How Pasteurization Works and What It Actually Kills

What Pasteurization Is and Is Not

Pasteurization is a heat treatment process designed to kill pathogenic microorganisms — bacteria, viruses, and parasites that cause disease — in food and beverages while preserving the product's flavor, nutritional value, and quality to the maximum possible extent. It is not sterilization: sterilization kills all microorganisms including heat-resistant bacterial spores, typically requiring much higher temperatures and pressures (as in autoclaving or ultra-high-temperature processing). Pasteurization kills most but not all microorganisms, and pasteurized products still contain living non-pathogenic bacteria. This is why pasteurized milk eventually spoils even under refrigeration and why pasteurized wine continues to ferment if left uncapped.

The distinction matters practically. Pasteurization extends shelf life and eliminates the pathogens responsible for illnesses like tuberculosis, salmonellosis, listeriosis, E. coli infection, brucellosis, and Q fever from commonly consumed foods. It does not produce a sterile, indefinitely shelf-stable product. Ultra-high-temperature (UHT) processing, which heats milk to approximately 140°C for two to four seconds, does achieve near-sterilization and produces the shelf-stable boxed milk common in Europe, but the higher heat creates off-flavors that many consumers find objectionable.

Louis Pasteur and the Origin of the Method

The process is named after French chemist and microbiologist Louis Pasteur, who in the 1860s demonstrated that heating wine and beer to moderate temperatures (around 57°C) for a controlled time could kill the microorganisms causing spoilage without significantly damaging the product. Pasteur's work built directly on his broader germ theory research, which established that fermentation, putrefaction, and infectious disease were caused by specific microorganisms rather than by spontaneous generation or chemical imbalances.

Pasteur was not treating milk — he was solving a commercial problem for the French wine and beer industries, which were losing enormous revenue to spoiled products. The application of his heat treatment to milk came later, primarily in response to the epidemic of milk-borne tuberculosis that killed thousands of children in industrializing cities of the late nineteenth and early twentieth centuries. Pasteur's contemporary Robert Koch had identified Mycobacterium tuberculosis as the causative agent of tuberculosis and shown that it could survive in unpasteurized milk from infected cattle. The public health campaign to mandate pasteurization of milk was one of the most significant and contentious food safety battles of the twentieth century.

The Science of Thermal Inactivation

The microbiology underlying pasteurization is the kinetics of thermal inactivation. At any given temperature, a given pathogen is killed at a rate that follows first-order kinetics: each minute at that temperature kills a constant fraction of the surviving population. The decimal reduction time (D-value) is defined as the time at a given temperature needed to kill 90% of a pathogen population — a one-log reduction. The D-value decreases (meaning killing is faster) as temperature increases.

A critical parameter is the z-value: the temperature increase needed to reduce the D-value by a factor of 10 (another one-log change). For most food pathogens, z-values are in the range of 7 to 10°C. This means that increasing the temperature by 7 to 10°C allows you to achieve the same log reduction in one-tenth the time — which is the basis for high-temperature short-time (HTST) processing. The pathogen used as the benchmark for pasteurization standards has historically been Coxiella burnetii (the agent of Q fever), which was found to be the most heat-resistant of the significant non-spore-forming dairy pathogens and thus set the thermal death target.

Methods of Pasteurization

Several pasteurization protocols are used commercially, varying in temperature and time:

• Low-temperature long-time (LTLT) or batch pasteurization: 63°C for 30 minutes. The original method, still used for some artisanal dairy products and specialty beverages. Milk is held in vats at this temperature. The long time at lower temperature achieves the required pathogen kill with minimal equipment complexity, but throughput is low.
• High-temperature short-time (HTST) pasteurization: 72°C for 15 seconds. The dominant method for commercial milk production. Milk flows continuously through a plate heat exchanger at precise temperature and velocity. Throughput is high, the short exposure time minimizes flavor and nutritional changes, and the method is highly automated.
• Ultra-high-temperature (UHT) processing: 135-140°C for 2-4 seconds. Achieves near-sterilization. Produces shelf-stable milk without refrigeration until opened. Common in Europe, South America, and Asia; less common in the US where refrigerated distribution is well-established.
• Flash pasteurization: 79°C for 15 seconds to 96°C for 0.5 seconds, depending on product. Used for juices, beer, and wine to minimize heat damage to flavor while achieving sufficient pathogen reduction.

What Pasteurization Kills

HTST pasteurization (72°C for 15 seconds) reliably kills the major milk-borne pathogens at the required safety margins:

• Mycobacterium tuberculosis (tuberculosis): destroyed rapidly at pasteurization temperatures
• Salmonella species: D-value at 71°C is approximately 0.8 seconds — effectively eliminated
• Listeria monocytogenes: more heat-resistant than Salmonella but reliably killed under HTST conditions
• Escherichia coli O157:H7: killed within the standard treatment
• Brucella species (brucellosis): destroyed at pasteurization temperatures
• Coxiella burnetii (Q fever): the thermal reference pathogen; HTST conditions provide a sufficient safety margin above its inactivation requirements

What pasteurization does NOT kill: heat-resistant spore-forming bacteria like Bacillus cereus and Clostridium species, whose spores can survive boiling temperatures. These do not cause rapid milk spoilage under normal refrigeration because their growth is temperature-dependent, but they are the reason UHT milk can still eventually spoil if warmed or kept for very long periods after opening.

What Pasteurization Does to Nutritional Content

One of the most persistent claims against pasteurization is that it destroys important nutrients. The nutritional impact of standard HTST pasteurization is modest. Milk proteins (casein and whey proteins) are denatured to a small degree but remain nutritionally intact and digestible. Fat-soluble vitamins (A, D, E, K) are largely unaffected. Some water-soluble vitamins are reduced: vitamin B12 may be reduced by 0 to 10%, folate by 0 to 10%, and vitamin C by approximately 25%, though raw milk is not a significant dietary source of vitamin C to begin with.

The claim that pasteurization destroys beneficial enzymes is true — it does denature many native milk enzymes — but the nutritional relevance is minimal, as digestive enzymes produced by the human gastrointestinal tract are far more important for digestion than any enzymes present in milk. The claim that raw milk contains beneficial probiotic bacteria that pasteurization destroys is also partially true but cuts both ways: raw milk also contains the pathogens that pasteurization is designed to eliminate, and the pathogenic bacteria outnumber the beneficial bacteria in most instances.

The Raw Milk Debate

Raw (unpasteurized) milk remains legal for sale in some US states and in certain other countries, and has a small but vocal constituency who believe it offers nutritional and health benefits. The scientific and public health consensus is strongly opposed. Outbreaks linked to raw milk are documented and recurrent: the CDC tracks several dozen per year in the United States alone, causing hundreds of illnesses and periodic deaths, with the most vulnerable being children, pregnant women, the elderly, and immunocompromised individuals. The claimed health benefits (enhanced gut microbiome support, reduced allergy risk, better digestibility) are not supported by randomized controlled trials and are outweighed by the documented infectious disease risk. The farm milk hypothesis — that farm exposure in early childhood reduces allergic disease — is real, but is attributable to environmental microbial exposure rather than milk consumption specifically.