Steam Production and Distribution
Steam is produced in large tube and chest heat exchangers, called water tube boilers if the water is in the tubes, surrounded by the flame, or fire tube boilers if the opposite is true. The pressure inside a boiler is usually high, 300-800 kPa. The steam temperature is a function of this pressure. The steam, usually saturated or of very high quality, is then distributed to the heat exchanger where it is to be used, and it provides heat by condensing back to water (called condensate) and giving up its latent heat. The temperature desired at the heat exchanger can be adjusted by a pressure reducing valve, which lowers the pressure to that corresponding to the desired temperature. After the steam condenses in the heat exchanger, it passes through a steam trap (which only allows water to pass through and hence holds the steam in the heat exchanger) and then the condensate (hot water) is returned to the boiler so it can be reused. The following image is a schematic of a steam production and distribution cycle.
Figure: Steam production and distribution cycle.
STEAM GENERATOR: Boiler
The boiler used in this industry is rated as follows:
Type: Dosing pump beta-1601 PPE with chemical tank.
• For chemical dosing, on/off feed water regulation.
• For boiler with maximum design pressure: 12 Bar
Liquid end: Polypropylene
Chemical tank: Polyethylene
|Max. outlet pressure:||16 bar|
|Max. inlet pressure:||8 bar|
|Capacity at 16 bar:||1.1 l/h|
|Stroke at 16 bar:||0.10 ml|
|Capacity at 8 bar:||1.5 l/h|
|Stroke at 8 bar:||0.14 ml|
|Max. frequency:||180 stroke/min|
|Suction lift:||6 m WG|
|Max. Working temperature at max. counter pressure:||50°C|
|Medium power drain:||20 W|
|Peak power drain:||1.9 A|
|Fuse (placed behind the control panel):||0.8 AT|
|Enclosure rating:||IP 65|
|Motor:||1 phase AC|
|Control supply:||115/230 V 50/60 Hz|
Figure: Flow of steam around and inside the boiler
Figure: Photographic image of Cleaver and Brooks fire tube boiler.
The recent power supply in Bangladesh is not continuous and load shedding is a common phenomena. But food industries like MILK VITA require continuous supply of electricity for refrigeration and other production purpose. There are several machines where sudden hamper of electrical supply may cause solemn problem. So generator is a must in this industry. The generator used in this industry is a diesel engine 6 cylinder in line IC engine.
Figure: Different Parts of Generator Used In MILK VITA (continue)
The several parts pointed on the figure are
1. Filler cap for the lubricating oil
2. Fuel filter
3. Lubricating oil cooler
4. Fuel injection pump
5. Lubricating oil dipstick
6. Drain plug for the lubricating oil
7. Crankshaft pulley
8. Drive belt
9. Coolant pump
11. Coolant outlet
12. Front lift bracket
Figure: Different Parts of Generator Used In MILK VITA
14. Induction manifold
16. Lubricating oil filter
17. Fuel lift pump
18. Lubricating oil sump
19. Starter motor
20. Flywheel housing
23. Exhaust manifold
24. Rear lift bracket
The specifications of the boiler used are as follows:
6 Cylinder arrangements in line
Cycle Four stroke
Turbocharged Induction system with Intercooler
Combustion system direct injection
Nominal bore 100 mm (3.937 in)
Stroke 127 mm (5.00 in)
Compression ratio 16.0:1
Firing order 1, 5, 3, 6, 2, 4
Cubic capacity 6 liters (365 in3)
Inlet 0.20 mm (0.008 in)
Exhaust 0.45 mm (0.018 in)
Lubricating oil pressure (minimum at maximum engine speed and normal engine temperature)
Engines without piston cooling jets 207 Kpa (30 lbf/in2) 2,1 kgf/cm2
Engines with piston cooling jets 280 Kpa (40 lbf/in2) 2,5 kgf/cm
Direction of rotation Clockwise from the front
While performing our training we found that there are several sections to improve. They can increase their production capability if they focus on those sections and try to optimize those sections.
The most important section that draws our attention is the UHT section. The production capability of UHT section is 10,000 liter/hr. We think that this huge amount of plant was not necessary for that industry. They can buy a smaller unit as consumption of milk in our country is not that high. This extra capable plant consumes much more electrical power which is in fact increasing cost. Moreover the manufacturer of the UHT plant suggested running the plant at full condition. But it is obvious that most often the plant is not run full.
Another recommendation can be given that a packing machine from Elecster is out of order. This machine is highly capable of faster production. This machine can produce 7200 pouches of milk per hour. If this machine is repaired then they will be capable of producing more product than before which will increase their production capability and also reduce their power consumption and thus reduce cost of production.
Like all other governmental industries there is a presence of bureaucracy in the MILK VITA industry. This causes some unexpected delay in some critical decision-making situation. This sometime directly effects the production. This should be eliminated for shake of MILK VITA itself.
This industrial training provides us the opportunity to get familiar with the various mechanical components used in the real life. The experience we gather during this training will help us in our professional life obviously. During the training period we got the chance to gather real life experience about refrigeration cycle, heat exchanger and so other components. We got idea about how these things are used in the real life. We think that the practical knowledge we gather during this training will surely enhance our engineering knowledge.
During the preparation of this industrial report helps from different sources are taken. Especially the manuals supplied by the personnel of engineering department of the MILK VITA industry were very helpful. Besides these we took help from the internet and several websites they are
Time to time help from Microsoft Encarta Encyclopedia 2008® was also taken.