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Fermentation is a
metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such as Cell signaling, signaling and self-sustaining ...

metabolic
process that produces chemical changes in organic substrates through the action of
enzyme Enzymes () are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins diff ...

enzyme
s. In
biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided into three fields: , and . Over the last decades of the 20th century, biochemistry has beco ...

biochemistry
, it is narrowly defined as the extraction of energy from
carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common ex ...
s in the absence of
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
. In food production, it may more broadly refer to any process in which the activity of
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
s brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as
zymology 300px, fermenting Fermentation is a metabolism, metabolic process that produces chemical changes in organic Substrate (chemistry), substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy f ...
. In microorganisms, fermentation is the primary means of producing
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properti ...

adenosine triphosphate
(ATP) by the degradation of organic nutrients
anaerobically
anaerobically
. Humans have used fermentation to produce foodstuffs and beverages since the
Neolithic The Neolithic period is the final division of the Stone Age The Stone Age was a broad prehistoric Prehistory, also known as pre-literary history, is the period of human history Human history, also known as world history, is t ...
age. For example, fermentation is used for preservation in a process that produces
lactic acid Lactic acid is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, conta ...

lactic acid
found in such sour
food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal origin, and contains essential nutrients, such as carbohydrates, fats, protein (nutrient), proteins, vi ...
s as
pickled cucumber A pickled cucumber (commonly known as a pickle in the United States and Canada, and a gherkin in Britain, Ireland, Australia, South Africa and New Zealand) is a cucumber that has been pickled in a brine, vinegar, or other solution and left ...

pickled cucumber
s,
kombucha Kombucha (also tea mushroom, tea fungus, or Manchurian mushroom when referring to the culture Culture () is an umbrella term which encompasses the social behavior and Norm (social), norms found in human Society, societies, as well as the kno ...

kombucha
,
kimchi ''Kimchi'' (; ko, 김치, gimchi, ), a staple food 215px, Unprocessed seeds of spelt, a historically important staple food A staple food, food staple, or simply a staple, is a food Food is any substance consumed to provide Nutrient, ...

kimchi
, and
yogurt Yogurt (; , from tr, yoğurt) also spelled yoghurt, yogourt or yoghourt, is a food produced by bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning ...

yogurt
, as well as for producing alcoholic beverages such as wine and
beer Beer is one of the oldest and most widely consumed alcoholic drink An alcoholic drink is a drink A drink (or beverage) is a liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continual ...

beer
. Fermentation also occurs within the gastrointestinal tracts of all animals, including humans.


Definitions

Below are some definitions of fermentation. They range from informal, general usages to more scientific definitions. # Preservation methods for food via microorganisms (general use). # Any large-scale microbial process occurring with or without air (common definition used in industry). # Any process that produces alcoholic beverages or acidic dairy products (general use). # Any energy-releasing metabolic process that takes place only under anaerobic conditions (somewhat scientific). # Any metabolic process that releases energy from a sugar or other organic molecule, does not require oxygen or an electron transport system, and uses an organic molecule as the final electron acceptor (most scientific).


Biological role

Along with
aerobic respiration Aerobic means "requiring Earth's atmosphere, air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Cellular respiration#Aerobic r ...
, fermentation is a method to extract energy from molecules. This method is the only one common to all bacteria and
eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...

eukaryote
s. It is therefore considered the oldest
metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. ...
, suitable for primeval environmentsbefore plant life on Earth, that is, before oxygen in the atmosphere.
Yeast Yeasts are eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classifie ...

Yeast
, a form of
fungus A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ve ...

fungus
, occurs in almost any environment capable of supporting microbes, from the skins of fruits to the guts of insects and mammals to the deep ocean. Yeasts convert (break down) sugar-rich molecules to produce ethanol and carbon dioxide. Basic mechanisms for fermentation remain present in all cells of higher organisms.
Mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
ian
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
carries out fermentation during periods of intense exercise where oxygen supply becomes limited, resulting in the creation of
lactic acid Lactic acid is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, conta ...

lactic acid
. In
invertebrates Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebrat ...
, fermentation also produces
succinate Succinic acid () is a dicarboxylic acidA dicarboxylic acid is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical co ...

succinate
and
alanine Alanine (symbol Ala or A) is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine, amine group and a carboxylic acid, carboxylic acid group, both attached to the central carbon atom which also carries a methyl group ...

