An anaerobic cellular process in which an organic food is converted into simpler compounds, and chemical energy (ATP) is produced
Table of Contents
What is fermentation? Fermentation is the breaking down of sugar molecules into simpler compounds to produce substances that can be used in making chemical energy. Chemical energy, typically in the form of ATP, is important as it drives various biological processes. Fermentation does not use oxygen; thus, it is “anaerobic”.
Apart from fermentation, living things produce chemical energy by degrading sugar molecules (e.g. glucose) through aerobic respiration and anaerobic respiration. Aerobic respiration uses oxygen, hence, the term ”aerobic”. It has three major steps. First, it begins with glycolysis wherein the 6-carbon sugar molecule is lysed into two 3-carbon pyruvate molecules. Next, each pyruvate is converted into acetyl coenzyme A to be broken down to CO2 through the citric acid cycle. Along with this, the hydrogen atoms and electrons from the carbon molecules are transferred to the electron-carrier molecules, NADH, and FADH2. Then, these electron carriers shuttle the high-energy electrons to the electron transport chain to harness the energy and synthesize ATP. The final electron acceptor in the chain is oxygen. As for anaerobic respiration, this form of respiration does not require oxygen. However, it is similar to aerobic respiration in a way that the electrons are passed along the electron transport chain to the final electron acceptor. In anaerobic respiration, the bottom of the chain is not oxygen but other molecules, for example, sulfate ion (SO4–2) or nitrate ion (NO3–).
Some people consider fermentation as an example or part of anaerobic respiration as both of them do not use oxygen, and therefore, are anaerobic. However, anaerobic respiration and fermentation are two different processes. Fermentation skips the electron transport chain system. After glycolysis, pyruvate (in lactic acid fermentation) or acetaldehyde (in alcohol fermentation) serves as the final electron acceptor.
The type of fermentation depends on its byproducts. For example, lactic acid fermentation is a type of fermentation that produces lactic acid. Alcohol fermentation produces alcohol, such as ethanol, aside from CO2.
Fermentation occurs in prokaryotes and eukaryotes, including humans. Our body resorts to fermentation when there is a high energy demand while the oxygen supply becomes limited. An example of this is when we do strenuous exercise. The muscle cells generate ATP to supply energy via aerobic respiration. But when the ATP demand in the muscle cells outruns the blood supply of oxygen, the muscle cells resort to lactic acid fermentation so that they can continue providing energy while the supply of oxygen is limited. When the oxygen level returns to normal, they go back to aerobic respiration.
While fermentation is only an alternative pathway in generating ATP, some organisms, such as obligate anaerobes, rely on fermentation to biosynthesize ATP. The genus Neocallimastix is an example of obligate anaerobes. The fungi in this genus are found in the rumen of herbivorous animals. As symbionts, they help digest cellulose through fermentation. (Ref. 1) Another example of obligate anaerobe is the genus Bacteroides. This genus consists of obligate anaerobes that are part of human colonic flora. (Ref. 2) They degrade sugar derivatives from plant materials and generate energy through fermentation.
Then, there are certain facultative anaerobes that will favor fermentation over aerobic respiration even in the presence of oxygen, especially when pyruvate is building up faster than it is metabolized. Baker’s yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) are examples of organisms that will ferment rather than respire even in the presence of oxygen. In contrast, Kluyveromyces lactis is an example of a yeast species that will ferment only in a completely anaerobic environment and will favor respiration over fermentation if oxygen becomes available.
Microbial fermentation is used commercially by certain industries. Lactic acid fermentation by certain fungi and bacteria, for instance, is used by the dairy industry to make yogurt and cheese. Alcohol fermentation by yeasts is used in making wine and liquor.
