Louis Pasteur did not invent a method for keeping farm foods completely clean of any bacteria — sterilization methods existed long before him. His breakthrough came in reducing the level of bacteria to levels deemed safe, while keeping the nutrients and taste that often disappeared from sterilized products. Pasteur had already demonstrated proof of the germ theory — that fermentation caused the growth of harmful microorganisms — when he turned his attention to proving those microorganisms exist in beverages as well.
On this day, April 20, in 1862 Lois Pasteur and his fellow scientists Claude Bernard, while researching a way to preserve the shelf life of milk and beer, discovered that heating up the products can destroy those microorganisms.
The removal of microbes from food was quickly found to have the added benefit of making them safer to eat, and the new method came to be called pasteurization, in honor of its inventor. Pasteur, meanwhile went on to have another happy accident by which he discover an anthrax vaccine — the chickens he was raising fell ill with the disease, but recovered, leading Pasteur to develop a way to to artificially replicate their immunity.