FERMENTATION: HISTORY, PROCESS, TYPES, EXAMPLES - BIOLOGY - 2024

The fermentation is a chemical process by which one or more organic compounds are degraded to simpler compounds in the absence of oxygen (anaerobically). It is carried out by many types of cells to produce energy in the form of ATP.

Today, organisms capable of “fermenting” molecules in the absence of oxygen are very important at an industrial level, as they are exploited for the production of ethanol, lactic acid and other commercially relevant products that are used to make wine, beer, cheese and yogurt., etc.

Bread and beer, two products of the alcoholic fermentation of yeasts (Image by PublicDomainImages at www.pixabay.com)

The word fermentation derives from the Latin word fervere, which means "to boil" and was coined alluding to the bubbling observed in the first fermented beverages, very similar in appearance to the boiling of a hot liquid.

Today, as Gay-Lussac suggested in 1810, it is the general term used to refer to the anaerobic breakdown of glucose or other organic nutrients in order to produce energy in the form of ATP.

Since the first living things to emerge on earth probably lived in an atmosphere without oxygen, the anaerobic breakdown of glucose is probably the oldest metabolic way among living things to obtain energy from organic molecules.

History of fermentation

Human knowledge of the phenomenon of fermentation is as old, perhaps, as is agriculture, since for thousands of years man has promoted the conversion of crushed sweet grape juice into effervescent wine or the conversion of wheat doughs into bread..

However, for the first societies, the transformation of these "basic" elements into fermented foods was considered a kind of "mystery" or "miraculous" event, since it was not known what caused it.

The progress of scientific thought and the invention of the first microscopes undoubtedly set an important precedent in the field of microbiology and, with it, allowed the solution of the fermentative “mystery”.

Lavoisier and Gay-Lussac experiments

Graphic portrait of Antoine Lavoisier (Source: H. Rousseau (graphic designer), E.Thomas (engraver) Augustin Challamel, Desire Lacroix Via Wikimedia Commons)

Lavoisier, a French scientist, in the late 1700s showed that in the process of transforming sugars into alcohol and carbon dioxide (as happens during wine production), the weight of the substrates consumed was equal to that of the products. synthesized.

Later, in 1810, Gay-Lussac summarized these claims in the following chemical reaction:

C6H12O6 (glucose) → 2CO2 (carbon dioxide) + 2C2H6O (ethanol)

However, for many years it was argued that these chemical changes observed during fermentation were the product of molecular vibrations emitted by decomposing matter, that is, by dead cells.

In simpler words: all the researchers were convinced that fermentation was a secondary effect of the death of some organism and not a necessary process for a living being.

Yeasts in action

Louis Pasteur in his laboratory. Via Wikimedia Commons

Later, Louis Pasteur, in 1857, marked the birth of microbiological chemistry when he associated fermentation with microorganisms such as yeasts, from which the term was related to the idea of ​​the existence of living cells, with the production of gases and some organic compounds.

Later, in 1920 it was discovered that in the absence of oxygen, some mammalian muscle extracts catalyzed the formation of lactate from glucose, and that many of the compounds produced during grain fermentation were also produced by muscle cells.

Thanks to this discovery, fermentation was generalized as a way of using glucose and not as an exclusive process of yeast and bacteria.

Many later studies considerably refined the knowledge related to the phenomenon of fermentation, since the metabolic routes and the enzymes involved were elucidated, which allowed their exploitation for different industrial purposes.

General fermentation process

As we have said, fermentation is a chemical process that involves the anaerobic transformation (without oxygen) of an organic substrate into simpler organic compounds, which cannot be metabolized “downstream” by enzymatic systems without the intervention of oxygen.

It is carried out by different enzymes and is normally observed in microorganisms such as molds, yeasts or bacteria, which produce a series of secondary products that man has used for commercial purposes for many centuries.

In the chemical reactions that take place during fermentation, enzymes (proteins capable of accelerating different chemical reactions) hydrolyze their substrates and decompose or “digest” them, yielding simpler molecules and more assimilable nutrients, metabolically speaking.

It is worth mentioning that fermentation is not an exclusive process of microorganisms, as it can occur in some animal cells (such as muscle cells, for example) and in some plant cells under certain conditions.

What substrates are fermentable?

At the beginning of scientific research related to fermentation, it was thought that the essential molecules for this process were carbohydrates.

However, it was soon understood that many organic acids (including amino acids), proteins, fats and other compounds are fermentable substrates for different types of microorganisms, since they can function as a source of food and energy for them.

It is important to clarify that anaerobic metabolism does not yield the same amount of energy as aerobic metabolism, since the substrates, in general, cannot be completely oxidized, so not all the possible energy is extracted from them.

Consequently, anaerobic microorganisms tend to consume much larger amounts of substrates in order to extract the same energy that a similar microorganism would extract under aerobic conditions (in the presence of oxygen).

What is fermentation about?

When respiration cannot occur, either due to the absence of an external electron acceptor or due to some defect in the cellular respiratory chain, fermentation is the catabolic pathway used to produce energy from glucose or other carbon sources.

