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Χρήστης:C messier/πρόχειρο/ζυμομύκητες

Από τη Βικιπαίδεια, την ελεύθερη εγκυκλοπαίδεια

Οι ζυμομύκητες, γνωστοί και ως βλαστομύκητες, ζύμες ή μαγιά, είναι μονοκύτταροι οργανισμοί οι οποίοι ανήκουν στο βασίλειο των μυκήτων. Μέχρι σήμερα έχουν αναγνωριστεί περίπου 1.500 είδη.[1][2][3] Υπολογίζεται ότι αποτελούν το 1% όλων των ειδών μυκήτων. Οι ζυμομύκητες είναι μονοκύτταροι οργανισμοί οι οποίοι εξελίχθηκαν από πολυκύτταρους μύκητες, με κάποια είδη να μπορούν να αναπτύξουν πολυκυττάριες μορφές με μορφή ίνας, αποτελούμενες από συνδεδεμένα κύτταρα, γνωστά ως ψευδοϋφές.[4] Το μέγεθος των ζυμομυκήτων είναι ποικίλο και εξαρτάται από το είδος και το περιβάλλον. Συνήθως έχουν διάμετρο 3-4 μm, αν και κάποια είδη φτάνουν τα 40 μm.[5] Οι περισσότεροι ζυμομύκητες αναπαράγονται ασεξουαλικώς με μίτωση, αν και πολλοί μπορούν με ασύμμετρο διαχωρισμό γνωστό και ως εκβλάστηση.

Με τη διαδικασία της ζύμωσης, ο ζυμομύκητας Σακχαρομύκητας, γνωστός και ως μαγιά, μετατρέπει τους υδατάνθρακες σε διοξείδιο του άνθρακα και αλκοόλη. Για χιλιάδες χρόνια, το διοξείδιο του άνθρακα χρησιμοποιείται στην αρτοποιία και το αλκοόλ στα οινοπνευματώδη ποτά.[6] Επίσης, αποτελεί σημαντικό οργανισμό μοντέλο στην σύγχρονη έρευνα στην κυτταρική βιολογία και συγκεκριμένα των ευκαρυωτικών οργανισμών (και εν τέλει της ανθρώπινης βιολογίας) και είναι ένας από τους καλύτερα μελετημένους ευκαρυωτικούς μικροοργανισμούς.[7] Άλλοι είδη ζυμομυκήτων, όπως η Candida albicans, είναι ευκαιριακά παθογόνα και μπορούν να προκαλέσουν λοιμώξεις στους ανθρώπους. Οι ζυμομύκητες έχουν χρησιμοποιηθεί για την παραγωγή ηλεκτρισμού σε μικροβιακές κυψελίδες καυσίμων[8] και αιθανόλης για βιοκαύσιμο.

Οι ζυμομύκητες δεν αποτελούν μια ταξινομική ή φυλογενετική ομάδα. Ο όρος ζυμομύκητας συχνά χρησιμοποιείται ως συνώνυμο του Σακχαρομύκητα,[9] αλλά η φυλογενετική ποικιλία των ζυμομυκήτων φαίνεται από την ταξινόμησή τους σε δύο φύλα, τους Ασκομύκητες και τους Βασιδιομύκητες. Η ονομασία μαγιά πέρα από τους ζυμομύκητες μπορεί να αναφέρεται στο ένζυμο πυτιά.

Ιστορία

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Yeast microbes are probably one of the earliest domesticated organisms. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeast-raised bread, as well as drawings of 4,000-year-old bakeries and breweries.[10] In 1680, Dutch naturalist Anton van Leeuwenhoek first microscopically observed yeast, but at the time did not consider them to be living organisms, but rather globular structures.[11] Researchers were doubtful whether yeasts were algae or a fungi,[12] but in 1837 Theodor Schwann recognized them as fungi.[13][14]

In 1857, French microbiologist Louis Pasteur proved in the paper "Mémoire sur la fermentation alcoolique" that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst.[10][15] Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, but fermentation was inhibited – an observation later called the "Pasteur effect".

