In the 1850’s Louis Pasteur, a chemist, enthusiastically accepted the challenge of studying how alcohol arises from grape juice. Biologists had already observed that when grape juice is held in large vats, alcohol and carbon dioxide are produced and the number of yeast cells increases. They argued that the multiplying yeast cells convert the sugar in the juice to alcohol and carbon dioxide. Pasteur agreed, but he could not convince two very powerful and influential German chemists, Justus von Liebig and Friedrich Wöhler, who refused to believe that the activities of microorganisms caused the breakdown of sugar. Both men lampooned the hypothesis and tried to discredit it by publishing pictures of yeast cells looking like miniature animals taking in grape juice through one orifice and eliminating carbon dioxide and alcohol through the other.

Pasteur studied the relationship between yeast and alcohol production using a strategy commonly employed by scientists today, that is, simplifying the experimental system so that relationships can be more easily identified. First, he prepared a clear solution of sugar, ammonia, mineral salts, and trace elements. He then added a few yeast cells. As the yeast grew, the sugar level decreased and the alcohol level increased, indicating that the sugar was being converted to alcohol as the cells grew. This strongly suggested that living cells caused the chemical transformation. Liebig, however, still would not believe the process was actually occurring inside microorganisms. To convince him, Pasteur tried to extract something from inside the yeast cells that would convert sugar into alcohol and carbon dioxide. He failed, like many others before him.

In 1897, Eduard Buchner, a German chemist, showed that ground-up yeast cells could convert sugar to ethanol and CO₂. We now know that the active ingredients of the ground-up cells that carried out this transformation were enzymes. For these pioneering studies, Buchner was awarded the Nobel Prize in 1907. He was the first of many investigators who received Nobel Prizes for studies on the processes by which cells degrade sugars.