In the case of glucose, for example, its partial oxidation is carried out through the glycolytic pathway, through which pyruvate, ATP and NADH are produced (these products vary according to the energy substrate).

Under aerobic conditions, pyruvate is further oxidized when it enters the Krebs cycle and the products of this cycle enter the electron transport chain. NAD + is also regenerated during these processes, which allows the continuity of the glycolytic pathway to be maintained.

When there is no oxygen, that is, in anaerobiosis, the pyruvate derived from oxidative reactions (or the other resulting organic compounds) undergoes a reduction. This reduction allows the regeneration of NAD +, a fundamental event for the fermentation process.

The reduction of pyruvate (or other oxidative product) marks the beginning of the synthesis of waste products, which can be alcohols, gases or organic acids, which are excreted into the extracellular environment.

How much energy is produced?

While the complete oxidation of one mole of glucose to carbon dioxide (CO2) and water under aerobic conditions generates 38 moles of ATP, fermentation produces between 1 and 3 moles of ATP for every mole of glucose consumed.

Types of fermentation

There are different types of fermentation, many times defined not only by the end products of the process, but also by the energetic substrates that are used as “fuel”. Many of these will be defined particularly in the industrial context.

As a note to the reader, it is probably a good idea to review some aspects of energy metabolism first, especially in relation to carbohydrate catabolism (glycolysis), the Krebs cycle, and the electron transport chain (respiration), in order to understand this topic with greater depth.

5 types of fermentation can be mentioned:

- Alcoholic fermentation

- Lactic or lactic acid fermentation

- Propionic fermentation

- Butyric fermentation

- Mixed acid fermentation

Alcoholic fermentation

When referring to this type of fermentation, it is usually understood that it has to do with the production of ethanol (CH3CH2OH or C2H6O), which is a type of alcohol (of which alcoholic beverages such as wine and beer, for example).

Industrially speaking, the main microorganism exploited by man to obtain alcoholic beverages is the yeast-like fungus belonging to the species Saccharomyces cerevisiae.

Alcoholic fermentation (Source: The author of the original version is User: Norro. / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons)

Yeasts are actually aerobic organisms that can grow as facultative anaerobes, that is, if conditions warrant, they change their metabolism and adapt to the absence of oxygen to live.

As we discussed in the previous section, energy performance in anaerobic conditions is much lower than in aerobic conditions, so growth is slower.

Alcoholic fermentation involves the conversion of pyruvate to ethanol, which takes place in a two-step process: first the transformation of pyruvate to acetaldehyde and then from acetaldehyde to ethanol.

The first reaction, the pyruvate to acetaldehyde conversion reaction, is a decarboxylation where one molecule of CO2 is released for each molecule of pyruvate and is catalyzed by the enzyme pyruvate decarboxylase, which needs a cofactor known as thiamine pyrophosphate or TPP.

The acetaldehyde thus produced is reduced to ethanol by means of the alcohol dehydrogenase enzyme, which uses one molecule of NADH2 as a cofactor for each molecule of acetaldehyde, releasing ethanol and NAD +.

NAD + can be reused for the reduction of glyceraldehyde 3-phosphate in one of the steps of the glycolytic pathway, allowing the synthesis of ATP to continue.

At an industrial level, different strains of S. cerevisiae are exploited for different purposes, as some have been “specialized” for the production of wine, beer, bread, etc., which is why they may present some distinctive metabolic differences.

Lactic or lactic acid fermentation

This type of fermentation can be subdivided into two: homofermentative and heterofermentative. The first has to do with the production of lactic acid as the only fermentative product of the reduction of glycolytic pyruvate and the second involves the production of lactic acid and ethanol.

- Homolactic fermentation

The pyruvate produced by the glycolytic route is converted directly into lactic acid thanks to the enzymatic action of a lactic acid dehydrogenase. In this reaction, as in the second reaction of alcoholic fermentation, a molecule of NAD + is regenerated to oxidize glyceraldehyde 3-phosphate in glycolysis.

For every molecule of glucose that is consumed, then, two molecules of pyruvate are produced, so the result of lactic fermentation corresponds to two molecules of lactic acid per molecule of glucose (and two molecules of NAD +).

This type of fermentation is very common in certain types of bacteria called lactic acid bacteria and is the simplest type of fermentation that exists.

Lactic acid can also be produced by some muscle cells, since pyruvate, through the action of lactate dehydrogenase (which uses NADH2), is converted into lactic acid.

- Heterolactic fermentation

In this type of fermentation, the two pyruvate molecules derived from glycolysis are not used to synthesize lactic acid. Instead, for each molecule of glucose, one pyruvate turns into lactic acid and the other turns into ethanol or acetic acid and CO2.

Bacteria that metabolize glucose in this way are known as heterofermentative lactic acid bacteria.

They do not produce pyruvate throughout the glycolytic pathway, but instead use a part of the pentose phosphate pathway to produce glyceraldehyde 3-phosphate, which is then metabolized to pyruvate by glycolytic enzymes.

