The middle years 1862-1877

Louis Pasteur’s work raised a new set of research questions, such as " Where do fermentation agents come from ? " and " Do they originate from germs similar to themselves or do they appear spontaneously as explained by the spontaneous generation theory ? "


Spontaneous generation - the big debate

At the time the spontaneous generation theory was widely accepted in scientific circles. Louis Pasteur decided to approach the issue via his experimental method.

This required the use of swan-necked flasks. Water in the flask was brought to the boil for a few minutes until the steam escaped from the open end of the flask. It was then left to cool. While cooling, the air entering the flask deposited dust and germs on the first bend. Although in contact with outside air the liquid remained unaltered because germs could not get through.

Louis Pasteur showed that microbes were omnipresent - in water, in air, on objects, on the skin – and that some were responsible for diseases.

After some memorable struggles against his opponents, notably the famous biologist and fierce defender of the spontaneous generation theory, Félix Pouchet, in his 1862 paper Louis Pasteur was able to claim that :

  • airborne dust contained microorganisms which develop and multiply.
  • even the most putrescible liquids remained unadulterated if kept away from air (and hence these microorganisms) after heating.

He recommended ways of preventing and fighting these germs, and thus the habits essential for personal and social hygiene. This notably included the use of aseptic procedures, i.e. the various measures to be taken to prevent invasion of live tissue or inert environments by exogenous microorganisms or viruses. He advocated the importance of sterilization of linen and dressings, passing instruments through a flame and clean hands. These recommendations led to the widespread advent of modern surgery.

So how does fermentation work ?

But Louis Pasteur still had ferments in mind. He pondered on fermentation and how ferments work. While studying butyric fermentation he discovered a new class of living organisms capable of living without air.

He used the term " anaerobic " to describe ferments able to live without air and " aerobic " for microorganisms requiring the presence of free oxygen to grow.

He came to the conclusion that fermentation is the consequence of life without air.

He applied his microbiological method to industry and agriculture to eradicate ancient diseases affecting crops and products.

To the rescue of industry and agriculture


He studied the formation of vinegar and the conversion of alcohol into acetic acid by Mycoderma aceti, which fixes oxygen from the air onto the alcohol. He showed vinegar makers how to produce vinegar of consistent quality by avoiding contamination by harmful mycoderma.

Wine diseases

Wine was France flagship industry and a difficult business in many respects. Winemakers had difficulty guaranteeing the quality of their production which was affected by diseases of no known cause or cure. The crisis was nothing new but risked damaging exports and above all trade agreements in place with England. Emperor Napoleon III called on Louis Pasteur to seek a solution.
First he showed that each wine disease was due to a particular ferment.
He developed a protocol to fight the diseases, heating the wine to between 55°C and 60°C, a temperature at which it does not deteriorate and its bouquet is preserved. This method is now known worldwide as pasteurization.


Just like wine, beer is infected by microorganisms transmitted by airborne dust. Louis Pasteur taught brewers to preserve the wort from the impurities and to heat the beer to 55° to prevent disease.


Silkworm diseases

In 1865, disease hit the silk industry. In France, this posed a threat to the economy of an entire region and the disease spread further afield to other silk-producing countries such as Italy, Austria and Asia Minor.
Louis Pasteur discovered that silkworms were affected by two diseases - silkworm nosema disease and flacherie.

Under the microscope, Louis Pasteur noticed that the worms with nosema disease developed shiny corpuscles, and showed that the disease was both hereditary and contagious.
He developed the cellular egg production method to enable the preservation of healthy silkworm eggs. He isolated the female moths to allow them to lay their eggs separately. After laying, he ground the female moths and examined them under the microscope. If the shiny corpuscles were observed he destroyed the eggs, otherwise he kept them for breeding.
As for flacherie, he introduced the "specific terrain" concept, i.e. the physiological condition of the infected host favoring outbreak of the disease. A few hygiene rules, good ventilation and quarantine of the suspect batches sufficed to prevent contamination.
These simple processes saved the silk industry from doom. But the research was of considerable value, paving the way for the study of contagious diseases. For the first time problems of heredity and contagion were scientifically proven and prophylaxis rules were established.
The time had now come for Louis Pasteur to address human diseases.


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