Fermentation is an anaerobic process performed by a cell to generate chemical energy (e.g. ATP) from pyruvate (a product of glycolysis) but without going through the citric acid cycle and the electron transport chain system as cellular respiration does. Etymology: from Latin fermentātiō, fermentātiōnem, fermentum, meaning “that which causes fermentation, yeast”)
|Table 1: Comparison of Fermentation, Anaerobic Respiration, and Aerobic Respiration
|Does not use the electron transport chain system to pass the electrons to the final electron acceptor
|Uses the electron transport chain system to pass the electrons to the final electron acceptor
|Uses the electron transport chain system to pass the electrons to the final electron acceptor
|Number of ATP gained: 2 per glucose molecule (by substrate-level phosphorylation)
|Number of ATP gained: varies
|Number of ATP gained: ~38 per glucose molecule (by substrate-level phosphorylation and oxidative phosphorylation)
|Final electron acceptor: organic molecule, e.g. pyruvate (lactic acid fermentation) or acetaldehyde (alcohol fermentation)
|Final electron acceptor: inorganic compounds, e.g. sulfate ion (SO4–2), nitrate (NO3–)and ferric ion (Fe3+) or organic compounds, e.g. dimethyl sulfoxide, fumarate, and trimethylamine N-oxide (Ref. 3)
|Final electron acceptor: oxygen
|Examples: lactic acid fermentation, alcohol fermentation, fermentative hydrogen production, mixed acid fermentation, butanediol fermentation, butyrate fermentation
|Examples: denitrification, fumarate respiration, sulfate respiration, methanogenesis, sulfur reduction, acetogenesis, dehalorespiration, iron reduction, cobalt reduction
|Examples: glycolysis + acetyl-CoA metabolism, fatty acid catabolism (beta oxidation) + acetyl-CoA metabolism
|Final product: lactic acid, alcohol, hydrogen gas, CO2
|Final product: varies, for example, N2 (in denitrification), succinate (in fumarate respiration), HS− (in sulfate respiration and sulfur reduction), methane (in methanogenesis), acetate (in acetogenesis), halide ions, and dehalogenated compound (in dehalorespiration), Fe(II) in iron reduction, Co(II) in cobalt reduction
|Final product: water, CO2
Function of Fermentation
What is the function of fermentation? Fermentation enables cells to produce chemical energy from the breakdown of sugar, e.g. glucose, without the help of oxygen. That gives anaerobic (obligate, facultative, or aerotolerant) organisms the advantage of thriving in anoxic (without oxygen) environments that would rather be harsh for aerobic organisms. Examples of anoxic environments are mud, soil, and hydrothermal vents deep under the sea. The anaerobic bacteria that can thrive in these environments are essential for their ecological niche. They ferment molecules to derive energy and, in return, they produce byproducts released into the environment. Their byproducts may be used by other organisms or may be returned to the environment as a form of nutrient cycling. Thus, having them in these environments could be essential for their distinctive ecological niche.
Apart from these habitats, there are also microbes that inhabit living organisms, such as the gastrointestinal tract of mammals. Ruminants, such as cattle, harbor normal gut flora that can ferment dietary food that the animals cannot digest by themselves. That is because the microbes living in their gut can synthesize enzymes needed in digesting celluloses and residual starch. Humans also have normal flora in the gut for a similar purpose. They help degrade undigested sugars in the large intestine. Too much fructose, for instance, may cause fructose to reach the large intestine. When it does, it is used in fermentation by the colonic flora. Byproducts, such as lactic acid, methane, hydrogen, and carbon dioxide, are produced. (Ref. 4) Fermentation is the major source of intestinal gas, which can cause flatulence, bloating, gastrointestinal pain, or diarrhea.
Some bacteria, though, are pathogenic (disease-causing) if they infect a human body. An example is Clostridium perfingens. This bacterial species can cause gas gangrene in humans.
Our body also carries out fermentation. When we are doing an energy-demanding activity, our body will keep on sustaining energy (ATP). If aerobic respiration is no longer able to meet up the energy demand, the body has lactic acid fermentation as an alternative. The cells will resort to it as a quick way to generate ATP. Truly, aerobic respiration is better at producing more ATP than fermentation as there are ~38 ATPs released per glucose molecule through aerobic respiration as opposed to only 2 ATP via fermentation. However, aerobic respiration is a longer process. Fermentation lets our cells, such as skeletal muscle cells, to quickly obtain the power they need to carry out a task. The purpose of lactic acid fermentation, in this regard, is chiefly to regenerate NAD+, which is essential for glycolysis to proceed again. NAD+ is regenerated when pyruvate (the end product of glycolysis) accepts electrons from NADH. (Ref. 5)
Fermentation is also the pathway used by certain cells in our body lacking in mitochondria. Our red blood cells, in particular, no longer possess mitochondria at maturity. Mitochondria are the organelles where the citric acid cycle and electron transport chain redox reactions occur. Fermentation entails glycolysis and the transferring of electrons from NADH to pyruvate or its derivatives (to regenerate NAD+). These processes occur in the cytosol. Therefore, mature red blood cells circulating in our blood generate chemical energy through lactic acid fermentation. This ensures that the red blood cells will not use any of the oxygen they transport. (Ref. 5)
In the food industry, fermentation is an important process in making bread, wine, cheese, soy sauce, and other foods and beverages. In particular, the yeasts ferment the sugars in the dough, releasing CO2 in the process. The CO2 helps the bread to rise. As for wines and other liquors, yeasts are added to the fruit juice (e.g. grape juice). The yeasts ferment the sugar in the juice into alcohol. Cheese is a product of bacteria fermenting milk or cream.
