Fermentation

Fermentation is a type of anaerobic metabolism that harnesses the redox potential of the reactants to make adenosine triphosphate and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and their electrons are transferred to other organic molecules. Anaerobic glycolysis is a related term used to describe the occurrence of fermentation in organisms when aerobic respiration cannot keep up with the ATP demand, due to insufficient oxygen supply or anaerobic conditions.
Fermentation is important in several areas of human society. Humans have used fermentation in the production and preservation of food for 13,000 years. It has been associated with health benefits, unique flavor profiles, and making products have better texture. Humans and their livestock also benefit from fermentation from the microbes in the gut that release end products that are subsequently used by the host for energy. Perhaps the most commonly known use for fermentation is at an industrial level to produce commodity chemicals, such as ethanol and lactate. Ethanol is used in a variety of alcoholic beverages while lactate can be neutralized to lactic acid and be used for food preservation, curing agent, or a flavoring agent.
This complex metabolism utilizes a wide variety of substrates and can form nearly 300 different combinations of end products. Fermentation occurs in both prokaryotes and eukaryotes. The discovery of new end products and new fermentative organisms suggests that fermentation is more diverse than what has been studied.

Definition

A variety of definitions have been proposed throughout the years, but the simplest definition and most recent definition of fermentation proposed is "catabolism where organic compounds are both the electron donor and acceptor." This definition distinguishes fermentation from aerobic respiration and types of anaerobic respiration. However, this definition does not encompass all forms of fermentation. For example, propionate fermentation which uses H₂ as an electron donor, or the second step of butyrate fermentation where CO₂ can act as an electron acceptor. Thus, it is simplest to use this definition while acknowledging that protons and CO₂ can also be used as electron donors and acceptors, respectively.
In 1876, before the discovery of anaerobic respiration, Louis Pasteur described it as "la vie sans air". It was also common for fermentation to be defined based on how fermentation forms ATP which was catabolism that forms ATP through only substrate-level phosphorylation.
Industrial fermentation is another type of fermentation that is defined loosely as a large-scale biological manufacturing process; however, this definition focuses on the process of manufacturing rather than metabolic details.

Biological role and prevalence

Fermentation can be used by organisms to generate a net gain of ATP from exogenous sources of organic molecules, such as glucose. It was not a net source of energy in the earliest forms of life because they were mostly single cell organisms living in the ocean and the ocean does not contain significant concentrations of complex organic molecules.
Because fermentation does not need an exogenous electron acceptor, it is able to occur regardless of the environmental conditions. However, the primary disadvantage of fermentation is that fermentation is relatively inefficient and produces between 2 and 5 ATP molecules per glucose versus 32 ATP molecules during aerobic respiration.
Over 25% of bacteria and archaea carry out fermentation. Fermentation is especially prevalent in prokaryotes of the phylum Bacillota, but is most rare in Actinomycetota, according to phylogenetic analysis. The fermenting microbes are most frequently found in host-associated habitats such as the gastrointestinal tract, but also sediments, food, and other habitats. Both bacteria and archaea share the capacity for fermentation, leading to a wide variety of organic end products. The most common fermentation products include lactate, acetate, ethanol, carbon dioxide, succinate, hydrogen, propionate, and butyrate.
In humans, fermentation pathways occur in health, as in exercising, and in disease, as in sepsis and hemorrhagic shock, providing energy for a period ranging from 10 seconds to 2 minutes. During this time, it can augment the energy produced by aerobic metabolism, but is limited by the buildup of lactate. Rest eventually becomes necessary.

Substrates and products of fermentation

Like many biochemical reactions, fermentation is an enzyme catalyzed reaction with the goal of either changing the initial substrate or forming a useful byproduct. When naturally occurring fermentation is carried out by microbes, the goal is usually to obtain useful metabolic products such as ATP, pyruvate, or lactic acid. The substrates used in this type of fermentation are often simple sugars that serve as a carbon source and this type of fermentation can be carried out by microbes and humans.
Food as a substrate for fermentation is the most common and oldest anthropogenic use of fermentation as it was a method to preserve food. This includes cereal, dairy products, rice, honey, bread, and beers. This type of naturally occurring fermentation continues to be harnessed by humans for preservative effects, flavor profiles, and texture profiles. Advances in fermentation has led to the engineering and industrialization of specific microbes and substrates in order to obtain certain flavor and texture profiles – this is most obvious when observing beer fermentation.

