We know that the milk has around 85% water; hence, it can be condensed manifolds by reducing the water content for the preparation of special dairy products like condensed milk and milk powders. Milk is normally surplus during winter months,while there is shortage during summer months. By reducing the bulk the shelf life of the milk is also enhanced. This process will help in augmenting the supply during lean months.
i. Milk Evaporators
These are the equipment to concentrate milk usually up to 40% total solids. This
is done by boiling the milk under vacuum. When the milk is boiled under atmosphericpressure, it boils at 100.5 o C. This temperature is so high that most of the nutrients will be lost. When the milk is boiled under vacuum, the boiling occurs at much lower temperature say around 60 o C. At this temperature, most of properties of milk are retained while the water gets evaporated. Milk evaporators are mainly of two types.
Batch-type Milk Evaporator: It is also called as vacuum pan. It is like a storage vat with provision of indirect heating and vacuum. Milk remains in the container for longer duration. Initially, a large amount of milk is loaded and slowly the batch gets concentrated. It is then cooled and stored. Since the evaporation occurs under vacuum the chemical properties of milk remain unchanged and only partial water is removed. Fresh milk is loaded and the heating tubes are submerged. Steam heats tubes are surrounded by milk and vapours formed. A vacuum device removes these vapours. Concentrated milk is taken out from the bottom outlet.
Continuous type: As the capacity of handling increases, there is need to get concentrated milk out in the continuous manner. Also in the batch system, the milk residence time is very high. It does affect the quality of the product. Long tube(rising or falling film) type equipment are available in the market. These units are very tall and sophisticated in terms of their operation and control. The residence time of milk during evaporation is very short, i.e., usually few seconds only. Thus,the product quality is superior. Milk travels from one heat exchanger (calendria) to another in a continuous flow. The flow of milk is such that the velocity of milk is very high giving very high rate of heat transfer. Whole system is under vacuum. Milk flow inside the tube could be either in rising or in falling manner. Accordingly,the evaporator is named. Another way of classifying is, the number of heat exchangers involved. Single, double, triple or even multi-stage systems are available.This helps in economizing the steam consumption. Figure 3.11 shows a classical single effect evaporator. Product enters the heater, and the heated milk is directed into a large calendria for evaporation. The concentrated milk falls into the vapour separator, from where the vapours are either recompressed with help of thermo-compressor or sent into condenser cum ejector system.
ii. Dryers
Drum dryer: It has drums heated internally with high-pressure steam. These drums revolve with low speed in the direction indicated The milk is supplied through feed pipe. It gets filled in between the drums. A layer of milk gets adhered on the surface of drum and it starts drying till it reaches the knife. The knife scraps out the thin layer and drops the dried flakes into the conveyor. The
dried product is taken out. The vapours generated over drums are sucked out through vapour hood. Milk during drying remains in contact with hot drum for relatively longer, causing denaturation of protein. Hence, powder obtained from this technique usually contains higher sediments. This powder is most suitable for indigenous milk product manufacture such as Gulabjamun. These dryers are getting obsolete due to high-energy requirement.
Spray dryer: They dry partially concentrated milk and are considered energy efficient systems. In this equipment, fine spray of concentrated milk is supplied into a chamber. The spray encounters with hot incoming air. Direct contact with air takes away all moisture from the spray drops. Within seconds the drops turn into a dried powder particle. The particles are heavier than air; hence, they settle down and are collected. The air gets saturated with moisture and is directed towards the exit. While diverting the air, care is taken that milk particles do not get carried away. In this system, the residence time and the rise in temperature of the particle are very less. This reduces the damage to the milk particle. This powder, therefore,gets dissolved in water without much of sediments. Hence, the spray-dried powder is considered excellent for reconstitution purposes. A typical schematic diagram of a spray dryer. Feed (concentrated milk) is sprayed through atomizer. It falls in the drying chamber. The dried product is discharged from the bottom. Fresh hot air is supplied through steam heaters and directed into the drying chamber. The exhaust fan draws the exit air through a cyclone separator. Fine particles carried away, are arrested in the cyclone separator while the clean,saturated air is delivered into atmosphere.