By the late 18th century, two yeast strains used in brewing had been identified: Saccharomyces cerevisiae (top-fermenting yeast) and S. carlsbergensis (bottom-fermenting yeast). S. cerevisiae has been sold commercially by the Dutch for bread-making since 1780; while, around 1800, the Germans started producing S. cerevisiae in the form of cream. In 1825, a method was developed to remove the liquid so the yeast could be prepared as solid blocks.[16] The industrial production of yeast blocks was enhanced by the introduction of the filter press in 1867. In 1872, Baron Max de Springer developed a manufacturing process to create granulated yeast, a technique that was used until the first World War.[17] In the United States, naturally occurring airborne yeasts were used almost exclusively until commercial yeast was marketed at the Centennial Exposition in 1876 in Philadelphia, where Charles L. Fleischmann exhibited the product and a process to use it, as well as serving the resultant baked bread.[18]

Nutrition and growth

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Yeasts are chemoorganotrophs, as they use organic compounds as a source of energy and do not require sunlight to grow. Carbon is obtained mostly from hexose sugars, such as glucose and fructose, or disaccharides such as sucrose and maltose. Some species can metabolize pentose sugars such as ribose,[19] alcohols, and organic acids. Yeast species either require oxygen for aerobic cellular respiration (obligate aerobes) or are anaerobic, but also have aerobic methods of energy production (facultative anaerobes). Unlike bacteria, no known yeast species grow only anaerobically (obligate anaerobes). Yeasts grow best in a neutral or slightly acidic pH environment.

Yeasts vary in regard to the temperature range in which they grow best. For example, Leucosporidium frigidum grows at −2 to 20 °C (28 to 68 °F), Saccharomyces telluris at 5 to 35 °C (41 to 95 °F), and Candida slooffi at 28 to 45 °C (82 to 113 °F).[20] The cells can survive freezing under certain conditions, with viability decreasing over time.

In general, yeasts are grown in the laboratory on solid growth media or in liquid broths. Common media used for the cultivation of yeasts include potato dextrose agar or potato dextrose broth, Wallerstein Laboratories nutrient agar, yeast peptone dextrose agar, and yeast mould agar or broth. Home brewers who cultivate yeast frequently use dried malt extract and agar as a solid growth medium. The antibiotic cycloheximide is sometimes added to yeast growth media to inhibit the growth of Saccharomyces yeasts and select for wild/indigenous yeast species. This will change the yeast process.

The appearance of a white, thready yeast, commonly known as kahm yeast, is often a byproduct of the lactofermentation (or pickling) of certain vegetables, usually the result of exposure to air. Although harmless, it can give pickled vegetables a bad flavor and must be removed regularly during fermentation.[21]

Ecology

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Yeasts are very common in the environment, and are often isolated from sugar-rich materials. Examples include naturally occurring yeasts on the skins of fruits and berries (such as grapes, apples, or peaches), and exudates from plants (such as plant saps or cacti). Some yeasts are found in association with soil and insects.[22][23] The ecological function and biodiversity of yeasts are relatively unknown compared to those of other microorganisms.[24] Yeasts, including Candida albicans, Rhodotorula rubra, Torulopsis and Trichosporon cutaneum, have been found living in between people's toes as part of their skin flora.[25] Yeasts are also present in the gut flora of mammals and some insects[26] and even deep-sea environments host an array of yeasts.[27][28]

