Fermentation: Definition, Process, and Examples

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Fermentation Definition

Fermentation is a Latin word which means causing fermentation. Fermentation is a type of metabolic process, in which the organism are responsible for breaking the sugar into alcohol or acid. These processes require chemical energy which is obtained from ATP, to carry out the reaction.

What is Fermentation?

Fermentation can take place in both aerobic in presence of oxygen and anaerobic condition in the absence of oxygen. Degradation of molecules such as sugar produces energy through aerobic and anaerobic respiration. The steps of aerobic respiration are starting from glycolysis where the 6-carbon sugar molecule is broken into a 3-carbon molecule called the pyruvate.

Then acetyl coenzyme A is formed from pyruvate. Further as the acetyl coenzyme A gets broken down to form CO2 from the citric acid cycle. To the NADH and FADH2 the carbon and the hydrogen atoms get transferred. From these two carrier the energy then reaches the electron transport chain from which the energy is obtain for ATP synthesis.

Oxygen is the final electron acceptor which is not required in anaerobic fermentation and gets replaced with sulphate and nitrate molecule. Although people consider fermentation and anaerobic respiration similar but electron transport chain step is absent in fermentation.

The last step is conversion of pyruvate to acetaldehyde. Depending on the fermentation, the byproducts are formed. Lactic acid is formed in lactic acid fermentation. In alcohol fermentation, along with alcohol, CO2 is also formed.

In prokaryotes and as well as in eukaryotes, fermentation takes place. When the amount of oxygen is very minimal, fermentation is used to obtain energy. For example, when we perform exercise ATP is generated from muscle cell to provide energy through aerobic respiration. However, when ATP supply competes with the oxygen supply, muscle cell carries out lactic acid fermentation to obtain energy in limited oxygen content. Once the level is restored, aerobic respiration again begins.

To synthesize ATP, fermentation is carried out by obligate anaerobes. An example is Neocallimastix. However, there are organism that will undergo fermentation even when the oxygen is present are called as facultative anaerobes. Example are yeast, Kluyveromyces lactis and baker’s yeast are few examples.

In commercial industries, to produce wine and other alcohol, yeast is used to carry out fermentation, whereas in dairy industry bacteria and fungi are used.

Importance of Fermentation

Due to the breakdown of sugar, chemical energy is produced, that is an advantage to the anerobic organism in the anerobic environment such as hydrothermal vent, soil and mud. These type of organism are very vital as they obtain energy by fermenting molecule and excrete the by-product directly into the environment, which can be used other organism or enter the nutrient cycle.

There are various organism residing in other living organism such as various microbiota in the human gut. Within cattle as well, organism are present that cannot breakdown their food, thus reside within the cattle to obtain it and in return have enzymes which aid in digesting starch and cellulose. Similarly, even humans have such organism present, these flora get shelter and food and in return they synthesize vitamins required.

Although these microflora reside within human and other living organism. They can also be opportunistic if found at the wrong position within the humans and animals.

When the oxygen content is limited and we are performing strenuous activities then body has to meet up the need and thus, has lactic acid fermentation to generate ATP. Through aerobic respiration, using one glucose molecule 38 ATPs are formed, whereas through fermentation only two molecules are formed. Thus, aerobic respiration is a lengthier process. Only when quick energy is required, fermentation is an alternative.

As the red blood cells mature, they no longer contain mitochondria. Thus, fermentation is carried out by such cells which do not contain mitochondria. Electron transport chain redox reaction and citric acid cycle occurs in the mitochondria. Thus, glycolysis takes place where electrons are transferred and this happens in the cytosol. So that the mature RBCs do not use the oxygen they transport, they obtain energy from lactic acid fermentation.

To make wine, cheese, bread, beverages, soy sauce and others fermentation is important. For example, for wine production, to the grapes the yeast is added for fermentation to occur. In case of making of bread, dough is prepared and allow to ferment after the yeast is added which will secrete CO2 and the bread will rise. Similarly, bacteria undergoes fermentation with milk to form cheese.

Fermentation Process

There are several steps in fermentation and they are: Glycolysis which means breaking of sugar molecule. The 6-carbon sugar molecule breaks into 3 carbon compound pyruvates, which uses chemical energy such as ATP , thus called the energy investment step. In the next step, ATP is produced by substrate level phosphorylation, along with NADH. In the electron transfer step, pyruvate is used, which gets NAD+ and is again reobtained as it was lost in the first step of glycolysis. After glycolysis citric acid cycle does not take place, thus only 2 ATP molecules are gained. This process takes place in the cytosol.

