تخمر

Phylogenetic tree of bacteria and archaea, highlighting those that carry out fermentation. Their end products are also highlighted. Figure modified from Hackmann (2024).[1]
التخمر أثناء تطوره: فقاعات CO2 تشكل رغوة فوق خليط التخمر.

التخمر Fermentation ، هي عملية كيميائية يتم فيها تحلل المواد العضوية. يحدث التخمر بفعل ميكروبات مثل البكتيريا والعفن والخميرة. وعلى سبيل المثال نجد أن الفطريات أو العفن، تعمل على خليط السكر مع الأملاح المعدنية فينتج البنسلين. وتقوم الخميرة بتحليل السكر الناتج عن الحبوب المنقوعة في الماء إلى غاز الكحول الإيثيلي وثاني أكسيد الكربون عند صناعة البيرة. ويتحلل السكر في عصير العنب بنفس الطريقة عند صناعة النبيذ. كذلك يعتبر التخمر جوهرياً في إنتاج الخُبز والجُبن واللبن الرائب. ولكنه قد يكون مُضرًّا في بعض الحالات، مثلما يحدث عندما يصبح الحليب المتخمر حليباً فاسداً.

وتُصنع المُنتجات المُخْتمرة النافعة لبني البشر بكميات كبيرة. وبالرغم من أن أنواعاً مختلفة من المواد تُنْتَج بوساطة التخمُّر، إلا أن العمليات الأساسية المُتَّبعة في ذلك تبقى متماثلة. فأولاً، تُملأ صهاريج كبيرة من الفولاذ المُقاوم للصدأ، بمحلول مائي من المواد الغذائية. ويْعقَّم هذا المحلول بالبخار لقتل الجراثيم غير المرغوبة، ثم تُضاف ميكروبات معيّنة إلى المحلول، لتقوم بتخمير المواد الغذائية خلال بضعة أيام. يتحكم المشرفون على عملية التخمير في درجة حرارة ونوعية حمض المواد في داخل الصهاريج. وأخيراً تُصفَّى الصهاريج من السائل، وتُفْصل المُنتجات المرغوب فيها عن بقية الخليط، إما بوساطة الاستخلاص أو الترشيح، أو ببعض الوسائل الأخرى. وفي معظم الحالات تُشكل المنتجات المرغوب فيها حوالي 5% فقط من الخليط الموجود في الصهاريج، ولذلك تُعتبر عملية التنقية ـ في الغالب ـ عملية معقدة إلى حد بعيد.

Fermentation is a type of redox metabolism carried out in the absence of oxygen.[1][2] During fermentation, organic molecules (e.g., glucose) are catabolized and donate electrons to other organic molecules. In the process, ATP and organic end products (e.g., lactate) are formed.

Because oxygen is not required, it is an alternative to aerobic respiration. Over 25 % of bacteria and archaea carry out fermentation.[2][3] They live in the gut, sediments, food, and other environments. Eukaryotes, including humans and other animals, also carry out fermentation.[4]

Fermentation is important in several areas of human society.[2] Humans have used fermentation in production of food for 13,000 years.[5] Humans and their livestock have microbes in the gut that carry out fermentation, releasing products used by the host for energy.[6] Fermentation is used at an industrial level to produce commodity chemicals, such as ethanol and lactate. In total, fermentation forms more than 50 metabolic end products[2] with a wide range of uses.

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Biological role and prevalence

Fermentation is used by organisms to generate ATP energy for metabolism.[1] One advantage is that it requires no oxygen or other external electron acceptors, and thus it can be carried when those electron acceptors are absent. A disadvantage is that it produces relatively little ATP, yielding only between 2 to 4.5 per glucose[1] compared to 32 for aerobic respiration.[7]

Over 25% of bacteria and archaea carry out fermentation.[2][3] This type of metabolism is most common in the phylum Bacillota, and it is least common in Actinomycetota.[2] Their most common habitat is host-associated ones, such as the gut.[2]

