Fermentation in Biology: Types, Reactions & Examples

Fermentation is considered to be an incomplete oxidation procedure of glucose, that reaction occurs under anaerobic conditions. The breakdown of glucose is the outcome of fermentation process. Many of the food products that we consume on regular basis like idli, bread, cheese, dosa, etc. are all fermented food. Fermentation is an old traditional process that was strongly followed by our ancestors as well. This article will discuss all the aspects associated with fermentation. ‌Follow this article to ‌find‌ ‌out‌ ‌more‌ ‌interesting‌ ‌facts‌ ‌about‌ fermentation.

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Fermentation: Definition

Fermentation is defined as an anaerobic breakdown of carbohydrates and other organic compounds to form alcohol and organic acids with the help of microorganisms or their enzymes.

1. The branch of an applied science that deals with study of fermentation process and its application is referred to as zymology.
2. The chambers in which fermentation is carried out are called fermenters or bio fermenters or bioreactors.

Fermentation: History

In the ancient times applied fermentation was used to make different products like wine, cheese, etc. Fermentation was used long before its biochemical reactions were understood by the human race. In the \(1850\rm{s}\) and \(1860\rm{s}\), the scientist Louis Pasteur became the first person to discover the process of fermentation and is now known as the “father of fermentation”.

Fermentation: Process

The cellular respiration is of two types, i.e. aerobic respiration and anaerobic respiration.

1. Aerobic respiration: It is a process when glucose is broken down to carbon dioxide in the presence of oxygen to produce energy in the form of ATP.
2. Anaerobic respiration: It is a process when glucose is broken down in the absence of oxygen. It is also called fermentation.

Fermentation is an anaerobic biochemical process that takes place in the absence of oxygen. In fermentation, the first process is the same as the cellular respiration process, which is the formation of pyruvic acid by glycolysis where \(2\) ATP molecules are synthesized. In the second step, pyruvate is reduced to lactic acid, ethanol, or other products. In this step, \({\bf{NA}}{{\bf{D}}^ + }\) is regenerated to be utilized in the glycolysis process.

Fermentation: Diagrammatic Representation

The fate of glucose under an anaerobic environment or during the fermentation process in various microorganisms is as shown in the figure below: 

Fermentation: Types based on Respiration

The first step of fermentation is glycolysis.

Glycolysis

1. Origin of word: The word glycolysis is derived by combination of two Greek words,  Glykos meaning sugar and lysis meaning breakdown or dissolution.
2. Definition: Glycolysis is a process in which glucose is broken down and converted into pyruvic acid in presence of certain enzymes.
3. This pathway is also known as EMP Pathway, as it was discovered by three German scientists, Embden, Meyerhof, and Parnas.
4. Location of Glycolysis: Cytoplasm of the cell
5. Type of pathway: It is an anaerobic oxidative process because it occurs in the absence of oxygen, and there is a loss of hydrogen.
6. Overall equation: \({\bf{Glucose}} + {\bf{2}}\,{\bf{ADP}} + {\bf{2}}{\rm{ }}{\bf{Pi}} + {\bf{2}}\,{\bf{NA}}{{\bf{D}}^ + } \to {\bf{2}}{\rm{ }}{\bf{Pyruvate}} + {\bf{2}}\,{\bf{ATP}} + {\bf{2}}\,{\bf{NADH}} + {\bf{2}}\,{{\bf{H}}^ + }\)
7. It is a common pathway for both aerobic respiration and anaerobic respiration.

After glycolysis, in absence of molecular oxygen, further breakdown of pyruvate takes place to regenerate \({\rm{NA}}{{\rm{D}}^ + }.\) There are two major types of anaerobic fermentation: Alcoholic fermentation and lactic acid fermentation. The fermentation process restores \({\rm{NA}}{{\rm{D}}^ + }\) concentration to allow a cell to continue generating ATP through glycolysis.

Alcoholic Fermentation

The alcoholic fermentation process is involved in brewing industries for producing beverages such as whisky, rum, beer, etc.

The organism involved in alcoholic fermentation: Brewer yeast, i.e. Saccharomyces cerevisiae.

Steps of Alcoholic Fermentation

1. Decarboxylation of pyruvic acid in the presence of an enzyme pyruvate decarboxylase, coenzyme thiamine pyrophosphate (TPP) and \({\rm{Z}}{{\rm{n}}^{2 + }}\) as a cofactor. It produces acetaldehyde.
2. Acetaldehyde accepts 2H-atoms from \({\bf{NADH}} + {{\bf{H}}^ + }\) in the presence of enzyme ethanol dehydrogenase and forms ethanol.
3. Thus, during alcoholic fermentation, one glucose molecule produces molecules of ethyl alcohol.
4. Yeast poisoning: Accumulation of alcohol formed by fermentation in a culture of yeast stops the multiplication of yeast cells and may even lead to the death of cells. This is called yeast poisoning and happens at \(13\%\) of alcoholic concentration.

