Methods and Equipment for Pasteurization in Food Preservation

Pasteurization is one of the most important heat treatments in the food industry. Named after French microbiologist Louis Pasteur, who demonstrated in the 1860s that heating could deactivate unwanted microorganisms in wine, the process has since become central to making milk, juices, and other liquid foods safe for consumption. The core idea is straightforward: apply controlled heat to destroy harmful bacteria and inactivate spoilage enzymes – without significantly altering the food’s taste, nutrition, or texture. In this post, we’ll break down the major pasteurization techniques, the equipment involved, and the specific temperature-time combinations used for different products.

What is pasteurization? 🔗

Pasteurization is a mild heat treatment in which liquid foods are heated to temperatures typically below 100°C for a specific duration. The goal is to eliminate pathogenic microorganisms – bacteria, viruses, yeasts, and moulds – while keeping the product’s sensory and nutritional qualities largely intact. Unlike sterilization, which aims to destroy all microorganisms (including spores), pasteurization targets specific pathogens and spoilage organisms. The treated product is not sterile and still requires proper storage, usually refrigeration, to remain safe.

According to the University of Guelph’s Dairy Science and Technology eBook, pasteurization serves two primary purposes: a public health function (making food safe by destroying pathogens) and a keeping quality function (extending shelf life by reducing spoilage organisms and inactivating enzymes). The extent of microbial destruction depends on the combination of temperature and holding time, which food safety regulators define precisely for each product category.

Key pasteurization methods for liquid foods 🔗

Several pasteurization methods exist, each suited to different production scales, food types, and shelf-life requirements. The three most widely used are Low Temperature Long Time (LTLT), High Temperature Short Time (HTST), and Ultra-High Temperature (UHT) treatment. But before a food processor chooses a method, they also need to decide whether the food will be pasteurized before or after being packaged – because this determines which equipment is appropriate.

Low Temperature Long Time (LTLT) or vat pasteurization 🔗

This is the oldest pasteurization method. The food product – usually in a jacketed vat – is heated to around 63°C and held at that temperature for at least 30 minutes. According to the Dairy Processing Handbook by Tetra Pak, the vat method is still used in the ice cream industry for dissolving and blending ingredients during the heating stage. It is also favoured by small-scale and artisanal producers who prioritise flavour preservation. However, LTLT is slow, energy-intensive, and not practical for large commercial operations.

High Temperature Short Time (HTST) pasteurization 🔗

HTST is the dominant method in commercial dairy and beverage processing today. It involves heating the liquid to at least 72°C for a minimum of 15 seconds, followed by rapid cooling. This approach destroys harmful pathogens – including Coxiella burnetii, the most heat-resistant non-sporulating pathogen found in milk – while having minimal impact on flavour and nutrients. We’ll cover HTST in much greater detail below, as it is the focus of modern pasteurization technology.

Ultra-High Temperature (UHT) treatment 🔗

UHT processing heats the product to 135-150°C for just 2-5 seconds. This results in a commercially sterile product that can be stored at room temperature for months when packaged aseptically. UHT is widely used for long-life milk, cream, and fruit juices, especially in markets where refrigeration infrastructure is limited. The trade-off is that the extreme heat can cause slight flavour changes that some consumers notice.

Water bath pasteurization 🔗

Water bath pasteurization is one of the simplest and most traditional methods, particularly suited for foods that are already packaged in sealed containers such as glass bottles, jars, or cans. The principle is basic: sealed containers are submerged in a tank of temperature-controlled hot water, typically between 60°C and 85°C depending on the product, and held there for a set duration.

The process begins by filling a large stainless steel tank with water and heating it – usually with steam coils or electric heaters – to the target temperature. Food containers are then lowered into the bath, either manually or via an automated loading system. Complete submersion is essential to ensure uniform heat distribution across every part of the container. Temperature sensors and timers monitor the process to ensure compliance with food safety parameters.

Once the required holding time is complete, the containers are transferred to a cooling bath or cooled within the same tank by introducing cold water. Rapid cooling is critical – it stops the thermal process, prevents overcooking, and reduces the risk of recontamination.