An Indian study of seven bee species and 9 plant species found 45 species from 16 genera colonise the nectaries of flowers and honey stomachs of bees. Most were members of the Candida genus; the most common species in honey stomachs was Dekkera intermedia and in flower nectaries, Candida blankii.[29] Yeast colonising nectaries of the stinking hellebore have been found to raise the temperature of the flower, which may aid in attracting pollinators by increasing the evaporation of volatile organic compounds.[24][30] A black yeast has been recorded as a partner in a complex relationship between ants, their mutualistic fungus, a fungal parasite of the fungus and a bacterium that kills the parasite. The yeast has a negative effect on the bacteria that normally produce antibiotics to kill the parasite, so may affect the ants' health by allowing the parasite to spread.[31]

Certain strains of some species of yeasts produce proteins called yeast killer toxins that allow them to eliminate competing strains. (See main article on killer yeast.) This can cause problems for winemaking but could potentially also be used to advantage by using killer toxin-producing strains to make the wine. Yeast killer toxins may also have medical applications in treating yeast infections (see "Pathogenic yeasts" section below).[32]

Reproduction

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The yeast cell's life cycle:
1. Budding
2. Conjugation
3. Spore
Δείτε επίσης: Mating of yeast

Yeasts, like all fungi, may have asexual and sexual reproductive cycles. The most common mode of vegetative growth in yeast is asexual reproduction by budding.[33] Here, a small bud (also known as a bleb), or daughter cell, is formed on the parent cell. The nucleus of the parent cell splits into a daughter nucleus and migrates into the daughter cell. The bud continues to grow until it separates from the parent cell, forming a new cell.[34] The daughter cell produced during the budding process is generally smaller than the mother cell. Some yeasts, including Schizosaccharomyces pombe, reproduce by fission instead of budding,[33] thereby creating two identically sized daughter cells.

In general, under high-stress conditions such as nutrient starvation, haploid cells will die; under the same conditions, however, diploid cells can undergo sporulation, entering sexual reproduction (meiosis) and producing a variety of haploid spores, which can go on to mate (conjugate), reforming the diploid.[35]

The haploid fission yeast Schizosaccharomyces pombe is a facultative sexual microorganism that can undergo mating when nutrients are limiting.[36][3] Exposure of S. pombe to hydrogen peroxide, an agent that causes oxidative stress leading to oxidative DNA damage, strongly induces mating and the formation of meiotic spores.[37] The budding yeast Saccharomyces cerevisiae reproduces by mitosis as diploid cells when nutrients are abundant, but when starved, this yeast undergoes meiosis to form haploid spores.[38] Haploid cells may then reproduce asexually by mitosis. Katz Ezov et al.[39] presented evidence that in natural S. cerevisiae populations clonal reproduction and selfing (in the form of intratetrad mating) predominate. In nature, mating of haploid cells to form diploid cells is most often between members of the same clonal population and out-crossing is uncommon.[40] Analysis of the ancestry of natural S. cerevisiae strains led to the conclusion that out-crossing occurs only about once every 50,000 cell divisions.[40] These observations suggest that the possible long-term benefits of outcrossing (e.g. generation of diversity) are likely to be insufficient for generally maintaining sex from one generation to the next.[41] Rather, a short-term benefit, such as recombinational repair during meiosis,[42][43] may be the key to the maintenance of sex in S. cerevisiae.

Some pucciniomycete yeasts, in particular species of Sporidiobolus and Sporobolomyces, produce aerially dispersed, asexual ballistoconidia.[44]

Uses

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The useful physiological properties of yeast have led to their use in the field of biotechnology. Fermentation of sugars by yeast is the oldest and largest application of this technology. Many types of yeasts are used for making many foods: baker's yeast in bread production, brewer's yeast in beer fermentation, and yeast in wine fermentation and for xylitol production.[45] So-called red rice yeast is actually a mold, Monascus purpureus. Yeasts include some of the most widely used model organisms for genetics and cell biology.[46]

Alcoholic beverages

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Alcoholic beverages are defined as beverages that contain ethanol (C2H5OH). This ethanol is almost always produced by fermentation – the metabolism of carbohydrates by certain species of yeasts under anaerobic or low-oxygen conditions. Beverages such as mead, wine, beer, or distilled spirits all use yeast at some stage of their production. A distilled beverage is a beverage containing ethanol that has been purified by distillation. Carbohydrate-containing plant material is fermented by yeast, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. Components other than ethanol are collected in the condensate, including water, esters, and other alcohols, which (in addition to that provided by the oak in which it may be aged) account for the flavour of the beverage.