Briefly, these bacteria "cut" xylulose 5-phosphate (synthesized from glucose) into glyceraldehyde 3-phosphate and acetyl phosphate using a TPP-linked pentose phosphate ketolase enzyme, producing glyceraldehyde 3-phosphate (GAP) and acetyl phosphate.

GAP enters the glycolytic pathway and is converted into pyruvate, which is then transformed into lactic acid thanks to an enzyme lactate dehydrogenase, while acetyl phosphate can be reduced to acetic acid or ethanol.

Lactic acid bacteria are very important for man, as they are used to produce different fermented milk derivatives, among which yogurt stands out.

They are also responsible for other fermented foods such as fermented cabbage or "sauerkraut", pickles and fermented olives.

- Propionic fermentation

This is carried out by propionibacteria, capable of producing propionic acid (CH3-CH2-COOH) and that inhabit the rumen of herbivorous animals.

It is a type of fermentation in which bacteria use glucose glycolytically to produce pyruvate. This pyruvate is carboxylated to oxaloacetate, which is then reduced in two steps to succinate, using the reverse reactions of the Krebs cycle.

The succinate is then converted to succinyl-CoA and this, in turn, to methyl malonyl-CoA by the enzyme methyl malonyl mutase, which catalyzes an intramolecular rearrangement of succinyl-CoA. The methyl malonyl-CoA is then decarboxylated to yield propionyl-CoA.

This propionyl-CoA yields propionic acid through a CoA-succinate transfer reaction, catalyzed by a CoA-transferase. Lactic acid bacteria and propionibacteria are used to produce Swiss cheese, as propionic acid gives it a special flavor.

- Butyric fermentation

Butyric fermentation. Source: Bellwasthow / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

It is carried out by spore-forming bacteria that are obligate anaerobes and generally belong to the genus Clostridium. Depending on the species, these bacteria can also produce butanol, acetic acid, ethanol, isopropanol, and acetone (carbon dioxide is always a product).

These bacteria break down glucose through the glycolytic pathway and produce pyruvate, which is decarboxylated to form acetyl-CoA.

In some bacteria, two acetyl-CoA molecules are condensed by a thiolase enzyme, producing acetoacetyl-CoA and releasing a CoA. Acetoacetyl-CoA is dehydrogenated by the enzyme β-hydroxybutyryl-CoA dehydrogenase to form P-hydroxybutyryl-CoA.

This last product gives rise to Crotonil-CoA through the action of the enzyme crotonase. Crotonyl-CoA is reduced again by a butyryl-CoA dehydrogenase associated with FADH2, producing butyryl-CoA.

Finally, butyryl-CoA is converted to butyric acid by removing the CoA portion and adding a water molecule. Under alkaline (high pH) conditions, some bacteria can convert butyric acid to n-butanol

- Mixed acid fermentation

It is common in bacteria known as Enterobacteriaceae, which can grow with or without oxygen. It is called "mixed acid" because different types of organic acids and neutral compounds are produced as a result of fermentation.

Summary scheme of mixed acid fermentation (Source: The original uploader was NicolasGrandjean at French Wikipedia. / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/) via Wikimedia Commons)

Depending on the species, formic acid, acetic acid, succinic acid, lactic acid, ethanol, CO2, butanediol, etc. can be produced.

It is also often known as formic acid fermentation, since under anaerobic conditions, some bacteria can form formic acid and acetyl-CoA from pyruvate by the action of the enzyme formic acid-pyruvate lyase.

Examples of processes in which there is fermentation

There are many examples of fermentation processes and their products. Some of these examples could include:

Yogurt, a fermentation product (Image by Imo Flow at www.pixabay.com)

- Salami (fermented meat), produced by lactic fermentation of lactic acid bacteria

- Yogurt (fermented milk), also produced by lactic acid bacteria

- Cheese (fermented milk), produced by lactic acid bacteria and propionibacteria through lactic and propionic fermentation

Cheese, product of the fermentation of lactic acid bacteria and propionibacteria (Image by lipefontes0 at www.pixabay.com)

- Bread (fermentation of gluten from wheat dough), produced by yeasts through alcoholic fermentation

- Wine and beer (fermentation of sugars in grape juice and sugars in grains), produced by yeasts through alcoholic fermentation

- Coffee and cocoa (fermentation of the sugars present in the mucilage of the fruit), produced by lactic acid bacteria and yeasts by lactic and alcoholic fermentation.

References

1. Ciani, M., Comitini, F., & Mannazzu, I. (2013). Fermentation.
2. Junker, B. (2000). Fermentation. Kirk-Othmer Encyclopedia of Chemical Technology.
3. Fruton, J. (2006). Fermentation: vital or chemical process ?. Brill.
4. Doelle, HW (1975). Fermentation. Bacterial metabolism, 559-692.
5. Nelson, DL, Lehninger, AL, & Cox, MM (2008). Lehninger principles of biochemistry. Macmillan.
6. Barnett, JA (2003). Beginnings of microbiology and biochemistry: the contribution of yeast research. Microbiology, 149 (3), 557-567.