French chemist who studied the effect of tartaric acid on polarized light. Under a microscope in 1848, he discovered that there were two types of tartrate crystals which were mirror images of each other. This contradicted Mitscherlich who had observed only a single type of crystal. It was later found that Pasteur had serendipitously prepared his crystals in a particular way which would produce large crystals of both isomers. He separated the two types by hand, and, using a Nicol prism, noticed that they rotated the plane of polarization in opposite directions. Pasteur is therefore the father of stereochemistry. In 1858, he showed that a plant mold only used one isomer of racemic acid, demonstrating that living tissues only organic compounds of right-handedness. Optical rotation was subsequently observed in solutions which contained no crystals. This result was explained by van't Hoff and Le Bel by asymmetry in the solute molecule themselves. Pasteur also investigated the reason that wine and beer could go sour. Under the microscope, he found two different shapes of yeast in the good and sour alcohol.

Pasteur showed the yeast to be an organism which, however, did not require oxygen for fermentation to occur. This showed Justus von Liebig who had maintained the fermentation was purely chemical, to have been mistaken. Pasteur demonstrated that mild heating applied after fermentation would kill the microorganisms and prevent souring. This gentle heating has come to be known as pasteurization. With a specially-constructed bent flask, Pasteur demonstrated conclusively that decay was produced by air-borne microorganisms. This refuted the doctrine of spontaneous generation. He also discovered the parasite responsible for killing silkworms, and saved the French silk industry by recommending that all infected worms and mulberries by destroyed.

Pasteur's work with silkworm parasites and air-borne germs led him to propose the germ theory of disease. He pressed doctors to disinfect their instruments by boiling and steaming. Pasteur found that septicemia was caused by an anaerobic bacterium. He urged surgeons to use clean instruments, wash their hands, and disinfect their gauze and bandages in "The Germ Theory and its Application to Medicine and Surgery," much as Lister was to do. He also confirmed Koch's discovery of the anthrax germ. Pasteur noticed that chickens were immune to anthrax, and postulated that it was because chickens have high body temperatures of 43-44° C. To test this hypothesis, he lowered the body temperature of a chicken to 37° C and found it to be susceptible to anthrax. Then, drawing from the work of Jenner, Pasteur heated anthrax germs and inoculated 25 sheep. When 50 sheep were then injected with anthrax, all 25 control sheep died while all inoculate sheep survived. Therefore, Pasteur demonstrated that the weakened anthrax lost its virulence but still could confer immunity. Pasteur termed this technique "vaccination," and applied it to chicken cholera and rabies.