Fermentation is a metabolic process by which microorganisms like yeast and bacteria convert sugars into acids, gases, and alcohol. Key points: - Glucose breaks down into ethanol and carbon dioxide during alcoholic fermentation by yeast. Lactic acid bacteria produce lactic acid through lactic acid fermentation. - Fermentation occurs without oxygen and generates ATP through substrate-level phosphorylation rather than oxidative phosphorylation. It allows regeneration of NAD+ to continue glycolysis. - Humans have harnessed fermentation for thousands of years to produce important foods and beverages like bread, wine, beer, yogurt, and pickles. Louis Pasteur's work established fermentation as a microbial process.

1. Fermentation process
Fermentation in progress: Bubbles of CO2 form a froth on top of the fermentation
mixture.
Overview of ethanol fermentation. One glucose molecule breaks down into two
pyruvate molecules (1). The energy from this exothermic reaction is used to bind
inorganic phosphates to ADP and convert NAD+ to NADH. The two pyruvates are
then broken down into two acetaldehyde molecules and give off two CO2
molecules as a waste product(2). The acetaldehyde is then reduced into ethanol
using the energy and hydrogen from NADH; in this process the NADH is oxidized
into NAD+ so that the cycle may repeat (3).

2. Fermentation is a metabolic process that converts sugar to acids, gases, and/or
alcohol. It occurs in yeast and bacteria, but also in oxygen-starved muscle cells, as
in the caseof lactic acid fermentation. Fermentation is also used more broadly to
refer to the bulk growth of microorganisms on a growth medium, often with the
goal of producing a specific chemical product. French microbiologist Louis Pasteur
is often remembered for his insights into fermentation and its microbial causes.
The science of fermentation is known as zymology.
Fermentation takes place in the lack of oxygen (when the electron transport chain
is unusable) and becomes the cell’s primary means of ATP (energy) production.[1]
It turns NADH and pyruvate produced in the glycolysis step into NAD+ and
various small molecules depending on the type of fermentation (see examples
below). In the presence of O2, NADH and pyruvate are used to generate ATP in
respiration. This is called oxidative phosphorylation, and it generates much more
ATP than glycolysis alone. For that reason, cells generally benefit from avoiding
fermentation when oxygen is available. Exceptions include obligate anaerobes,
which cannot tolerate oxygen.
The first step, glycolysis, is common to all fermentation pathways:
C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi → 2 CH3COCOO− + 2 NADH + 2 ATP
+ 2 H2O + 2H+
Pyruvate is CH3COCOO−. Pi is phosphate. Two ADP molecules and two Pi are
converted to two ATP and two water molecules via substrate-level
phosphorylation. Two molecules of NAD+ are also reduced to NADH.[2]
In oxidative phosphorylation the energy for ATP formation is derived from an
electrochemical protongradient generated across the inner mitochondrial
membrane (or, in the case of bacteria, the plasma membrane) via the electron
transport chain. Glycolysis has substrate-level phosphorylation (ATP generated
directly at the point of reaction).
Fermentation has been used by humans for the productionof food and beverages
since the Neolithic age. For example, fermentation is employed for preservation in
a process that produces lactic acid as found in such sour foods as pickled
cucumbers, kimchi and yogurt (see fermentation in food processing), as well as for
producing alcoholic beverages such as wine (see fermentation in winemaking) and
beer. Fermentation can even occurwithin the stomachs of animals, such as
humans. Auto-brewery syndrome is a rare medical condition where the stomach