Beer

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Yeast ring used by Swedish homebrewers in the 19th century to preserve yeast between brewing sessions.
Κύριο λήμμα: Brewing
Δείτε επίσης: Barm
Bubbles of carbon dioxide forming during beer-brewing[7]

Brewing yeasts may be classed as "top-cropping" (or "top-fermenting") and "bottom-cropping" (or "bottom-fermenting").[47] Top-cropping yeasts are so called because they form a foam at the top of the wort during fermentation. An example of a top-cropping yeast is Saccharomyces cerevisiae, sometimes called an "ale yeast".[48] Bottom-cropping yeasts are typically used to produce lager-type beers, though they can also produce ale-type beers. These yeasts ferment well at low temperatures. An example of bottom-cropping yeast is Saccharomyces pastorianus, formerly known as S. carlsbergensis.

Decades ago, taxonomists reclassified S. carlsbergensis (uvarum) as a member of S. cerevisiae, noting that the only distinct difference between the two is metabolic. Lager strains of S. cerevisiae secrete an enzyme called melibiase, allowing them to hydrolyse melibiose, a disaccharide, into more fermentable monosaccharides. Top- and bottom-cropping and cold- and warm-fermenting distinctions are largely generalizations used by laypersons to communicate to the general public.[49]

The most common top-cropping brewer's yeast, S. cerevisiae, is the same species as the common baking yeast.[50] Brewer's yeast is also very rich in essential minerals and the B vitamins (except B12).[51] However, baking and brewing yeasts typically belong to different strains, cultivated to favour different characteristics: baking yeast strains are more aggressive, to carbonate dough in the shortest amount of time possible; brewing yeast strains act slower but tend to produce fewer off-flavours and tolerate higher alcohol concentrations (with some strains, up to 22%).

Dekkera/Brettanomyces is a genus of yeast known for its important role in the production of 'lambic' and specialty sour ales, along with the secondary conditioning of a particular Belgian Trappist beer.[52] The taxonomy of the genus Brettanomyces has been debated since its early discovery and has seen many reclassifications over the years. Early classification was based on a few species that reproduced asexually (anamorph form) through multipolar budding.[53] Shortly after, the formation of ascospores was observed and the genus Dekkera, which reproduces sexually (teleomorph form), was introduced as part of the taxonomy.[54] The current taxonomy includes five species within the genera of Dekkera/Brettanomyces. Those are the anamorphs Brettanomyces bruxellensis, Brettanomyces anomalus, Brettanomyces custersianus, Brettanomyces naardenensis, and Brettanomyces nanus, with teleomorphs existing for the first two species, Dekkera bruxellensis and Dekkera anomala.[55] The distinction between Dekkera and Brettanomyces is arguable, with Oelofse et al. (2008) citing Loureiro and Malfeito-Ferreira from 2006 when they affirmed that current molecular DNA detection techniques have uncovered no variance between the anamorph and teleomorph states. Over the past decade, Brettanomyces spp. have seen an increasing use in the craft-brewing sector of the industry, with a handful of breweries having produced beers that were primarily fermented with pure cultures of Brettanomyces spp. This has occurred out of experimentation, as very little information exists regarding pure culture fermentative capabilities and the aromatic compounds produced by various strains. Dekkera/Brettanomyces spp. have been the subjects of numerous studies conducted over the past century, although a majority of the recent research has focused on enhancing the knowledge of the wine industry. Recent research on eight Brettanomyces strains available in the brewing industry focused on strain-specific fermentations and identified the major compounds produced during pure culture anaerobic fermentation in wort.[56]

