Which French scientist showed that fermentation is caused by living microorganisms?

The French scientist who showed that fermentation is caused by living microorganisms was Louis Pasteur.

Pasteur began studying fermentation in Lille in 1856 after a local manufacturer asked for help with spoiled beetroot alcohol. In investigations published in 1857 and 1858, he argued that different fermentations were associated with different living organisms: yeast produced alcoholic fermentation, while another microorganism could produce lactic acid and sour a product. This challenged chemical explanations that treated fermentation as simple decomposition.

His work also linked microorganisms to food and beverage spoilage. Pasteur showed that unwanted microbes could contaminate wine, beer, and milk, helping establish the scientific basis for pasteurization. His experiments with swan-neck flasks further supported the idea that microorganisms came from contamination in the environment rather than appearing spontaneously.

Pasteur was not the first person to suspect that yeast was alive; Charles Cagniard de la Tour, Theodor Schwann, and Friedrich Traugott Kützing had reached similar conclusions in the 1830s. Pasteur’s major contribution was demonstrating the specific role of microorganisms in fermentation and extending that insight into germ theory. Later, Eduard Buchner showed that cell-free yeast extracts could ferment sugar, revealing that enzymes produced by living cells drive the chemical reactions.