Fermentation
Fermentation is a process of energy production in a cell in an anaerobic environment (with no oxygen present). In common usage fermentation is a type of anaerobic respiration, however a more strict definition exists which defines fermentation as respiration in an anaerobic environment with no external electron acceptor.
In its strictest sense, fermentation (formerly called zymosis) is the anaerobic metabolic breakdown of a nutrient molecule, such as glucose, without net oxidation. Fermentation does not release all the available energy in a molecule; it merely allows glycolysis (a process that yields two ATP per glucose) to continue by replenishing reduced coenzymes. Depending on which organism it is taking place in, fermentation may yield lactate, acetic acid, ethanol, or other reduced metabolites. Yeast produces ethanol and CO2; human muscle (under anaerobic conditions) produces lactic acid.
Fermentation is also used much more broadly to refer to the bulk growth of microorganisms on a growth medium. No distinction is made between aerobic and anaerobic metabolism when the word is used in this sense.
Fermentation usually implies that the action of the microorganisms is desirable. Occasionally wines are enhanced through the process of cofermentation. When fermentation stops prior to complete conversion of sugar to alcohol, a stuck fermentation is said to have occurred.
Sugars are the common substrate of fermentation, and typical examples of fermentation products are ethanol, lactic acid, and hydrogen. However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone. Yeast famously 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. Anaerobic respiration in mammalian muscle under periods of intense exercise (which has no external electron acceptor) is, under the strict definition, a type of fermentation.
History
French chemist Louis Pasteur was the first zymologist, when in 1857 he connected yeast to fermentation. 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.".
The German Eduard Buchner, winner of the 1907 Nobel Prize in chemistry, later determined that fermentation was actually caused by a yeast secretion that he termed zymase.
muscles - 1920s
The research efforts undertaken by the Danish Carlsberg scientists greatly accelerated the gain of knowledge about yeast and brewing. The Carlsberg scientists are generally acknowledged with jump-starting the entire field of molecular biology?
Reaction
The reaction of fermentation differs according to the sugar being used and the product produced. Below the sugar will be glucose (C6H12O6) the simplest sugar, and the product will be ethanol (2C2H5OH). This is one of the fermentation reactions carried out by yeast, and is used in food production.
Chemical Equation
- C6H12O6 → 2C2H5OH + 2CO2 + 2 ATP (Energy Released:118 kJ mol−1)
Word Equation
- Sugar (glucose, fructose, or sucrose) → Alcohol (ethanol) + Carbon Dioxide + Energy (ATP)
Energy source in anaerobic conditions
Fermentation is thought to have been the primary means of energy production in earlier organisms before oxygen was at high concentration in the atmosphere and thus would represent a more ancient form of energy production in cells.
Fermentation products contain chemical energy (they are not fully oxidized) but are considered waste products since they cannot be metabolised further without the use of oxygen (or other more highly-oxidized electron acceptors). A consequence is that the production of ATP by fermentation is less efficient than oxidative phosphorylation, where pyruvate is fully oxidized to carbon dioxide. Fermentation produces two ATP molecules per molecule of glucose compared to approximately 36 by aerobic respiration.
Aerobic glycolysis is a method employed by muscle cells for the production of lower-intensity energy over a longer period of time when oxygen is plentiful. Under low-oxygen conditions, however, vertebrates use the less-efficient but faster anaerobic glycolysis to produce ATP. The speed at which ATP is produced is about 100 times that of oxidative phosphorylation.[citation needed] While fermentation is helpful during short, intense periods of exertion, it is not sustained over extended periods in complex aerobic organisms. In humans, for example, lactic acid fermentation provides energy for a period ranging from 30 seconds to 2 minutes.
The final step of fermentation, the conversion of pyruvate to fermentation end-products, does not produce energy. However, it is critical for an anaerobic cell since it regenerates nicotinamide adenine dinucleotide (NAD+), which is required for glycolysis. This is important for normal cellular function, as glycolysis is the only source of ATP in anaerobic conditions.
