Fermentation: Process and Types

The fermentation It is a metabolic process that some organisms use to obtain energy and nutrients from certain organic compounds. An important characteristic of fermentation is that it is an anaerobic reaction, which means that it occurs in the absence of oxygen.

Many microorganisms use fermentation as a mechanism of energy production in the form of ATP. The energy is obtained through the degradation of organic molecules, such as starch or sugar, through fermentation.

Fermentation: Process and Types

The yeasts carry out the fermentation of sugars and convert them into alcohols, while the bacteria convert certain carbohydrates into lactic acid. Fermentation also occurs in fruits, fungi and mammalian muscles.

This natural process of fermentation has been widely used by modern man to obtain products of interest, such as beer, wine, yogurt and cheeses, among others. The study of fermentation is called cimology.


  • 1 Fermentation process
  • 2 Types of fermentation
    • 2.1 Alcoholic fermentation
    • 2.2 Lactic fermentation
  • 3 Microorganisms involved in food fermentations
    • 3.1 Bacteria
    • 3.2 Yeasts
    • 3.3 Mold
  • 4 References

Fermentation process

Like other metabolic processes of obtaining energy, fermentation begins with glycolysis. This metabolic reaction is based on the degradation of glucose molecules to obtain important energy molecules. During this process the glucose is degraded by oxidation and molecules of NADH and pyruvate are generated.

In aerobic reactions (which use oxygen), NADH and pyruvate participate in a mechanism called oxidative phosphorylation, a process that takes place in the membrane of the mitochondria and is highly efficient in producing energy in the form of ATP molecules.

Conversely, fermentation does not lead to such an efficient production of energy because some molecules, such as NADH, can not release their electrons to become again NAD +, which is the oxidized form of the molecule and which is required to help generate more ATP molecules.

As a result, other metabolic reactions occur that ensure that the NADH molecules donate their electrons to another organic molecule, such as pyruvate from glycolysis. This oxidation of NADH to NAD + allows glycolysis to continue functioning.

Types of fermentation

Alcoholic fermentation

In alcoholic fermentation the NADH molecules donate their electrons to other molecules derived from pyruvate, and thus an alcohol is produced. The alcohol that is produced is specifically ethanol, or ethyl alcohol, and it is a process that occurs in two steps.

In the first step, a carboxyl group is released from pyruvate, which is released in the form of carbon dioxide, thus leaving behind a two-carbon molecule called alketaldehyde.

In the second step, the NADH passes its electrons to the acetaldehyde produced previously, with which the ethanol is produced and the NAD + is regenerated, which is necessary to maintain glycolysis and, consequently, the supply of pyruvate.

The net chemical equation for the production of ethanol from glucose is:

C6H12O6 (glucose) → 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide)

The yeasts carry out the alcoholic fermentation that is used in the production of common alcoholic beverages, such as beer and wine, as well as in the production of breads.

It is important to note that alcohol is toxic in large quantities, both for yeasts and humans, which has established tolerance levels ranging from 5 to 21% approximately.

Lactic fermentation

In the fermentation of lactic acid NADH transfers its electrons directly to pyruvate, thus generating a lactate molecule. The bacteria that produce yogurt do so through lactic fermentation, as well as red blood cells in the human body.

The following equation describes the production of lactic acid from glucose:

C6H12O6 (glucose) → 2 CH3CHOHCOOH (lactic acid)

The production of lactic acid can also occur from lactose and water, as indicated in the following summary equation:

C12H22O11 (lactose) + H2O (water) → 4 CH3CHOHCOOH (lactic acid)

Lactic fermentation can also occur in muscle cells, but only under certain conditions; for example, when physical exercise is very intense and there is little oxygen supply.

The lactic acid produced in the muscles is transported by the bloodstream to the liver, where it is converted back to pyruvate to be reused in other reactions of energy production.

Microorganisms involved in food fermentations

The most common groups of microorganisms involved in food fermentation are the following:


The lactic acid bacteria of the genera Lactobacillus , Pediococcus , Streptococcus Y Oenococcus , are the most important bacteria in fermented foods, followed by species of Acetobacter , which oxidize alcohol in acetic acid.

Fermentation of acetic acid has been widely used to produce fruit vinegars, including cider vinegar. A third group of bacteria important in fermentation are the species of Bacillus subtilis , B. licheniformis Y B. pumilus , which increase the pH of the medium.

Bacillus subtilis is the dominant species in the production of molecules that increase the alkalinity of the medium, such as ammonia. This makes the environment unsuitable for the growth of decomposer organisms, which helps to preserve food.

Alkaline fermentations are more common in protein-rich foods such as soybeans and other legumes, although they have also been carried out with plant seeds. For example, watermelon seeds and sesame seeds.


Like bacteria and molds, yeasts can have beneficial and non-beneficial effects in the fermentation of food. Some of the yeasts like Pichia deteriorate the food, while the Candida It is used for the production of proteins of interest.

The most beneficial yeast in terms of desirable food fermentations is the family Saccharomyces . It's about the S. cerevisiae involved in making bread and alcohol in wine fermentations. The variety carlbergenisis of the family Saccharomyces cerevisiae It is the yeast involved in the production of beer.

The variety ellipsoideus of the family Saccharomyces cerevisiae It is widely used in winemaking. On the other hand, Schizosaccharomyces pombe Y S. boulderi they are the dominant yeasts in the production of traditional fermented beverages, especially those derived from corn and millet.

It has been found that the species Schizosaccharomyces pombe It has the ability to degrade malic acid in ethanol and carbon dioxide, and has been used successfully to reduce acidity in grape and plum musts.


Molds are also important organisms in food processing, both in degradation and in conservation. Many molds have the ability to produce enzymes of commercial importance, such as pectinase from Aspergillus niger .

The species of Aspergillus they are involved in the production of citric acid from the remains of apple pulp. The species of Aspergillus They are often responsible for undesirable changes in foods that cause spoilage.

On the other hand, the species of Penicillium are associated with the development of maturation and taste in cheeses, while the species of Ceratocystis they are involved in the production of the flavor of the fruit. At the same time, Penicillium is the causative agent for the production of toxins such as patulin.


  1. Berg, J., Tymoczko, J., Gatto, G. & Strayer, L. (2015). Biochemistry (8th ed.). W. H. Freeman and Company.
  2. Hogg, S. (2005). Essential Microbiology (1st ed.). Wiley
  3. Ray, R. & Montet, D. (2014). Microorganisms and Fermentation of Traditional Foods (1st ed.). CRC Press.
  4. Simon, E. (2014). Biology: The Core (1st ed.). Pearson.
  5. Solomon, E., Berg, L. & Martin, D. (2004). Biology (7th ed.) Cengage Learning.
  6. Voet, D., Voet, J. & Pratt, C. (2016). Fundamentals of Biochemistry: Life at the Molecular Level (5th ed.). Wiley

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