Philosopher Aristotle coined the idea of the spontaneous generation theory in 4th century BCE, 22 centuries before time of Louis Pasteur. This theory stated that living matter could arise from non-living matter spontaneously. One of the most famous examples of this theory is that maggots could appear on decaying piece of meat.

This idea went on to persist for a very long time. This is due largely to the fact that it easily explained how mold grow on bread or that flies appear on spoiled food.

But this idea did not align to many scientists, many of them tried to disprove this idea, including Louis Pasteur.

Early challenges to spontaneous generation

For a millennium, Aristotle’s theory of spontaneous generation was widely believed around the world. Was this because of the lack of technology that science enjoys today? For example, microscopes were far from being invented to allow researchers to observe and study microorganisms. Hence, experiments to test theories were not really much of a thing back then.

By the 1600, scientists and scholars have started questioning the factualness of the theory. One of these individuals who challenged the theory was Italian physician Francesco Redi. He showed that maggots do not spontaneously arise from decaying meat by doing the so-called “Redi experiment” in 1668.

In this experiment, Redi set up 3 jars of various conditions. The first jar was open and let flies to enter the jar. The second jar was tightly to prevent flies from entering. And the last jar was covered with a mesh. After letting the jars sit for a short period, maggots appeared in the open jar and mesh-covered jar, but not the tightly sealed one.

Redi concluded that flies laid eggs that would hatch into maggots. This result suggested that living matters like maggots come from other living matters, and do not arise spontaneously. Although the Redi experiment demonstrated that living matters could only arise from pre-existing living matters, this was not sound enough to disprove the spontaneous generation theory.

Hence, the debate continued.

Antonie Van Leeuwenhoek contribution

Antonie Van Leeuwenhoek was a Dutch scientist known as the Father of Microbiology. He developed microscopes during the 17th century that were considered advance during that time. Leeuwenhoek made about 500 microscopes in this career. One of these could magnify objects up to 300 times. This capability was unrivaled back then. In comparison, the microscope English physicist Robert Hooke developed could only magnify up to 50 times. This magnification could only reveal basic details on minute organisms.

Leeuwenhoek’s, on the other hand had greater magnification. This allowed him to study various microorganisms in greater detail. Leeuwenhoek described bacteria, yeasts, and other microorganisms. Their shapes, movement, and behavior were documented for the first time. But his discovery of the existence of microorganisms was not solid evidence to dispel the spontaneous generation theory.

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When his work was made public, scientist still wondered the same question. Do microorganisms come from pre-existing living things? Or they just generate spontaneously from non-living things? Yes, the scope of microbiology back then was very limited. However, Leeuwenhoek’s contribution to understanding microorganisms paved the way for another scientist to disprove the theory of spontaneous generation at once, Louis Pasteur.

Louis Pasteur’s works prior to debunking the spontaneous generation theory

Louis Pasteur was a French chemist and microbiologist. He lived during the 19th century. At this point, the debate on the spontaneous generation theory was at its peak. Prior to disproving the theory, he already worked on fermentation, and pasteurization.

In 1850s, Pasteur studied extensively the process of fermentation. Fermentation is a preservation method wherein sugar in food is converted into alcohol or acid. Prior to Pasteur’s research on the process, it was widely accepted that fermentation was solely a chemical process. The belief was that fermentation would occur because components in food decompose in the absence of air. Hence, microorganisms were not believed to be responsible in fermentation.

But Pasteur’s work changed this when he studied spoilage in wine and beer. In the mid-19th century, the brewing industry in France was suffering from economic losses due to spoilage of wine. The losses were massive that it hit wine exports badly. To resolve the problem, Napoleon III and the French government asked for help from Pasteur. He then presented clear evidence that undesirable or spoilage microorganisms were responsible for the off-flavor and souring in wine.

What Pasteur did was preheat the wine at between 122°F (50°C) and 140°F (140). This prevented souring and extended the shelf life of wine.

Based on his research on microorganisms, spoilage microorganisms found in wine are heat sensitive. Hence, he hypothesized that treating the wine with elevated heat high enough to destroy these microbes would effectively extend the shelf life of wine. The temperature range he used was well thought of because not only it killed unwanted microbes, but it was also not high enough to preserve the flavor of the wine. This heat treatment is now called pasteurization.

Pasteur’s Swan-Neck Flask experiment debunked the spontaneous generation

Louis Pasteur became aware of the spontaneous generation when he came to know fellow Frenchmen Felix Archimède Pouchet, a strong follower of the spontaneous generation theory. Pasteur had been very skeptical about the theory, and the French Academy of Sciences opened a competition called Alhumbert Prize to ultimately put an end to this debate. Pasteur took up the challenge and performed an experiment that would ultimately debunk the theory— the Swan-Neck flask experiment.

In this experiment, Pasteur gathered a number of long, curved S-shaped flasks that looked like swan’s neck, hence the name of the experiment. He filled each flask with an infusion or nutrient rich broth. After that, he pasteurized the flasks to destroy the harmful microorganisms that were present in the broth.

After letting the pasteurized broth in the flask to sit for some time he observed what happened. And just as he predicted, the broth did not change in appearance or appear to have been contaminated. The unique S-shape of the flask prevented contaminated to happen here. The curve neck allowed air to flow through, but not dust and any other elements that may contaminate the broth.

But if the curved long neck of the flasks were removed, or the flask were tilted that the broth got into contact with the curve neck, airborne microorganisms would have been introduced to the broth and contaminate it.

The Swan-Neck flask experiment by Pasteur ultimately debunked the spontaneous generation theory. Because of this, he was awarded the Alhumbert prize, which also carried a value of 2,500 francs. This was considered a huge sum already in 1862.

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