REPORT OF INDUSTRIAL TRAINING MILK VITA
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) |
| Finland | 183.9 | 19.1 | 5.3 |
| Sweden | 145.5 | 18.5 | 1.0 |
| Ireland | 129.8 | 10.5 | 2.9 |
| Netherlands | 122.9 | 20.4 | 3.3 |
| Norway | 116.7 | 16.0 | 4.3 |
| Spain (2005) | 119.1 | 9.6 | 1.0 |
| Switzerland | 112.5 | 22.2 | 5.6 |
| United Kingdom (2005) | 111.2 | 12.2 | 3.7 |
| Australia (2005) | 106.3 | 11.7 | 3.7 |
| Canada (2005) | 94.7 | 12.2 | 3.3 |
| European Union (25 countries) | 92.6 | 18.4 | 4.2 |
| Germany | 92.3 | 22.4 | 6.4 |
| France | 92.2 | 23.9 | 7.3 |
| New Zealand (2005) | 90.0 | 7.1 | 6.3 |
| United States | 83.9 | 16.0 | 2.1 |
| Austria | 80.2 | 18.8 | 4.3 |
| Greece | 69.0 | 28.9 | 0.7 |
| Argentina (2005) | 65.8 | 10.7 | 0.7 |
| Italy | 57.3 | 23.7 | 2.8 |
| Mexico | 40.7 | 2.1 | N/A |
| China (2005) | 8.8 | N/A | N/A |
Table 1. Per Capita Consumption of Milk and Milk Products in Various Countries, 2006 data.
| United States | 82,462 |
| India | 39,759 |
| China | 31,934 |
| Russia | 31,100 |
| Germany | 27,955 |
| Brazil | 25,750 |
| France | 24,195 |
| New Zealand | 15,000 |
| United Kingdom | 14,359 |
| Ukraine | 13,287 |
| Poland | 11,970 |
| Italy | 11,186 |
| Netherlands | 10,995 |
| Mexico | 10,352 |
| Argentina | 10,250 |
| Turkey | 10,000 |
| Australia | 9550 |
| Canada | 7854 |
Table 2. Cow milk production (‘000 tones) in selected countries in the world (2006).
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)
· Lactose 4.6%
· 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.
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.
2. Ultra Heat Treated Milk.
3. Flavored Milk (Chocolate, strawberry and mango).
4. Ice cream.(Cup, Stick and container)
5. Butter
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.
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:
1. Milk Collection & Transport.
2. Refrigeration System
3. Vehicle Refrigeration System
4. Liquid milk processing and packing.
5. Ice cream section.
6. Utilities
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) |
Fuel type |
| TATA DS-356 | 10,000 | 24 | 8 | Diesel |
| TATA DS-175 | 5,000 | 32 | 6 | Diesel |
| ASHOK LEYLAND | 10,000 | 6 | 8 | Diesel |
| TOYOTA | 2,000 | 16 | 6 | Diesel |
| SUZUKI MARUTI T-45 | 500 | 42 | 4 | CNG |
| TOYOTA | 250 | 36 | 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:
1. An evaporator,
2. A compressor,
3. A condenser, and
4. 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.
| Condenser |
| Compressor |
| Evaporator |
| Expansion Valve |
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
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:
Evaporator
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
1. Reciprocating and
2. 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:
| Flywheel |
| Motor |
| Vessel |
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
Service Continuous
Speed 1480 rpm
Suction pressure 1 Bar
Temperature -35’C
Oil Pressure 4.8 Bar
Discharge Pressure 9.5 Bar
Intermediate Pressure 2 Bar
Powered by 3 phase induction motor
Condenser
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
· Refrigeration 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.
| Timing relay |
| Contact Relay |
| Selector Switch
(Generator/Supply) |
| Power Distribution Relay |
| Transformer |
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.
1. Compressor
2. Condenser
3. Expansion valve
4. Evaporator
| Compressor |
| Coupling Belt |
| Generator |
| Solenoid Valve |
| Fan |
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
· Seal type
· Open type
· 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
1. Beer type
2. Plate type
3. Fin type
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.
| Expansion
Valve |
| Plate type
Coil |
| Evaporator
Case |
Figure: Typical Evaporator Used in Vehicle Refrigeration
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 ?m 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)
· standardizing
· 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.
Figure: Separation
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.
Figure: Clarification
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:
1) 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.
PASTEURIZATION
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:
1. Public Health Aspect – to make milk and milk products safe for human consumption by destroying all bacteria that may be harmful to health (pathogens)
2. 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
| Temperature | Time |
| 150°F (66°C)
(vat pasteurization) |
30 minutes |
| 161°F (72°C) 15 seconds
(high temperature, short time pasteurization) |
15 seconds |
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 thermometerand arecorder-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:
1. Milk is taken through two regenerators in a heat exchanger where it takes heat from steam which comes from a boiler.
2. From regenerators the milk can be taken out for fat separation and homogenization and then again sent to heater.
3. After heating it is held in a holder for 15 seconds.
4. 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.
5. 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.
| Storage tank |
| Overflow Tank |
| Heat exchanger Plate |
| Pump |
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 differentia