Products
Although human metabolism is primarily aerobic, in the partial or complete absence of oxygen (for example, in overworked muscles that are starved of oxygen or in infarcted heart muscle cells), pyruvate can be converted to the waste product lactate, donating its hydrogen to pyruvate. (The lactic-acid buildup in our muscles causes "the burn" we associate with intense exercise.) This reaction, which is an example of a fermentation, is a solution to maintaining the metabolic flux through glycolysis in the absence of oxygen or when oxygen levels are low.
There are several types of fermentations in which pyruvate and NADH are anaerobically metabolized to yield any of a variety of products with an organic molecule acting as the final hydrogen acceptor. For example, the bacteria involved in making yogurt simply reduce pyruvate to lactic acid. In organisms such as brewers' yeast, a carboxyl group is first removed from pyruvate to form acetaldehyde and carbon dioxide; the acetaldehyde is then reduced to yield ethanol and NAD+. Anaerobic bacteria are capable of using a wide variety of compounds other than oxygen as terminal electron acceptors.
Products produced by fermentation are actually waste products produced during the reduction of pyruvate to regenerate NAD+ in the absence of oxygen. Bacteria generally produce acids. Vinegar (acetic acid) is the direct result of bacterial metabolism (Bacteria need oxygen to convert the alcohol to acetic acid). In milk, the acid coagulates the casein, producing curds. In pickling, the acid preserves the food from pathogenic and putrefactive bacteria.
When yeast ferments, it breaks down the glucose (C6H12O6) into exactly two molecules of ethanol (C2H6O) and two molecules of carbon dioxide (CO2).
- Ethanol fermentation (performed by yeast and some types of bacteria) breaks the pyruvate down into ethanol and carbon dioxide. It is important in bread-making, brewing, and wine-making. When the ferment has a high concentration of pectin, minute quantities of methanol can be produced. Usually only one of the products is desired; in bread the alcohol is baked out, and in alcohol production the carbon dioxide is released into the atmosphere.
- Lactic acid fermentation breaks down the pyruvate into lactic acid. It occurs in the muscles of animals when they need energy faster than the blood can supply oxygen. It also occurs in some bacteria and some fungi. It is this type of bacteria that convert lactose into lactic acid in yogurt, giving it its sour taste.
In vertebrates, during intense exercise, cellular respiration will deplete oxygen in the muscles faster than it can be replenished. An associated burning sensation in muscles has been attributed lactic acid causing a decrease in the pH during a shift to anaerobic glycolysis.
Fermentation in food production
Fermentation typically refers to the conversion of sugar to alcohol using yeast under anaerobic conditions. A more general definition of fermentation is the chemical conversion of carbohydrates into alcohols or acids. The science of fermentation is known as zymology.
The process is used to produce wine, beer, and vinegar, but fermentation is also employed in preservation to create lactic acid in sour foods such as pickled cucumbers, kimchi and yogurt. Bacteria, often in combination with yeasts and molds, are used in the preparation of fermented foods such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine, and yogurt.
History
Since fruits ferment naturally, fermentation precedes human history. Since prehistoric times, however, humans have been taking control of the fermentation process. The earliest evidence of winemaking dates from 6000 B.C.E., in Georgia, the former Soviet Republic.[1] 7000 year old jars of wine have been excavated in the Zagros Mountains, which are now on display at the University of Pennsylvania.[2] There is strong evidence that people were fermenting beverages in Babylon circa 5000 B.C.E.,[citation needed] ancient Egypt circa 3000 B.C.E.,[citation needed] pre-Hispanic Mexico circa 2000 B.C.E.,[citation needed] and Sudan circa 1500 B.C.E.[citation needed] There is also evidence of leavened bread in ancient Egypt circa 1500 B.C.E.[citation needed] and of milk fermentation in Babylon circa 3000 B.C.E.[citation needed] The Chinese were probably the first to develop vegetable fermentation.
