Types of Fermentation

All living beings need energy to perform various important functions. This energy is derived from respiration. Respiration may or may not occur in the presence of oxygen. When respiration takes place in the presence of oxygen, it is termed aerobic respiration and when it takes place in the absence of oxygen it is called anaerobic respiration. Most living organisms derive energy from aerobic respiration, but a few organisms like bacteria, yeast, etc. respire anaerobically. These organisms derive energy in the absence of oxygen by converting starch or sugar to alcohol or an acid. This enzyme-catalyzed metabolic process is called fermentation.

Fermentation is a biochemical process taking place in some organisms. The beginning of this process is similar to cellular respiration. That is the formation of pyruvic acid by the process of glycolysis where net 2 ATP molecules are synthesized. Further, pyruvate is reduced to lactic acid, ethanol, or other products. The NAD+ formed in the process is further re-utilized in the process of glycolysis.

Fermentation is a chemical process that breaks down compounds such as glucose anaerobically. Fermentation, in a broader sense, is the foaming that happens during the production of wine and beer, a process that has been around for at least 10,000 years. The foaming is caused by the development of carbon dioxide gas, which was not discovered until the 17th century. In the nineteenth century, French chemist and microbiologist Louis Pasteur used the term fermentation narrowly to describe the changes caused by yeasts and other microorganisms growing in the absence of air (anaerobically); he also recognized that ethyl alcohol and carbon dioxide are not the only products of fermentation.

During the 1920s, a revelation was that, in the absence of air, the synthesis of lactate was accelerated by muscle extracts from glucose, and that muscle produces the same intermediate chemicals as grain fermentation. As a result, an important generalization emerged: fermentation processes are not exclusive to yeast activity, but also occur in many other cases of glucose use.

Glycolysis, or sugar breakdown, was first characterized about 1930 as the breakdown of sugar into lactate. It can also be defined as the type of fermentation that occurs in cells in general, in which the six-carbon sugar glucose is broken down into two molecules of the three-carbon organic acid, pyruvic acid (the nonionized form of pyruvate), and chemical energy is transferred to the synthesis of adenosine triphosphate (ATP). In the presence of oxygen, the pyruvate can be oxidized via the tricarboxylic acid cycle, or in the absence of oxygen, it can be reduced to lactic acid, alcohol, or other compounds. The road from glucose to pyruvate is sometimes referred to as the Embden–Meyerhof pathway, after two German biochemists who proposed and tested the crucial steps in that chain of events in the late 1920s and 1930s.

Industrial fermentation processes begin with appropriate microorganisms and certain circumstances, such as precise nutrient content adjustment. Alcohol, glycerol, and carbon dioxide are produced by yeast fermentation of various sugars; butyl alcohol, acetone, lactic acid, monosodium glutamate, and acetic acid are produced by bacteria; and mold fermentation yields citric acid, gluconic acid, and so on. Ethyl alcohol, which is created by the fermentation of starch or sugar, is a significant source of liquid biofuel.

Fermentation occurs primarily in cells of yeast and bacteria, and in mammalian muscles as well. 

Cellular respiration refers to the respiration that occurs at the most basic level of our bodies, namely the cell. Any kind of cellular respiration begins with glycolysis, which results in the formation of a 3-C molecule, pyruvic acid, as the end product.

Different cells deal with pyruvate in two ways, one of which is fermentation. Let's take a closer look at fermentation, its many forms, and anaerobic respiration.

Fermentation is an anaerobic mechanism, which is seen in most prokaryotes and unicellular eukaryotes. Glucose is partly oxidized in this process to generate acids and alcohol.

Pyruvic acid produced by partial oxidation of glucose is transformed to ethanol and carbon dioxide in organisms such as yeast (CO₂). This anaerobic state is known as alcoholic or ethanol fermentation. The enzymes pyruvic acid decarboxylase and alcohol dehydrogenase catalyzes the whole process. Lactate dehydrogenase converts pyruvic acid to lactic acid in some bacteria and mammalian muscle cells under anaerobic circumstances. Because of the end products of these anaerobic routes, they are dangerous processes. For example, yeast cells that make alcohol in concentrations of more than 13% might kill themselves.

