Unit 4 | Principles of Food Science and Nutrition | 5th Semester

Principles of Food Science and Nutrition

Unit IV

Unit - IV Syllabus
Pasteurization, sterilization, bactofugation, UHT, and homogenization of milk, cooling and chilling of milk. Filtration and clarification of milk.

Notes by Agricorn.in


Pasteurization is a process that involves heating milk to a specific temperature for a specified period of time to destroy any harmful bacteria or pathogens that may be present in it. This process was developed by French microbiologist Louis Pasteur in the 19th century, and it is widely used in the dairy industry to ensure the safety and quality of milk.

The two main types of pasteurization are:

  1. Low-Temperature Long-Time (LTLT) Pasteurization: also called batch method. Low-temperature long-time (LTLT) pasteurization is a method of milk pasteurization that involves heating raw milk to a temperature of 63°C for 30 minutes, followed by rapid cooling to below 10°C. This process is typically used for small-scale dairy processing operations and is considered a relatively mild form of pasteurization. This method is widely used for milk pasteurization in most countries. The process may not be sufficient to eliminate all the bacteria and spores that can cause spoilage, which is why refrigeration and proper storage are still necessary. The main advantage of LTLT pasteurization is that it causes minimal damage to the milk proteins, enzymes, and vitamins, which can have a positive effect on the sensory properties and nutritional value of the milk.
  2. High-temperature, short-time (HTST) pasteurization: In this process, milk is heated to a temperature of 72°C (161.6°F) for 15 seconds, followed by rapid cooling to below 4°C (39.2°F). This process destroys most of the harmful bacteria present in milk while preserving its nutritional value and taste. This method can reduce the microbial load in milk by 99.999% and can extend the shelf life of milk up to 2-3 weeks if properly stored. HTST pasteurization is widely used in the dairy industry as it provides an effective means of reducing the risk of foodborne illness without affecting the taste, texture, or nutritional value of milk. It is also more efficient than other methods of pasteurization, such as batch pasteurization, as it can process large quantities of milk quickly and continuously.
  3. Ultra-high-temperature (UHT) pasteurization: In this process, milk is heated to a temperature of 135°C (275°F) for 2-5 seconds, followed by aseptic packaging to prevent contamination. This process can extend the shelf life of milk up to several months and is commonly used for products like long-life milk, cream, and condensed milk.

Some of the benefits of pasteurization include:

  1. Increased shelf life: Pasteurization can extend the shelf life of milk by several days, depending on the type of process used.
  2. Improved safety: Pasteurization destroys harmful bacteria such as Salmonella, E. coli, and Listeria, which can cause serious illness in humans.
  3. Improved taste and texture: Pasteurization does not significantly affect the taste or texture of milk, making it more palatable and enjoyable to consume.

However, some critics of pasteurization argue that it can also have some negative effects, including:

  1. Reduction in nutritional value: Some studies suggest that pasteurization can reduce the nutritional value of milk, particularly its vitamin and mineral content.
  2. Changes in taste and texture: Some people claim that pasteurized milk has a slightly different taste and texture compared to raw milk.
  3. Destruction of beneficial bacteria: Pasteurization can also destroy beneficial bacteria in milk that are important for gut health and immune function.

Despite these concerns, pasteurization is still widely used in the dairy industry as a way to ensure the safety and quality of milk for human consumption.


Sterilization is a process of heating milk to a high temperature to destroy all microorganisms present in it. It is done to increase the shelf life of milk, prevent spoilage and reduce the risk of foodborne illnesses caused by consuming contaminated milk. 

The process of milk sterilization involves heating milk to a temperature of 121°C for 15-20 seconds, which is referred to as the "high-temperature short-time" (HTST) process. This high temperature is maintained for a sufficient period to kill all microorganisms present in the milk, including both beneficial and harmful ones. The milk is then rapidly cooled to prevent any recontamination.