Uses
The primary benefit of fermentation is the conversion of sugars and other carbohydrates, e.g., converting juice into wine, grains into beer, carbohydrates into carbon dioxide to leaven bread, and sugars in vegetables into preservative organic acids.
According to Steinkraus (1995), food fermentation serves five main purposes:
- 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, essential fatty acids, and vitamins.
- Detoxification during food-fermentation processing.
- A decrease in cooking times and fuel requirements.
Fermentation has some uses exclusive to foods. Fermentation can produce important nutrients or eliminate antinutrients. Food can be preserved by fermentation, since fermentation uses up food energy and can make conditions unsuitable for undesirable microorganisms. For example, in pickling the acid produced by the dominant bacteria inhibit the growth of all other microorganisms. Depending on the type of fermentation, some products (e.g., fusel alcohol) can be harmful to people's health.
Fermented foods by region
- Worldwide: alcohol, wine, vinegar, olives, yogurt, bread
- Asia
- East and Southeast Asia: amazake, asinan, bai-ming, belacan, burong mangga, dalok, doenjang (된장), douchi, jeruk, lambanog, kimchi (김치), kombucha, leppet-so, narezushi, miang, miso, nata de coco, nata de pina, natto, naw-mai-dong, pak-siam-dong, paw-tsaynob in snow (雪裡蕻), prahok, sake, seokbakji, soy sauce, stinky tofu, szechwan cabbage (四川泡菜), tai-tan tsoi, takuan, tape, tempeh, totkal kimchi, yen tsai (醃菜), zha cai (榨菜)
- Central Asia: kumis (mare milk), kefir, shubat (camel milk)
- India: achar, appam, dosa, dhokla, dahi, gundruk, idli, mixed pickle
- Africa: garri, hibiscus seed, hot pepper sauce, injera, lamoun makbouss, laxoox, mauoloh, msir, mslalla, oilseed, ogi, ogili, ogiri
- Americas: cheese, chicha, elderberry wine, kombucha, pickling (pickled vegetables), sauerkraut, lupin seed, oilseed, chocolate, vanilla, tabasco, tibicos
- Middle East: kushuk, lamoun makbouss, mekhalel, torshi, boza
- Europe: cheese, rakfisk, sauerkraut, surströmming, soured milk products such as quark, kefir, filmjölk, crème fraîche, smetana, skyr, mead elderberry wine.
- Oceania: poi, kaanga pirau (rotten corn)
Fermentation vs. putrefaction and rancidification
There are a number of animal-based foods from different parts of the world that are described as being "fermented." However, the term is erroneous when applied to such foods because fermentation properly means the decomposition of carbohydrates, and since animal tissues are composed of proteins and lipids, and contain at most only traces of carbohydrates, the operative processes in the transformation undergone by these foods are actually putrefaction and rancidification.
The difference is more than technical since the end products of these processes are quite different from those of fermentation, and also because putrefied/rancidified foods are often dangerous for human consumption. For instance, Alaska, despite its small population, witnesses more cases of botulism than any other U.S. state [1]. This is caused by the traditional Eskimo practice of allowing animal products such as whole fish, fish heads, walrus, sea lion and whale flippers, beaver tails, seal oil, birds, etc., to decompose for an extended period of time before being consumed raw. The risk is exacerbated when a plastic container is used for this purpose instead of the old-fashioned method, a grass-lined hole, as the botulinum bacteria thrive in the anaerobic conditions created by the former method.
Other putrefied/rancidified foods include fish sauce from Southeast Asia, Icelandic hákarl, fermented Baltic herring and certain speciality sausages from Sweden, and Limburger cheese. Most putrefied/rancified foods are considered to have an exceptionally foul odor, but if the process of decomposition is allowed to reach completion, the smell is greatly diminished, as with some varieties of fish sauce.
ReferencesISBN links support NWE through referral fees
- Steinkraus, K. H., Ed. 1995. Handbook of Indigenous Fermented Foods. New York, NY: Marcel Dekker, Inc.
- Stryer, L. 1995. Biochemistry, 4th edition. New York, NY: W.H. Freeman.
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