Editing Fermentation theory (section)

== History ==
Prior to Pasteur's research on fermentation, there existed some preliminary competing notions of it. One scientist who had a substantial degree of influence on the theory of fermentation was [[Justus von Liebig]]. Liebig believed that fermentation was largely a process of [[decomposition]] as a consequence of the exposure of yeast to air and water.<ref name=":0">{{Cite book|title=Conant, James Bryant ; Nash, Leonard K. ; Roller, Duane ; Roller, Duane H.D.: Harvard Case Histories in Experimental Science. Volume II.|isbn=978-0-674-59871-3|location=Cambridge, Mass.|oclc=979880864}}</ref> This theory was corroborated by Liebig's observation that other decomposing matter, such as rotten plant and animal parts, interacted with sugar in a similar manner as yeast. That is, the decomposition of albuminous matter (i.e. water-soluble proteins) caused sugar to transform to alcohol.<ref name=":0" /><ref name=":3">{{Cite book|last=Ben-Menahem, Ari.|title=Historical encyclopedia of natural and mathematical sciences|date=2009|publisher=Springer|isbn=978-3-540-68832-7|location=Berlin|oclc=318545341}}</ref> Liebig held this view until his death in 1873.<ref name=":0" /> A different theory was supported by [[Charles Cagniard de la Tour]] and cell theorist [[Theodor Schwann]], who claimed that alcoholic fermentation depended on the biological processes carried out by brewer's yeast.<ref name=":4">{{Cite book|last=Geison, Gerald L., 1943-|title=The private science of Louis Pasteur|date=14 July 2014|isbn=978-1-4008-6408-9|location=Princeton, New Jersey|oclc=889252696}}</ref><ref name=":3" />

Louis Pasteur's interest in fermentation began when he noticed some remarkable properties of [[amyl alcohol]]&mdash;a by-product of lactic acid and alcohol fermentation&mdash;during his biochemical studies. In particular, Pasteur noted its ability to “rotate the [[Plane of polarization|plane of polarized light]]”, and its “unsymmetric arrangement of atoms."<ref name=":0" /> These behaviors were characteristic of organic compounds Pasteur had previously examined, but also presented a hurdle to his own research about a "law of hemihedral correlation".<ref name=":4" /><ref name=":1">{{Cite book|last=Dubos, René J. (René Jules), 1901-1982.|title=Pasteur and modern science|date=1998|publisher=ASM Press|others=Brock, Thomas D.|isbn=1-55581-144-2|location=Washington, D.C.|oclc=39538952}}</ref> Pasteur had previously been attempting to derive connections between substances' chemical structures and external shape, and the optically active amyl alcohol did not follow his expectations according to the proposed 'law'.<ref name=":4" /> Pasteur sought a reason for why there happened to be this exception, and why such a chemical compound was generated during the fermentation process in the first place.<ref name=":4" /> In a series of lectures later in 1860, Pasteur attempted to link optical activity and molecular asymmetry to organic origins of substances, asserting that no chemical processes were capable of converting symmetric substances (inorganic) into asymmetric ones (organic).<ref name=":4" /> Hence, the amyl alcohol observation provided some of the first motivations for a biological explanation of fermentation.

In 1856, Pasteur was able to observe the microbes responsible for alcoholic fermentation under a microscope, as a professor of science in the [[University of Lille]].<ref name=":0" /><ref name=":4" /> According to a legend originating in the 1900 biography of Pasteur, one of his chemistry students—an owner of a beetroot alcohol factory in Lille—sought aid from him after an unsuccessful year of brewing.<ref name=":4" /> Pasteur performed experiments at the factory in observation of the fermentation process, noticing that yeast globules became elongated after lactic acid was formed, but round and full when alcohol was fermenting correctly.<ref name=":4" />

In a different observation, Pasteur inspected particles originating on grapevines under the microscope and revealed the presence of living cells. Leaving these cells immersed in grape juice resulted in active alcoholic fermentation. This observation provided evidence for ending the distinction between ‘artificial’ fermentation in wine and ‘true’ fermentation in yeast products.<ref name=":0" /> The previous incorrect distinction had stemmed in part from the fact that yeast had to be added to beer wort in order to provoke desired alcoholic fermentation, while the fermenting catalysts for wine occurred naturally on grapevines; the fermentation of wine had been viewed as 'artificial' since it did not require additional catalyst, but the natural catalyst had been present on the grapevine itself.<ref>{{Cite book|last=Tyndall|first=John|title=Essays on the floating-matter of the air, in relation to putrefaction and infection|publisher=D. Appleton|year=1892|location=New York}}</ref> These observations provided Pasteur with a working hypothesis for future experiments.<ref name=":4" /><ref name=":1" />

One of the chemical processes that Pasteur studied was the fermentation of sugar into lactic acid, as occurs in the souring of milk. In an 1857 experiment, Pasteur was able to isolate microorganisms present in lactic acid ferment after the chemical process had taken place.<ref name=":2">{{Cite web|url=https://lemelson.mit.edu/resources/louis-pasteur|title=Louis Pasteur {{!}} Lemelson-MIT Program|website=lemelson.mit.edu|access-date=2020-02-16}}</ref> Pasteur then cultivated the microorganisms in a [[Microbiological culture|culture]] with his laboratory. He was then able to accelerate the lactic acid fermentation process in fresh milk by administering the cultivated sample to it.<ref name=":1" /> This was an important step in proving his hypothesis that lactic acid fermentation was catalyzed by microorganisms.<ref name=":1" /><ref name=":2" />

Pasteur also experimented with the mechanisms of brewer's yeast in the absence of organic nitrogen.<ref name=":4" /> By adding pure brewer's yeast to a solution of cane sugar, ammonium salt, and yeast ash, Pasteur was able to observe the alcoholic fermentation process with all of its usual byproducts: [[Glycerol|glycerin]], [[succinic acid]], and small amounts of [[cellulose]] and fatty matters.<ref name=":4" /> However, if any of the ingredients were removed from the solution, no fermentation would occur. To Pasteur, this was proof that yeast required the nitrogen, minerals, and carbon from the medium for its metabolic processes, releasing carbonic acid and ethyl alcohol as byproducts.<ref name=":3" /><ref name=":4" /> This also disproved Liebig's theory, since there was no albuminous matter present in the medium; the decomposition of the yeast was not the driving force for the observed fermentation.<ref name=":3" /><ref name=":4" />[[File:Fermentation lock.JPG|thumb|left|A fermentation lock, an example of a curved neck apparatus used in brewing today]]