Lactic acid fermentation
In the 19th century, several chemists discovered some fundamental concepts of the domain of organic chemistry. One of them was the French chemist Joseph Louis Gay-Lussac, who was especially interested in fermentation processes.  He also passed this fascination to one of his best students, Justus von Liebig. With a difference of some years, each of them described the chemical structure of the lactic acid molecule as we know it today. Both Gay-Lussac and von Liebig had a purely chemical understanding of the fermentation process:  in their view, the process  can be optimized with chemical catalyzers; neither of them was interested in seeing it with a  microscope. In 1857, the French chemist Louis Pasteur first described lactic acid as the product of a microbial fermentation. During this time, he worked at the university of Lille, where a local distillery asked him for advice concerning some fermentation problems. Per chance and with the badly equipped laboratory he had at that time, he discovered that in this distillery, two fermentations were taking place, a lactic acid one and an alcoholic one, both induced by microorganisms. He then continued the research on these discoveries in Paris, where he also published his theories which contradicted  the purely chemical version represented by Liebig and his followers. Even though Pasteur described some concepts that are still accepted today, Liebig refused to accept them. But even Pasteur himself wrote that he was "driven" to a completely new understanding of this chemical phenomenon. Even if Pasteur didn't find every detail of this process, he still discovered the main mechanism of how the microbial lactic acid fermentation works. He was the first to describe fermentation as a "form of life without air."