What is the process of fermentation? Does fermentation require oxygen? Fermentation is an anaerobic process. It does not use oxygen. The fermentation reaction entails two major steps: (1) glycolysis and (2) electron transfer from NADH to pyruvate or its derivatives. The first step — glycolysis — is similarly the first step in cellular respiration. Glycolysis means “splitting of sugar“. That’s because, glucose, a 6-carbon sugar molecule is split into two pyruvates (a 3-carbon compound) after glycolysis.
In glycolysis, glucose is oxidized to pyruvate to harvest chemical energy. The first phase is called an energy-investment phase because the process uses ATP molecules. The next phase is an energy-payoff phase. That’s because ATP is now produced via substrate-level phosphorylation.
Aside from ATP, NADH, another high-energy molecule, is produced. NADH is produced when glyceraldehyde phosphate (product of the energy-investment phase) is oxidized and then the H+ and the electrons are transferred to NAD+.
The end product of the energy-payoff phase is pyruvate. Pyruvate is, then, used in the next step of fermentation, which is the electron transfer from NADH to pyruvate or its derivatives. This step regenerates NAD+, which is important because it is used in glycolysis during the energy-payoff phase, as mentioned above.
How much ATP does fermentation produce? Because fermentation skips the citric acid cycle after glycolysis, the energy gain is two ATP molecules per glucose molecule. But what about the NADH produced in glycolysis? As described above, NADH is consumed in the second step when the electron from NADH is transferred to pyruvate or its derivatives, e.g. acetaldehyde. Thus, there is no net NADH production during fermentation. This is also why there is no ATP production through oxidative phosphorylation but only substrate-level. In cellular respiration, NADH enters the electron transport chain to transfer the electron along the chain and produce ATP through oxidative phosphorylation.
Where does fermentation occur? Glycolysis and the electron transfer from NADH to pyruvate or its derivatives occur in the cytoplasm (particularly, the cytosol).
What causes fermentation? The presence of pyruvate coming from glycolysis incites fermentation. Some cells that respire aerobically (e.g. muscle cells) may resort to fermentation when oxygen is scarce and yet there is high energy demand. The muscle cells seem to “buy time” by using fermentation to generate energy quickly until such time that the muscle cell can respire again when the oxygen supply is no longer limited. (Ref. 5)
Types of Fermentation
What are the 3 types of fermentation? There are many types of fermentation. But the three types of fermentation that are commonly used in the industry are lactate fermentation, ethanol fermentation, and acetic acid fermentation. In brief, lactate fermentation produces lactate, ethanol fermentation produces ethanol, and acetic acid fermentation produces acetic acid. The first two types of fermentation are further described in the preceding sections.
Fermentation activity occurs in both prokaryotes and eukaryotes. Nevertheless, bacterial fermentation and yeast fermentation are the most commercially valuable. They are used in the food industry. Below are examples of some of the commercial applications of fermentation.
Ethanol fermentation is a type of fermentation wherein the end product is ethanol (or ethyl alcohol). It is a three-step process. First, glucose is oxidized by glycolysis, producing two pyruvate molecules. Second, each pyruvate releases carbon dioxide to produce acetaldehyde. Third, acetaldehyde takes the hydrogen ions from NADH, consequently producing ethanol and converting NADH back to NAD+. The enzymes that catalyze the second and third steps are pyruvate carboxylase and alcohol dehydrogenase, respectively.
Yeasts (e.g. Saccharomyces cerevisiae, Schizosaccharomyces) and certain anaerobic bacteria (e.g. Zymomonas mobilis) are capable of ethanol fermentation. These microscopic organisms are used by the food industry in making alcoholic beverages and causing bread dough to rise. Certain fish groups (e.g. goldfish and crucian carp) can also ferment and produce ethanol especially when their environment becomes anoxic (oxygen-deficient). These fish species of the Cyprinid family form ethanol in their myotomal muscles. Apart from ethanol fermentation, they are also capable of lactic acid fermentation. (Ref. 6)
Lactic Acid Fermentation
Lactic acid fermentation is a biological process wherein sugars are converted into lactate to yield energy. Where does lactic acid fermentation occur? Similar to ethanol fermentation, lactic acid fermentation occurs in the cytosol of the cell.
There are two forms of lactic acid fermentation: (1) homolactic fermentation and (2) heterolactic fermentation. Homolactic fermentation is when the endproduct is only lactate. When there are other endproducts apart from lactate, for example, ethanol and carbon dioxide, it is a heterolactic type. Nevertheless, both of them begin in glycolysis and ultimately produce two pyruvates with each glucose molecule.
In homolactic fermentation, no carbon dioxide is released. Also, the pyruvate is reduced directly by NADH. This results in lactate (an ionized form of lactic acid) formation and NAD+ regeneration. The enzyme responsible for this reaction is lactate dehydrogenase.
