 The turning point came when Louis Pasteur (1822–1895), during the 1850s and 1860s, repeated Schwann's experiments and showed that fermentation is initiated by living organisms in a series of investigations.In 1857, Pasteur showed that lactic acid fermentation is caused by living organisms.In 1860, he demonstrated that bacteria cause souring in milk, a process formerly thought to be merely a chemical change, and his work in identifying the role of microorganisms in food spoilage led to the process of pasteurization.