Pasteurization started as a fix for sour wine, not safe milk
On a summer holiday in Arbois in 1864, Louis Pasteur tackled a local complaint: aged wines kept turning acidic. He found that briefly warming young wine to around 50 to 60 °C killed the microbes without ruining how it matured. Beer and wine came first; milk would wait many years for the treatment.
Pasteur was not the first to heat food for keeping. Wine was being warmed to keep it in China by 1117, and a diary kept by a succession of Japanese monks between 1478 and 1618 records the practice. In 1768 the Italian priest Lazzaro Spallanzani boiled broth for an hour, sealed it at once and found it stayed free of microbes. Then in 1795 the French military dangled a 12,000-franc prize for a new way to preserve rations. The Paris confectioner Nicolas Appert spent some fourteen or fifteen years sealing food in corked glass jars and boiling them, won in January 1810 and opened the world's first bottling factory at Massy. That same year Peter Durand patented the idea using tin cans, although cans stayed awkward until Robert Yeates invented an opener in 1855; before that, people used a hammer and chisel.
Milk became the big target as cities grew. Cows had once been kept in town so milk was drunk fresh, but longer supply chains brought days-old milk and disease. In England and Wales around 65,000 people died between 1912 and 1937 of tuberculosis caught from milk, a link obscured by the illness's long incubation. In 1892 the chemist Ernst Lederle showed that milk from infected cows gave guinea pigs the disease, and as New York City's health commissioner in 1910 he made pasteurization compulsory there. Milton Rosenau set early standards of 60 °C for 20 minutes, and in 1973 U.S. federal rules required it for milk sold across state lines.
The method is deliberately mild, usually under 100 °C, so flavour and nutrition shift only slightly. It kills or disables most bacteria and spoilage enzymes, though bacterial spores largely survive, which is why pasteurized milk still needs the fridge. Standard high-temperature short-time treatment holds milk at 71.5 °C for 15 seconds, giving about two weeks of chilled life, while ultra-high-temperature processing at 135 °C for a second or two, with sterile packaging, lets it sit unrefrigerated for months.
In a modern dairy the liquid flows through heating plates, a holding tube and a cooler. If it misses its target temperature or time, a diversion valve sends it back to be treated again.
Source: Pasteurization
