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 case of lactic acid
fermentation. Fermentation is also used more
broadly to refer to the bulk growth of microorganisms
on a growth medium. 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 absence of
oxygen and becomes the cell’s primary means
of ATP production. It turns NADH and pyruvate
produced in the glycolysis step into NAD+
and various small molecules. In the presence
of O2, NADH and pyruvate are used in respiration;
this is oxidative phosphorylation, it generates
a lot more ATP in addition to that created
by glycolysis, and 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.
In oxidative phosphorylation the energy for
ATP formation is derived from an electrochemical
proton gradient generated across the inner
mitochondrial membrane via the electron transport
chain. Glycolysis has substrate-level phosphorylation.
Fermentation has been used by humans for the
production of 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,
as well as for producing alcoholic beverages
such as wine and beer. Fermentation can even
occur within the stomachs of animals, such
as humans. Auto-brewery syndrome is a rare
medical condition where the stomach contains
brewers yeast that break down starches into
ethanol; which enters the blood stream.
Examples
Fermentation does not necessarily have to
be carried out in an anaerobic environment.
For example, even in the presence of abundant
oxygen, yeast cells greatly prefer fermentation
to aerobic respiration, as long as sugars
are readily available for consumption. The
antibiotic activity of hops also inhibits
aerobic metabolism in yeast.
Fermentation reacts NADH with an endogenous,
organic electron acceptor. 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:
homolactic fermentation is the production
of lactic acid exclusively
heterolactic fermentation is the production
of lactic acid as well as other acids and
alcohols.
Sugars are the most common substrate of fermentation,
and typical examples of fermentation products
are ethanol, lactic acid, carbon dioxide,
and hydrogen gas. 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
production of large quantities of carbon dioxide.
Fermentation occurs in mammalian muscle during
periods of intense exercise where oxygen supply
becomes limited, resulting in the creation
of lactic acid.
Chemistry
Fermentation products contain chemical energy,
but are considered waste products, since they
cannot be metabolized further without the
use of oxygen.
Ethanol fermentation
The chemical equation below shows the alcoholic
fermentation of glucose, whose chemical formula
is C6H12O6. 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.
Lactic acid fermentation
Homolactic fermentation 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 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
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-product is mostly
lactic acid, or
Heterolactic fermentation, where some lactate
is further metabolized and results in ethanol
and carbon dioxide, acetate, or other metabolic
products, e.g.: C6H12O6 → CH3CHOHCOOH +
C2H5OH + CO2
If lactose is fermented, it is first converted
into glucose and galactose: C12H22O11 + H2O
→ 2 C6H12O6
Heterolactic fermentation is in a sense intermediate
between lactic acid fermentation, and other
types, e.g. alcoholic fermentation. 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 who
are unadapted to the acidity; as a result
the food will have a longer shelf-life; however,
beyond a certain point, the acidity starts
affecting the organism that produces it.
The high concentration of lactic acid drives
the equilibrium backwards, 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 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, 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, the amount
of acidity produced per glucose consumed will
decrease, as with ethanol, allowing faster
growth.
Aerobic respiration
In aerobic respiration, the pyruvate produced
by glycolysis is oxidized completely, generating
additional ATP and NADH in the citric acid
cycle and by oxidative phosphorylation. However,
this can occur only 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.
Hydrogen gas production in fermentation
Hydrogen gas is produced in many types of
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 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 carbon dioxide:
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 of the carboxylic group
to the methyl group of acetic acid to respectively
produce CO2 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, 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.
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 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, during the 1850s and 1860s,
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 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. 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 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. 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
fourteenth century in alchemy, but only in
a broad sense. It was not used in the modern
scientific sense until around 1600.
See also
References
External links
Works of Louis Pasteur Pasteur Brewing.
The chemical logic behind fermentation and
respiration
