Heat Processing Methods in Dairy: Pasteurization, Sterilization, and More

Raw milk is one of nature’s most nutritious foods – and also one of the most perishable. Without proper treatment, it can harbour dangerous pathogens like Salmonella, Listeria, and E. coli O157:H7, which are responsible for a significant share of foodborne illness outbreaks linked to dairy worldwide. Heat processing is the dairy industry’s primary defence against these risks. But it is far more than just “boiling milk.” Different heat treatments serve different purposes – some are designed to make milk safe to drink today, others to keep it shelf-stable for months without refrigeration, and still others to prepare milk for concentrated or dried products. This post covers the major heat processing methods used in dairy: pasteurization, sterilization, forewarming, condensing, drying, and Ultra High Temperature (UHT) processing.

Why heat processing matters 🔗

Milk is an excellent growth medium for microorganisms. When stored at ambient temperature, bacteria proliferate rapidly, making raw milk one of the most hazardous food products when improperly handled. Heat processing works by denaturing the proteins within microbial cells, destroying their structure and rendering them inactive. The key principle is the inverse relationship between temperature and time: higher temperatures require shorter exposure times to achieve the same level of microbial destruction. This allows processors to tailor treatments to their specific goals – whether eliminating pathogens while keeping flavour intact, or achieving full commercial sterility for ambient storage.

Pasteurization 🔗

Pasteurization is the most widely used heat treatment in the dairy industry. Named after scientist Louis Pasteur, it applies controlled heat to every particle of milk in properly designed equipment to destroy pathogenic microorganisms, without drastically altering the milk’s nutritional or sensory qualities. It does not sterilize milk – some non-pathogenic bacteria and heat-resistant spores survive – so refrigeration is still required after treatment.

Low Temperature Long Time (LTLT) pasteurization 🔗

Also known as vat pasteurization, this is the original method. Milk is heated in a large tank and held for at least 30 minutes. The LTLT process typically operates at 65°C for 30 minutes, which is sufficient to destroy all significant pathogens. Today, LTLT is used primarily for small-scale or artisanal operations, and for preparing starter cultures in cheese, yogurt, and buttermilk production. Its slow, gentle heat makes it suitable for preserving enzyme activity where that is desirable in speciality processing.

High Temperature Short Time (HTST) pasteurization 🔗

The most common commercial pasteurization method today is HTST, which uses metal plates and hot water to raise milk temperatures to at least 72°C (161°F) for no less than 15 seconds, followed by rapid cooling. The continuous-flow design means large volumes can be processed efficiently. HTST-pasteurized milk typically has a refrigerated shelf life of two to three weeks. Modern HTST systems are equipped with flow-rate restrictions and divert valves to ensure every particle of milk reaches the required temperature – it is common for temperatures to slightly exceed 72°C as a built-in safety margin.

Sterilization 🔗

Where pasteurization reduces harmful bacteria to safe levels, sterilization goes further. Sterilized milk is defined as homogenized milk that has been heated to 100°C or above for periods sufficient to keep it fit for human consumption for at least 7 days at room temperature. Under India’s Food Safety and Standards regulations, sterilization means heating milk in a sealed container to either 115°C for 15 minutes, or at least 130°C for one second or more in continuous flow, then packing under aseptic conditions in hermetically sealed containers – ensuring preservation at room temperature for no less than 15 days.

The classic in-container sterilization process involves filling milk into glass bottles or cans, sealing them, and passing them through a steam chamber. The bottles are heated to 113-130°C for approximately 10-12 minutes and then rapidly cooled to prevent further thermal degradation. The high temperatures involved cause some Maillard browning – the reaction between lactose and milk proteins – which gives sterilized milk its characteristic slightly caramelized flavour and off-white colour. Commercially sterilized milk is rarely sterile in the strict bacteriological sense, because the temperatures needed for absolute sterility conflict with the consumer preference for acceptable colour and flavour – highly heat-resistant spore-forming bacteria can sometimes survive and may eventually cause spoilage.

Forewarming (preheating) 🔗

Forewarming is a preparatory heat treatment used specifically in the manufacture of concentrated milk products, particularly evaporated milk. Its primary role is not pathogen destruction on its own, but rather to stabilize the milk’s proteins and improve its heat stability before the more intensive processing stages that follow.

Forewarming serves several functions: it enhances the heat stability of the concentrated product, inactivates enzymes, kills a significant proportion of bacterial spores, and helps the milk feed smoothly into the vacuum evaporator. The temperature-time range for forewarming is wide – from 82-93°C for 5-15 minutes, up to 116-149°C for 0.5 to 5 minutes – and the modern trend is towards high-temperature short-time heating. Research has demonstrated that high-temperature short-hold forewarming can increase the heat stability of evaporated milk by as much as six times compared to conventional lower-temperature preheating. Tubular heat exchangers – double-tube or shell-and-tube designs – are commonly used for this step.

Condensing (vacuum evaporation) 🔗

After forewarming, milk destined for concentrated products undergoes condensing – the removal of water under vacuum. The principle is straightforward: by reducing atmospheric pressure inside the evaporator chamber, the boiling point of milk drops significantly. Under vacuum conditions, water evaporates at temperatures as low as 40-45°C, compared to 100°C at normal atmospheric pressure, which preserves nutritional quality and avoids developing a pronounced cooked flavour.

