• Thermal processing is a process in which packaged and non-packaged foods are treated with mild heat, usually less than 100 °C (212 °F) or around it, to eliminate pathogens and extend shelf life
• The process safeguards foods by destroying or inactivating organisms that contribute to spoilage, including vegetative bacteria but not bacterial spores
• The process was named after the French scientist Louis Pasteur, whose research in the 1880s demonstrated that thermal processing would inactivate unwanted microorganisms in wine
• Today, pasteurization is used widely in the dairy industry and other food processing industries to achieve food preservation and food safety

Thermal processing of milk

Initially, the target organism was the bacterium that caused tuberculosis (Mycobacterium bovis or M. tuberculosis). In the 1950’s, the minimum pasteurization temperature was increased to destroy a slightly more heat-resistant organism that was associated with raw milk, Coxiella burnetti.

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• Thermization: Heating the milk to between 57°C to 68°C and hold for 15 m. It targets pathogenic bacteria while leaving the beneficial bacteria in the product and do not affects the structure and taste of the milk

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• Batch pasteurization: Also known as low-temperature long time (LTLT) pasteurization. Heat the milk to 63°C for 30 m. The extended holding time causes the alteration in the milk protein structure and taste

Types of thermal processing methods

• Flash pasteurization: Also known as high-temperature short time (HTST) pasteurization. Heat the milk to between 72°C to 74°C for 15 to 20 s

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• Ultra-high temperature (UHT) pasteurization: Heat the milk to between 135°C to 151°C for 1- 2 s. The extreme heat targets Coxiella burnetii. The heat kills all the vegetative forms of bacteria and the milk can survive for up to 9 months

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• Canned sterilization: This is a wet treatment of canned milk products in an autoclave or similar specialized treatment chamber. Heat to between 115°C to 121°C for 10 to 20 m

Types of thermal processing methods

Types of thermal processing methods

• Milk pasteurization is the process of heating milk (or milk product) to a predetermined temperature for a specified period without re-contamination during the entire process
• The temperature usually depends on the heat resistance of microorganisms that the pasteurization is targeting
• Pasteurisation refers to the process of heating every particle of milk to at least 63^°C and holding at such temperature continuously for at least 30 m or heating it to at least 71.5^°C continuously for at least 15 s or an approved temperature time combination that will serve to give a negative Phosphatase test
• All pasteurised milk must be cooled immediately to a temperature of 7 ^°C, or less

Pasteurization of milk

• Pasteurization is defined as a process applied with the aim of avoiding public health hazards arising from pathogenic microorganisms associated with milk, by heat treatment which is consistent with minimal chemical, physical and organoleptic changes in the product (IDF, 1986)

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• Pasteurization of milk and cream must result in a negative alkaline phosphatase reaction immediately after the treatment. For milk, the minimum pasteurization conditions are those having bactericidal effects equivalent to heating every particle of the milk to 72°C for 15 s or 63°C for 30 m or other equivalent time and temperature conditions -- Codex Alimentarius

• The French scientist Louis Pasteur invented pasteurization in 1860
• To remedy the frequent acidity of the local wines, he found out experimentally that it is sufficient to heat a young wine to only about 50-60˚C (122-140˚F) for a brief time to kill the microbes
• Pasteurization was originally used as a way of preventing wine and beer from souring
• Pasteurization of milk was suggested by Franz von soxhlet in 1886

HISTORY OF PASTEURIZATION

• When more rigorous heat treatments might harm the quality of products
• When the aim is to kill pathogens as with market milk and other dairy products
• When the main spoilage organisms are not very heat resistant, such as the yeasts in fruit juices
• When any spoilage organisms will be taken care of by additional preservative or by any other means
• When competing organisms are to be killed

PASTEURIZATION IS USED FOR

VAT PASTEURIZATION

• The first widely used pasteurization process for milk involved heating the milk in large tanks or vats to 60˚C for at least 20 m
• The heating and holding method was subsequently changed to 61.7˚C for 30 m
• This is not a continuous process and is referred to as vat pasteurization

