Fermentation

In biochemistry, fermentation is an enzyme-catalyzed, energy-generating process in which organic compounds act as both donors and acceptors of electrons. Fermentation can occur in the absence of oxygen; in fact, the term can also be defined as the pathway in cells by which fuel molecules are broken down anaerobically.

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.

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, such as butyric acid and acetone.

Glycolysis is an important type of fermentation that is common to muscle cells, yeast, some bacteria, and plants. In glycolysis, the six-carbon sugar glucose (Glc) is oxidized to two molecules of pyruvic acid (Pyr), yielding a small net gain of chemical energy (ATP) to power cellular function. There are two main pathways for the pyruvate end-product in the absence of oxygen:

• Ethanol fermentation (performed by yeast and some types of bacteria) breaks the pyruvate down into ethanol and carbon dioxide. Yeast famously carries out fermentation in the production of ethanol in beers, wines and other alcoholic drinks.
• Lactic acid fermentation breaks down the pyruvate into lactic acid. It occurs in some bacteria and fungi – for example, lactic acid fermentation occurs in the production of yogurt when bacteria convert lactose into lactic acid, giving yogurt its sour taste. In vertebrates, during periods of intense exercise, cellular respiration may deplete oxygen in the muscles faster than it can be replenished. The shift to glycolysis produces lactic acid, which is associated with an accompanying burning sensation in muscles.

In an industrial context, the term fermentation is used 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. There are also many microbiological processes that go on in the presence of air while yielding incomplete oxidation products. Good examples are the formation of acetic acid (vinegar) from alcohol by vinegar bacteria, and of citric acid from sugar by certain molds (for example, Aspergillus niger). These microbial processes, too, have gained industrial importance, and are often referred to as fermentations, even though they do not conform to L. Pasteur's concept of fermentation as a decomposition in the absence of air.

History

Alcoholic fermentation is a process that was known to antiquity. Before 2000 B.C.E. the Egyptians apparently knew that crushed fruits stored in a warm place would produce a substance with a pleasant intoxicating power. By 1500 B.C.E. the production of beer from germinating cereals (malt) and the preparation of wines from crushed grapes were established arts in most of the Middle East. Aristotle believed that grape juice was an infantile form of wine and that fermentation was, therefore, the maturation of the grape extract. Interest in the process of fermentation has continued through the ages, and much of modern biochemistry, especially enzyme studies, has emerged directly from early studies on the fermentation process. One of the earliest laboratories established for the study of biological chemistry was that founded in Copenhagen in 1875 and financed by the brewing family of Jacob Christian Jacobsen.

Germ theory

Louis Pasteur demonstrated that the fermentation process is caused by the growth of microorganisms, and that the growth of microorganisms in nutrient broths is not due to spontaneous generation.

He exposed boiled broths to air in vessels that contained a filter to prevent all particles from passing through to the growth medium, and even in vessels with no filter at all, with air being admitted via a long tortuous tube that would not allow dust particles to pass. Nothing grew in the broths; therefore, the living organisms that grew in such broths came from outside, as spores on dust, rather than spontaneously generated within the broth. Thus, Pasteur dealt the death blow to the theory of spontaneous generation and supported germ theory.

While Pasteur was not the first to propose germ theory (Girolamo Fracastoro, Agostino Bassi, Friedrich Henle and others had suggested it earlier), he developed it and conducted experiments that clearly indicated its correctness and managed to convince most of Europe it was true. Today he is often regarded as the father of germ theory and bacteriology, together with Robert Koch.

Pasteur's research also showed that some microorganisms contaminated fermenting beverages. With this established, he invented a process in which liquids such as milk were heated to kill most bacteria and molds already present within them. He and Claude Bernard completed the first test on April 20, 1862. This process was soon afterwards known as pasteurization.

French chemist Louis Pasteur was the first zymologist (reference to yeast-oriented fermentation), 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. Buchner's experiment for which he won the Nobel Prize consisted of producing a cell free extract of yeast cells and showing that this "press juice" could ferment sugar. This dealt yet another blow to vitalism by showing that the presence of living yeast cells was not needed for fermentation. The cell free extract was produced by combining dry yeast cells, quartz and kieselguhr and then pulverizing the yeast cells with a mortar and pestle. This mixture would then become moist as the yeast cells' contents would come out of the cells. Once this step was done, the moist mixture would be put through a press and the resulting "press juice" had glucose, fructose, or maltose added and carbon dioxide was seen to evolve, sometimes for days. Microscopic investigation revealed no living yeast cells in the extract. One interesting thing is that Buchner hypothesized that yeast cells secrete proteins into their environment in order to ferment sugars, instead of the fermentation occurring inside the yeast cells, which is the actual mechanism.

Louis Pasteur in the 19th century used the term fermentation in a narrow sense to describe the changes brought about by yeasts and other microorganisms growing in the absence of air (anaerobically); he also recognized that ethyl alcohol and carbon dioxide are not the only products of fermentation. In the 1920s it was discovered that extracts of muscle catalyze, in the absence of air, the formation of lactate from glucose and that the same intermediate compounds formed in the fermentation of grain are produced by muscle. An important generalization thus emerged: that fermentation reactions are not peculiar to the action of yeast but also occur in many other instances of glucose utilization.

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?Jacobsen set up the Carlsberg Laboratory in 1875 which worked on scientific problems related to brewing. It featured a Department of Chemistry and a Department of Physiology. The species of yeast used to make pale lager, Saccharomyces carlsbergensis, was isolated at the Laboratory and was named after it. The concept of pH was developed there as well as advances in protein chemistry.

Reaction

The reactions of fermentation vary according to the fuel molecule and end-product involved. In the chemical equation below, the sugar is glucose (C₆H₁₂O₆) and the end-product is ethanol (2C₂H₅OH). Chemical Equation

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP (Energy Released:118 kJ mol⁻¹)

Verbal description

Sugar (glucose, fructose, or sucrose) → Alcohol (ethanol) + Carbon Dioxide + Energy (ATP)

Energy yield

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 (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. 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.

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.

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

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.

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.

Fermentation in food production

File:MM ferment.jpg

Yeast fermenting the wort at Maker's Mark distillery, a step in the production of a distilled beverage.

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 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.

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:

1. Enrichment of the diet through development of a diversity of flavors, aromas, and textures in food substrates.
2. Preservation of substantial amounts of food through lactic acid, alcohol, acetic acid, and alkaline fermentations.
3. Biological enrichment of food substrates with protein, essential amino acids, essential fatty acids, and vitamins.
4. Detoxification during food-fermentation processing.
5. 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.

References

ISBN 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.

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

• Fermentation_(biochemistry)101030396  ^history
• 101230254  ^history

The history of this article since it was imported to New World Encyclopedia:

• History of "Fermentation"

Note: Some restrictions may apply to use of individual images which are separately licensed.