Biochemical overview

When an organic compound is fermented, it is broken down to a simpler molecule and releases electrons. The electrons are transferred to a redox cofactor, which, in turn, transfers them to an organic compound. ATP is generated in the process, and it can be formed via substrate-level phosphorylation or by ATP synthase.
When glucose is fermented, it enters glycolysis or the pentose phosphate pathway and is converted to pyruvate. From pyruvate, pathways branch out to form a number of end products. At several points, electrons are released and accepted by redox cofactors. At later points, these cofactors donate electrons to their final acceptor and become oxidized. ATP is also formed at several points in the pathway.

Biochemistry of individual products

Ethanol

Yeast and other anaerobic microorganisms can convert the pyruvate produced from the oxidation of glucose by a glycolysis pathway to ethanol and. In ethanol fermentation, one glucose molecule is converted into two ethanol molecules and two carbon dioxide molecules. It is used to make bread dough rise: the carbon dioxide forms bubbles, expanding the dough into a foam. The ethanol is the intoxicating agent in alcoholic beverages such as wine, beer and liquor. Fermentation of feedstocks, including sugarcane, maize, and sugar beets, produces ethanol that is added to gasoline. In some species of fish, such as carp, it provides energy when oxygen is scarce.
Before fermentation, a glucose molecule breaks down into two pyruvate molecules. The energy from this exothermic reaction is used to bind inorganic phosphates to ADP, which converts it to ATP, and convert NAD⁺ to NADH. The pyruvates break down into two acetaldehyde molecules and give off two carbon dioxide molecules as waste products. The acetaldehyde is reduced into ethanol using the energy and hydrogen from NADH, and the NADH is oxidized into NAD⁺ so that the cycle may repeat. The reaction is catalyzed by the enzymes pyruvate decarboxylase and alcohol dehydrogenase.

History of bioethanol fermentation

The history of ethanol as a fuel spans several centuries and is marked by a series of significant milestones. Samuel Morey, an American inventor, was the first to produce ethanol by fermenting corn in 1826. However, it was not until the California Gold Rush in the 1850s that ethanol was first used as a fuel in the United States. Rudolf Diesel demonstrated his engine, which could run on vegetable oils and ethanol, in 1895, but the widespread use of petroleum-based diesel engines made ethanol less popular as a fuel. In the 1970s, the oil crisis reignited interest in ethanol, and Brazil became a leader in ethanol production and use. The United States began producing ethanol on a large scale in the 1980s and 1990s as a fuel additive to gasoline, due to government regulations. Today, ethanol continues to be explored as a sustainable and renewable fuel source, with researchers developing new technologies and biomass sources for its production.

1826: Samuel Morey, an American inventor, was the first to produce ethanol by fermenting corn. However, ethanol was not widely used as a fuel until many years later.
1850s: Ethanol was first used as a fuel in the United States during the California gold rush. Miners used ethanol as a fuel for lamps and stoves because it was cheaper than whale oil.
1895: German engineer Rudolf Diesel demonstrated his engine, which was designed to run on vegetable oils, including ethanol. However, the widespread use of diesel engines fueled by petroleum made ethanol less popular as a fuel.
1970s: The oil crisis of the 1970s led to renewed interest in ethanol as a fuel. Brazil became a leader in ethanol production and use, due in part to government policies that encouraged the use of biofuels.
1980s–1990s: The United States began to produce ethanol on a large scale as a fuel additive to gasoline. This was due to the passage of the Clean Air Act in 1990, which required the use of oxygenates, such as ethanol, to reduce emissions.
2000s–present: There has been continued interest in ethanol as a renewable and sustainable fuel. Researchers are exploring new sources of biomass for ethanol production, such as switchgrass and algae, and developing new technologies to improve the efficiency of the fermentation process.