DIFFERENT METHODS OF PASTEURIZATION

BATCH PASTEURIZATION (LTLT)

• Milk is heated at 63˚C for 30 m
• Can heat milk in vat or in continuous flow
• Here, milk is heated in plate heat exchanger and send to the vat
• Agitator is provided in the vat to prevent cream formation and to ensure uniform heat distribution
• Then, the milk is cooled and again passed through the plate heat exchanger and cooled using chilled water

CONTINUOUS PASTEURIZATION (HTST)

• HTST involves heating milk to 71.7°C (160°F) for 15 s to kill C. burnetii, which is the most heat-resistant pathogen in raw milk
• This method, commonly used in the dairy industry, ensures the destruction of harmful microorganisms while preserving the milk's taste and nutritional quality to a greater extent compared to older methods
• Since, it is technically impossible to bring the milk to that exact temperature, it is always safe to work with a range of temperatures. To be safe, we can heat the milk to between 72°C to 74°C for 15 to 20 s
• Flash pasteurized milk can be kept for 16 - 21 days safely

Fig. Pasteurization unit for market milk with partial homogenization.

1. Balance tank

2. Product feed pump

3. Flow controller

4. De-aerator

5. Separator

6. Constant pressure valve

7. Density transmitter

8. Flow transmitter

9. Regulating valve

10. Shut-off valve

11. Check valve

12. Homogenizer

13. Booster pump

14. Holding tube

15. Flow diversion valve

16. Plate heat exchanger

17. Process control

Stages in HTST

1. Heat regeneration

2. Heating system

3. Holding section

4. Cooling section

PROCESS

1. Cold raw milk is fed into the pasteurization plant and passes into the regeneration heating system of the plate heat exchanger

2. The plate heat exchanger is basically a series of stainless steel plates stacked together with some space in between forming chambers to hold

the milk as it passes through

3. In the regeneration section, cold milk is pumped through the BALANCE TANK, while milk that already been heated and pasteurized is pumped through the FLOW DIVERSION LINE

4. The heat from hot milk passes to the cold milk through steel plates

5. This warms the milk to 57-68˚C

6. Next the milk passes into the heating section of plate heat exchanger. Here, hot water in the chamber heats the milk to at least 72˚C which is the goal temperature for HTST

7. Hot milk is then passed though a holding tubes, it takes milk at least 15 s to pass through the tube

8. Milk is officially pasteurized once its passes through the holding tube

9. Now, the pasteurized milk is sent back through the regenerative section, where it warms the incoming cold milk

10. This cools the pasteurized milk to about 32˚C

ULTRA HIGH TEMPERATURE (UHT)

• Heat treatment processes in excess of pasteurization for milk and milk products have been designated as very high temperature (VHT) and ultra high temperature (UHT)
• It involves heating milk or cream to between 135°C to 150°C for 1- 2 s, then chilling it immediately and aseptically packaging in a hermetic container
• Despite the risk of Millard browning, UHT remains the most popular milk preservation method for safe and stable milk
• UHT offers long shelf life without refrigeration but results in a cooked taste, reduced nutritional value (vit B and C), potential age gelation, and a altered protein structure that may affect digestibility

Age gelation in UHT milk is a storage defect where the liquid milk transforms into a custard-like gel over time, limiting its shelf life. It is primarily caused by proteolytic enzymes breaking down proteins or physico-chemical reactions involving whey protein and casein micelles.

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Contributing factors:

• Storage temperature: Higher temperatures (e.g., 20°C−30°C) accelerate the process compared to lower temperatures
• Raw milk quality: High levels of bacteria producing proteases increase risk
• Control includes using high-quality raw milk, optimizing heat treatment parameters, and sometimes adding stabilizers

• Sterilization of milk is aimed at killing all MO present, including bacterial spores, so that the packaged product can be stored for a long period at ambient temperature, without spoilage

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• Since molds and yeasts are readily killed, bacteria are of prime concerned during the process

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• Sterilized milk may be defined as milk which has been heated to a temperature of 100°C or above for such lengths of time that it remain fit for human consumption for at least 7 days at room temperature

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• Commercially sterilized milk is rarely sterile in the strict bacteriological sense. This is because the requirement for complete sterility conflict with the consumer preference for normal color and flavor in the sterilized product

STERILIZATION OF MILK

Commercial sterility means the absence of microorganisms capable of growing in the food at normal non-refrigerated conditions at which the milk is likely to be held during manufacture, distribution and storage.