There are several methods used for milk sterilization, including:

  1. Ultra-High Temperature (UHT) Sterilization: This is the most common method of sterilizing milk, and it involves heating the milk to a temperature of 135-150°C for 1-2 seconds. This process kills all microorganisms in the milk, including spores, and the milk can be stored at room temperature for several months without refrigeration.
  2. In-container sterilization: This method involves sterilizing the milk in the container in which it will be sold. The milk is first heated to a temperature of 82-85°C for 15-30 seconds and then cooled rapidly to below 20°C to prevent any recontamination. This process ensures that the milk is sterile when it is sold to the consumer.
  3. Batch sterilization: In this method, milk is heated to a temperature of 63°C for 30 minutes or 72°C for 15 seconds. This process kills most of the bacteria in the milk but may not be effective against spores. The milk must be stored at low temperatures to prevent the growth of any surviving bacteria.
  4. Continuous sterilization: This is similar to batch sterilization, but the milk is heated at a higher temperature (above 90°C) for a shorter period of time (less than 1 second). This process is more effective in killing bacteria and spores but requires specialized equipment and is not commonly used.

Sterilization of milk can affect the quality of the milk. High-temperature treatments can cause changes in the texture, taste, and nutritional content of the milk. For example, UHT-treated milk has a slightly cooked taste and may have a slightly brown colour. It also has a longer shelf life, but some nutrients may be lost during the heating process. In-container sterilization is considered to be the most effective method of sterilizing milk without affecting its quality.


Increased shelf life: Sterilized milk has a longer shelf life compared to raw milk, as it is free from microorganisms that cause spoilage.

Increased safety: Sterilization kills harmful microorganisms such as bacteria, viruses, and parasites, which can cause diseases such as typhoid, cholera, and tuberculosis.

Convenience: Sterilized milk can be stored at room temperature without the need for refrigeration, making it convenient for consumers who do not have access to refrigeration or have limited space in their refrigerator.

Nutritional value: Sterilization does not significantly affect the nutritional value of milk, unlike some other preservation methods such as ultra-high temperature (UHT) treatment.


Flavor: Sterilization can cause a cooked flavor in milk, which may be undesirable to some consumers.

Nutritional loss: Sterilization can cause a slight loss of certain heat-sensitive vitamins such as vitamin C and thiamine.

Cost: Sterilization requires specialized equipment and additional processing time, which increases the cost of production compared to other preservation methods such as pasteurization.

Environmental impact: Sterilization requires more energy and resources compared to pasteurization, which may have a negative impact on the environment.


Bactofugation is a process used to remove bacteria from milk. It is a physical separation process based on the principle of centrifugation. The process uses a high-speed centrifuge to separate the bacteria and other solid impurities from milk. The centrifuge applies a high gravitational force to the milk, which causes the heavier bacteria and other impurities to separate from the milk and collect at the bottom of the centrifuge.

The bactofugation process is typically used after the milk has been pasteurized. This is because pasteurization reduces the bacterial count in the milk, but does not completely eliminate all the bacteria. Bactofugation is used to further reduce the bacterial count and improve the microbiological quality of the milk.

The bactofugation process begins with the milk being pre-heated to a temperature of about 50-60°C. This helps to reduce the viscosity of the milk and improve the separation efficiency of the centrifuge. The milk is then pumped into the centrifuge at a high flow rate. The centrifuge spins the milk at a very high speed, typically around 15,000 to 20,000 rpm.

As the milk is spun, the heavier bacteria and other impurities are forced to the outer edge of the centrifuge bowl. A series of nozzles positioned near the outer edge of the bowl spray a hot water solution into the bowl, which washes the bacteria and impurities out of the bowl and into a collection chamber. The purified milk flows out of the centrifuge through a separate outlet.

The effectiveness of bactofugation in removing bacteria from milk depends on various factors such as the flow rate of the milk, the centrifuge speed, the temperature of the milk, and the properties of the bacteria. However, bactofugation can achieve a bacterial reduction of up to 99.9% in milk.

Overall, bactofugation is an effective method for removing bacteria from milk, which helps to improve the microbiological quality and safety of milk. However, the process is relatively expensive and requires specialized equipment, which limits its use to larger-scale milk processing operations.

UHT of Milk

UHT (Ultra High Temperature) treatment is a thermal processing technique that is used to sterilize milk and other liquid foods. In UHT treatment, milk is heated to a high temperature of about 135-150°C for a short duration of 2 to 5 seconds, which effectively kills all the microorganisms present in it, including spores. The milk is then rapidly cooled and aseptically packaged in a sterile container to prevent recontamination.