Modern dairy plants use multiple-effect falling-film evaporators for large-scale continuous operation, where the milk passes through steam-heated tubes in multiple stages, each operating at progressively lower pressures and temperatures. The result is a concentrated milk with roughly double the solids content of fresh milk. To produce standard evaporated milk, concentration continues until the product reaches approximately 25-31% total solids, after which stabilizers may be added and the product is hermetically sealed in cans for in-container sterilization.

Drying 🔗

Drying takes heat processing to its logical conclusion – complete removal of water to produce milk powder. This creates a product with an extremely long shelf life and convenient storage properties. Spray drying is the dominant commercial method. Liquid milk is atomized into fine droplets and exposed to hot air at temperatures typically between 160-220°C. Because the droplets are tiny, water evaporates almost instantaneously, minimizing the actual time the milk solids spend at high temperature and thereby limiting heat damage to proteins and nutrients.

Heat treatment before and during drying is carefully calibrated for the intended use of the powder. Whole milk powder is produced from milk heated at 90-95°C for 30-60 seconds – conditions designed to denature whey proteins and generate antioxidant sulfhydryl groups that protect the fat in the powder from oxidation during storage. Medium-heat powders are used in confectionery and bakery products, while high-heat powders are suited for recombined evaporated and sweetened condensed milk. Controlling the drying parameters precisely is essential to avoid protein denaturation, lactose caramelization, and the development of off-flavours that would reduce powder quality.

Ultra High Temperature (UHT) processing 🔗

UHT processing represents the most advanced form of commercial heat sterilization for fluid milk. It sterilizes liquid milk by heating it above 140°C – the temperature required to kill bacterial endospores – for two to five seconds. The combination of very high temperature and very short time achieves complete commercial sterility while minimizing the chemical changes – Maillard browning, vitamin loss, protein denaturation – that would occur with longer lower-temperature sterilization.

Direct and indirect UHT systems 🔗

UHT treatment is a continuous process involving heating raw materials at temperatures of 138-145°C for a holding time of 1-10 seconds, usually 3-5 seconds. Two main system types are used commercially. In indirect heating, milk flows through a plate or tubular heat exchanger, never coming into direct contact with the heating medium – this is more energy-efficient but imparts a slightly greater heat load, which can subtly affect flavour. In direct heating (steam injection or steam infusion), steam is mixed directly into the milk, raising its temperature almost instantaneously. Milks produced by direct heating with short holding periods can have flavours almost indistinguishable from HTST-pasteurized milk.

Aseptic packaging 🔗

UHT processing only delivers its full benefit when paired with aseptic packaging. After heat treatment and cooling, the product is filled into pre-sterilized containers in an aseptic environment and hermetically sealed to prevent contamination throughout the distribution chain. UHT milk packaged in a sterile container has a typical unrefrigerated shelf life of six to nine months. Ultra-high-temperature pasteurization has little effect on the nutritional value of milk due to the speed of the process, though some loss of vitamin B12, vitamin C, and thiamin can occur, and UHT milk contains significantly less folate than pasteurized milk.

Comparing the methods: purpose and outcome 🔗

Each heat processing method occupies a specific place in dairy production based on its goals. LTLT pasteurization suits small-scale and speciality production where gentle treatment is preferred. HTST pasteurization is the standard for commercial fresh milk, balancing safety and quality with a refrigerated shelf life of two to three weeks. In-container sterilization delivers ambient-stable products with a shelf life of several months, at the cost of some colour and flavour change. Forewarming is not a standalone preservation method but an essential preparatory step that makes condensed milk production possible and controls final product viscosity. Vacuum condensing concentrates milk for products like evaporated and sweetened condensed milk. Drying produces shelf-stable powders with storage lives measured in years. And UHT processing achieves full commercial sterility with minimal quality impact, making it the method of choice for markets where cold-chain infrastructure is limited or where long shelf life is commercially essential – across Europe, seven out of ten people regularly consume UHT milk.

Understanding the differences between these methods matters for more than academic reasons. Each choice reflects a trade-off between safety, nutrition, shelf life, cost, and sensory quality. As consumer demand grows for minimally processed, nutrient-retentive dairy foods, conventional heat processing is increasingly being evaluated alongside emerging non-thermal technologies – from high-pressure processing to pulsed electric fields – that aim to achieve the same safety outcomes with a lighter thermal footprint.

What do you think? Given that UHT milk requires no refrigeration and has a shelf life of six to nine months, why do you think consumers in some countries still strongly prefer fresh pasteurized milk despite its much shorter shelf life? And how do you think advances in non-thermal processing technologies might change the way dairy products are heat-treated in the coming decades?

References

1. https://en.wikipedia.org/wiki/Pasteurization
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC12026572/
3. https://www.idfa.org/pasteurization
4. http://ecoursesonline.iasri.res.in/mod/page/view.php?id=6160
5. https://www.dairyknowledge.in/dkp/article/sterilization
6. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/sterilized-milk
7. https://ebooks.inflibnet.ac.in/ftp04/chapter/technology-of-concentrated-milk-products-evaporated-milk-sweetened-condensed-milk/
8. https://www.sciencedirect.com/article/pii/S0022030243927560
9. https://www.sciencedirect.com/topics/food-science/condensed-milk
10. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/evaporated-milk
11. https://www.mdpi.com/2673-8392/1/4/98
12. https://en.wikipedia.org/wiki/Ultra-high-temperature_processing
13. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/ultra-high-temperature-processing
14. https://dairynutrition.ca/en/milk-quality/uht-milk/ultra-high-temperature-pasteurized-milk