alanine
. Fermentative bacteria play an essential role in the production of methane in habitats ranging from the
rumen The rumen, also known as a paunch, is the largest stomach compartment in ruminant Ruminants (suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recog ...
s of cattle to sewage digesters and freshwater sediments. They produce hydrogen, carbon dioxide,
formate Formate (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the authoritative pow ...

formate
and
acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetal image:Simple Periodic Table Chart-en.svg, upright=1.75, Nonmetals (and metalloids) in the periodic table: Metalloids are i ...

acetate
and
carboxylic acid A carboxylic acid is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acid ...
s. Then consortia of microbes convert the carbon dioxide and acetate to methane. Acetogenic bacteria oxidize the acids, obtaining more acetate and either hydrogen or formate. Finally,
methanogens Methanogens are microorganisms that produce methane Methane ( or ) is a chemical compound with the chemical formula (one atom of carbon and four atoms of hydrogen). It is a group-14 hydride and the simplest alkane , the simplest alkane In ...

methanogens
(in the domain ''
Archea Archaea ( ; singular archaeon ) constitute a domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacteria, recei ...
'') convert acetate to methane.


Biochemical overview

Fermentation reacts
NADH Nicotinamide adenine dinucleotide (NAD) is a Cofactor (biochemistry), coenzyme central to metabolism. Found in all living cell (biology), cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate ...
with an
endogenous Endogenous substances and processes are those that originate from within a system such as an organism, Tissue (biology), tissue, or Cell (biology), cell. Endogenous substances and processes contrast with exogenous ones, such as Drug, drugs, which ...
,
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or ...
electron acceptor An electron acceptor is a chemical entity that accepts electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle phy ...
. Usually this is
pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is a key intermediate in several metabolic pathways throughout the c ...

pyruvate
formed from sugar through
glycolysis Glycolysis is the metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of on ...

glycolysis
. The reaction produces and an organic product, typical examples being
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
,
lactic acid Lactic acid is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, conta ...

lactic acid
, and , and often also
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
. However, more exotic compounds can be produced by fermentation, such as
butyric acid Butyric acid (from grc, βούτῡρον, meaning "butter"), also known under the systematic name butanoic acid, is a straight-chain alkyl In organic chemistry, an alkyl substituent is an alkane missing one hydrogen. The term alkyl is inten ...

butyric acid
and
acetone Acetone, or propanone, is an organic compound with the chemical formula, formula (methyl group, CH3)2carbonyl, CO. It is the simplest and smallest ketone. It is a colourless, highly volatile and flammable liquid with a characteristic pungent od ...

acetone
. Fermentation products are considered waste products, since they cannot be metabolized further without the use of oxygen. Fermentation normally occurs in an
anaerobic environment Hypoxia refers to low oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, react ...
. In the presence of O2, NADH, and pyruvate are used to generate ATP in
respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellular ...

respiration
. This is called
oxidative phosphorylation Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the in which s use s to s, thereby releasing chemical energy in order to produce (ATP). In , this takes place inside . Almost all s car ...

oxidative phosphorylation
. This generates much more ATP than glycolysis alone. It releases the chemical energy of O2. For this reason, fermentation is rarely used when oxygen is available. However, even in the presence of abundant oxygen, some strains of
yeast Yeasts are eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classifie ...

yeast
such as ''
Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () is a species of yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of millions of years ago, and at le ...

Saccharomyces cerevisiae
'' prefer fermentation to
aerobic respiration Aerobic means "requiring Earth's atmosphere, air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Cellular respiration#Aerobic r ...
as long as there is an adequate supply of
sugars Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides o ...

sugars
(a phenomenon known as the
Crabtree effect The Crabtree effect, named after the English biochemist Herbert Grace Crabtree, describes the phenomenon whereby the yeast, ''Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () is a species of yeast Yeasts are eukaryotic, single-celle ...
). Some fermentation processes involve
obligate anaerobe{{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym '' facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organism that cannot survive without oxygen * Obligate anaerobe, an organism th ...
s, which cannot tolerate oxygen. Although
yeast Yeasts are eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classifie ...

yeast
carries out the
fermentation Fermentation is a metabolism, metabolic process that produces chemical changes in organic Substrate (chemistry), substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in ...
in the production of
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
in
beer Beer is one of the oldest and most widely consumed alcoholic drink An alcoholic drink is a drink A drink (or beverage) is a liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continual ...

