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7.2.2.3.1. Alcoholic or ethyl fermentation

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 ethanolThe 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.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Aragon.  June 2018, option A, question 5 .

Biotechnology: (2 points)

a) What do the making of beer and bread have in common? (0.5 points)

b) What is and where does the starting molecule come from? What is and where does the molecule resulting from the basic reaction of these industrial processes go? (1.25 points)

c) Which organism is responsible for this reaction? (0.25 points)

Aragon. June 2009, option B. 5. (2 points).

Read this text carefully and answer the questions indicated at the end.

Beer is an industrial product made from barley. Barley seeds are moistened to germinate, so that when germination begins, their own hydrolytic enzymes break the reserve polysaccharides, ( mainly starch ), into monosaccharides.

After stopping this process by heat, the malt is obtained. The malt obtained is subjected to the action of yeasts, which initially multiply using the sugars in the medium as a source of carbon and energy. When the oxygen present is consumed, the yeasts start the process that leads to the production of beer.

a) What monosaccharide will originate as a result of the hydrolysis of starch ? What types of bonds should the hydrolytic enzymes discussed above break? (0.5 points).

b) What will be the end products of the use of sugars by yeasts, once the oxygen has been consumed? (0.5 points).

c) Compare the efficiency from the energy point of view, of the stage in which the yeasts have oxygen available and the stage in which it has already been consumed. (1 point).

Basque Country, July 2018, option A, question 5.

Bioethanol obtained by fermentation of carbohydrates promises to be the current substitute for gasoline.

a) What types of organisms are capable of producing ethanol from glucose and what is the metabolic advantage of this production? Give reasons for your answer. (0.5 points)

b) Using a diagram, detail the biochemical process to obtain ethanol from glucose. (1 point)

c) What effect would the presence of oxygen have on the process? (0.5 points)

Basque Country, July 2018, option B, question 3.

Applications of microorganisms in Biotechnology:

a) Indicates what the obtaining of bread and wine have in common, at the biochemical process level. Give reasons for your answer. (0.5 points)

b) What biomolecule is used in both processes as starting material and what product is it converted into? What effect would the presence of air have on these processes? Reason for your answers. (1 point)

c) Indicates what type of microorganism is responsible for the biochemical transformations necessary to obtain bread and beer. Is it prokaryotic or eukaryotic ? Give reasons for your answer. (0.5 points)

Basque Country, July 2019, option B, question 3

In relation to microorganisms and their applications:

a) (0.5 points). Briefly state what types of microorganisms are used to make bread, beer, and wine.

b) (1 point). It indicates what type of biochemical reactions convert sugars and starches into ethanol. Where does the CO 2 produced in these reactions come from?

c) (0.5 points). Are they aerobic or anaerobic reactions? Reason for your answers.

Valencian Community, July 2021, question 7.3.

Regarding the industrial production of beer, he explains:

a) Its relationship with fermentation;

b) The microorganisms involved;

c) The substrate on which they act, under what conditions and the final products (3 points)

Fundamental ideas about alcohol fermentation

Alcoholic or ethyl fermentation

  • The alcoholic fermentation or ethylic is anaerobic degradation of glucose to ethanol, originating CO2 as a byproduct.
  • The alcoholic fermentation is a catabolic process of  fermentation in the absence of oxygen, consisting of the anaerobic degradation of glucose to ethanol, originating CO2 as a byproduct.
  • It is oxidized is glucose  (from the hydrolysis of starch in bread or barley in the case of beer) that has entered glycolysis,  where pyruvate, NADH and  ATP have been formed .  The pyruvate , after  decarboxylation  (e shows CO2) and reduced (is the reoxidized  NADH to NAD) is converted into ethanol. 
  • It is made by some yeasts (Fungi kingdom), such as Saccharomyces cerevisiae (beer yeasts) that intervenes in the production of wine, beer, and in the manufacture of bread.
  • Energy yield: 2 ATP.
  • It is carried out in the cytosol