The UHT treatment process has several advantages over traditional pasteurization. Firstly, it ensures a much longer shelf life for milk, as it is effectively sterilized and free of microorganisms that could cause spoilage. UHT-treated milk can be stored at room temperature for several months without refrigeration, making it a convenient option for consumers in areas with limited refrigeration facilities. Secondly, UHT treatment also preserves the nutritional quality of milk, as it does not result in significant losses of heat-sensitive nutrients like vitamins and proteins.

However, UHT treatment also has some drawbacks. The high temperatures used in the process can cause some changes in the sensory qualities of milk, such as a slightly cooked flavour and a darker colour. Additionally, UHT-treated milk is more expensive than traditionally pasteurized milk, due to the higher capital and operating costs associated with the UHT processing equipment.

Overall, UHT treatment is an effective method for producing sterile milk with an extended shelf life, which is suitable for use in areas with limited refrigeration facilities or for long-term storage.

Homogenization of Milk

Homogenization is a process used in milk processing to break down fat globules and disperse them uniformly throughout the milk, resulting in a stable and consistent product. Here is a detailed explanation of the homogenization process:

  1. Raw milk is collected and stored in refrigerated tanks before processing.
  2. The milk is then preheated to a temperature of 50-60°C to increase its fluidity and reduce its viscosity.
  3. The milk is then passed through a homogenizer, which consists of a high-pressure piston pump, a small orifice valve, and a homogenizing chamber.
  4. As the milk is forced through the small valve, it is subjected to high pressure, typically around 15-25 MPa.
  5. The high pressure causes the milk fat globules to break down into smaller particles, typically less than 1 micron in size.
  6. The smaller fat particles are then dispersed throughout the milk, creating a uniform distribution of fat.
  7. The homogenized milk is then cooled and stored until it is ready for further processing or packaging.

The homogenization process has several benefits, including:

  1. Improved stability: Homogenization creates a more stable milk product by preventing the cream from separating from the milk.
  2. Improved texture: Homogenized milk has a smoother and creamier texture compared to non-homogenized milk.
  3. Improved colour: Homogenized milk has a whiter and more consistent colour than non-homogenized milk.
  4. Improved flavour: Homogenized milk has a slightly sweeter taste compared to non-homogenized milk due to the smaller fat globules.
  5. Improved shelf-life: Homogenized milk has a longer shelf-life compared to non-homogenized milk, as it is less prone to spoilage and bacterial growth.

However, there are some potential drawbacks to homogenization. One concern is that it may lead to the oxidation of the milk fat, which can result in off-flavours and the breakdown of some vitamins. Additionally, some studies suggest that homogenization may increase the risk of heart disease by making it easier for cholesterol to be absorbed into the bloodstream. However, the evidence for this is not conclusive, and more research is needed to fully understand the effects of homogenization on health. Other demarits are: Production costs, Nutritional loss etc.

Notes by Agricorn.in

Cooling and chilling of milk

Cooling and chilling of milk are important steps in the processing of raw milk to maintain its quality and safety. The main objective of cooling and chilling is to reduce the temperature of the milk as quickly as possible to slow down the growth of bacteria and other microorganisms. This also helps to reduce the rate of enzymatic reactions that can lead to the spoilage of milk.

The cooling process is usually done immediately after milking to reduce the temperature of the milk from body temperature (37°C) to a lower temperature that inhibits bacterial growth. The recommended temperature for milk storage is between 2°C and 4°C. 


After pasteurization or sterilization, milk is heated to a high temperature which results in denaturation of proteins and other changes. Cooling the milk to a low temperature helps to prevent microbial growth, maintain freshness, and preserve the nutritional value of the milk. The cooling process involves reducing the temperature of the milk quickly and efficiently to the desired level. 


Chilling is the process of further reducing the temperature of the milk to a lower level for storage. Chilling milk also slows down the growth of bacteria, reduces spoilage, and extends the shelf life of the milk. 

Cooling of milk is done st 5-10°C (chilling - below 5° C).

