How Does Pasteurisation Work? Killing Enough Without Cooking It
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Sterilising milk would make it safe and would also taste like something else. Pasteurisation solves the problem differently, applying just enough heat for just long enough to destroy the organisms that cause disease while leaving the product recognisable, which means it is deliberately incomplete and the remaining organisms are the reason pasteurised milk still goes off.
What it does and does not kill
Pasteurisation is defined by a time and temperature combination validated to achieve a specified reduction in the most heat-resistant relevant pathogen. Historically that reference organism was the bacterium causing tuberculosis, and the standard has since been set against Coxiella burnetii, which is more heat resistant, so a process that destroys it destroys everything less resistant including salmonella, listeria, campylobacter and pathogenic strains of E. coli. What survives is a population of heat-tolerant spoilage organisms and bacterial spores, which cannot cause disease in this context but will eventually multiply and sour the product, which is why pasteurised milk requires refrigeration and has a shelf life of days rather than months. That is the essential difference from sterilisation, which destroys everything including spores and produces a product stable at room temperature for months at the cost of a noticeably cooked flavour.
The processes in use
Several time and temperature combinations achieve equivalent lethality, and the choice trades throughput against flavour:
- •Low temperature long time, holding milk at around sixty-three degrees for thirty minutes, the original batch method, which is gentle and slow
- •High temperature short time, around seventy-two degrees for fifteen seconds in a continuous flow through a plate heat exchanger, which is the standard commercial process and which recovers heat from outgoing product to warm incoming product
- •Ultra high temperature treatment, around one hundred and forty degrees for a few seconds followed by aseptic packaging, which is sterilisation rather than pasteurisation and gives months of unrefrigerated shelf life with a distinctly different taste
- •Extended shelf life processing at intermediate conditions, which sits between the two
- •Equivalent processes for other products, including beer, fruit juice, liquid egg, wine and canned goods, each with its own validated parameters
- •Non-thermal alternatives including high pressure processing, which inactivates organisms with pressure rather than heat and preserves fresh flavour at considerable equipment cost
Where it came from
Louis Pasteur developed the process in the 1860s, and the original problem was not milk but wine and beer spoiling during storage and transport, which was a substantial commercial loss in France. Pasteur demonstrated that spoilage was caused by microorganisms rather than by spontaneous chemical change, and that gentle heating destroyed them without ruining the product. Application to milk followed later and was driven by public health rather than commerce, because urban milk supplies in the nineteenth and early twentieth centuries were a major route for tuberculosis, typhoid, scarlet fever and diphtheria, and infant mortality from milk-borne infection was severe. Compulsory pasteurisation was resisted for decades by dairy interests and by campaigners who considered it adulteration, and its adoption is associated with sharp declines in the diseases concerned. The same period saw the introduction of tuberculosis testing of herds and refrigerated transport, so the improvement had several causes, with pasteurisation the measure that worked regardless of conditions on the farm.
The raw milk argument
Sale of unpasteurised milk is permitted in some jurisdictions, restricted in others and prohibited in some, and the debate recurs. The claims made for raw milk include better flavour, which is genuinely a matter of taste, and health benefits including reduced allergy and improved digestion, which have weaker support: some epidemiological studies have found associations between farm milk consumption in childhood and lower rates of allergic disease, though disentangling that from the broader farm environment is difficult, and claims about enzymes and nutrients largely do not survive examination, since pasteurisation has modest effects on nutritional content. The risks are better established and quantified, with outbreak data consistently showing that unpasteurised dairy accounts for a share of foodborne illness enormously disproportionate to its share of consumption, with severe outcomes concentrated in children, pregnant women and the immunocompromised. The regulatory approaches reflect a genuine disagreement about how far adults should be permitted to accept a known risk, complicated by the fact that the people most severely harmed frequently did not make the choice themselves.
The takeaway
Pasteurisation applies a validated time and temperature that destroys disease-causing organisms while leaving heat-tolerant spoilage bacteria and spores alive, which is why pasteurised milk still needs refrigerating and still sours. High temperature short time at around seventy-two degrees for fifteen seconds is the commercial standard, while ultra high temperature treatment sterilises and changes the flavour. Pasteur developed it for wine and beer in the 1860s, and its use on milk followed to stop tuberculosis and typhoid transmission.