Animals, including humans, also carry out fermentation.[4] The product of fermentation in humans is lactate, and it is formed during anaerobic exercise or in cancerous cells. No animal is known to survive on fermentation alone, even as one parasitic animal (Henneguya zschokkei) is known to survive without oxygen.[8]


Substrates and products of fermentation

The most common substrates and products of fermentation. Figure modified from Hackmann (2024).[1]

Fermentation uses a range of substrates and forms a variety of metabolic end products. Of the 55 end products formed, the most common are acetate and lactate.[1][2] Of the 46 chemically-defined substrates that have been reported, the most common are glucose and other sugars.[1][2]

استعراض الكيمياء الحيوية

Comparison of aerobic respiration and most known fermentation types in eucaryotic cell.[9] Numbers in circles indicate counts of carbon atoms in molecules, C6 is glucose C6H12O6, C1 carbon dioxide CO2. Mitochondrial outer membrane is omitted.

Fermentation reacts NADH with an endogenous, organic electron acceptor.[10] 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).[11] 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.

الكيمياء الحيوية للمنتجات الفردية

إيثانول

استعراض تخمر الإيثانول.

In ethanol fermentation, one glucose molecule is converted into two ethanol molecules and two carbon dioxide (CO2) molecules.[12][13] It is used to make bread dough rise: the carbon dioxide forms bubbles, expanding the dough into a foam.[14][15] The ethanol is the intoxicating agent in alcoholic beverages such as wine, beer and liquor.[16] Fermentation of feedstocks, including sugarcane, maize, and sugar beets, produces ethanol that is added to gasoline.[17] In some species of fish, including goldfish and carp, it provides energy when oxygen is scarce (along with lactic acid fermentation).[18]

Before fermentation, a glucose molecule breaks down into two pyruvate molecules (glycolysis). The energy from this exothermic reaction is used to bind inorganic phosphates to ADP, which converts it to ATP, and convert NAD+ to NADH. The pyruvates break down into two acetaldehyde molecules and give off two carbon dioxide molecules as waste products. 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 catalyzed by the enzymes pyruvate decarboxylase and alcohol dehydrogenase.[12]

تاريخ استخدام التخمر

اِستُخْدِم التخمُّر في صناعة الخمور مُنذ عهد بعيد. قام سكان وادي نهر النيل بتخمير البيرة حوالي عام 3000 قبل الميلاد. ومع ذلك ظلت حقيقة التخمر مجهولة حتى العِقْد الأول من القرن التاسع عشر الميلادي، عندما توصل العلماء وخاصة العالم الفرنسي لويس باستير، إلى اكتشاف كيف تُحدِثْ الميكروبات التخمر في البيرة والحليب والنبيذ.

لويس پاستير في معمله

وخلال القرن العشرين طُوِّرَت أنواع أخرى من التخمر. ونتج عن تخمر أحد أنواع البكتيريا مُكوِّناتٌ للمواد المتفجرة خلال الحرب العالمية الأولى (1914- 1918). ومنذ عام 1943 كان أهم تطبيق عملي للتخمر، هو استخدامه في إنتاج المضادات الحيوية (الأدوية القاتلة للمرض) وخاصة البنسلين. واستُخْدِم التخمير أيضاً في أدوية معينة أخرى، وفي الفيتامينات وفي بعض أنواع الكيميائيات.

  • 1826: Samuel Morey, an American inventor, was the first to produce ethanol by fermenting corn. However, ethanol was not widely used as a fuel until many years later. (1)
  • 1850s: Ethanol was first used as a fuel in the United States during the California Gold Rush. Miners used ethanol as a fuel for lamps and stoves because it was cheaper than whale oil. (2)
  • 1895: German engineer Rudolf Diesel demonstrated his engine, which was designed to run on vegetable oils, including ethanol. However, the widespread use of diesel engines fueled by petroleum made ethanol less popular as a fuel. (3)
  • 1970s: The oil crisis of the 1970s led to renewed interest in ethanol as a fuel. Brazil became a leader in ethanol production and use, due in part to government policies that encouraged the use of biofuels. (4)
  • 1980s–1990s: The United States began to produce ethanol on a large scale as a fuel additive to gasoline. This was due to the passage of the Clean Air Act in 1990, which required the use of oxygenates, such as ethanol, to reduce emissions. (5)
  • 2000s–present: There has been continued interest in ethanol as a renewable and sustainable fuel. Researchers are exploring new sources of biomass for ethanol production, such as switchgrass and algae, and developing new technologies to improve the efficiency of the fermentation process. (6)