This is the type of fermentation that occurs in the muscle cells during vigorous physical activity. Lactate is a waste product released by the muscle cell into the bloodstream to be carried by the blood to the liver. The liver cell takes up lactate from the blood to convert it back into pyruvate via the enzyme, lactate dehydrogenase — a process called the Cori cycle (Ref. 7) This means that the reaction can proceed in either direction.
The chemical equation of fermentation varies depending on the reactants and products involved. Let’s take a look at the following examples below.
The overall chemical formula for ethanol fermentation is:
C6H12O6 (glucose) → 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide) + energy
Because there are two pyruvates produced per one glucose molecule, there are two ethanol molecules and two carbon dioxide molecules produced after fermentation. The total ATP gain is two.
Lactic acid fermentation equation
The general chemical formula for lactic acid (homolactic) fermentation is as follows:
C6H12O6 (glucose) → 2 CH3CHOHCOO- (lactate) + energy
Because there are two pyruvates produced per one glucose molecule, there are two lactate molecules produced after fermentation. The total ATP gain is two.
Certain fermentative bacteria (e.g. Leuconostoc mesenteroides) are capable of further metabolizing lactate. As a result, the products of the fermentation are not just lactate but other metabolic products, such as alcohol and carbon dioxide. In this case, the formula is:
C6H12O6 (glucose) → CH3CHOHCOO- (lactate) + C2H5OH (alcohol) + CO2 (carbon dioxide) + energy
This is a sample of a heterolactic type of lactic acid fermentation. The total ATP gain in this example is 1 ATP.
The products of fermentation will depend on the enzymes involved. For instance, producing ethanol from pyruvate will require enzymes, pyruvate carboxylase, and alcohol dehydrogenase. Conversely, to produce lactate from pyruvate, the enzyme, lactate dehydrogenase is required. Apart from lactate (or lactic acid) and ethanol, other byproducts of fermentation are acetates, carbon dioxide, and hydrogen gas.
Acetic acid bacteria are a group of bacteria that will oxidize sugars or ethanol to produce acetic acid. One such important commercial application is vinegar making. Vinegar is produced by allowing the acetic acid bacteria to act on sugars or ethanol. The formula is as follows: CH3CH2OH (ethanol) + O2 (oxygen)→ CH3COOH (acetic acid)+ H2O (water). In this reaction, oxygen is utilized and made to react with ethanol to produce acetic acid and water. Thus, the production of vinegar is a combined process of fermentation and oxidation.
Fermentative hydrogen production, in turn, is a form of fermentation wherein an organic compound is converted into hydrogen gas (H2). Certain types of bacteria and protozoa have enzymes that enable this process. When light is not required, the process is referred to as dark fermentation. If light energy is required, the process is called photofermentation.
History of the Use of Fermentation
The practice of fermentation has existed in ancient history. People have been applying the basic steps of fermentation in their food and beverages. They were making beer from malted barley, wine from grapes, chicha from maize, and octli (now known as “pulque“) from agave, a type of cactus. (Ref. 8)
People were able to produce these beverages by placing them inside the tightly covered containers and then leaving them for over a certain period of time but no one knew how this practice worked. It was only in the 17th century that people began to understand the biology of it when microscopes and lenses were invented. Antoni van Leeuwenhoek, for instance, was able to see for the first time various microorganisms, including yeasts. As more powerful microscopes were contrived, scientists were able to learn more about multifarious microorganisms. Charles Cagniard de la Tour found out that yeasts are microorganisms and might have been associated with the fermentation process. He observed them multiplying by budding during alcoholic fermentation. However, our modern understanding of the biology and chemistry of fermentation comes from the work of Louis Pasteur, a French chemist and microbiologist. In the 1850s and 1860s, he was the first to demonstrate through experiments that living yeasts were the ones responsible for transforming glucose into ethanol in fermented beverages. And these yeasts were able to do so in the absence of oxygen. He described the process as “respiration without air”. (Ref. 8)
Pasteur also identified two types of fermentation: alcoholic fermentation which he attributed to the multiplying yeasts and lactic acid fermentation by the growing bacteria. (Ref. 8) This was based on his observations where he found out that sugars were converted into alcohol in the presence of live yeast and that the “souring” of the beet juice was due to the presence of live bacterial species, which led to the conversion of ethanol into acetic acid. (Ref. 9) Pasteur, however, did not know exactly how these organisms caused fermentation.
By the end of the 19th century, Eduard Buchner (German chemist) found that by pulverizing the yeasts cells and extracting “press juice” from the yeasts he was able to incite the conversion of sucrose to alcohol and carbon dioxide. He coined the term “zymase” to refer to the compound extracted from yeast that catalyzed the conversion in alcoholic fermentation. (Ref. 9)
Since then, more organisms have been identified to carry out fermentation, including the cells of human muscles. (Ref. 10)
Try to answer the quiz below to check what you have learned so far about fermentation.
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