The key idea here is that fermentation is driven by living organisms, specifically yeast. In 1857, Pasteur demonstrated through careful experiments that fermentation depends on these living cells: when a sugar solution is heated to kill microbes, fermentation stops; once yeast is added back, fermentation resumes. This showed that yeast, not some purely chemical reaction, is the agent transforming sugar into alcohol and carbon dioxide. This finding connected a microorganism to a biological process and helped shift thinking toward the germ theory of fermentation. Dates outside this time frame don’t match when this crucial link was first established. The 1840s were before his decisive experiments, and the later dates aren’t the moment when he proved yeast’s role.