حمض اللبن

Homolactic fermentation (producing only lactic acid) is the simplest type of fermentation. Pyruvate from glycolysis[19] undergoes a simple redox reaction, forming lactic acid.[20][21] Overall, one molecule of glucose (or any six-carbon sugar) 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 (such as lactobacilli) and some fungi. It is the type of bacteria that convert 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 to ethanol and carbon dioxide[20] (via the phosphoketolase pathway), acetate, or other metabolic products, e.g.:

C6H12O6 → CH3CHOHCOOH + C2H5OH + CO2

If 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 C6H12O6

Heterolactic fermentation is in a sense intermediate between lactic acid fermentation and other types, e.g. alcoholic fermentation. Reasons to go further and convert lactic acid into something else include:

  • 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 (one 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, forming 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 proceed 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.


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غاز الهيدروجين

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.[12] 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,[22] but hydrogen gas at a fairly high concentration can nevertheless be formed, as in flatus.[بحاجة لمصدر]

For example, Clostridium pasteurianum ferments glucose to butyrate, acetate, carbon dioxide, and hydrogen gas:[23] The reaction leading to acetate is:

C6H12O6 + 4 H2O → 2 CH3COO + 2 HCO3 + 4 H+ + 4 H2

Glyoxylate

Glyoxylate fermentation is a type of fermentation used by microbes that are able to utilize glyoxylate as a nitrogen source. [24]

Other

Other types of fermentation include mixed acid fermentation, butanediol fermentation, butyrate fermentation, caproate fermentation, and acetone–butanol–ethanol fermentation.[25][بحاجة لمصدر]

In the broader sense

In food and industrial contexts, any chemical modification performed by a living being in a controlled container can be termed "fermentation". The following do not fall into the biochemical sense, but are called fermentation in the larger sense:

Alternative protein

Fermentation is used to produce the heme protein found in the Impossible Burger.

Fermentation can be used to make alternative protein sources. It is commonly used to modify existing protein foods, including plant-based ones such as soy, into more flavorful forms such as tempeh and fermented tofu.

More modern "fermentation" makes recombinant protein to help produce meat analogue, milk substitute, cheese analogues, and egg substitutes. Some examples are:[26]

Heme proteins such as myoglobin and hemoglobin give meat its characteristic texture, flavor, color, and aroma. The myoglobin and leghemoglobin ingredients can be used to replicate this property, despite them coming from a vat instead of meat.[26][27]

Enzymes

Industrial fermentation can be used for enzyme production, where proteins with catalytic activity are produced and secreted by microorganisms. The development of fermentation processes, microbial strain engineering and recombinant gene technologies has enabled the commercialization of a wide range of enzymes. Enzymes are used in all kinds of industrial segments, such as food (lactose removal, cheese flavor), beverage (juice treatment), baking (bread softness, dough conditioning), animal feed, detergents (protein, starch and lipid stain removal), textile, personal care and pulp and paper industries.[28]

Modes of industrial 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.[29]

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.[30]:1 However, it can be expensive because the fermentor must be sterilized using high pressure steam between batches.[29] Strictly speaking, there is often addition of small quantities of chemicals to control the pH or suppress foaming.[30]:25

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.[30]:25

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.[30]:1[31]

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 °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.[29]

Continuous

In continuous fermentation, substrates are added and final products removed continuously.[29] 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.[31] 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.[29] Typically the fermentor must run for over 500 hours to be more economical than batch processors.[31]