• Sterilization means heating milk continuously to either 115°C for 15 m or at least 130°C for a period of 1 s or more in a continuous flow and then packed under aseptic condition in sealed containers to ensure preservation at room temperature for a period not less than 15 d
• Sterilized milk is kept for a long time so that it will show extensive gravity creaming if unhomogenized. Creaming as such is undesirable
• Besides, partial coalescence of fat globules form a cream plug, which is hard to mix in milk; oiling off may occur at elevated temperatures. Therefore, sterilized liquid milk is always homogenized
• Oiling off is a potential problem in unhomogenized sterilized milk or cream, where high temperatures can cause the fat emulsion to become unstable, leading to the separation of free, liquid fat

Advantages:

• Remarkable keeping quality; does not need refrigerated storage
• No cream layer/plug
• Forms a soft  digestible curd and hence useful for feeding infants and invalids
• Distinct rich  flavor due to homogenization
• Economical to use
• Less liable to develop oxidized off- flavours

Disadvantages:

• Increased cost of production
• More loss in nutritive value than pasteurization
• Gerber test by a normal procedure not so accurate

Fat test is less accurate for sterilized milk using normal procedures due to severe protein denaturation by higher heat, which interferes with fat separation. This results in a dark, turbid fat layer, often leading to lower of fat content. 

Key reasons for inaccuracy:

• Protein charring: Sterilized milk contains highly heat-denatured proteins. When mixed with conc. H₂SO₄ in Gerber process, these proteins tend to char, creating black particles and interferes with the fat column making reading difficult
• Reduced fat separation: Severe heat changes the stability of fat, making it harder for acid and centrifuge to completely separate it from the SNF.
• Turbidity: The resulting fat column is often not clear, which decreases the accuracy of the reading

Sterilization may be:

Batch Sterilization

These may either be rotary or non-rotary in type. The batch sterilizers are rectangular, horizontal, boiler shaped retorts with a steam inlet and condensate outlet, fitted with clamp-down covers, into which steam is adjusted for the required temperature and time for sterilization.

Advantages

• Simplicity and flexibility of operation
• Less initial capital and recurring expenditure

Disadvantages

• Usually produces a brownish appearance and cooked taste in the finished product
• Sterilization may be faulty
• Cooling has to be slow to avoid breakage
• Economic advantages of large-scale processing are not obtained

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In the batch-rotary type, the filled bottles are put in to holders which are rotated at 6-7 rpm. The sterilized milk is of a slightly better quality in rotary-type sterilizers than in non-rotary ones.

(ii)  Continuous sterilizers

In this type, the filled and sealed milk bottles are automatically placed by means of a slat conveyor in to the pockets of carrier cages.

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They then passed into water at or near boiling temperature; from there, they enter the sterilizing zone, which consists of a steam chamber at 108-111⁰C.

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Here the bottles remain for a pre-determined time, viz., 25-30 minutes, for milk sterilization

Loss of nutrients during sterilization

The nutritive value of pasteurized and UHT-sterilized milk changes little by the heat treatment and during storage. In-bottle sterilized milk shows a somewhat greater loss of nutritive value.

• decrease of available lysine
• total or partial loss of some vitamins
• Maillard reactions are responsible for the partial loss of lysine
• The losses of vitamin C and B vitamins, vit B1, B2, and B6

Loss of vitamins during storage can largely be avoided if O₂ is excluded. Vitamins C and B9 may completely disappear within a few days if much O₂ is present. The loss is accelerated by exposure to light, with riboflavin (vitamin B2) being a catalyst. Most of the riboflavin disappears on long-term exposure to light.