Louis Pasteur (1822–1895), a French chemist and bacteriologist, was a pioneer in the fields of bacteriology and preventive medicine. He had already established an international reputation as a chemist and won the Rumford Medal of the British Royal Society for his work on the structure of crystals when he made his first foray into bacteriology in 1854. Having recently been appointed a professor of chemistry in Lille, Pasteur was invited to solve a problem in the fermentation of beer that affected its taste and rendered it undrinkable. He showed that this was caused by bacteria that could be killed by heat. In this way he invented the process for heat treatment to kill harmful bacteria, first applied to the making of beer, then to milk. This process has been known ever since as pasteurization.