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How did Louis Pasteur explain why milk becomes sour?

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Painting by Albert Edelfelt showing Louis Pasteur in his laboratory, holding a glass flask and examining it intently, surrounded by scientific equipment on a workbench. — Louis Pasteur In the 19th century Louis Pasteur's work established the germ theory of disease, proving that microorganisms cause fermentation and sickness, which led to the development of pasteurization, aseptic techniques in medicine, and effective vaccines for diseases such as rabies. (Painting by Albert Edelfelt, 1885.)

Louis Pasteur explained that the souring of milk is a result of fermentation caused by the activity of specific microorganisms.12

Through his research in the mid-1850s, Pasteur investigated the production of lactic acid, identifying it as the compound responsible for the souring process. In 1857, he presented experimental evidence demonstrating that all fermentation processes are driven by living organisms and that each specific type of fermentation is associated with a particular microorganism. This discovery was a foundational element of his germ theory of fermentation.12

Building on these findings, Pasteur developed pasteurization, a process that involves heating liquids to a controlled temperature to destroy these microbes, thereby preventing spoilage and extending the shelf life of products like milk.324