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Fermentation reaction

The fermentation reaction

C6H12O6 → 2 C2H5OH + 2 CO2
          sugar  -> ethanol + carbonic gas

was established by Gay-Lussac in 1815 [V6]. Pasteur found in 1857 that microorganisms (yeasts) were responsible for the reaction. The alcohol created is ethanol (or ethylic alcohol, figure 2).

Sugars involved are glucose and fructose (figure 3). Both are fermentable (they can be used by yeasts for fermentation) and they share the same formula (C6H12O6).

fructose molecule glucose molecule
Fructose and glucose [B1]

This reaction is interesting for drunkards as it gives alcohol but it is of little interest to the yeast as it brings in 15 times less energy than respiration per molecule of sugar:

C6H12O6 + 6 O2 → 6 H2O + 6 CO2
           sugar + oxygen -> water + carbon. gas
In presence of oxygen (O2), the yeast will choose an aerobic mode (use of oxygen) rather than fermentation (anaerobic - without oxygen).
reactions involved in the fermentation process
Steps of fermentation [B1]

The figure below shows the main steps of fermentation (a complete chart can be found page 189 in V6). The reaction can be written in a more complete way [V6] :

C6H12O6 + 2 ADP + 4 H+ + 2 HPO42- → 2 C2H5OH + 2 CO2 + 2 ATP + 2 H2O

ATP (adenosine triphosphate) is the « battery » of yeast. The transformation of ADP (adenosine diphosphate) into ATP corresponds to « charging its batteries ». The reverse transformation is using the energy of these batteries. In order to initiate fermentation, the yeast must use 2 ATP per molecule of sugar. It will get them back just before the formation of pyruvate. At the end of the reaction, it will get 2 ATP. It is therefore necessary to invest 2 ATP in the first place to double the outlay: if the yeast is too weak when the fermentation starts, it will have a hard time starting the reaction and getting energy. Hence the interest of a good hydration and the use of a starter to avoid demanding that yeasts, already too weak, ferment sugar.

Other reactions

The fermentation reaction is simple and introduces protagonists. But one could argue that it is not the most important reaction: the purpose is not to produce alcohol, the purpose is to produce a drink. Mixing honey and water does not give mead. Mead also contains other products of the fermentation which are quantitatively small compared to alcohol but which are an integral part of the taste of mead. Yeast does not only produce ethanol and CO2 (carbonic gas), it also makes lots of components capital to the taste, flavor and aspect of mead. Different yeasts will produce different organoleptic compounds or in different proportions and this will result in different meads. Quantities involved are small but human senses can be very sensitive to some of these molecules. H2S (skunk odor) for instance -- which can be found in wine when grapes have been sulfated short before the harvest -- can been detected at levels as low as a few p. p. b. (parts per billion), that is a few molecules of H2S among a billion molecules of air are enough to waste a wine.


May 29th 2002