April 20, 2022

Louis Pasteur Invents Pasteurization

Louis Pasteur Invents Pasteurization

April 20,1862. French biologist Louis Pasteur invents a process of heating a liquid to kill potentially harmful bacteria and names it after himself: pasteurization.


Cold Open

It’s July 8th, 1921, at the Baltimore Hotel in downtown Kansas City, Missouri.

Outside, a large crowd jeers and hisses as a public health officer from New York makes his way through a mass of people. 

Back in May, the mayor’s office hired Dr. Charles North to help sanitize Kansas City’s dairy industry after a batch of contaminated milk caused widespread disease. Dr. North is a fierce advocate of a relatively new bacteria-killing process, known as “pasteurization”. But he’s been met with fierce opposition from dairy farmers here in Missouri, who believe “pasteurization” is a waste of time and money.

Dr. North bounds up the front steps of the hotel. And inside, a gaggle of reporters is waiting for him. A photographer steps forward and says: “smile for the Kansas City Star!”

With the camera flash still burning in his eyes, Dr. North blinks, looking around the lobby. And there, standing in the middle of the room, is a cow.

A reporter approaches Dr. North, hands him a bucket, and says: “Let’s see if you really are America’s leading authority on milk.” Dr. North approaches the cow and becomes acutely aware of the dairy farmers crowding the lobby – their narrow eyes boring into him as he gingerly takes a seat at the milking stool.

Dr. North removes his jacket and rolls up his sleeves. He knows that if these people are ever going to take him seriously, if they’re ever going to listen to his recommendations and implement mandatory pasteurization, he’s going to have to earn their respect – he’s going to have to milk this cow.

Dr. North clears his throat. He throws an apologetic glance at the animal. And then, with trembling fingers, he takes hold of an udder and starts to pull.

But nothing comes out.

As hostile murmurs ripple through the lobby, a droplet of sweat trickles down Dr. North’s brow. Growing desperate, he takes hold of a different udder and – muttering a silent prayer – gives it a firm yank. 

To Dr. North’s overwhelming delight, a thimbleful of milk dribbles from the udder and lands in the bucket at his feet. He continues pulling, producing a couple more thimblefuls, before standing triumphantly and turning to face the crowd.

In the early 20th century, America was at war… over milk. Health officials like Dr. Charles North found themselves locking horns with dairy farmers over controversial new food standard requirements, as they sought to prevent disease through regulations on milk production. This battle raged in towns and cities all across the country, but the conflict between science and skepticism actually started decades earlier in a laboratory in Paris, where a scientist named Louis Pasteur invented the process that would one day bear his name, on April 20th, 1862.


From Noiser and Airship, I’m Lindsay Graham and this is History Daily.

History is made every day. On this podcast—every day—we tell the true stories of the people and events that shaped our world.

Today is April 20th: Louis Pasteur Invents Pasteurization.

Act One: Swill

It’s May 22nd, 1858 in Manhattan, four years before Louis Pasteur his greatest discovery. 

A man named Frank Leslie sits behind his desk reading the newspaper. Frank is the founder and editor of Frank Leslie’s Illustrated Newspaper, a weekly current affairs magazine. Today, his paper has published a special report - one Frank has been working on for months.

The report begins: “For the midnight assassin, we have the rope and the gallows… but for those who murder our children by the thousands, we have neither reprobation nor punishment… They are not penal villains, but licensed traders, and though their traffic is in human life, the Government seems powerless or unwilling to interfere.”

Frank nods approvingly, his eyes ablaze with emotion. He hopes that with the publication of his report, the people of his city will finally see reason.

Because last year, nearly 8,000 children in New York died from a mysterious sickness. Doctors were perplexed. Until the New York Academy of Medicine carried out a series of tests, and traced the roots of the sickness back to one culprit: milk. Specifically, a new product being peddled by dairies across the city – a cheaper and more nutritious alternative to ordinary milk, perfect for children and adults alike. The product is called “Pure Country Milk,” and it’s been a resounding commercial success, comprising between 50% and 80% of the milk consumed by New Yorkers.

But this milk is deadly. In the early 1850s, children started suffering from uncontrollable diarrhea. Doctors put it down to diphtheria or tuberculosis, failing to make the connection between Pure Country Milk and child mortality – which seemed to be growing year on year.

Then, one morning in 1858, Frank Leslie, editor of Frank Leslie’s Illustrated Newspaper, noticed that his milk had an unusually thick consistency. On closer inspection, Frank discovered it was filled with blood and pus. This wasn’t just spoiled milk – this was something worse. So Frank sent his reporters to inspect the so-called “dairies” where the Pure Country Milk was coming from. Their discoveries were shocking.

Pure Country Milk was a devious piece of misbranding; the name given to the contaminated milk produced by malnourished, diseased cows. These sick cows had largely been fed the by-product of whiskey distillation – a substance known as “swill.”

