Pasteurization: Methods, Parts, and Thermal Resistance

Two glasses of milk look identical. One came from a carton on a shop shelf that had sat unrefrigerated for weeks. The other was fresh from a farm that morning. The shelf-stable one is safe. The fresh raw one can carry tuberculosis, Salmonella, Listeria, and the organism that causes Q fever. Pasteurization is the heat step that makes the difference. It does not sterilize the milk or change it much, yet it has prevented more disease than almost any other food process. Understanding it means understanding one balance: enough heat to kill the dangerous microbes, but not so much that the milk is ruined.

Pasteurization is a mild heat treatment used to make food safer and extend its shelf life. It is applied to milk, wine, juice, and other products. The heat destroys disease-causing (pathogenic) bacteria and greatly reduces the number of spoilage microbes, without trying to kill every microorganism present. This is what separates pasteurization from sterilization: pasteurization makes food safe and longer-lasting, while sterilization kills everything, including heat-resistant spores.

Figure: Schematic diagram of Pasteurization (4)

The heat also inactivates some of the food's natural enzymes that would otherwise cause spoilage, which further extends shelf life.

The method is named after Louis Pasteur, who showed in the 1860s that gently heating wine to about 55 to 60°C killed the microbes that spoiled it, without ruining the wine. The same idea was later applied to milk. By the early 20th century, milk pasteurization had spread worldwide and became one of the most important public-health measures in food processing.

What sets the pasteurization temperature

Pasteurization time and temperature are not arbitrary. They are set by one organism: Coxiella burnetii, the cause of Q fever. Among the disease-causing bacteria found in raw milk that do not form spores, C. burnetii is the most heat-resistant. The rule is simple: if a heat treatment reliably kills C. burnetii, it will also kill all the other non-spore-forming pathogens in milk, such as Mycobacterium tuberculosis, Salmonella, Listeria, and E. coli.

This is why C. burnetii is called the index organism for pasteurization. Every approved time-temperature combination is designed to achieve at least a 5-log reduction (a 100,000-fold decrease) of C. burnetii.

This also explains what pasteurization does not do. Spore-forming bacteria such as Bacillus and Clostridium survive pasteurization, because their spores tolerate far more heat than C. burnetii. Pasteurized milk is therefore safe from the dangerous vegetative pathogens but is not sterile, which is why most pasteurized milk still needs refrigeration.

The different methods below are simply different routes to the same target: the same lethal effect on C. burnetii, reached either with lower heat for a long time or higher heat for a short time.

Methods of Pasteurization

There are two broad approaches: batch pasteurization (slower, at lower temperature) and continuous pasteurization (faster, at higher temperature). Both reach the same lethal effect on Coxiella burnetii. The choice depends mainly on the scale of production.

Batch pasteurization

Batch pasteurization heats milk to 63°C and holds it at that temperature for 30 minutes. It is also called vat pasteurization or the holder method, and it follows the low-temperature-long-time (LTLT) principle. This time and temperature are enough to kill Coxiella burnetii, the most heat-resistant non-spore-forming pathogen in milk.

The defining feature of this method is the long holding time. Milk is pumped into a vat, heated to 63°C, and then held there for a full 30 minutes, which is the step that actually does the killing. After the hold, the milk is cooled quickly to below 10°C. In many designs, cold water is circulated through a jacket around the vat, and an agitator keeps the temperature even throughout.

Batch pasteurization suits small operations and low-volume products such as creams and flavored milk. It is not practical for large dairies, because holding every batch for 30 minutes is slow.

Except for small-scale industries, batch pasteurization is not generally useful as it is suitable only for low milk volumes. However, it can be useful for low-volume milk by-products like creams and flavored milk.

Continuous pasteurization

Continuous pasteurization is the most common process in pasteurizing milk and milk by-products. Two methods are involved in continuous pasteurization: High-temperature short time (HTST) and Ultra high temperature (UHT).

1. HTST is a continuous flow pasteurization process at 72 ℃ with a holding period of 15 seconds. The process is followed by immediate cooling below 10 ℃. The plate heat exchanger (PHE) is used for the indirect heating and cooling of the products. Hot water is used in the heating medium, and chilled water is in the cooling medium. The use of PHE makes this process suitable for large-scale industry, as the capacity of PHE for pasteurization is 400 to 5000 liters per hour.
2. UHT (ultra-high temperature) treatment heats milk to above 135°C for 2 to 5 seconds, then cools it immediately. This very high temperature destroys not only the vegetative pathogens but also almost all spores, making the milk commercially sterile. Sealed in an airtight carton, UHT milk keeps for several months at room temperature without refrigeration. This is why UHT milk sits on unrefrigerated shop shelves, while HTST milk must be kept cold. Heat is usually delivered by steam or superheated water, either mixed directly with the milk or through a heat exchanger.

