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Pasteurised milk, the idea that germs cause disease, and vaccines made from weakened microbes all lead back to one chemist. Louis Pasteur also left notebooks that show a more complicated picture than the legend. This post covers what he discovered, the experiments behind it, where his public reports differed from his private records, and what a student can learn from both.
Who was Louis Pasteur?
Pasteur was born in Dole, France, on 27 December 1822, and died in 1895, according to the Science History Institute. He failed the entrance exam for the École Normale Supérieure in Paris once, studied another year, and passed in 1843. He earned his PhD in 1847.
His first research was in chemistry. He studied crystals and solutions that rotate polarised light and traced this optical activity to asymmetry in their structure, work the Science History Institute credits with helping to define stereochemistry. That is the idea behind chirality and optical isomers in your organic chemistry chapters.
What did Pasteur show about fermentation?
In 1854 Pasteur became dean of the Faculty of Science at Lille, and his interest turned to fermentation. At the time the German chemist Justus von Liebig held that fermentation was a purely chemical breakdown.
Pasteur showed that living yeast was responsible for alcoholic fermentation, building on Theodor Schwann's earlier finding that yeasts are living organisms, the Science History Institute records. He showed that a living organism also turns sugar into lactic acid in milk, and that oxygen slows fermentation, now called the Pasteur effect.
In 1864 he found that heating wine to a minimum of 50 to 60 °C killed the microbes that spoil it. He patented the process, pasteurisation, which was later applied to milk. The NEET-UG syllabus covers this ground under "Microbes in human welfare" (NEET (UG) 2026 syllabus); our guide to microbes in human welfare covers what to learn for the exam.
How did the swan-neck flask test spontaneous generation?
Many people then believed that microbes could arise from non-living matter, an idea called spontaneous generation. Félix Pouchet argued for it in his 1859 book.
Pasteur's swan-neck flasks had long, curved necks that let air in but trapped dust and airborne microbes in the bend. Sterilised broth in the base of such a flask did not produce life, the Science History Institute notes. The design meant that air could reach the broth but microbes could not, so a lack of growth could not be blamed on a lack of air.
How did his vaccines work?
Edward Jenner had used cowpox to protect against smallpox in 1796; read our post on Edward Jenner and smallpox vaccination. Pasteur aimed to make vaccines from weakened, or attenuated, forms of the disease-causing microbe itself.
Anthrax, 1881. In a public trial at Pouilly-le-Fort, the 25 animals that received his vaccine survived infection, while unvaccinated animals died (Science History Institute).
Rabies, 1885. With Émile Roux he weakened the infectious agent by drying the spinal cords of infected rabbits. The Institut Pasteur records that in July 1885 nine-year-old Joseph Meister was bitten 14 times by a rabid dog. Dr Jacques Joseph Grancher persuaded Pasteur to treat him and gave the injections over ten days. The boy survived.
Patients then came from many countries, and the Institut Pasteur was inaugurated on 14 November 1888.
What did his notebooks reveal?
Pasteur's laboratory notebooks, studied by historians long after his death, show gaps between his "private science" and his public reports, the Science History Institute explains:
For anthrax, he reported weakening the bacterium by exposure to air, but his notes show he used the chemical potassium bichromate, a method close to that of his rival Jean Joseph Henri Toussaint.
For rabies, later accounts questioned how many dogs had really been tested, and how successfully, before the first human treatment.
He believed rabies was caused by a bacterium. We now know it is a virus.
None of this erases what he achieved. It does show that science is done by people who compete, sometimes cut corners, and need others to check their claims.
What can you learn from Pasteur?
Carry methods across fields. His crystal work trained him to see that small structural differences matter, and he took that habit into biology.
Design out the objection. The swan neck was built to answer the strongest argument against him.
Keep honest records. His notebooks are why we know both his successes and where he overstated them. Your own lab notes should say exactly what you did.
If you want to revise chirality or microbes concept by concept, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can move from optical isomers to vaccines without switching books.
Next, read how Robert Koch turned the germ idea into a test for any disease in our post on Koch's postulates.