3. contains brewers yeast that break down starches into ethanol; which enters the
blood stream.[3]
Definitions
To many people, fermentation simply means the production of alcohol: grains and
fruits are fermented to producebeer and wine. If a food soured, one might say it
was 'off' or fermented. Here are some definitions of fermentation. They range from
informal, general usage to more scientific definitions.[4]
1. Any spoilage of food by microorganisms (general use).
2. Any process that produces alcoholic beverages or acidic dairy products
(general use).
3. Any large-scale microbial process occurring with or without air (common
definition used in industry).
4. Any energy-releasing metabolic process that takes place only under
anaerobic conditions (becoming more scientific).
5. Any metabolic process that releases energy from a sugar or other organic
molecules, does not require oxygen or an electron transport system, and uses
an organic molecule as the final electron acceptor(most scientific).
Examples
Fermentation does not necessarily have to be carried out in an anaerobic
environment. Forexample, even in the presence of abundant oxygen, yeast cells
greatly prefer fermentation to aerobic respiration, as long as sugars are readily
available for consumption (a phenomenon known as the Crabtree effect).[5] The
antibiotic activity of hops also inhibits aerobic metabolism in yeast.
Fermentation reacts NADH with an endogenous, organic electron acceptor.[1]
Usually this is pyruvate formed from the sugar during the glycolysis step. During
fermentation, pyruvate is metabolized to various compounds through several
processes:
 ethanol fermentation, aka alcoholic fermentation, is the production of
ethanol and carbon dioxide
 lactic acid fermentation refers to two means of producing lactic acid:
1. homolactic fermentation is the productionof lactic acid exclusively
2. heterolactic fermentation is the production of lactic acid as well as other
acids and alcohols.

4. Sugars are the most common substrate of fermentation, and typical examples of
fermentation products are ethanol, lactic acid, carbon dioxide, and hydrogen gas
(H2). However, more exotic compounds can be produced by fermentation, such as
butyric acid and acetone. Yeast carries out fermentation in the production of
ethanol in beers, wines, and other alcoholic drinks, along with the productionof
large quantities of carbondioxide. Fermentation occurs in mammalian muscle
during periods of intense exercise where oxygen supply becomes limited, resulting
in the creation of lactic acid.[6]
Chemistry
Comparison of aerobic respiration and most known fermentation types in
eucaryotic cell.[7] Numbers in circles indicate counts of carbonatoms in molecules,
C6 is glucose C6H12O6, C1 carbondioxide CO2. Mitochondrial outer membrane is
omitted.
Fermentation products contain chemical energy (they are not fully oxidized), but
are considered waste products, since they cannot be metabolized further without
the use of oxygen.

5. Ethanol fermentation
Main article: Ethanolfermentation
The chemical equation below shows the alcoholic fermentation of glucose, whose
chemical formula is C6H12O6.[8] One glucose molecule is converted into two
ethanol molecules and two carbon dioxide molecules:
C6H12O6 → 2 C2H5OH + 2 CO2
C2H5OH is the chemical formula for ethanol.
Before fermentation takes place, one glucose molecule is broken down into two
pyruvate molecules. This is known as glycolysis.[8][9]
Lactic acid fermentation
Main article: Lactic acid fermentation
Homolactic fermentation (producing only lactic acid) is the simplest type of
fermentation. The pyruvate from glycolysis[10] undergoes a simple redox reaction,
forming lactic acid.[2][11] It is unique because it is one of the only respiration
processes to not producea gas as a byproduct. 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
can supply oxygen. It also occurs in some kinds of bacteria (such as lactobacilli)
and some fungi. It is this type of bacteria that converts lactose into lactic acid in
yogurt, giving it its sour taste. These lactic acid bacteria can carry out either
homolactic fermentation, where the end-productis mostly lactic acid, or
Heterolactic fermentation, where some lactate is further metabolized and results
in ethanol and carbon dioxide[2] (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, and other types, e.g. alcoholic fermentation (see below). The reasons
to go further and convert lactic acid into anything else are:

6.  The acidity of lactic acid impedes biological processes;this can be beneficial
to the fermenting organism as it drives out competitors who are unadapted to
the acidity; as a result the food will have a longer shelf-life (part of the
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 productof fermentation)
drives the equilibrium backwards (Le Chatelier's principle), decreasing the
rate at which fermentation can occur, and slowing down growth
 Ethanol, that lactic acid can be easily converted to, is volatile and will
readily escape, allowing the reaction to proceed easily. CO2 is also
produced, however it's 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 a lot of additional energy. It is a lighter molecule than lactic
acid, that forms fewer hydrogen bonds with its surroundings (due to having
fewer groups that can form such bonds), and thus more volatile and will also
allow the reaction to move forward more quickly.
 If propionic acid, butyric acid and longer monocarboxylic acids are
produced (see mixed acid fermentation), the amount of acidity produced per
glucose consumed will decrease, as with ethanol, allowing faster growth.
Aerobic respiration
In aerobic respiration, the pyruvate produced byglycolysis is oxidized completely,
generating additional ATP and NADH in the citric acid cycle and by oxidative
phosphorylation. However, this can occuronly in the presence of oxygen. Oxygen
is toxic to organisms that are obligate anaerobes, and is not required by facultative
anaerobic organisms. In the absence of oxygen, one of the fermentation pathways
occurs in order to regenerate NAD+; lactic acid fermentation is one of these
pathways.[2]
Hydrogen gas production in fermentation
Hydrogen gas is produced in many types of fermentation (mixed acid fermentation,
butyric acid fermentation, caproate fermentation, butanol fermentation, glyoxylate
fermentation), as a way to regenerate NAD+ from NADH. Electrons are transferred
to ferredoxin, which in turn is oxidized by hydrogenase, producing H2.[8] Hydrogen
gas is a substrate for methanogens and sulfate reducers, which keep the
concentration of hydrogen low and favor the productionof such an energy-rich