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History of the use of fermentation

The use of fermentation, particularly for beverages, has existed since the Neolithic and has been documented dating from 7000 to 6600 BCE in Jiahu, China,[32] 5000 BCE in India, Ayurveda mentions many Medicated Wines, 6000 BCE in Georgia,[33] 3150 BCE in ancient Egypt,[34] 3000 BCE in Babylon,[35] 2000 BCE in pre-Hispanic Mexico,[35] and 1500 BC in Sudan.[36] Fermented foods have a religious significance in Judaism and Christianity. The Baltic god Rugutis was worshiped as the agent of fermentation.[37][38] In alchemy, fermentation ("putrefaction") was symbolized by Capricorn Capricornus symbol (fixed width).svg ♑︎.[بحاجة لمصدر]

Louis Pasteur in his laboratory

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.[39][40]:6 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.[41]:108–109

The turning point came when Louis Pasteur (1822–1895), during the 1850s and 1860s, repeated Schwann's experiments and showed fermentation is initiated by living organisms in a series of investigations.[21][40]:6 In 1857, Pasteur showed lactic acid fermentation is caused by living organisms.[42] In 1860, he demonstrated how bacteria cause souring in milk, a process formerly thought to be merely a chemical change. His work in identifying the role of microorganisms in food spoilage led to the process of pasteurization.[43]

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".[44] He defined fermentation (incorrectly) as "Life without air",[45] yet he correctly showed how specific types of microorganisms cause specific types of fermentations and specific end-products.[بحاجة لمصدر]

Although showing fermentation resulted from the action of living microorganisms was a breakthrough, it did not explain the basic nature of fermentation; nor did it prove it is caused by microorganisms which appear to be always present. Many scientists, including Pasteur, had unsuccessfully attempted to extract the fermentation enzyme from yeast.[45]

Success came in 1897 when the German chemist Eduard Buechner ground up yeast, extracted a juice from them, then found to his amazement this "dead" liquid would ferment a sugar solution, forming carbon dioxide and alcohol much like living yeasts.[46]

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 fermentation is caused by enzymes produced by microorganisms.[47] In 1907, Buechner won the Nobel Prize in chemistry for his work.[48]

Advances in microbiology and fermentation technology have continued steadily up until the present. For example, in the 1930s, it was discovered 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.[49][50] Strain selection and hybridization developed as well, affecting most modern food fermentations.[بحاجة لمصدر]

بعد ع1930

The field of fermentation has been critical to the production of a wide range of consumer goods, from food and drink to industrial chemicals and pharmaceuticals. Since its early beginnings in ancient civilizations, the use of fermentation has continued to evolve and expand, with new techniques and technologies driving advances in product quality, yield, and efficiency. The period from the 1930s onward saw a number of significant advancements in fermentation technology, including the development of new processes for producing high-value products like antibiotics and enzymes, the increasing importance of fermentation in the production of bulk chemicals, and a growing interest in the use of fermentation for the production of functional foods and nutraceuticals.[بحاجة لمصدر]

The 1950s and 1960s saw the development of new fermentation technologies, such as the use of immobilized cells and enzymes, which allowed for more precise control over fermentation processes and increased the production of high-value products like antibiotics and enzymes. In the 1970s and 1980s, fermentation became increasingly important in the production of bulk chemicals like ethanol, lactic acid, and citric acid. This led to the development of new fermentation techniques and the use of genetically engineered microorganisms to improve yields and reduce production costs. In the 1990s and 2000s, there was a growing interest in the use of fermentation for the production of functional foods and nutraceuticals, which have potential health benefits beyond basic nutrition. This led to the development of new fermentation processes and the use of probiotics and other functional ingredients.[بحاجة لمصدر]

Overall, the period from 1930 onward saw significant advancements in the use of fermentation for industrial purposes, leading to the production of a wide range of fermented products that are now consumed around the world.[بحاجة لمصدر]

انظر أيضاً

المصادر

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