There are different methods of cooling and chilling milk, which include:

  1. Bulk tank cooling: This is a common method of cooling and chilling milk on farms. The milk is stored in a refrigerated bulk tank where it is cooled rapidly using a heat exchanger. The heat exchanger works by passing cold water or refrigerant through a network of tubes that are in contact with the milk.
  2. Plate cooler: A plate cooler is a heat exchanger that is used to cool milk as it flows through a series of plates. This is a type of heat exchanger that is used to cool milk immediately after milking. The milk is pumped through a series of stainless steel plates that are cooled by a refrigerant. As the milk flows through the plates, heat is transferred from the milk to the refrigerant, which cools the milk. 
  3. Pre-cooling: This is a method of cooling milk before it is transported to a central processing plant. It is usually done by passing the milk through a heat exchanger that is cooled by well water or a cooling tower.
  4. Cold storage: Once the milk has been cooled to the recommended temperature, it is stored in a refrigerated storage tank until it is transported to a processing plant.
  5. Direct expansion cooling: This method involves passing cold refrigerant directly through a heat exchanger in the milk. The refrigerant cools the milk by absorbing heat and carrying it away. This is a fast and efficient method of cooling milk.
  6. Ice bank cooling: In this method, milk flows through a series of plates that contain a refrigerant, which absorbs heat and cools the milk. The refrigerant is cooled by an ice bank, which is a large insulated tank filled with water and ice.
  7. Immersion cooling: This method involves immersing milk containers in a cold-water bath or tank to cool them.

In addition to slowing down bacterial growth and reducing the rate of enzymatic reactions, cooling and chilling of milk also help to maintain its quality attributes such as flavour, appearance, and nutritional value. It also helps to extend the shelf life of milk, which is important for both the dairy industry and consumers.

In summary, cooling and chilling of milk are critical steps in the preservation and storage of milk, and there are various methods available to achieve these objectives.

Filtration and clarification of milk

Filtration and clarification are two important processes in milk processing that help to improve the quality and safety of milk. Filtration refers to the physical separation of solids and liquids using a filter, while clarification is the removal of suspended solids, impurities, and microorganisms from milk.

Filtration of milk: The purpose of filtration is to remove impurities and particles from milk to improve its appearance, texture, and shelf life. There are several types of filtration techniques used in milk processing, including microfiltration, ultrafiltration, and nanofiltration.

  1. Microfiltration: This is the most commonly used filtration technique in milk processing. It involves the use of a membrane with a pore size of 0.1-1.0 µm to filter out bacteria, spores, and other solids from milk. Microfiltration helps to increase the shelf life of milk and improves its texture.
  2. Ultrafiltration: This technique uses a membrane with a pore size of 0.001-0.1 µm to remove smaller particles such as proteins, lactose, and minerals from milk. Ultrafiltration is commonly used in the production of concentrated milk products such as cheese, butter, and cream.
  3. Nanofiltration: This technique uses a membrane with a pore size of 0.001 µm to remove ions and other small particles from milk. Nanofiltration is used to produce high-quality, low-lactose milk products for people with lactose intolerance.

Clarification of milk: Clarification is the process of removing suspended particles and impurities from milk, which can affect its appearance, taste, and texture. There are several techniques used in milk clarification, including centrifugation, sedimentation, and the use of clarifying agents.

  1. Centrifugation: This technique involves the use of a centrifuge to separate milk into different layers based on its density. The heavier particles settle at the bottom, while the lighter particles remain at the top. Centrifugation is commonly used to remove fat from milk, to separate cream from milk, and remove impurities from whey.
  2. Sedimentation: This technique involves the natural settling of suspended particles in milk over time. Milk is left to stand for several hours, allowing the heavier particles to settle at the bottom. The clarified milk is then decanted from the top, leaving the sediment behind. This technique is commonly used in small-scale milk production.
  3. Clarifying agents: These are chemicals added to milk to coagulate and remove suspended particles and impurities. Common clarifying agents include calcium hydroxide, activated charcoal, and diatomaceous earth. Clarifying agents are commonly used in the production of cheese and other dairy products.

In summary, filtration and clarification are important processes in milk processing that help to remove impurities and particles from milk to improve its quality and safety. The choice of technique depends on the desired outcome, the type of milk product being produced, and the scale of production.

Notes by Agricorn.in

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