Types of Fermentation

There are 3 types of fermentation, ethanol fermentation, lactic acid fermentation and acetic acid fermentation. In both eukaryotes and prokaryotes, fermentation takes place. The most commercial fermentation are bacterial and yeast fermentation.

i. Ethanol Fermentation

In this fermentation, the product obtained is ethanol. The steps involved are glucose molecule forms two pyruvate molecules in the glycolysis step. Then it further forms acetaldehyde and excretes CO2. From the NADH, a hydrogen ion is released and combines with acetaldehyde to form alcohol particularly ethanol, with NAD+ left. Pyruvate carboxylase and alcohol dehydrogenase are the enzyme involved in the second and third step.

Anaerobic bacteria such as yeast can also produce ethanol. Example Saccharomyces cerevisiae. They have been used in commercial industry to make alcohol, bread and others. In anaerobic conditions, fishes also produce ethanol in their myotomal muscles. Lactic acid fermentation is also possible.

ii. Lactic Acid Fermentation

It takes place in the cells cytosol and sugars are converted to lactate. Lactic acid fermentation is of two types: Homolactic and Heterolactic fermentation. End-product is lactate in homolactic fermentation. In Heterolactic fermentation, there are other end products as well such as carbon dioxide.

Homolactic fermentation also begins with glycolysis, and pyruvate is directly reduced by NADH, thus forming lactate and obtains NAD+. Lactate dehydrogenase is the enzyme carrying out the reaction.

While performing strenuous activities, there is a waste product which is released from the muscle to the blood to the liver, where it will get converted to pyruvate. This is known as Cori cycle and the enzyme is lactate dehydrogenase. It is a bidirectional reaction.

Fermentation Equation: The equation for ethanol fermentation is:

C6H12O6 → 2C2H5OH+ 2CO2 + Energy

As the 6-carbon sugar molecule is broken into two 2 pyruvate molecules, thus there are two molecules of ethanol. The byproducts is carbon dioxide. There is a gain of two ATP molecules.

iii. Lactic Acid Fermentation

The equation for homolactic fermentation of lactic acid is:

C6H12O6 → 2 CH3CHOHCOO + Energy

As the 6-carbon sugar molecule is broken into two 2 pyruvate molecules, thus there are two molecules of lactate. There is a gain of two ATP molecules.

Lactate can be metabolized further by bacteria such as Leuconostoc mesenteroides, thus having by-products such as carbon, dioxide and etc.

C6H12O6 → CH3CHOHCOO + C2H5OH + CO2 + Energy

A heterolactic acid fermentation involves few byproducts as well. 1 ATP is gained in this fermentation.

Fermentation Products

Depending on the fermentation and the enzymes involved, the products will vary. For example, from pyruvate, ethanol is produced with the enzyme pyruvate carboxylase and alcohol dehydrogenase. To product lactate, pyruvate is required and the enzyme is lactate dehydrogenase. Carbon dioxide, energy, acetate, hydrogen gas are the by-products.

For the formation of acetic acid, bacteria will break down sugar to form acetic acid. example is vinegar. When acetic acid bacteria acts on sugars it results in the formation of vinegar.

CH3CH20H + O2 → CH3COOH + H20

Here ethanol reacts with water to form acetic acid, which is an oxidation and fermentation process. If fermentation requires light it is called as photo-fermentation and in the absence of light it is called as dark fermentation.

History of Fermentation

Since ancient times, fermentation has been carried out. Examples are making wine from grapes, octli from agave, from malted barley making beer and from maize chicha. At that time, they used wooden containers to store these beverages in them, but how they were aware of this concept is not well known. However, in 17th century people as people became aware about lenses and microscope.

All these microorganism could be seen microscope to open up the world. Through microscope it became clear that the microorganism associated with fermentation is yeast and they divide by budding in the fermentation process.

However, Louis Pasteur, was showed that yeast are living organism and transform sugar into alcohol. They could also carry out in the absence of oxygen. Alcoholic fermentation was termed by Louis Pasteur, where he said that the change in the sourness is due to the presence of living organism, thus converting into desired product.

In 19th century, a German chemist pulverized the yeast cell, to make alcohol from sucrose. The one responsible for catalyzing the reaction was termed as “zymase”. Further the research has kept on growing and many more organism have been discovered.

Fermentation Citations


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