Yeast & Fermentation

                                                     In the seventeenth century, a Dutch tradesman named Antoni van Leeuwenhoek developed high-quality lenses and was able to observe yeast for the first time. In his spare time Leeuwenhoek used his lenses to observe and record detailed drawings of everything he could, including very tiny objects, like protozoa,bacteria, and yeast. Leeuwenhoek discovered that yeast consist of globules floating in a fluid, but he thought they were merely the starchy particles of the grain from which the wort (liquid obtained from the brewing of whiskey and beer) was made (Huxley 1894). Later, in 1755, yeast were defined in the Dictionary of the English Language by Samuel Johnson as "the ferment put into drink to make it work; and into bread to lighten and swell it." At the time, nobody believed that yeast were alive; they were seen as just organic chemical agents required for fermentation.

In the eighteenth and nineteenth centuries, chemists worked hard to decipher the nature of alcoholic fermentation through analytical chemistry and chemical nomenclature. In 1789, the French chemist Antoine Lavoisier was working on basic theoretical questions about the transformations of substances. In his quest, he decided to use sugars for his experiments, and he gained new knowledge about their structures and chemical reactions. Using quantitative studies, he learned that sugars are composed of a mixture of hydrogen, charcoal (carbon), and oxygen.

                                                                 Lavoisier was also interested in analyzing the mechanism by which sugarcane is transformed into alcohol and carbon dioxide during fermentation. He estimated the proportions of sugars and water at the beginning of the chemical reaction and compared them with the alcohol and carbon dioxide proportions obtained at the end. For the alcoholic reaction to proceed, he also added yeast paste (or "ferment," as it was called). He concluded that sugars were broken down through two chemical pathways: Two-thirds of the sugars were reduced to form alcohol, and the other third were oxidized to form carbon dioxide (the source of the bubbles observedduring fermentation). Lavoisier predicted (according to his famous conservation-of-mass principle) that if it was possible to combine alcohol and carbon dioxide in the right proportions, the resulting product would be sugar. The experiment provided a clear insight into the basic chemical reactions needed to produce alcohol. However, there was one problem: Where did the yeast fit into the reaction? The chemists hypothesized that the yeast initiated alcoholic fermentation but did not take part in the reaction. They assumed that the yeast remained unchanged throughout the chemical reactions.

Yeast Are Microorganisms

                                                             In 1815 the French chemist Joseph-Louis Gay-Lussac made some interesting observations about yeast. Gay-Lussac was experimenting with a method developed by Nicolas Appert, a confectioner and cooker, for preventing perishable food from rotting. Gay-Lussac was interested in using the method to maintain grape juice wort in an unfermented state for an indefinite time. The method consisted of boiling the wort in a vessel, and then tightly closing the vessel containing the boiling fluid to avoid exposure to air. With this method, the grape juice remained unfermented for long periods as long as the vessel was kept closed. However, if yeast (ferment) was introduced into the wort after the liquid cooled, the wort would begin to ferment. There was now no doubt that yeast were indispensable for alcoholic fermentation. But what role did they play in the process?

                                                       When more powerful microscopes were developed, the nature of yeast came to be better understood. In 1835, Charles Cagniard de la Tour, a French inventor, observed that during alcoholic fermentation yeast multiply by gemmation (budding). His observation confirmed that yeast are one-celled organisms and suggested that they were closely related to the fermentation process. Around the same time, Theodor Schwann, Friedrich Kützing, and Christian Erxleben independently concluded that "the globular, or oval, corpuscles which float so thickly in the yeast [ferment] as to make it muddy" were living organisms (Barnett 1998). The recognition that yeast are living entities and not merely organic residues changed the prevailing idea that fermentation was only a chemical process. This discovery paved the way to understand the role of yeast in fermentation.

Pasteur Demonstrates the Role of Yeast in Fermentation

 

Our modern understanding of the fermentation process comes from the work of the French chemist Louis Pasteur.

                                                                  Pasteur was the first to demonstrate experimentally that fermented beverages result from the action of living yeast transforming glucose into ethanol. Moreover, Pasteur demonstrated that only microorganisms are capable of converting sugars into alcohol from grape juice, and that the process occurs in the absence of oxygen. He concluded that fermentation is a vital process, and he defined it as respiration without air (Barnett 2000; Pasteur 1876).

Pasteur performed careful experiments and demonstrated that the end products of alcoholic fermentation are more numerous and complex than those initially reported by Lavoisier. Along with alcohol and carbon dioxide, there were also significant amounts of glycerin, succinic acid, and amylic alcohol (some of these molecules were optical isomers — a characteristic of many important molecules required for life). These observations suggested that fermentation was an organic process. To confirm his hypothesis, Pasteur reproduced fermentation under experimental conditions, and his results showed that fermentation and yeast multiplication occur in parallel. He realized that fermentation is a consequence of the yeast multiplication, and the yeast have to be alive for alcohol to be produced. Pasteur published his seminal results in a preliminary paper in 1857 and in a final version in 1860, which was titled "Mémoire sur la fermentation alcoolique" (Pasteur 1857).

                                                                 In 1856, a man named Bigo sought Pasteur's help because he was having problems at his distillery, which produced alcohol from sugar beetroot fermentation. The contents of his fermentation containers were embittered, and instead of alcohol he was obtaining a substance similar to sour milk. Pasteur analyzed the chemical contents of the sour substance and found that it contained a substantial amount of lactic acid instead of alcohol. When he compared the sediments from different containers under the microscope, he noticed that large amounts of yeast were visible in samples from the containers in which alcoholic fermentation had occurred. In contrast, in the polluted containers, the ones containing lactic acid, he observed "much smaller cells than the yeast." Pasteur's finding showed that there are two types of fermentation: alcoholic and lactic acid. Alcoholic fermentation occurs by the action of yeast; lactic acid fermentation, by the action of bacteria.