The Lille Fermentation Observation

In 1856, a Lille manufacturer of beet-sugar alcohol presented Pasteur with a commercial problem: fermentation vats were souring, producing lactic acid instead of ethanol. The expected response was chemical analysis within the framework of Justus von Liebig — the most influential European chemist, whose theory held fermentation was purely chemical decomposition with living organisms as incidental passengers. Pasteur examined healthy and diseased vats under the microscope. Healthy vats contained yeast globules; diseased vats contained rod-shaped organisms associated with a grey deposit. The organisms had been there for anyone to see. What Pasteur possessed was a perceptual apparatus calibrated by a decade of crystallographic work to detect structural differences at the microscopic level — a capacity the chemical tradition had never cultivated. He recognized the organisms as agents, not passengers. The observation would become the foundation of the germ theory of disease.