Pasteur observed that fermentation does not require oxygen, but needs yeast, which is alive. Fermentation is a biological process, not a reduction and oxygen chemical process. He used two slender bottles. One of the bottles had a curved neck; this is called a swan neck. Pasteur poured liquid broth into the two bottles, and heated in the bottom of the bottles. When the liquid boiled, he let them cool. Pasteur observed that the broth in the curved bottle stayed clear, except when the bottle was shaken. 
Pasteur explained that the two bottles were filled with air, but the curved bottle could stop most of the particles in the air, and it kept its nature. However, the liquid in the other bottle degenerated. Therefore, he concluded that fermentation does not require oxygen, but needs the yeast. When yeast is allowed to grow over time, the substance will spoil or rot.
Pasteur's viewed fermentation as a type of vitalism. He observed that living organisms were responsible for the process of fermentation.