Guide to DIY Homemade Yoghurt: Fermentation is an enzyme-catalyzed, energy-generating process in which organic compounds act as both donors and acceptors of electrons.

French chemist Louis Pasteur was the first zymologist (an early term for a scientist who studies fermentation) when in 1857 he connected yeast to the process of fermentation. Pasteur demonstrated that fermentation is caused by the growth of microorganisms, and that the growth of microorganisms in nutrient broths is not due to spontaneous generation (a theory which held that complex, living organisms can be generated from decaying organic substances).In the nineteenth century, Pasteur used the term fermentation in a narrow sense to describe the changes brought about by yeasts and other microorganisms growing in the absence of air (anaerobically); he also recognized that ethyl alcohol and carbon dioxide are not the only products of fermentation. Pasteur originally defined fermentation as respiration without air. After performing careful research, he concluded, “I am of the opinion that alcoholic fermentation never occurs without simultaneous organization, development and multiplication of cells….If asked, in what consists the chemical act whereby the sugar is decomposed…I am completely ignorant of it.”The German Eduard Buchner, winner of the 1907 Nobel Prize in chemistry, later determined that fermentation was actually caused by a yeast secretion that he termed zymase. The experiment for which Buchner won the Nobel Prize consisted of producing a cell-free extract of yeast cells and showing that this “press juice” could ferment sugar. This finding dealt yet another blow to vitalism by demonstrating for the first time that fermentation could occur outside living cells.In the 1920s, scientists discovered that, in the absence of air, extracts of muscle catalyze the formation of lactate from glucose; in addition, they learned that the same intermediate compounds formed in the fermentation of grain are produced by muscle. This discovery revealed an underlying unity in biochemistry: fermentation reactions are not peculiar to the action of yeast but also occur in many other instances of glucose utilization. The elucidation of the glycolytic pathway in the early decades of the twentieth century involved the pioneering efforts of Gustav Embden, Otto Meyerhof, Carl Neuberg, Jacob Parnas, Otto Warburg, and Gerty and Carl Cori.