beer
s,
wine Wine is an alcoholic drink typically made from Fermentation in winemaking, fermented grapes. Yeast in winemaking, Yeast consumes the sugar in the grapes and converts it to ethanol and carbon dioxide, releasing heat in the process. Different ...

wine
s, and other alcoholic drinks, this is not the only possible agent:
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...

bacteria
carry out the fermentation in the production of
xanthan gum Xanthan gum () is a polysaccharide with many industrial uses, including as a common food additive. It is an effective thickening agent and stabilizer (food), stabilizer to prevent ingredients from separating. It can be produced from monosacchar ...
.


Products of fermentation


Ethanol

In ethanol fermentation, one glucose molecule is converted into two ethanol molecules and two
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

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 Sugarcane or sugar cane is a species of (often hybrid) tall, perennial A perennial plant or simply perennial is a plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, ...

sugarcane
,
corn Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American English, North American and Australian English), is a cereal grain first domesticated by indigenous peoples of the Americas, indige ...

corn
, and
sugar beets A sugar beet is a plant whose root contains a high concentration of sucrose Sucrose is a type of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated ...

sugar beets
, produces ethanol that is added to
gasoline Gasoline () or petrol () (see the #Etymology, etymology for naming differences and the use of the term ''gas'') is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignition engine, spark-ignite ...

gasoline
. In some species of fish, including
goldfish The goldfish (''Carassius auratus'') is a freshwater fish are common freshwater fish throughout temperate Eurasia. Freshwater fish are those that spend some or all of their lives in fresh water, such as river A river is a natural flowi ...

goldfish
and
carp Carp are various species of from the family , a very large group of native to and . While carp is consumed in many parts of the world, they are generally considered an in parts of Africa, Australia and most of the United States. Biology The ...

carp
, it provides energy when oxygen is scarce (along with lactic acid fermentation). The figure illustrates the process. Before fermentation, a glucose molecule breaks down into two pyruvate molecules (
Glycolysis Glycolysis is the metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of on ...

Glycolysis
). The energy from this
exothermic reaction In thermochemistry, an exothermic reaction is a "reaction for which the overall Standard enthalpy of reaction, standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat and entail the replacement of weak bonds wi ...
is used to bind inorganic
phosphate In chemistry, a phosphate is an anion, salt (chemistry), salt, functional group or ester derived from a phosphoric acids and phosphates, phosphoric acid. It most commonly means orthophosphate, a derivative of phosphoric acid, orthophosphoric a ...

phosphate
s to ADP, which converts it to ATP, and convert NAD+ to NADH. The pyruvates break down into two
acetaldehyde Ethanal (common name acetaldehyde) is an organic chemical compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds ...
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.


Lactic acid

''Homolactic fermentation'' (producing only lactic acid) is the simplest type of fermentation. Pyruvate from glycolysisIntroductory Botany: plants, people, and the Environment. Berg, Linda R. Cengage Learning, 2007. . p. 86 undergoes a simple
redox (mild reducing agent) are added to powdered potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4 and composed of potassium ion, K+ and permanganate, . It is a purplish-black crystalline salt, ...

redox
reaction, forming
lactic acid Lactic acid is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, conta ...

lactic acid
.AP Biology. Anestis, Mark. 2nd Edition. McGraw-Hill Professional. 2006. . p. 61A dictionary of applied chemistry, Volume 3. Thorpe, Sir Thomas Edward. Longmans, Green and Co., 1922. p.159 Overall, one molecule of glucose (or any six-carbon sugar) is converted to two molecules of lactic acid: :C6H12O6 → 2 CH3CHOHCOOH It occurs in the muscles of animals when they need energy faster than the
blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers t ...

blood
can supply oxygen. It also occurs in some kinds of
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the sm ...
(such as
lactobacilli The Lactobacillaceae are a family of lactic acid bacteria Lactobacillales are an order of gram-positive, GC-content, low-GC, acid-tolerant, generally nonsporulating, Aerotolerant anaerobe, nonrespiring, either rod-shaped (bacillus (shape), ba ...
) and some
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
. It is the type of bacteria that convert
lactose Lactose, a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-g ...