i. Milk Evaporators
These are the equipment to concentrate milk usually up to 40% total solids. This
is done by boiling the milk under vacuum. When the milk is boiled under atmosphericpressure, it boils at 100.5 o C. This temperature is so high that most of the nutrients will be lost. When the milk is boiled under vacuum, the boiling occurs at much lower temperature say around 60 o C. At this temperature, most of properties of milk are retained while the water gets evaporated. Milk evaporators are mainly of two types.
Batch-type Milk Evaporator: It is also called as vacuum pan. It is like a storage vat with provision of indirect heating and vacuum. Milk remains in the container for longer duration. Initially, a large amount of milk is loaded and slowly the batch gets concentrated. It is then cooled and stored. Since the evaporation occurs under vacuum the chemical properties of milk remain unchanged and only partial water is removed. Fresh milk is loaded and the heating tubes are submerged. Steam heats tubes are surrounded by milk and vapours formed. A vacuum device removes these vapours. Concentrated milk is taken out from the bottom outlet.
Continuous type: As the capacity of handling increases, there is need to get concentrated milk out in the continuous manner. Also in the batch system, the milk residence time is very high. It does affect the quality of the product. Long tube(rising or falling film) type equipment are available in the market. These units are very tall and sophisticated in terms of their operation and control. The residence time of milk during evaporation is very short, i.e., usually few seconds only. Thus,the product quality is superior. Milk travels from one heat exchanger (calendria) to another in a continuous flow. The flow of milk is such that the velocity of milk is very high giving very high rate of heat transfer. Whole system is under vacuum. Milk flow inside the tube could be either in rising or in falling manner. Accordingly,the evaporator is named. Another way of classifying is, the number of heat exchangers involved. Single, double, triple or even multi-stage systems are available.This helps in economizing the steam consumption. Figure 3.11 shows a classical single effect evaporator. Product enters the heater, and the heated milk is directed into a large calendria for evaporation. The concentrated milk falls into the vapour separator, from where the vapours are either recompressed with help of thermo-compressor or sent into condenser cum ejector system.
ii. Dryers
Drum dryer: It has drums heated internally with high-pressure steam. These drums revolve with low speed in the direction indicated The milk is supplied through feed pipe. It gets filled in between the drums. A layer of milk gets adhered on the surface of drum and it starts drying till it reaches the knife. The knife scraps out the thin layer and drops the dried flakes into the conveyor. The
dried product is taken out. The vapours generated over drums are sucked out through vapour hood. Milk during drying remains in contact with hot drum for relatively longer, causing denaturation of protein. Hence, powder obtained from this technique usually contains higher sediments. This powder is most suitable for indigenous milk product manufacture such as Gulabjamun. These dryers are getting obsolete due to high-energy requirement.
Spray dryer: They dry partially concentrated milk and are considered energy efficient systems. In this equipment, fine spray of concentrated milk is supplied into a chamber. The spray encounters with hot incoming air. Direct contact with air takes away all moisture from the spray drops. Within seconds the drops turn into a dried powder particle. The particles are heavier than air; hence, they settle down and are collected. The air gets saturated with moisture and is directed towards the exit. While diverting the air, care is taken that milk particles do not get carried away. In this system, the residence time and the rise in temperature of the particle are very less. This reduces the damage to the milk particle. This powder, therefore,gets dissolved in water without much of sediments. Hence, the spray-dried powder is considered excellent for reconstitution purposes. A typical schematic diagram of a spray dryer. Feed (concentrated milk) is sprayed through atomizer. It falls in the drying chamber. The dried product is discharged from the bottom. Fresh hot air is supplied through steam heaters and directed into the drying chamber. The exhaust fan draws the exit air through a cyclone separator. Fine particles carried away, are arrested in the cyclone separator while the clean,saturated air is delivered into atmosphere.
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