After working in the field of optics he moved on to the study of biology and medicine for which he was to become famous. Pasteur theorised and then demonstrated by experiment that the fermentation of liquids was a result of external causes: particles that entered the liquid from the surrounding air. This displaced the popular theory of the time, which was that fermentation was a spontaneous process within the liquid.
On 20th April, 1862, Pasteur and his friend Claude Bernard conducted an experiment to test Pasteur’s theory that heat could be used to kill moulds and bacteria in fermenting milk. The experiment was a success and the process of heating beverages to destroy harmful organisms was called pasteurization in honour of its inventor. Pasteur went on to work in the field of immunology before his death in 1895.
For a longer biography of Louis Pasteur, see this page on the Bio-medicine.org site.