lactose
into lactic acid in
yogurt Yogurt (; , from tr, yoğurt) also spelled yoghurt, yogourt or yoghourt, is a food produced by bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning ...

yogurt
, giving it its sour taste. These lactic acid bacteria can carry out either homolactic fermentation, where the end-product is mostly lactic acid, or ''heterolactic fermentation'', where some lactate is further metabolized to ethanol and carbon dioxide (via the phosphoketolase pathway), acetate, or other metabolic products, e.g.: :C6H12O6 → CH3CHOHCOOH + C2H5OH + CO2 If lactose is fermented (as in yogurts and cheeses), it is first converted into glucose and galactose (both six-carbon sugars with the same atomic formula): :C12H22O11 + H2O → 2 C6H12O6 Heterolactic fermentation is in a sense intermediate between
lactic acid fermentation One isomer of lactic acid Lactic acid fermentation is a metabolic process by which glucose or other hexose, six-carbon sugars (also, disaccharides of six-carbon sugars, e.g. sucrose or lactose) are converted into cellular energy and the metabolit ...

lactic acid fermentation
and other types, e.g. alcoholic fermentation. Reasons to go further and convert lactic acid into something else include: *The acidity of lactic acid impedes biological processes. This can be beneficial to the fermenting organism as it drives out competitors that are unadapted to the acidity. As a result, the food will have a longer shelf life (one reason foods are purposely fermented in the first place); however, beyond a certain point, the acidity starts affecting the organism that produces it. *The high concentration of lactic acid (the final product of fermentation) drives the equilibrium backwards (
Le Chatelier's principle Le Chatelier's principle (pronounced or ), also called Chatelier's principle, is a principle of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, mole ...
), decreasing the rate at which fermentation can occur and slowing down growth. *Ethanol, into which lactic acid can be easily converted, is volatile and will readily escape, allowing the reaction to proceed easily. is also produced, but it is only weakly acidic and even more volatile than ethanol. *Acetic acid (another conversion product) is acidic and not as volatile as ethanol; however, in the presence of limited oxygen, its creation from lactic acid releases additional energy. It is a lighter molecule than lactic acid, forming fewer hydrogen bonds with its surroundings (due to having fewer groups that can form such bonds), thus is more volatile and will also allow the reaction to proceed more quickly. *If
propionic acid Propionic acid (, from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population ...

propionic acid
,
butyric acid Butyric acid (from grc, βούτῡρον, meaning "butter"), also known under the systematic name butanoic acid, is a straight-chain alkyl In organic chemistry, an alkyl substituent is an alkane missing one hydrogen. The term alkyl is inten ...

butyric acid
, and longer monocarboxylic acids are produced (see
mixed acid fermentation Mixed acid fermentation is the biological process by which a six-carbon sugar e.g. glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of car ...
), the amount of acidity produced per glucose consumed will decrease, as with ethanol, allowing faster growth.


Hydrogen gas

Hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

Hydrogen
gas is produced in many types of fermentation as a way to regenerate NAD+ from NADH.
Electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

Electron
s are transferred to
ferredoxin Ferredoxins (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in re ...

ferredoxin
, which in turn is oxidized by
hydrogenaseA hydrogenase is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme c ...
, producing H2. Hydrogen gas is a
substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached **Substrate (locomotion), the surface over which an organism loco ...
for
methanogen Methanogens are microorganisms that produce methane Methane (, ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of n ...
s and sulfate reducers, which keep the concentration of hydrogen low and favor the production of such an energy-rich compound, but hydrogen gas at a fairly high concentration can nevertheless be formed, as in
flatus Flatulence is defined in the medical literature as "flatus expelled through the anus The anus (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin w ...
. For example, '' Clostridium pasteurianum'' ferments glucose to
butyrate The conjugate acids are in :Carboxylic acids. {{Commons category, Carboxylate ions, Carboxylate anions Carbon compounds Oxyanions ...

butyrate
,
acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetal image:Simple Periodic Table Chart-en.svg, upright=1.75, Nonmetals (and metalloids) in the periodic table: Metalloids are i ...

acetate
, carbon dioxide, and hydrogen gas: The reaction leading to acetate is: :C6H12O6 + 4 H2O → 2 CH3COO + 2 HCO3 + 4 H+ + 4 H2


