GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

14. TYPES OF FERMENTATION

14.2. ALCOHOLIC FERMENTATION

14.2.1. Ethanol Production by Yeasts

The Catabolism of glucose during its Fermentation to ethanol and СО₂ proceeds via The Glycolytic Pathway. Glucose is oxidized to Pyruvate. The conversion of pyruvate to ethanol occurs in two steps (Fig. 14.2):

1) pyruvate is decarboxylated by pyruvate decarboxylase to acetaldehyde;

2) acetaldehyde is reduced to ethanol by Alcohol dehydrogenase with the participation of NADH. This reaction transfers the hydrogen generated during the dehydrogenation of triose phosphate. Thus, the redox balance is maintained.

Fermentation types discovered by C. Neuberg. If bisulfite (which is non-toxic to Yeasts) is added to glucose-fermenting yeasts, a new product—glycerol—appears, while the yields of ethanol and СО₂ decrease simultaneously. Acetaldehyde binds with bisulfite to form acetaldehyde-bisulfite addition compound and therefore cannot serve as a hydrogen acceptor; thus, ethanol is not formed. Instead of acetaldehyde, dihydroxyacetone phosphate acts as this hydrogen acceptor. It is reduced to glycerol-3-phosphate and subsequently dephosphorylated to glycerol. Fermentation in the presence of bisulfite is used industrially to produce glycerol. This is Neuberg's second form of fermentation.

If sodium bicarbonate or disodium hydrogen phosphate is added to the solution during fermentation, glycerol is also produced. This occurs because acetaldehyde is converted into ethanol and acetic acid via a dismutation reaction, making it unavailable to serve as a hydrogen acceptor. This is Neuberg's third form of fermentation. The first form of fermentation is normal Yeast fermentation.

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Fig. 14.2. Alcoholic Fermentation

The Pasteur Effect. The fermentation of glucose by yeasts is an anaerobic process, even though yeasts are aerobic microorganisms.

Under anaerobic conditions, fermentation proceeds very intensively, but Cell growth is weak. Upon aeration, fermentation slows down, Respiration begins, and active growth takes place. In some yeasts, fermentation can be almost completely suppressed by vigorous aeration (the Pasteur effect). L. Pasteur discovered this effect over a century ago. It is characteristic not only of yeasts, but also of all facultatively anaerobic Cells, including higher animal tissue cells.