<P> The first solid evidence of the living nature of yeast appeared between 1837 and 1838 when three publications appeared by C. Cagniard de la Tour, T. Swann, and F. Kuetzing, each of whom independently concluded as a result of microscopic investigations that yeast is a living organism that reproduces by budding . It is perhaps because wine, beer, and bread were each basic foods in Europe that most of the early studies on fermentation were done on yeasts, with which they were made . Soon, bacteria were also discovered; the term was first used in English in the late 1840s, but it did not come into general use until the 1870s, and then largely in connection with the new germ theory of disease . </P> <P> Louis Pasteur (1822--1895), during the 1850s and 1860s, 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 . In 1877, working to improve the French brewing industry, Pasteur published his famous paper on fermentation, "Etudes sur la Bière", which was translated into English in 1879 as "Studies on fermentation". He defined fermentation (incorrectly) as "Life without air", but correctly showed that specific types of microorganisms cause specific types of fermentations and specific end - products . </P> <P> Although showing fermentation to be the result of the action of living microorganisms was a breakthrough, it did not explain the basic nature of the fermentation process, or prove that it is caused by the microorganisms that appear to be always present . Many scientists, including Pasteur, had unsuccessfully attempted to extract the fermentation enzyme from yeast . Success came in 1897 when the German chemist Eduard Buechner ground up yeast, extracted a juice from them, then found to his amazement that this "dead" liquid would ferment a sugar solution, forming carbon dioxide and alcohol much like living yeasts . Buechner's results are considered to mark the birth of biochemistry . The "unorganized ferments" behaved just like the organized ones . From that time on, the term enzyme came to be applied to all ferments . It was then understood that fermentation is caused by enzymes that are produced by microorganisms . In 1907, Buechner won the Nobel Prize in chemistry for his work . </P> <P> Advances in microbiology and fermentation technology have continued steadily up until the present . For example, in the 1930s, it was discovered that microorganisms could be mutated with physical and chemical treatments to be higher - yielding, faster - growing, tolerant of less oxygen, and able to use a more concentrated medium . Strain selection and hybridization developed as well, affecting most modern food fermentations . </P>

The fermentation of sugar to alcohol can take place only if living yeast cells are present