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Ask the question: “Who introduced the pasteurization of milk?” and I can pretty well guarantee that the answer will be: “Louis Pasteur.” I know because I have carried out that little experiment numerous times. Surprisingly, that answer is wrong. My story starts decades ago, with a visit to the Pasteur Museum in Paris that houses the apartment where Pasteur and his wife lived during the last seven years of his life, a collection of his scientific equipment, and the ornate crypt where his remains lie. Stunning mosaics in the crypt pay homage to Pasteur’s landmark discoveries, but as a chemist, my eyes were immediately drawn to the phrase “une dissymetrie dans les molecules” or, in English, “molecular dissymmetry.” Contrary to popular belief, Louis Pasteur was not a doctor or a biologist, he was a chemist. His early fascination was with crystals, particularly with salts of tartaric acid, which since antiquity have been known to form during the fermentation of grape juice. One day in 1848, Pasteur was examining a sample of such crystals under a magnifying glass and made a remarkable observation: There were two kinds of crystals and they were mirror images of each other. He laboriously separated the crystals with tweezers and discovered that they had exactly the same physical properties, save for one. When dissolved in water and placed in the beam of a special kind of light, known as plane polarized light, they rotated the beam in opposite directions. Since the crystals had been dissolved in water, Pasteur hypothesized that this behaviour was actually due to the individual molecules and that the two types of crystal must consist of mirror-image molecules. This was advanced thinking, but Pasteur could push it no further because at the time the structure of molecules was a mystery. Twenty-five years later, Jacobus van’t Hoff correctly interpreted Pasteur’s finding by proposing that carbon atoms in a molecule can form four bonds oriented toward the corners of a tetrahedron. If these corners are occupied by four different atoms, two possible arrangements are possible, giving rise to molecules that are alike in every respect except that, like our hands, they are non-superimposable mirror images of each other. Van’t Hoff’s explanation laid a cornerstone to the understanding of molecular structure and in 1901 he was duly rewarded with the first Nobel Prize in chemistry. One of the mosaics in the Pasteur crypt features mulberry trees and silkworms, alluding to his success in solving a mysterious disease that afflicted silkworms. Another portrays a dog, to symbolize Pasteur’s pioneering of a vaccine against rabies. There are also sheep grazing in a pasture that reflect his work on anthrax infections, and an image of a bird recalls his conquest of chicken cholera. Then there are grapevines, a testimonial to Pasteur’s study of fermentation and his realization that it is contamination with microbes that causes wine to go sour. As I took all this in, I glanced around looking for a depiction of a cow, which I was sure must be there in some fashion to commemorate his discovery of pasteurization, a process that has saved multitudes from getting sick from drinking raw milk. My search was in vain, as I discovered by looking into the history of pasteurization. Louis Pasteur had nothing to do with the pasteurization of milk. Back in 1886, Frans von Soxhlet, a German agricultural chemist, was the first person to suggest that milk sold to the public be “pasteurized.” The term “pasteurization” was indeed coined in recognition of Pasteur’s pioneering work on the destruction of microbes through heat treatment, but his area of interest was wine and beer, not milk. The fact that heat treatment made foods safer was known long before Pasteur, but the French chemist was the first to provide an explanation for the phenomenon. Pasteur realized that spoilage was due to chemical reactions initiated by living microbes, and that the reason heat treatment prevented spoilage was because of its destructive effect on these living organisms. If wine or beer turned sour, Pasteur maintained, it was because of contamination by acid-producing rogue yeasts after the alcohol producing yeast had done its job. Heating of beer or wine, he suggested, would destroy these invaders and preserve the beverage. After the Franco-Prussian War of 1870, Pasteur, a noted French patriot, proved his point by creating his “beer of revenge,” which would serve as a testimonial to the superiority of French brewing techniques over anything the Prussians could muster. Heat-treated French beer would keep indefinitely, while Prussian beer would go cloudy with time. As far as beer goes, this was mostly a cosmetic, not a health issue. But milk presents an altogether different scenario. Typhoid and scarlet fever, diphtheria, tuberculosis and various diarrheal diseases were all capable of being transmitted through the consumption of raw milk. But it seems that until von Soxhlet made his suggestion, nobody thought of pasteurizing milk on a large scale. In 1891, one in every four infants in New York City died, many from drinking tainted milk. This dropped to about one in 14 when pasteurization was introduced. The advent of this life-saving technology, though, was not without its critics. Some argued that heat treatment destroys vital nutrients in milk and produces a “burnt” flavour. The controversy about the pros and cons of pasteurization continues to this day. In 2024, RFK Jr. asserted on a post on X that raw milk “advances human health” and claimed that its sale is “aggressively suppressed” by the FDA, supposedly because the “healing powers” that Kennedy believes raw milk has may affect the sales of pharmaceutical companies. This is utter nonsense. There is also a lot of bluster from raw-milk advocates about pasteurization impairing the nutritional merits of milk, but the arguments are not backed by evidence. Milk contains thousands of naturally occurring compounds, and some of these certainly undergo chemical changes with heat. But that does not mean these changes have any health consequence. There is plenty of evidence about unpasteurized milk causing disease. While raw milk from an individual farm, where cleanliness is scrupulously maintained, can be safe to drink, raw milk pooled from many farms, as is the usual case today, can harbour a range of bacteria. Although Louis Pasteur did not advocate the pasteurization of milk, there is no question that the process named in his honour saves lives. Heating to 72 degrees C for 15 seconds or to 138 degrees C for 2 seconds destroys Listeria monocytogenes, Campylobacter, Salmonella and/or E. coli O157:H7, all of which can cause severe disease. Why take a chance? @JoeSchwarcz
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