'Pasteur discovers the law of fermentation'. Pasteur studied the process of souring of wine and beer. He was able to demonstrate that fermentation does not require oxygen, but that it nevertheless involves living organisms. He recognised that there are two kinds of yeast, one of which produces alcohol and the other lactic acid, which turns wine sour. He suggested that heating wine or beer at about 120 degrees Fahrenheit would kill the yeast, preventing further fermentation and souring due to lactic acid production. This heating process used to kill undesirable micro-organisms is known as pasteurization and is now also applied to milk.