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Lactic acid fermentation is a metabolic process by whichglucose or othersix-carbon sugars (also,disaccharides of six-carbon sugars, e.g.sucrose orlactose) are converted into cellular energy and the metabolitelactate, which is lactic acid in solution. It is ananaerobicfermentation reaction that occurs in some bacteria andanimal cells, such asmuscle cells.[1][2][3][page needed]
If oxygen is present in the cell, many organisms will bypass fermentation and undergocellular respiration; however,facultative anaerobic organisms will both ferment and undergo respiration in the presence of oxygen.[3] Sometimes even when oxygen is present and aerobic metabolism is happening in themitochondria, if pyruvate is building up faster than it can be metabolized, the fermentation will happen anyway.
Lactate dehydrogenase catalyzes the interconversion ofpyruvate andlactate with concomitant interconversion of NADH andNAD+.
Inhomolactic fermentation, one molecule of glucose is ultimately converted to two molecules of lactic acid.Heterolactic fermentation, by contrast, yieldscarbon dioxide andethanol in addition to lactic acid, in a process called thephosphoketolase pathway.[1]
Chemical analysis of archaeological finds show that milk fermentation had been used since prehistory; its first applications were probably a part of theNeolithic Revolution. Since milk naturally containslactic acid bacteria, the discovery of the fermentation process was quite evident, since it happens spontaneously at an adequate temperature. The problem of these firstfarmers was that fresh milk is nearly indigestible by adults, so they had an interest to discover this mechanism. In fact, lactic acid bacteria contain the neededenzymes to digest lactose, and theirpopulations multiply strongly during the fermentation. Therefore, milk fermented even a short time contains enough enzymes to digest the lactose molecules, after the milk is in the human body, which allows adults to consume it. Even safer was a longer fermentation, which was practiced forcheesemaking. This process was also discovered a very long time ago, which is proven by recipes for cheese production onCuneiform scripts, the first written documents that exist, and later inBabylonian and Egyptian texts. There is a theory ofcompetitive advantage related to fermented milk products. This theory suggests that the women of these first settled agricultural civilisations could shorten the time between two children thanks to the additional lactose uptake from milk consumption. This factor may have given them an important advantage to out-compete thehunter-gatherer societies.[4]
With the increasing consumption of milk products these societies developed alactase persistence byepigenetic inheritance, which means that the milk-digesting enzymelactase was present in their bodies during the whole lifetime, so they could drink unfermented milk as adults too. This early habituation to lactose consumption in the firstsettler societies can still be observed today in regional differences of this mutation's concentration. It is estimated that about 65% of world population still lacks it.[5] Since these first societies came from regions around easternTurkey to centralEurope, thegene appears more frequently there and in North America, as it was settled by Europeans. It is because of the dominance of this mutation thatWestern cultures believe it is unusual to have alactose intolerance, when it is in fact more common than themutation. On the contrary,lactose intolerance is much more present in Asian countries.[6]
Milk products and their fermentation have had an important influence on some cultures' development. This is the case inMongolia, where people often practice apastoral form of agriculture. The milk that they produce and consume in these cultures is mainlymare milk and has a long tradition. But not every part or product of the fresh milk has the same meaning. For instance, the fattier part on the top, the "deež", is seen as the most valuable part and is therefore often used to honor guests.