Water bath pasteurization works well for small-batch operations and specialty food producers because the equipment is relatively inexpensive and simple to operate. However, it has clear limitations: the process is slow, energy-inefficient compared to continuous systems, and not suited for high-volume production. For packaged foods in glass containers, hot water is preferred over steam to avoid thermal shock that could crack the glass.

Continuous water spray pasteurization 🔗

Continuous water spray pasteurization addresses many of the limitations of the batch water bath method. Instead of submerging containers in a stationary tank, this system moves containers – bottles, cans, or pouches – through a tunnel on a conveyor belt, exposing them to controlled water sprays at progressively different temperatures.

The equipment is divided into multiple zones. In the first zone, containers encounter warm water sprays that gradually raise their temperature. Subsequent zones increase the temperature further until the product reaches the target pasteurization temperature. After the holding zone, containers pass through cooling zones where increasingly cooler water sprays bring the temperature back down.

This gradual, incremental heating has two major advantages. First, it prevents thermal shock – particularly important for glass bottles, which can crack if exposed to sudden temperature changes. Second, as noted in research published on ScienceDirect, the incremental system allows surplus hot water to cascade into the next zone, reducing energy waste through built-in heat recovery.

Key process controls in water spray pasteurizers include temperature profiling (tracking product temperature through each zone), conveyor speed control (determining residence time in each zone), and water circulation systems (maintaining consistent water quality and temperature). The continuous flow design dramatically increases production capacity compared to batch systems, making it the preferred choice for large-scale pasteurization of beer, acidic beverages, and packaged foods.

HTST pasteurization: the modern standard 🔗

High Temperature Short Time pasteurization has become the backbone of commercial liquid food processing. Unlike water bath or spray systems – which pasteurize food inside sealed containers – HTST is designed for unpackaged liquid foods that flow continuously through the system before being filled into containers under hygienic conditions.

The method uses a plate heat exchanger as its core equipment, paired with a balance tank, feed pump, holding tube, flow diversion valve, and associated controls. Here’s how the process works step by step:

The plate heat exchanger 🔗

The plate heat exchanger (PHE) is the heart of every HTST system. It consists of a series of thin, corrugated stainless steel plates clamped together in a frame. The corrugations create turbulent flow patterns that maximise heat transfer efficiency. The liquid product flows on one side of each plate, while the heating or cooling medium (typically hot water or steam) flows on the opposite side. Gaskets between the plates define channels and prevent leakage.

A typical HTST plate heat exchanger has three main sections: a regeneration section, a heating section, and a cooling section.

How product flows through an HTST system 🔗

Cold raw product – say, milk at about 4°C – enters the system from a constant level (balance) tank. This tank maintains a steady liquid level to ensure consistent pressure and flow rate into the system, preventing air bubbles or flow interruptions that could compromise the heat treatment.

A feed pump (usually a positive displacement pump) then pushes the product at a precisely controlled flow rate into the regeneration section of the plate heat exchanger. Here, the incoming cold product is pre-warmed by the outgoing hot pasteurized product flowing on the other side of the plates. According to AGC Heat Transfer, this regenerative process saves significant energy – modern systems recover up to 90-95% of the heat.

After pre-warming, the product enters the heating section, where hot water or steam raises its temperature to the pasteurization target – typically 72°C or higher. The heated product then passes through a holding tube, which is a pipe of calculated length that ensures the product remains at the required temperature for the minimum holding time (at least 15 seconds for milk at 72°C). The length of this tube and the flow rate are precisely calibrated.

A temperature sensor at the end of the holding tube checks whether the product has reached the required temperature. If it has, a flow diversion valve (FDV) directs the product forward to the cooling section. If the temperature is below the set point, the valve automatically diverts the under-processed product back to the balance tank for reprocessing. This safety mechanism is critical for ensuring every particle of product receives adequate heat treatment.

In the cooling section, the pasteurized product is rapidly cooled – first by transferring its heat to the incoming cold product (regeneration), and then by chilled water or glycol, bringing the product down to about 4°C before it is packaged.

Why plate heat exchangers are preferred 🔗

Plate heat exchangers offer several advantages that make them the standard for HTST pasteurization. Their compact design provides a large surface area for heat transfer in a small footprint. The counter-current flow arrangement – where the product and heating medium move in opposite directions – maximises thermal efficiency. They are easy to disassemble for inspection and cleaning, and additional plates can be added to increase capacity. As described in the Dairy Processing Handbook, maintaining higher pressure on the pasteurized side of the heat exchanger is essential to prevent contamination of pasteurized product with raw product in case of any pinhole leak in the plates.