Alternative protein

Fermentation can be applied to generate alternative protein sources. For instance, plant based protein foods such as
tempeh Tempeh or tempe (; jv, ꦠꦺꦩ꧀ꦥꦺ, témpé, ) is a traditional Javanese cuisine, Javanese food made from fermentation (food), fermented soybeans. It is made by a natural culturing and controlled fermentation (food), fermentation process t ...

tempeh
are produced using fermentation. However, fermentation can also be used to
culture Culture () is an umbrella term which encompasses the social behavior and Norm (social), norms found in human Society, societies, as well as the knowledge, beliefs, arts, laws, Social norm, customs, capabilities, and habits of the individuals i ...
animal products made from non-living material in vitro. Eggs, honey, cheese and milk are all examples which are made of various proteins. These proteins can be produced using this particular application of fermentation. Substances that are made using fermentation and which resemble milk are called
milk substitute A milk substitute is any substance that resembles milk Milk is a nutrient-rich liquid food produced by the mammary gland A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mam ...
s. Substances that resemble cheese are called cheese analogue and substances that resemble eggs are called
egg substitutes Egg substitutes are food products which can be used to replace eggs in cooking and baking. Types Commercial There are many commercial substitutes on the market today for people who wish to avoid eggs. Most of these products are devoid of all anim ...
. Some companies have started providing fermentation services to farmers ( Farming as a Service). Heme is a protein which gives meat its characteristic texture, flavour and aroma. Impossible Foods used fermentation to generate a particular strand of heme derived from soybean roots, called soy leghemoglobin, which was integrated into the Impossible Burger to mimic meat flavor and appearance.


Other

Other types of fermentation include
mixed acid fermentation Mixed acid fermentation is the biological process by which a six-carbon sugar e.g. glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of car ...
, butanediol fermentation, butyrate fermentation, caproate fermentation, acetone–butanol–ethanol fermentation, and glyoxylate fermentation.


Modes of operation

Most industrial fermentation uses batch or fed-batch procedures, although continuous fermentation can be more economical if various challenges, particularly the difficulty of maintaining sterility, can be met.


Batch

In a batch process, all the ingredients are combined and the reactions proceed without any further input. Batch fermentation has been used for millennia to make bread and alcoholic beverages, and it is still a common method, especially when the process is not well understood. However, it can be expensive because the fermentor must be sterilized using high pressure steam between batches. Strictly speaking, there is often addition of small quantities of chemicals to control the pH or suppress foaming. Batch fermentation goes through a series of phases. There is a lag phase in which cells adjust to their environment; then a phase in which exponential growth occurs. Once many of the nutrients have been consumed, the growth slows and becomes non-exponential, but production of ''secondary metabolites'' (including commercially important antibiotics and enzymes) accelerates. This continues through a stationary phase after most of the nutrients have been consumed, and then the cells die.


Fed-batch

Fed-batch fermentation is a variation of batch fermentation where some of the ingredients are added during the fermentation. This allows greater control over the stages of the process. In particular, production of secondary metabolites can be increased by adding a limited quantity of nutrients during the non-exponential growth phase. Fed-batch operations are often sandwiched between batch operations.


Open

The high cost of sterilizing the fermentor between batches can be avoided using various open fermentation approaches that are able to resist contamination. One is to use a naturally evolved mixed culture. This is particularly favored in wastewater treatment, since mixed populations can adapt to a wide variety of wastes. Thermophile, Thermophilic bacteria can produce lactic acid at temperatures of around 50 °Celsius, sufficient to discourage microbial contamination; and ethanol has been produced at a temperature of 70 °C. This is just below its boiling point (78 °C), making it easy to extract. Halophiles, Halophilic bacteria can produce bioplastics in hypersaline conditions. Solid-state fermentation adds a small amount of water to a solid substrate; it is widely used in the food industry to produce flavors, enzymes and organic acids.


Continuous

In continuous fermentation, substrates are added and final products removed continuously. There are three varieties: chemostats, which hold nutrient levels constant; turbidostats, which keep cell mass constant; and Plug flow reactor model, plug flow reactors in which the culture medium flows steadily through a tube while the cells are recycled from the outlet to the inlet. If the process works well, there is a steady flow of feed and effluent and the costs of repeatedly setting up a batch are avoided. Also, it can prolong the exponential growth phase and avoid byproducts that inhibit the reactions by continuously removing them. However, it is difficult to maintain a steady state and avoid contamination, and the design tends to be complex. Typically the fermentor must run for over 500 hours to be more economical than batch processors.


