Alcoholic or ethyl fermentation
The alcoholic fermentation is a catabolic process of fermentation in the absence of oxygen, by the activity of some microorganisms which process the glucose to obtain as final products: one alcohol, as ethanol (whose chemical formula is CH3-CH2-OH), carbon dioxide (CO2) in the form of gas and adenosine triphosphate (ATP) molecules that are consumed by the microorganisms themselves in their anaerobic cellular energy metabolism. the ethanol. The resulting product is used in the production of some alcoholic beverages, such as wine, beer, cider, cava, etc.
Some yeasts (Fungi kingdom), such as Saccharomyces cerevisiae (beer yeasts) are capable of transforming pyruvic acid into ethanol and carbon dioxide, oxidizing NADH.
All fermentations take place in the cytosol. In alcoholic fermentation, the substrate that is oxidized is glucose that has entered glycolysis, where pyruvate, NADH and ATP have been formed. The pyruvate, after decarboxylation (thus follows CO2) and reduced (in the reoxidized NADH to NAD) is converted into ethanol.
The production of ethanol is carried out thanks to the enzyme pyruvate decarboxylase, in plant cells, fungi and bacteria.
The alcoholic fermentation or ethylic is anaerobic degradation of glucose to ethanol, originating CO 2 as a byproduct.
The glucose is oxidized to pyruvic acid, generating NADH. Later two processes happen:
- Pyruvic acid decarboxylation, which gives rise to acetaldehyde and CO2 is released.
- Reduction of acetaldehyde to ethanol, thanks to the alcohol-dehydrogenase enzyme, and by consuming the NADH produced in glycolysis.
The energy yield is 2 ATP, the same as in lactic fermentation.
Alcoholic fermentation is carried out mainly by yeasts, highlighting Saccharomyces cerevisiae, which is involved in the production of wine, beer, and in the manufacture of bread. They use glucose, which comes from the hydrolysis of starch in bread and barley in the case of beer.
Logically the bread does not contain alcohol, since the ethanol evaporates with cooking and the CO2 escapes making the bread fluffy.
In a winery, in which the grape must is fermenting, care must be taken with the CO2 that is produced, since if it is a closed space without ventilation, CO2 can displace oxygen and cause suffocation.
Yeasts are facultative aerobic organisms in which the Pasteur effect occurs. If there is O2, fermentation is inhibited and the pyruvic is fully degraded to CO2 and water, increasing the energy obtained to 38 ATP per mole, compared to the 2 ATP obtained in fermentation.