As New York City expanded during the 1800s, it became impossible to operate dairy farms in urban areas. There was no room left for cows to graze in the city, so milk vendors were forced to transport their product into the city from rural farms upstate. But this proved problematic: milk would spoil in over-heated railcars, and vendors soon realized they needed to find an alternative solution.

In the end, it was provided by an unlikely source: distilleries. Opportunistic distillery owners, capitalizing on the lack of pastureland in New York, decided to make use of the waste by-product from their distillation facilities. It was an ingenious scheme. Cows fed on swill produced 25% more milk than grass-fed cows. They didn’t need pastures to graze on and could be kept in large, windowless milking sheds, where the grain sludge was piped into feeding troughs.

Vendors were making a fortune. But there were some drawbacks. Swill lacks nutrients and as a result, the cows grew sick; they developed stomach ulcers and their tails fell off. Too diseased to stand, these scrawny animals were held up by slings. And the milk they produced was thin, watery, and bluish in color. So the enterprising vendors used thickening agents, such as chalk, flour, and plaster-of-Paris.

All the while, babies in New York were dying by the thousands, and nobody was making the connection between swill milk and infant mortality. Until, in 1858, when Frank Leslie publishes his scathing report.

The 5,000-word exposé is accompanied by illustrations depicting the Grim Reaper serving milk to needy children. New York’s distilleries are labeled “milk murderers” and accused of distributing “liquid poison.” Frank even provides maps detailing the precise locations of the guilty distilleries.

Frank’s campaign has an immediate effect. Angry mobs gather outside the distilleries, demanding to inspect the premises. Bowing to public pressure, the City Council agrees to send a team of aldermen to carry out a formal inspection. But it soon becomes clear that local politicians are in cahoots with the distilleries, and profiting from the swill milk racket themselves. In late 1858, a committee of city officials votes in favor of keeping the distilleries open.

Frank is enraged by the City Council’s inaction. He publishes more scathing articles and illustrations. He takes out ads in rival newspapers with the line: “are you aware what kind of milk you are drinking?” And soon, sales of Pure Country Milk plummet. The New York Academy of Medicine carries out tests, proving the link between Swill Milk and infant mortality. Eventually, in 1862, unable to ignore the public outcry any longer, the New York State legislature bans the sale of swill milk.

Frank Leslie has triumphed in his campaign to illegalize swill milk. But there’s about to be another breakthrough. Because at this very moment, thousands of miles away in Europe, a scientist named Louis Pasteur is about to make a discovery that will revolutionize milk consumption – and save countless lives.

Act Two: Germ Theory

It’s April 20th, 1862, in Paris.

A 40-year-old French scientist is stooped over a microscope in his laboratory. A clock on the wall shows how late the hour is. But the scientist doesn’t care. He believes he’s on the verge of a major breakthrough. 

Louis Pasteur is working on his theory of fermentation. At this time in the 19th century, despite the widespread popularity of beer and wine, almost nothing is known about the scientific process behind these beverages. The consensus view is that a chemical reaction occurs when certain sugar molecules are added to certain liquids, producing alcohol.

But Louis Pasteur rejects this theory. He believes that fermentation is essentially a biological process, in which yeast – a living microorganism – feeds on the sugar molecules and then converts them into alcohol.

Pasteur was ridiculed last year when he published his “germ theory” of disease, the hypothesis that the world is filled with living microorganisms; that every surface, even the air that humans breathe, is teeming with microscopic bacteria, some of which are responsible for illnesses. Until now, disease in humans has been explained by the theory of “spontaneous generation”, in which bacteria materializes from oxygen in the air.

Pasteur’s “germ theory” will eventually become the accepted explanation for disease, but in 1862, Pasteur is still trying to prove his hypothesis. He hopes that the principles of germ theory could also explain fermentation in beer and wine. And if Pasteur can verify that fermentation is the result of microorganisms, and not mere chemical reactions – his germ theory might also finally gain the respect it deserves.

So now as midnight approaches, Pasteur’s mind is filled with the mocking taunts of his fellow scientists when he first proposed germ theory. But it’s more than just professional pride driving Pasteur’s research. He has the weight of an entire nation’s economic fortunes on his shoulders.

Not long ago, the Emperor of France, Napoleon III, requested a meeting with Pasteur to discuss a concerning blight afflicting the nation’s wine exports: spoilage. Crates and crates of French wine were being shipped to Britain. But upon arrival in England, the wine would taste sour and vinegary. Napoleon asked Pasteur to find a solution to the spoilage problem, and Pasteur accepted.

So this evening, in his laboratory in Paris, Pasteur is conducting an experiment on blood, raising its temperature under a microscope and monitoring the amount of bacteria present. Suddenly, Pasteur lifts his head from the microscope, his eyes wide.

