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How did Louis Pasteur show that microorganisms cause fermentation?

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Diagram showing how epinephrine binding to a beta-adrenergic receptor on a cell membrane activates a G-protein, which then stimulates adenylyl cyclase to convert ATP into cyclic AMP (cAMP), leading to physiological effects such as increased heart rate, blood vessel dilation, and glycogen breakdown. — Biochemistry In biochemistry, researchers study the chemical substances and processes that occur in living organisms and cells. In particular, various processes by which cells gain energy, such as epinephrine-stimulated cAMP synthesis, have been deduced through the study of biochemistry.

In 1857, Louis Pasteur provided experimental evidence that fermentation is not a spontaneous chemical process but one driven by living organisms. Through his research on alcohol production and the souring of milk, he demonstrated that specific microorganisms are associated with each type of fermentation.12

Pasteur further substantiated his germ theory by studying butyric acid fermentation, where he discovered that certain organisms function only in the absence of oxygen. This led him to define the terms aerobic and anaerobic to describe life-forms based on their oxygen requirements.13

To definitively disprove the theory of spontaneous generation, Pasteur used "swan-neck" flasks to show that sterilized broth remained free of microbial growth unless exposed to airborne contaminants. By proving that fermentation and putrefaction were caused by specific germs rather than spontaneous generation, he established the foundation for modern bacteriology and techniques such as pasteurization.42