Fermentation Equation

\({{\bf{C}}_{\bf{6}}}{{\bf{H}}_{{\bf{12}}}}{{\bf{O}}_{\bf{6}}} + {\bf{2}}\,{\bf{ADP}} + {\bf{2}}\,{\bf{Pi}} \to {\bf{2}}\,{{\bf{C}}_{\bf{2}}}{{\bf{H}}_{\bf{5}}}{\bf{OH}} + {\bf{2}}\,{\bf{C}}{{\bf{O}}_{\bf{2}}} + {\rm{ }}{\bf{2}}\,{\bf{ATP}}\)

Lactic Acid Fermentation

1. It is used in the milk industry and even occurs in the white muscle fibres of the vertebrates or even during strenuous muscle activity.
2. In the milk industry, it occurs in the presence of lactic acid bacteria, Lactobacillus acidophilus.
3. In this type of fermentation, pyruvic acid acts as an H-acceptor and receives two H-atoms from \({\rm{NADH}} + {{\rm{H}}^ + }\) and changes into lactic acid.
4. It occurs in the presence of \({\rm{FMN}}\) as a coenzyme and \({\rm{Z}}{{\rm{n}}^{2 + }}\) as a cofactor.
5. So, one glucose molecule produces two molecules of lactic acid.
6. Lactic acid is excreted out so a large amount of energy of glucose is lost in the lactic acid. In this, carbon dioxide is not produced.

Fermentation Equation

\({{\bf{C}}_{\bf{6}}}{{\bf{H}}_{{\bf{12}}}}{{\bf{O}}_{\bf{6}}} + {\bf{2}}\,{\bf{ADP}} + {\bf{2}}\,{\bf{Pi}}{\rm{ }} \to {\bf{2}}\,{{\bf{C}}_{\bf{3}}}{{\bf{H}}_{\bf{6}}}{{\bf{O}}_{\bf{3}}} + {\bf{2}}\,{\bf{ATP}} + {\bf{2}}\,{{\bf{H}}_{\bf{2}}}{\bf{O}}\)

Lactic Acid Fermentation: Types

1. Homolactic fermentation: The fermentation in which only the lactic acid is produced is called homo-lactic fermentation. No byproducts are formed during the reaction.
2. Heterolactic fermentation: The fermentation in which the lactic acid is produced along with some by-products like gases.

Did You Know: Muscle cramps arise due to the accumulation of lactic acid.

Types of Fermentation Based on Supply of Nutrient Medium

Based on culture media, fermentation is divided into three types, i.e., batch, fed-batch, and continuous fermentation.

Batch Fermentation

1. A batch fermentation is a closed culture type of system because the initial and limited amount of sterilized nutrient medium is introduced into the fermenter.
2. The medium used in this process is inoculated with a suitable microorganism and incubated for a definite period for fermentation to proceed under optimal physiological conditions.
3. The fermentation is run for a definite period or until the nutrients are exhausted.
4. The fermentation process obtains a characteristic Sigmoid or S-shaped curve.

Fed-Batch Fermentation

1. A fed-batch is a modification of batch fermentation.
2. In this fermentation process, the substrate is added periodically in instalments as the fermentation progresses, due to which the substrate is always at an optimal concentration.
3. This is essential as some secondary metabolites are subjected to catabolite repression by a high concentration of either glucose or other carbohydrate or nitrogen compounds present in the medium.
4. It reduces the poisoning of fermenting microbes and also productivity is higher than batch fermentation.

Continuous Fermentation

1. Continuous fermentation is a closed system of fermentation, run for an indefinite period.
2. In this fermentative method, a fresh nutrient medium is added continuously or intermittently to the fermenter and an equivalent amount of used medium with microorganisms is withdrawn continuously or intermittently for the recovery of cells or fermentation products.
3. The productivity is maximum in such a fermentation.
4. The fermentation process follows a characteristic exponential or J-shaped curve.

Significance of Fermentation

Some of the functions of fermentation are given below:

1. Net gain of \(2\) \({\rm{ATP}}\) molecules from one molecule of glucose.
2. The main function of fermentation is to convert \({\rm{NADH}}\) back into the coenzyme \({\rm{NA}}{{\rm{D}}^ + }\) so that it can be used again for glycolysis.
3. Fermentation is important to produce various food products like curd, cheese, yoghurt, dosa, idli, etc.
4. Anaerobic respiration occurs in muscles to generate energy during strenuous activities.
5. RBCs also undergo anaerobic respiration to obtain energy.
6. Fermentation is used for preservation in a process that produces lactic acid as found in such sour foods as pickled cucumbers, kimchi, etc.
7. This process is also used for producing alcoholic beverages such as wine and beer.

Summary

Fermentation is the chemical transformation of organic substances into simpler compounds in absence of oxygen by the action of enzymes, complex organic catalyst, which are produced by a microorganism such as yeast, bacteria, etc., Through this article, we understood how fermentation is being used from the olden days to this new modern era. Fermentation is the process used in various industries and has wide applications towards obtaining varied products.

Frequently Asked Questions (FAQs) on Fermentation

Frequently asked questions related to fermentation are listed as follows:

Q.1. What are the 3 types of fermentation?
Ans: The 3 types of fermentation are lactic acid fermentation, ethanol fermentation, and acetic acid fermentation. 

Q.2. What exactly is fermentation?
Ans: Fermentation is defined as a metabolic process that forms chemical changes in organic substrates through the action of different enzymes in absence of oxygen.

Q.3. Give an example of cells or tissue in humans undergoing anaerobic respiration.
Ans: RBCs, white muscle fibres, muscles under strenuous exercise, etc. are few examples of cells or tissues undergoing anaerobic respiration in humans.

Q.4. What are the microorganisms that are used in the fermentation process?
Ans: The common microorganisms that are used in the fermentation process are lactic acid bacteria such as Lactobacillus, Lactococcus. Different species of Yeast like Saccharomyces cerevisiae, Monascus purpureus, etc. are also used. 

Q.5. What is the use or significance of fermentation?
Ans: The fermentation process is used in breweries to produce wines, beers, etc. and it is used in preparing bread, pizza dough, idli, etc.

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