History of the use of fermentation

The use of fermentation, particularly for alcoholic beverage, beverages, has existed since the
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and has been documented dating from 7000 to 6600 BCE in Jiahu, Neolithic China, China, 5000 BCE in India, Ayurveda mentions many Medicated Wines, 6000 BCE in Georgia, 3150 BCE in ancient Egypt, 3000 BCE in Babylon, 2000 BCE in pre-Hispanic Mexico, and 1500 BC in Sudan.Dirar, H., (1993), The Indigenous Fermented Foods of the Sudan: A Study in African Food and Nutrition, CAB International, UK Fermented foods have a religious significance in Chametz, Judaism and Christianity and alcohol, Christianity. The Baltic mythology, Baltic god Rugutis was worshiped as the agent of fermentation. In 1837, Charles Cagniard de la Tour, Theodor Schwann and Friedrich Traugott Kützing independently published papers concluding, as a result of microscopic investigations, that yeast is a living organism that reproduces by budding. Schwann boiled grape juice to kill the yeast and found that no fermentation would occur until new yeast was added. However, a lot of chemists, including Antoine Lavoisier, continued to view fermentation as a simple chemical reaction and rejected the notion that living organisms could be involved. This was seen as a reversion to vitalism and was lampooned in an anonymous publication by Justus von Liebig and Friedrich Wöhler. The turning point came when Louis Pasteur (1822–1895), during the 1850s and 1860s, repeated Schwann's experiments and showed fermentation is initiated by living organisms in a series of investigations. In 1857, Pasteur showed lactic acid fermentation is caused by living organisms. In 1860, he demonstrated how bacteria cause souring in milk, a process formerly thought to be merely a chemical change. His work in identifying the role of microorganisms in food spoilage led to the process of pasteurization. In 1877, working to improve the French brewing industry, Pasteur published his famous paper on fermentation, "''Etudes sur la Bière''", which was translated into English in 1879 as "Studies on fermentation". He defined fermentation (incorrectly) as "Life without air",Modern History Sourcebook: Louis Pasteur (1822–1895): Physiological theory of fermentation, 1879. Translated by F. Faulkner, D.C. Robb. yet he correctly showed how specific types of microorganisms cause specific types of fermentations and specific end-products. Although showing fermentation resulted from the action of living microorganisms was a breakthrough, it did not explain the basic nature of fermentation; nor, prove it is caused by microorganisms which appear to be always present. Many scientists, including Pasteur, had unsuccessfully attempted to extract the fermentation
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. Success came in 1897 when the German chemist Eduard Buechner ground up yeast, extracted a juice from them, then found to his amazement this "dead" liquid would ferment a sugar solution, forming carbon dioxide and alcohol much like living yeasts. Buechner's results are considered to mark the birth of biochemistry. The "unorganized ferments" behaved just like the organized ones. From that time on, the term enzyme came to be applied to all ferments. It was then understood fermentation is caused by enzymes produced by microorganisms. In 1907, Buechner won the Nobel Prize in chemistry for his work. Advances in microbiology and fermentation technology have continued steadily up until the present. For example, in the 1930s, it was discovered microorganisms could be mutation, mutated with physical and chemical treatments to be higher-yielding, faster-growing, tolerant of less oxygen, and able to use a more concentrated medium. Strain Selection (biology), selection and Hybrid (biology), hybridization developed as well, affecting most modern fermentation (food), food fermentations.


Etymology

The word "ferment" is derived from the Latin verb ''fervere'', which means to boil. It is thought to have been first used in the late 14th century in alchemy, but only in a broad sense. It was not used in the modern scientific sense until around 1600.


See also

* List of fermented foods * Acetone-butanol-ethanol fermentation * Dark fermentation * Fermentation in food processing * Fermentation lock * Gut fermentation syndrome * Industrial fermentation * Non-fermenter * Photofermentation * Aerobic fermentation * Cultured Meat


References


External links


Works of Louis Pasteur
Pasteur Brewing.

{{Authority control Fermentation, Anaerobic digestion Oenology Fermented drinks, * Brewing Food science Metabolism Food preservation Alchemical processes Mycology Catalysis