Very important with often a traditional meaning as well are fermentation products of mare milk, like for example the slightly-alcoholic yogurtkumis. Consumption of these peaks during cultural festivities such as theMongolian lunar new year (in spring). The time of this celebration is called the "white month", which indicates that milk products (called "white food" together withstarchy vegetables, in comparison to meat products, called "black food") are a central part of this tradition. The purpose of these festivities is to "close" the past year – clean the house or theyurt, honor the animals for having provided their food, and prepare everything for the coming summer season – to be ready to "open" the new year. Consuming white food in this festive context is a way to connect to the past and to a national identity, which is theMongol Empire personified byGenghis Khan. During the time of this empire, the fermented mare milk was the drink to honor and thank warriors and leading persons, it was not meant for everybody. Although it eventually became a drink for normal people, it has kept its honorable meaning. Like many other traditions, this one feels the influence ofglobalization. Other products, like industrialyogurt, coming mainly from China and western countries, have tended to replace it more and more, mainly in urban areas. However, in rural and poorer regions it is still of great importance.[7]
Although thischemical process had been used in food production for thousand of years, microbial lactic acid fermentation was not properly described before much later. During the 19th century, several chemists discovered some fundamental concepts oforganic chemistry. One of these was the French chemistJoseph Louis Gay-Lussac, who was especially interested in fermentation processes, and he passed this fascination to one of his best students,Justus von Liebig. With a difference of some years, each of them described, together with colleagues, thechemical structure of the lactic acidmolecule as we know it today. They had a purely chemical understanding of the fermentation process; it could not be observed using amicroscope, and could only be optimized bychemical catalyzers. In 1857, the French chemistLouis Pasteur first describedlactic acid as the product of a microbial fermentation. During this time, he worked at theUniversity of Lille, where a localdistillery asked him for advice concerning some fermentation problems. Per chance and with the badly equipped laboratory he had at that time, he was able to discover that in this distillery, two fermentations were taking place, a lactic acid one and analcoholic one, both induced bymicroorganisms. He then continued the research on these discoveries in Paris, where he also published his theories that presented a stable contradiction to the purely chemical version represented by Liebig and his followers. Even though Pasteur described some concepts that are still accepted today, Liebig refused to accept them. But even Pasteur himself wrote that he was "driven" to a completely new understanding of this chemical phenomenon. Although Pasteur didn't find every detail of this process, he still discovered the main mechanism of how microbial lactic acid fermentation works. He was the first to describe fermentation as a "form of life without air".[8][9]
Homofermentative bacteria convert glucose to two molecules of lactate and use this reaction to performsubstrate-level phosphorylation to make two molecules ofATP:
Heterofermentative bacteria produce less lactate and less ATP, but produce several other end products:
Examples includeLeuconostoc mesenteroides,Lactobacillus bifermentous, andLeuconostoc lactis.
Bifidobacterium bifidum utilizes a lactic acid fermentation pathway that produces more ATP than either homolactic fermentation or heterolactic fermentation:
Some major bacterial strains identified as being able to ferment lactose are in thegeneraEscherichia, Citrobacter, Enterobacter andKlebsiella. All four of these groups fall underneath thefamily ofEnterobacteriaceae. These four genera are able to be separated from each other by using biochemical testing, and simple biological tests are readily available. Apart from whole-sequencegenomics, common tests includeH2S production,motility andcitrate use,indole,methyl red andVoges-Proskauer tests.[10]
Lactic acid fermentation is used in many areas of the world to produce foods that cannot be produced through other methods.[11][12] The most commercially importantgenus of lactic acid-fermenting bacteria isLactobacillus, though other bacteria and evenyeast are sometimes used.[11] Two of the most common applications of lactic acid fermentation are in the production of yogurt and sauerkraut.
Pickling in brine is a very common way to use lactic acid fermentation to aid in the preservation of food. Lactic acid bacteria (LAB) already exists as part of the natural flora in most vegetables, so by creating a selective environment of oxygen-poor brine, LAB will dominate in growth and convert sugars to lactic acid.