Temperature-time combinations for different products 🔗

Different food products require specific temperature-time combinations to achieve safe pasteurization while maintaining quality. These parameters are determined by the most heat-resistant target pathogen in each product and are strictly regulated. Here are the standard combinations for key food categories:

Milk: The standard HTST treatment is 72°C for 15 seconds. For the batch (LTLT) method, it is 63°C for 30 minutes. Products with fat content of 10% or more, or with added sweeteners, require the temperature to be increased by 3°C (5°F), as noted by Zwirner Equipment. UHT treatment uses 135°C for 2-5 seconds for extended shelf life.

Cream and high-fat dairy products: Because fats can have a protective effect on microorganisms, cream typically requires temperatures above 80°C for 1-5 seconds using HTST processing, or adjusted LTLT parameters.

Ice cream mix and eggnog: These require higher temperature-time combinations due to their rich composition. Common standards include 69°C for 30 minutes (batch) or 80°C for 25 seconds (continuous).

Fruit juices (e.g., apple juice): Typically pasteurized at 71-74°C for 15-30 seconds to destroy pathogens like E. coli O157:H7 while preserving vitamin C and flavour.

Liquid egg products: Egg whites are treated at 55-57°C for 3.5-4 minutes at lower temperatures to preserve protein functionality. Egg yolks require 60-63°C for about 3.5 minutes, with careful temperature control to prevent coagulation.

Beer and wine: These are often flash-pasteurized at 71.5-74°C for 15-30 seconds, or tunnel-pasteurized in sealed containers using water spray systems.

Effects of pasteurization on food quality 🔗

While pasteurization is essential for food safety, it can affect the product if not properly controlled. Heat-sensitive vitamins – particularly vitamin C and some B vitamins – may experience minor losses. However, these losses are generally small, typically less than 10%, and are far outweighed by the safety benefits.

Flavour changes are usually minimal with properly calibrated pasteurization. LTLT methods tend to preserve taste best, while UHT processing may introduce slight cooked flavour notes. Modern HTST systems, with their rapid heating and rapid cooling, minimise both nutrient loss and flavour changes by reducing the total time the product spends at elevated temperatures.

Protein denaturation can be a concern in products like liquid eggs and certain dairy items. This is why these products use carefully optimised, lower-temperature treatments that balance microbial safety with functional quality – for example, maintaining the foaming properties of egg whites or preventing coagulation in yolks.

Choosing the right pasteurization method 🔗

The choice of pasteurization method depends on several practical factors: the type of product, production scale, desired shelf life, packaging format, and available infrastructure. Small-batch producers working with packaged products often find water bath or spray tunnel systems sufficient. Large-scale liquid food processors – especially in the dairy and juice industries – overwhelmingly rely on HTST systems with plate heat exchangers for their efficiency, speed, and consistent quality.

UHT is the go-to option for shelf-stable products intended for distribution without refrigeration, such as long-life milk commonly seen in markets across India and other regions. Emerging non-thermal technologies like High-Pressure Processing (HPP) and Pulsed Electric Fields (PEF) are also gaining traction as alternatives that preserve product freshness even further, though they are not yet as widely adopted as heat-based methods.

What do you think? Given the balance between food safety and quality preservation, which pasteurization method do you think offers the best trade-off for liquid foods in your region? And as non-thermal pasteurization technologies become more accessible, could they eventually replace traditional heat-based methods entirely?

References

1. https://en.wikipedia.org/wiki/Pasteurization
2. https://books.lib.uoguelph.ca/dairyscienceandtechnologyebook/chapter/pasteurization/
3. https://dairyprocessinghandbook.tetrapak.com/chapter/heat-exchangers
4. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/pasteurizers
5. https://thermtest.com/applications-of-heat-exchangers-in-the-dairy-industry
6. https://www.agcheattransfer.com/blog/how-heat-exchangers-work-in-dairy-pasteurization
7. https://www.zwirnerequipment.com/blog/what-is-htst-pasteurization/