7. compound,[12] but hydrogen gas at a fairly high concentration can nevertheless be
formed, as in flatus.
As an example of mixed acid fermentation, bacteria suchas Clostridium
pasteurianum ferment glucose producing butyrate, acetate, carbondioxide and
hydrogen gas:[13] The reaction leading to acetate is:
C6H12O6 + 4 H2O → 2 CH3COO- + 2 HCO3
- + 4 H+ + 4 H2
Glucose could theoretically be converted into just CO2 and H2, but the global
reaction releases little energy.
Methane gas production in fermentation
Acetic acid can also undergo a dismutation reaction to produce methane and
carbondioxide:[14][15]
CH3COO– + H+ → CH4 + CO2 ΔG° = -36 kJ/reaction
This disproportionation reaction is catalysed by methanogen archaea in their
fermentative metabolism. One electron is transferred from the carbonyl function
(e– donor) of the carboxylic group to the methyl group (e– acceptor)of acetic acid
to respectively produceCO2 and methane gas.
History
The use of fermentation, particularly for beverages, has existed since the Neolithic
and has been documented dating from 7000–6600 BCE in Jiahu, China,[16] 6000
BCE in Georgia,[17] 3150 BCE in ancient Egypt,[18] 3000 BCE in Babylon,[19] 2000
BCE in pre-Hispanic Mexico,[19] and 1500 BC in Sudan.[20] Fermented foods have
a religious significance in Judaism and Christianity. The Baltic god Rugutis was
worshiped as the agent of fermentation.[21][22]
The first solid evidence of the living nature of yeast appeared between 1837 and
1838 when three publications appeared by C. Cagniard de la Tour, T. Swann, and
F. Kuetzing, each of whom independently concluded as a result of microscopic
investigations that yeast is a living organism that reproduces by budding. It is
perhaps because wine, beer, and bread were each basic foods in Europe that most
of the early studies on fermentation were done on yeasts, with which they were
made. Soon, bacteria were also discovered; the term was first used in English in

8. the late 1840s, but it did not come into general use until the 1870s, and then largely
in connection with the new germ theory of disease
Louis Pasteur in his laboratory
Louis Pasteur (1822–1895), during the 1850s and 1860s, showed that fermentation
is initiated by living organisms in a series of investigations.[11] In 1857, Pasteur
showed that lactic acid fermentation is caused by living organisms.[24] In 1860, he
demonstrated that bacteria cause souring in milk, a process formerly thought to be
merely a chemical change, and his work in identifying the role of microorganisms
in food spoilage led to the process of pasteurization.[25] In 1877, working to
improve the French brewing industry, Pasteur published his famous paper on
fermentation, "Etudessur la Bière", which was translated into English in 1879 as
"Studies on fermentation".[26] He defined fermentation (incorrectly) as "Life
without air",[27] but correctly showed that specific types of microorganisms cause
specific types of fermentations and specific end-products.
Although showing fermentation to be the result of the action of living
microorganisms was a breakthrough, it did not explain the basic nature of the
fermentation process, orprove that it is caused by the microorganisms that appear
to be always present. Many scientists, including Pasteur, had unsuccessfully
attempted to extract the fermentation enzyme from yeast.[27] Success came in 1897
when the German chemist Eduard Buechner ground up yeast, extracted a juice
from them, then found to his amazement that this "dead" liquid would ferment a
sugar solution, forming carbon dioxide and alcohol much like living yeasts.[28]
Buechners 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 that