NADH+H+ is the reducing agent that is reduced to NAD+ during alcoholic and lactic acid fermentation. Both processes release extremely little energy, and the total number of ATP molecules created during fermentation is two, which is quite low when compared to aerobic respiration. However, this is used commercially in the food and beverage sectors, as well as the pharmaceutical businesses.

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Types of Fermentation

Fermentation is of two types, depending upon the number of end products.

Homo Fermentation - In this type, only one end product is formed.

Hetero Fermentation - In this type, more than one end product is formed.

Different end products are formed at the end of fermentation and depending upon the type of end product formed, fermentation is categorized into various types:

1. Lactic Acid Fermentation

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Lactic acid fermentation is a process by which glucose and other sugars containing six carbon atoms turn into metabolite lactate and cellular energy. This type of anaerobic fermentation mostly occurs in bacteria, certain animal cells, mammalian red blood cells, and sometimes in skeletal muscles under conditions of insufficient oxygen supply to allow continuity of aerobic respiration. In this process lactic acid forms from pyruvate produced during glycolysis. NAD+ is generated from NADH and lactate dehydrogenase catalyzes the reaction. The lactic acid formed by anaerobic respiration during exercise gets accumulated in the cell and causes fatigue.

2. Alcohol Fermentation

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Alcohol fermentation is also called ethanol fermentation because the end product in this case is ethanol. It is mostly carried out by yeast to convert simple sugars into carbon dioxide and ethanol. It is widely used in industries to produce wine, beer, biofuel, etc. In this process, pyruvic acid is broken down into acetaldehyde and the carbon dioxide is released. The acetaldehyde thus formed, gives ethanol. NAD+ is also formed from NADH which later enters glycolysis. Two enzymes catalyze the two steps of the process. These are pyruvic acid decarboxylase and alcohol dehydrogenase.

3. Acetic Acid Fermentation

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Acetic acid fermentation is the process used in the formation of vinegar. The two-step process involves the following steps:Formation of ethyl alcohol anaerobically from sugar by using yeast.Oxidation of ethyl alcohol to give acetic acid using acetobacter by aerobic respiration.

4. Butyric Acid Fermentation

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The bacteria carrying out butyric acid fermentation are obligately anaerobic and spore-forming bacteria belonging to the genus Clostridium. It is also known as mixed acid fermentation because along with butyric acid, n-butanol, acetic acid, ethanol, isopropanol, and acetone are also formed depending upon the species carrying out the process.

This type of fermentation is used to carry out the following processes:

• Retting of jute fiber

• Rancid butter

• Tobacco processing

• Tanning of leather

In the first step, oxidation of sugar gives pyruvate by the process of glycolysis. In the next step, pyruvate is oxidized to form acetyl-CoA with the help of the oxidoreductase enzyme and produces CO₂ and H₂. Finally, acetyl-CoA is further reduced to form butyric acid. Butyric Acid fermentation yields more energy with the formation of net 3 molecules of ATP.

Uses of Fermentation

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Fermentation is one of the oldest metabolic processes commonly used by prokaryotes and eukaryotes. It is also a process used in industries to manufacture various products. Various types of fermentation are used to produce the desired end product for use. Common products we use in our day to day life produced by fermentation include:

• Wine

• Beer

• Biofuels

• Yogurt

• Pickles

• Bread

• Lactic Acid containing sour foods

• Certain vitamins and antibiotics

• VinegarCondiments like apple cider vinegar, kombucha, etc.

Advantages of Fermentation

• Fermented food tastes better, is easy to digest and nutritious for the body. Benefits of consuming fermented food are as follows:

• Fermented food helps maintain the stomach bacteria, thus helping digestion.

• It has an anti-carcinogenic effect.

• It is good for the immune system.

• It is also beneficial for lactose-intolerant people.

There are many more applications of fermentation than industrial and domestic. For example - It is used to produce methane in sewage treatment plants.