The bacteria in the blood sample was killed when heated to a precise temperature. He conducts similar experiments with wine, finding that when warmed to 131 degrees Fahrenheit, the microbes causing the wine to spoil are also killed.

It’s a eureka moment, and worthy of celebration. But Pasteur is a stern and serious man. He maintains scholarly calm as he documents his findings. But even the stoic Pasteur cannot stop his imagination leaping forward, picturing the revolutionary impact his discovery could someday have.

And three years later, in 1865 – after conducting many further experiments – Pasteur patents his discovery, naming the process after himself: pasteurization. The wine industry has adopted the process. And now, shipments of French wine to Britain remain fresh and unspoiled. Napoleon III sends Pasteur a personal note of thanks.

And while Pasteurization will come to revolutionize the wine and beer industries, it won’t be until 1886, nearly 20 years later, that a German chemist named Franz von Soxhlet will propose the same technique could be applied to milk. Soxhlet’s suggestion will take years to gain traction in the dairy industry, as many farmers rebel against the addition of costly new elements to the milk production process.

Furthermore, scientists and health officials will argue that pasteurized milk is less nutritious than so-called raw milk. This opinion prevails among consumers too, even when it becomes obvious that raw milk is more likely to carry disease. In the end, it will take the owner of one of America’s largest department stores to introduce pasteurization to the milk industry in the United States; single-handedly saving an estimated half a million lives.

Act Three: Pure Milk

It’s the summer of 1893; thirty years after Louis Pasteur invented pasteurization.

A line has formed outside a small wooden kiosk in New York City. Anxious mothers soothe crying children. People jostle to get ahead in line. A sign above the storefront reads: Nathan Straus Milk Depot. The prices are also listed: four cents a liter, and just one cent for a glass.

Behind the counter, dairymaids serve the grateful customers, who stagger away clutching overflowing milk churns in each hand. Overseeing this busy operation is a kindly-looking white-haired man in a dark three-piece suit: Nathan Straus.

Nathan is the owner of a Macy’s department store on Sixth Avenue. As well as being exceptionally wealthy, Nathan is a keen philanthropist, and there’s one cause he’s championed above all others: providing clean, safe milk to the children of America.

Ever since the swill milk scandal in New York in the 1850s – when contaminated milk was sold under the misnomer “Pure Country Milk” – health issues surrounding fresh dairy products have persisted. When a cow on Nathan Straus’ upstate farm died from bovine tuberculosis, Nathan realized the danger of raw milk. In 1892, he opened a laboratory in Philadelphia to conduct research into pasteurization: the process of heating milk to kill dangerous microorganisms, invented by Louis Pasteur thirty years prior.

Then in 1893, Nathan opens his first milk depot in New York, selling pasteurized milk to the masses. Soon, Nathan opens dozens more, catering to the poorest parts of the city. In 1891, 24 percent of infants in New York died before the age of one. But of the 20,000 children fed on Nathan Straus’ pasteurized milk, only 6 died prematurely.

And then in 1898, following the success of his depots, Nathan is appointed the head of the New York City board of health. From that position, Nathan promotes the use of pasteurization across the entire country. But still, Nathan faces resistance. Dairy farmers resent being forced to acquire costly new pasteurization equipment. Even some health experts question the benefits of pasteurized milk. The American Pediatric Society, for instance, warns that feeding babies pasteurized milk could give them scurvy.

But Nathan persists. At his own expense, he opens 297 milk stations across thirty-six cities. In Chicago, with infant mortality rates down by 50%, the city passes America’s first mandatory pasteurization laws in 1908. New York City follows suit in 1914.

The impact of pasteurization is immense. The national death rate for children in 1891 was 125 per thousand; by 1925, that figure falls to less than 16 per thousand. Still, it won’t be until 1924 that pasteurization becomes a recommended federal policy in the United States. Even today, almost 120 years after Nathan Straus opened his first milk depot, the debate still rages over the relative merits of pasteurized versus raw milk. Still, there is no doubt the process has saved countless lives, thanks to the diligence of scientists and health advocates like Dr. Charles North, Frank Leslie, Franz von Soxhlet, Nathan Strauss, and, perhaps most importantly, Louis Pasteur, who invented pasteurization on April 20th, 1862.


Next onHistory Daily: April 21st, 1934. The Daily Mailpublishes an alleged photo of the so-called “Loch Ness Monster”, sparking an international sensation around one of the world’s most enduring modern legends. 

From Noiser and Airship, this is History Daily, hosted, edited, and executive produced by me, Lindsay Graham.

Audio editing and sound design by Mollie Baack.

Music by Lindsay Graham.

This episode is written and researched by Joe Viner.

Executive Producers are Steven Walters for Airship, and Pascal Hughes for Noiser.