The steam-based method uses steam directly or indirectly to heat the milk. In direct heating, mixing of steam with milk directly can help achieve the required temperature with minimal chemical change in milk. In the indirect method, tubular or plate heat exchangers have low heat-transfer rates, causing more chemical change.

The chemical change in direct and indirect methods causes a difference in flavor, color, and nutritional loss, with direct heating being suitable.

Pasteurization methods compared

All three reach the same lethal effect on Coxiella burnetii. Higher temperature simply lets the holding time drop sharply, which is what makes continuous processing fast enough for large dairies.

Advantages of Food Pasteurization

Pasteurization is an effective method of improving food quality, especially for milk and other dairy products. Further, we discuss the advantages of consuming pasteurized food.

1. Pasteurization reduces the level of microorganisms and prevents food from spoilage.
2. It protects us from food-borne illnesses by destroying pathogenic bacteria.
3. Some food products are breeding grounds for microorganisms. Pasteurization maintains sanitation in food by killing them without degrading the nutritional value of food.
4. Some bacteria degrade the aroma and flavor of food. By destroying these microbes, pasteurization helps protect the food's natural flavor and keeps it fresh for longer.
5. Degradation of pathogenic bacteria reduces the risk of illness, which is especially important for children and pregnant women.

Pasteurizer

Pasteurization occurs in the heating section of the equipment known as a pasteurizer. The milk travels through various parts of the pasteurizer for the completion of pasteurization. Here the heating and cooling of milk occurs simultaneously. The apparatus is made from stainless steel, and a low-speed agitator motor is installed.

Major Parts of Pasteurizer

Some major parts of the pasteurizer involved in the pasteurization procedure are as follows:

Balance tank:  A balance tank is a vessel that maintains the consistency of the product in the pump inlet. A float-controller valve regulates the milk flow and maintains a constant level in the balance tank.

Figure: Balance tank

Regenerative preheating: Regenerative preheating is a procedure where the warming of incoming raw milk by hot pasteurized milk and simultaneously cooling the pasteurized milk. The heated and cooled liquids are always in a counter-flow, and the temperature difference between the two liquids (after heat exchange) remains constant. The regenerative procedure saves energy within the system.

Figure: Heating and cooling section

Timing pump: The timing pump is also known as a flow-controlling device. It draws the product from the regenerator and pushes the milk through the pasteurized regenerator. This device controls the flow rate of products in the holding tube so that each product particle is held for the required time.

Cream separator: It is a centrifugal separator that uses spinning force to separate cream from milk based on density. The denser skim milk is thrown outward, while the lighter fat globules (cream) move toward the center, the axis of rotation, where they are collected.

Figure: Cream separator

Holding tube: It is the tube where the milk is held for at least 15 seconds at 72°C to complete HTST pasteurization. The holding tube must slope upwards 0.25 inch/ft in the product flow direction to prevent the risk of air entrapment.

Figure: Holding tube (5)

Flow diversion device (FDD): FDD is also known as flow diversion valve (FDV). The purpose of FDD is to divert the milk flow for reprocessing if the milk temperature during pasteurization does not meet the pre-set temperature.

Cooling section: As mentioned previously, cooling of pasteurized milk occurs mainly by regenerative heat exchange. The temperature must be 4 ℃ or below for storing the pasteurized milk. As regeneration efficiency is 94-95 %, ice water is preferable option to reduce the temperature of the milk.

Figure: Cooling section

How pasteurization is verified: the phosphatase test

Heating equipment can fail, temperatures can drift, and pasteurized milk can be accidentally mixed with raw milk. So how do you confirm that a batch was properly pasteurized? The standard check is the alkaline phosphatase (ALP) test.

Alkaline phosphatase is an enzyme naturally present in raw milk. By a useful coincidence, it is destroyed at a temperature just slightly higher than the temperature needed to kill Coxiella burnetii. This makes it an almost perfect indicator.