9. fermentation is caused by enzymes that are produced by microorganisms.[29] In
1907, Buechner won the Nobel Prize in chemistry for his work.[30]
Advances in microbiology and fermentation technology have continued steadily up
until the present. Forexample, in the late 1970s, it was discovered that
microorganisms could be mutated with physical and chemical treatments to be
higher-yielding, faster-growing, tolerant of less oxygen, and able to use a more
concentrated medium.[31] Strain selection and hybridization developed as well,
affecting most modern food fermentations.
Fermentation in food processing
Beer fermenting at a brewery
Fermentation in food processing is the conversion of carbohydrates to alcohols and
carbondioxide or organic acids using yeasts, bacteria, or a combination thereof,
under anaerobic conditions. Fermentation usually implies that the action of
microorganisms is desirable. The science of fermentation is also known as
zymology or zymurgy.
The term "fermentation" is sometimes used to specifically refer to the chemical
conversion of sugars into ethanol, a process which is used to producealcoholic
beverages such as wine, beer, and cider. Fermentation is also employed in the
leavening of bread (CO2 produced byyeast activity); in preservation techniques to
producelactic acid in sour foods suchas sauerkraut, dry sausages, kimchi, and
yogurt; and in pickling of foods with vinegar (acetic acid).

10. See also: History of wine and History of beer
Natural fermentation precedes human history. Since ancient times, however,
humans have been controlling the fermentation process.The earliest evidence of an
alcoholic beverage, made from fruit, rice, and honey, dates from 7000–6600 BCE,
in the Neolithic Chinese village of Jiahu,[1] and winemaking dates from 6000
BCE, in Georgia, in the Caucasus area.[2] Seven-thousand-year-old jars containing
the remains of wine, now on display at the University of Pennsylvania, were
excavated in the Zagros Mountains in Iran. [3] There is strong evidence that people
were fermenting beverages in Babylon circa 3000 BC,[4] ancient Egypt circa 3150
BC,[5] pre-Hispanic Mexico circa 2000 BC,[4] and Sudan circa 1500 BC.[6]
French chemist Louis Pasteur was the first known zymologist, when in 1856 he
connected yeast to fermentation.[7] Pasteur originally defined fermentation as
"respiration without air". Pasteur performed careful research and concluded:
I am of the opinion that alcoholic fermentation never occurs without simultaneous
organization, development, and multiplication of cells, ... . If asked, in what
consists the chemical act whereby the sugar is decomposed, ... , I am completely
ignorant of it.
Contributions to biochemistry
Main articles: History of biochemistry and NADH § History
When studying the fermentation of sugar to alcohol by yeast, Louis Pasteur
concluded that the fermentation was catalyzed by a vital force, called "ferments,"
within the yeast cells. The "ferments" were thought to function only within living
organisms. "Alcoholic fermentation is an act correlated with the life and
organization of the yeast cells, not with the death or putrefaction of the cells,"[8]
he wrote.
Nevertheless, it was known that yeast extracts can ferment sugar even in the
absence of living yeast cells. While studying this process in 1897, Eduard Buchner
of Humboldt University of Berlin, Germany, found that sugar was fermented even
when there were no living yeast cells in the mixture,[9] by a yeast secretion that he
termed zymase.[10] In 1907 he received the Nobel Prize in Chemistry for his
research and discovery of "cell-free fermentation."
One year prior, in 1906, ethanol fermentation studies led to the early discovery of
NAD+.[11]

11. Uses
Beer and bread, two major uses of fermentation in food
The primary benefit of fermentation is the conversion of sugars and other
carbohydrates into preservative organic acids, e.g. converting juice into wine,
grains into beer, carbohydrates into carbondioxide to leaven bread, and sugars in
vegetables.
Food fermentation has been said to serve five main purposes:[12]
 Enrichment of the diet through development of a diversity of flavors,
aromas, and textures in food substrates
 Preservation of substantial amounts of food through lactic acid, alcohol,
acetic acid, and alkaline fermentations
 Biological enrichment of food substrates with protein, essential amino acids,
and vitamins
 Elimination of antinutrients
 A decrease in cooking time and fuel requirement
Fermented foods by region
Nattō, a Japanese fermented soybean food

