Fermentation is a metabolic process that produces
chemical changes in organic substrates through
the action of enzymes.
In biochemistry, it is narrowly defined as
the extraction of energy from carbohydrates
in the absence of oxygen.
In the context of food production, it may
more broadly refer to any process in which
the activity of microorganisms brings about
a desirable change to a foodstuff or beverage.
The science of fermentation is known as zymology.
In microorganisms, fermentation is the primary
means of producing ATP by the degradation
of organic nutrients anaerobically.
Humans have used fermentation to produce foodstuffs
and beverages since the Neolithic age.
For example, fermentation is used for preservation
in a process that produces lactic acid found
in such sour foods as pickled cucumbers, kimchi,
and yogurt, as well as for producing alcoholic
beverages such as wine and beer.
Fermentation 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 process that produces alcoholic beverages
or acidic dairy products (general use).
Any large-scale microbial process occurring
with or without air (common definition used
in industry).
Any energy-releasing metabolic process that
takes place only under anaerobic conditions
(becoming more 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 photosynthesis and aerobic respiration,
fermentation is a way of extracting energy
from molecules, but it is the only one common
to all bacteria and eukaryotes.
It is therefore considered the oldest metabolic
pathway, suitable for an environment that
does not yet have oxygen.
Yeast, a form of fungus, occurs in almost
any environment capable of supporting microbes,
from the skins of fruits to the guts of insects
and mammals and the deep ocean, and they harvest
sugar-rich materials to produce ethanol and
carbon dioxide.The basic mechanism for fermentation
remains present in all cells of higher organisms.
Mammalian muscle carries out the fermentation
that occurs during periods of intense exercise
where oxygen supply becomes limited, resulting
in the creation of lactic acid.
In invertebrates, fermentation also produces
succinate and alanine.Fermentative bacteria
play an essential role in the production of
methane in habitats ranging from the rumens
of cattle to sewage digesters and freshwater
sediments.
They produce hydrogen, carbon dioxide, formate
and acetate and carboxylic acids; and 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 (which are in the domain
Archea) convert acetate to methane.
== Biochemical overview ==
Fermentation reacts NADH with an endogenous,
organic electron acceptor.
Usually this is pyruvate formed from sugar
through glycolysis.
The reaction produces NAD+ and an organic
product, typical examples being ethanol, lactic
acid, carbon dioxide, and hydrogen gas (H2).
However, more exotic compounds can be produced
by fermentation, such as butyric acid and
acetone.
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.
Fermentation normally occurs in an anaerobic
environment.
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, fermentation is rarely utilized
when oxygen is available.
However, even in the presence of abundant
oxygen, some strains of yeast such as Saccharomyces
cerevisiae prefer fermentation to aerobic
respiration as long as there is an adequate
supply of sugars (a phenomenon known as the
Crabtree effect).
Some fermentation processes involve obligate
anaerobes, which cannot tolerate oxygen.
Although yeast carries out the fermentation
in the production of ethanol in beers, wines,
and other alcoholic drinks, this is not the
only possible agent: bacteria carry out the
fermentation in the production of xanthan
gum.
== Products ==
=== 
Ethanol ===
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,
corn, and sugar beets, produces ethanol that
is added to gasoline.
In some species of fish, including goldfish
and 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.
The energy from this exothermic reaction is
used to bind inorganic phosphates to ATP and
convert NAD+ to NADH.
The pyruvates break down into two acetaldehyde
molecules and give off two carbon dioxide
molecules as a waste product.
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 catalysed by the enzymes pyruvate
decarboxylase and alcohol dehydrogenase.
=== Lactic acid ===
Homolactic fermentation (producing only lactic
acid) is the simplest type of fermentation.
The pyruvate from glycolysis undergoes a simple
redox reaction, forming lactic acid.
It is unique because it is one of the only
respiration processes to not produce a gas
as a byproduct.
Overall, one molecule of glucose (or any six-carbon
sugar) is converted to two molecules of lactic
acid:
C6H12O6 → 2 CH3CHOHCOOHIt 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 the 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-product
is mostly lactic acid, or Heterolactic fermentation,
where some lactate is further metabolized
and results in ethanol and carbon dioxide
(via the phosphoketolase pathway), acetate,
or other metabolic products, e.g.:
C6H12O6 → CH3CHOHCOOH + C2H5OH + CO2If 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 C6H12O6Heterolactic
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:
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 (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 product of fermentation) drives the
equilibrium backwards (Le Chatelier's principle),
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.
CO2 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,
that forms 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 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.
=== Hydrogen gas ===
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.
Hydrogen gas is a substrate for methanogens
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.
As an example of mixed acid fermentation,
bacteria such as Clostridium pasteurianum
ferment glucose producing butyrate, acetate,
carbon dioxide, and hydrogen gas: The reaction
leading to acetate is:
C6H12O6 + 4 H2O → 2 CH3COO− + 2 HCO3−
+ 4 H+ + 4 H2Glucose could theoretically be
converted into just CO2 and H2, but the global
reaction releases little energy.
== 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.
Thermophilic bacteria can produce lactic acid
at temperatures of around 50 degrees 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.
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
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 human use ==
The use of fermentation, particularly for
beverages, has existed since the Neolithic
and has been documented dating from 7000–6600
BCE in Jiahu, 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.
Fermented foods have a religious significance
in Judaism and Christianity.
The 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 that fermentation is
initiated by living organisms in a series
of investigations.
In 1857, Pasteur showed that lactic acid fermentation
is caused by living organisms.
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.
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", 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, or prove
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.
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.
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 that fermentation is
caused by enzymes that are 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
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.
Strain selection and hybridization developed
as well, affecting most modern 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