The logic is simple:

• If the milk was heated enough to destroy C. burnetii, it was also hot enough to destroy alkaline phosphatase.
• So if the enzyme is absent, the milk was properly pasteurized.
• If the enzyme is still active, the milk was under-heated or contaminated with raw milk, and it is not safe.

In the test, a substrate is added to a milk sample. If active phosphatase is present, it produces a colored product, a positive result, meaning pasteurization failed. No color, a negative result, means the enzyme is gone and pasteurization was adequate. The test is fast, cheap, and does not require growing any bacteria, which is why it is used worldwide as the routine check on pasteurized milk.

This is a good example of an indirect test: instead of trying to detect a dangerous organism directly, it measures an enzyme that disappears under the same conditions that kill the organism.

Thermal Resistance of Microorganisms

Thermal resistance refers to how well a microorganism survives heat. It varies widely between organisms and can be affected by the food and conditions around them. It is not true that most bacteria are heat-loving; most are killed by pasteurization temperatures. The important exceptions are spore-forming bacteria such as Bacillus and Clostridium, whose spores survive heat that easily kills ordinary cells. This is why pasteurization removes the dangerous vegetative pathogens but does not make milk sterile.

Two values are used to describe thermal resistance, and both appear in the table below.

D-value: the time (at a given temperature) needed to kill 90% of the microbes, that is, to reduce their number by one log (a factor of 10). A smaller D-value means the organism dies faster. Z-value: the number of degrees the temperature must rise to cut the D-value to one-tenth, in other words, how much extra heat is needed to speed killing tenfold. Together, these two numbers describe how quickly an organism dies at a given temperature and how sensitive that killing is to raising the temperature.

How to Remember

One organism runs the whole show: Coxiella burnetii.
It is the toughest non-spore pathogen in milk, so it sets the temperature. Kill it, and you have killed everything else that matters. Every method (LTLT, HTST, UHT) is just a different way to reach the same lethal dose for this one organism.

LTLT vs HTST: trade time for temperature.
Low Temperature, Long Time (63°C, 30 min) and High Temperature, Short Time (72°C, 15 sec) do the same job. Turn the heat up, and the time needed drops fast. The names literally tell you the trade.

The phosphatase test, "the enzyme that quits just after the germ dies."
Alkaline phosphatase is destroyed at a temperature just above what kills Coxiella. So if the enzyme is gone (negative test), the germ is certainly gone too. Enzyme still there (positive test) = under-heated milk. Positive is bad news here, which trips students up.

UHT vs the rest: sterile and shelf-stable.
UHT goes hot enough (above 135°C) to kill spores too, so it needs no fridge. LTLT and HTST leave spores alive, so their milk stays in the cold case.

Key exam facts in one table

Where Students Get Confused

Does pasteurization sterilize milk?
No. Pasteurization kills disease-causing bacteria and most spoilage microbes, but not heat-resistant spores. Pasteurized milk is safe but not sterile, which is why HTST and LTLT milk still need refrigeration. Only UHT goes hot enough to be shelf-stable.

Why is a positive phosphatase test bad?
Because a positive result means the enzyme alkaline phosphatase is still active, and that enzyme is destroyed by proper pasteurization. If it survived, the milk was under-heated (or mixed with raw milk) and is unsafe. Here, "positive" means failure, the opposite of what students expect from most tests.

Is LTLT about heating and cooling quickly?
No. The defining feature of LTLT (batch) pasteurization is the long hold, 30 minutes at 63°C. That hold is what kills the pathogens. Confusing it with "rapid" steps misses the whole principle. Rapid heating and short holds belong to HTST.

Why does one organism decide the temperature?
Because Coxiella burnetii is the most heat-resistant non-spore-forming pathogen likely to be in milk. If the heat kills it, everything else non-sporing dies too. Setting the standard to the toughest target guarantees the rest are covered.

Are most bacteria heat-loving (thermophilic)?
No. Most bacteria, including the milk pathogens, are killed by pasteurization heat. Only spore-forming bacteria survive, and they do so because of their tough spores, not because the bacteria themselves love heat.

References

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3. Tetra Pak. Dairy Processing Handbook. Lund: Tetra Pak Processing Systems; 2015.
4. European Food Safety Authority (EFSA). The use of alkaline phosphatase and possible alternative testing to verify pasteurisation of raw milk. EFSA Journal. 2021;19(4):6576. https://doi.org/10.2903/j.efsa.2021.6576
5. Lucey JA. Raw milk consumption: risks and benefits. Nutr Today. 2015;50(4):189–193.
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