Silage fermentation uses the same principle of creating an anaerobic environment. Different types of LAB will produce different types of silage fermentation.[13]
Kimchi also uses lactic acid fermentation.[14]
Lactic acid fermentation is also used in the production ofsauerkraut. The main type of bacteria used in the production of sauerkraut is of the genusLeuconostoc.[1][15]
As in yogurt, when the acidity rises due to lactic acid-fermenting organisms, many otherpathogenic microorganisms are killed. The bacteria produce lactic acid, as well as simple alcohols and otherhydrocarbons. These may then combine to formesters, contributing to the unique flavor of sauerkraut.[1]
In some Asian cuisines, fish is traditionally fermented with rice to produce lactic acid that preserves the fish. Examples of these dishes includeburong isda of thePhilippines;narezushi ofJapan; andpla ra ofThailand. The same process is also used for shrimp in the Philippines in the dish known asbalao-balao.[16][17][18]
Lactic acid is a component in the production ofsour beers, includingLambics andBerliner Weisses.[19]
The main method of producingyogurt is through the lactic acid fermentation of milk with harmless bacteria.[11][20] The primary bacteria used are typicallyLactobacillus bulgaricus andStreptococcus thermophilus, and United States as well as European law requires all yogurts to contain these two cultures (though others may be added as probiotic cultures).[20] These bacteria produce lactic acid in the milk culture, decreasing itspH and causing it to congeal. The bacteria also produce compounds that give yogurt its distinctive flavor. An additional effect of the lowered pH is the incompatibility of the acidic environment with many other types of harmful bacteria.[11][20]
For aprobiotic yogurt, additional types of bacteria such asLactobacillus acidophilus are also added to the culture.[20]
Lactobacillus fermentation and accompanying production of acid provides a protective vaginalmicrobiome that protects against the proliferation of pathogenic organisms.[21]
The vaginal environment is heavily influenced by lactic acid producing bacteria.Lactobacilli spp. that live in the vaginal canal assist in pH control. If the pH in the vagina becomes too basic, more lactic acid will be produced to lower the pH back to a more acidic level. Lactic acid producing bacteria also act as a protective barrier against possible pathogens such as bacterial vaginosis and vaginitis species, different fungi, and protozoa through the production of hydrogen peroxide, and antibacterial compounds. It is unclear if further use of lactic acid, through fermentation, in the vaginal canal is present.[22]
Human (and other eukaryote) cells can produce ATP from glucose without oxygen in a process calledglycolysis. This is not as efficient as respiration, but provides a high instantaneous output, and is hence used by some muscle cells. Glycolysis consumes ADP, Pi, glucose, and NAD+ to produce ATP, pyruvate, and NADH. Through lactate fermentation, pyruvate and NADH are turned into lactate and NAD+, thereby regenerating the NAD+ required for more glycolysis.
During the 1990s, the lactic acid hypothesis was created to explain why people experienced burning or muscle cramps that occurred during and after intense exercise. The hypothesis proposes that a lack of oxygen in muscle cells results in a switch from cellular respiration to fermentation. Lactic acid created as a byproduct of fermentation of pyruvate from glycolysis accumulates in muscles causing a burning sensation and cramps.
Research from 2006 has suggested that acidosis isn't the main cause of muscle cramps. Instead cramps may be due to a lack ofpotassium in muscles, leading to contractions under high stress.
Animals, in fact, do not produce lactic acid during fermentation. Despite the common use of the term lactic acid in the literature, the byproduct of fermentation in animal cells is lactate.[23]
Another change to the lactic acid hypothesis is that when sodium lactate is inside of the body, there is a higher period of exhaustion in the host after a period of exercise.[24]
In small amounts, lactic acid is good for the human body by providing energy and substrates while it moves through the cycle. In lactose intolerant people, the fermentation of lactose to lactic acid has been shown in small studies to help lactose intolerant people. The process of fermentation limits the amount of lactose available. With the amount of lactose lowered, there is less build up inside of the body, reducing bloating. Success of lactic fermentation was most evident in yogurt cultures. Further studies are being conducted on other milk products like acidophilus milk.[25]
Metabolism map | ||
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 Majormetabolic pathways inmetro-style map. Click any text (name of pathway or metabolites) to link to the corresponding article. Single lines: pathways common to most lifeforms. Double lines: pathways not in humans (occurs in e.g. plants, fungi, prokaryotes).  Orange nodes:carbohydrate metabolism. Violet nodes:photosynthesis. Red nodes:cellular respiration. Pink nodes:cell signaling. Blue nodes:amino acid metabolism. Grey nodes:vitamin andcofactor metabolism. Brown nodes:nucleotide andprotein metabolism. Green nodes:lipid metabolism. | ||