12.  Worldwide: alcohol, wine, vinegar, olives, yogurt, bread, cheese
 Asia
o East and Southeast Asia: amazake, atchara, bai-ming, belacan, burong
mangga, com ruou, dalok, doenjang, douchi, jeruk, lambanog, kimchi,
kombucha, leppet-so, narezushi, miang, miso, nata de coco,nata de
pina, natto, naw-mai-dong, oncom, pak-siam-dong, paw-tsaynob,
prahok, ruou nep, sake, seokbakji, soju, soy sauce, stinky tofu,
szechwan cabbage, tai-tan tsoi, chiraki, tape, tempeh, totkal kimchi,
yen tsai, zha cai
o Central Asia: kumis (mare milk), kefir, shubat (camel milk)
o South Asia: achar, appam, dosa, dhokla, dahi (yogurt), idli, kaanji,
mixed pickle, ngari, hawaichaar, jaand (rice beer), sinki, tongba,
paneer
 Africa: fermented millet porridge, garri, hibiscus seed, hot pepper sauce,
injera, lamoun makbouss, laxoox, mauoloh, msir, mslalla, oilseed, ogi, ogili,
ogiri, iru
 Americas: sourdough bread, cultured milk, chicha, elderberry wine,
kombucha, pickling (pickled vegetables), sauerkraut, lupin seed, oilseed,
chocolate, vanilla, tabasco, tibicos, pulque, mikyuk (fermented bowhead
whale)
 Middle East: kushuk, lamoun makbouss, mekhalel, torshi, boza
 Europe: rakfisk, sauerkraut, pickled cucumber, surströmming, mead,
elderberry wine, salami, prosciutto, cultured milk products suchas quark,
kefir, filmjölk, crème fraîche, smetana, skyr.
 Oceania: poi, kaanga pirau (rotten corn), sago
Fermented foods by type
Bean-based[edit]
Cheonggukjang, doenjang, miso, natto, soysauce, stinky tofu, tempeh, oncom,
soybean paste, Beijing mung bean milk, kinama, iru
Grain-based[edit]

13. Batter made from rice and lentil (Vigna mungo) prepared and fermented for baking
idlis and dosas
Amazake, beer, bread, choujiu, gamju, injera, kvass, makgeolli, murri, ogi, sake,
sikhye, sourdough, sowans, rice wine, malt whisky, grain whisky, idli, dosa, vodka
Vegetable based[edit]
Kimchi, mixed pickle, sauerkraut, Indian pickle, gundruk
Fruit based[edit]
Wine, vinegar, cider, perry, brandy, atchara, nata de coco, burongmangga, asinan,
pickling, vişinată
Honey based[edit]
Mead, metheglin
Dairy based[edit]
Cheese, kefir, kumis (mare milk), shubat (camel milk), cultured milk products such
as quark, filmjölk, crème fraîche, smetana, skyr, yogurt
Fish based[edit]

14. Bagoong, faseekh, fish sauce, Garum, Hákarl, jeotgal, rakfisk, shrimp paste,
surströmming, shidal
Meat based[edit]
Chin som mok is a northern Thai speciality made with grilled, banana leaf-
wrapped pork (both skin and meat) that has been fermented with glutinous rice
Jamón ibérico, Chorizo, Salami, Pepperoni, Nem chua, Som moo
Tea based[edit]
Pu-erh tea, Kombucha
Risks of consuming fermented foods[edit]
Alaska has witnessed a steady increase of cases of botulism since 1985.[13] It has
more cases of botulism than any other state in the United States of America. This is
caused by the traditional Eskimo practice of allowing animal products suchas
whole fish, fish heads, walrus, sea lion, and whale flippers, beaver tails, seal oil,
birds, etc., to ferment for an extended period of time before being consumed. The
risk is exacerbated when a plastic container is used for this purposeinstead of the
old-fashioned, traditional method, a grass-lined hole, as the botulinum bacteria
thrive in the anaerobic conditions created by the air-tight enclosure in plastic.[13]
The World Health Organization has classified pickled foods as a possible
carcinogen, based on epidemiological studies.[14] Other research found that
fermented food contains a carcinogenic by-product, ethyl carbamate
(urethane).[15][16] "A 2009 review of the existing studies conducted across Asia
concluded that regularly eating pickled vegetables roughly doubles a person's risk
for esophageal squamous cell carcinoma.