Wine

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Κύριο λήμμα: Yeast in winemaking

Yeast is used in winemaking, where it converts the sugars present (glucose and fructose) in grape juice (must) into ethanol. Yeast is normally already present on grape skins. Fermentation can be done with this endogenous "wild yeast",[57] but this procedure gives unpredictable results, which depend upon the exact types of yeast species present. For this reason, a pure yeast culture is usually added to the must; this yeast quickly dominates the fermentation. The wild yeasts are repressed, which ensures a reliable and predictable fermentation.[58]

Most added wine yeasts are strains of S. cerevisiae, though not all strains of the species are suitable.[58] Different S. cerevisiae yeast strains have differing physiological and fermentative properties, therefore the actual strain of yeast selected can have a direct impact on the finished wine.[59] Significant research has been undertaken into the development of novel wine yeast strains that produce atypical flavour profiles or increased complexity in wines.[60][61]

The growth of some yeasts, such as Zygosaccharomyces and Brettanomyces, in wine can result in wine faults and subsequent spoilage.[62] Brettanomyces produces an array of metabolites when growing in wine, some of which are volatile phenolic compounds. Together, these compounds are often referred to as "Brettanomyces character", and are often described as "antiseptic" or "barnyard" type aromas. Brettanomyces is a significant contributor to wine faults within the wine industry.[63]

Researchers from the University of British Columbia, Canada, have found a new strain of yeast that has reduced amines. The amines in red wine and Chardonnay produce off-flavors and cause headaches and hypertension in some people. About 30% of people are sensitive to biogenic amines, such as histamines.[64]

Baking

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 Η ενότητα αυτή δεν τεκμηριώνεται επαρκώς με παραπομπές. Παρακαλούμε βοηθήστε προσθέτοντας την κατάλληλη τεκμηρίωση. Ατεκμηρίωτο υλικό μπορεί να αμφισβητηθεί και να αφαιρεθεί.
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Κύριο λήμμα: Baker's yeast

Yeast, the most common one being S. cerevisiae, is used in baking as a leavening agent, where it converts the food/fermentable sugars present in dough into the gas carbon dioxide. This causes the dough to expand or rise as gas forms pockets or bubbles. When the dough is baked, the yeast dies and the air pockets "set", giving the baked product a soft and spongy texture. The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Most yeasts used in baking are of the same species common in alcoholic fermentation. In addition, Saccharomyces exiguus (also known as S. minor), a wild yeast found on plants, fruits, and grains, is occasionally used for baking. In breadmaking, the yeast initially respires aerobically, producing carbon dioxide and water. When the oxygen is depleted, fermentation begins, producing ethanol as a waste product; however, this evaporates during baking.[65]

A block of compressed fresh yeast

It is not known when yeast was first used to bake bread. The first records that show this use came from Ancient Egypt.[6] Researchers speculate a mixture of flour meal and water was left longer than usual on a warm day and the yeasts that occur in natural contaminants of the flour caused it to ferment before baking. The resulting bread would have been lighter and tastier than the normal flat, hard cake.

Active dried yeast, a granulated form in which yeast is commercially sold

Today, there are several retailers of baker's yeast; one of the best-known in North America is Fleischmann's Yeast, which was developed in 1868. During World War II, Fleischmann's developed a granulated active dry yeast which did not require refrigeration, had a longer shelf life than fresh yeast, and rose twice as fast. Baker's yeast is also sold as a fresh yeast compressed into a square "cake". This form perishes quickly, so must be used soon after production. A weak solution of water and sugar can be used to determine whether yeast is expired. In the solution, active yeast will foam and bubble as it ferments the sugar into ethanol and carbon dioxide. Some recipes refer to this as proofing the yeast, as it "proves" (tests) the viability of the yeast before the other ingredients are added. When a sourdough starter is used, flour and water are added instead of sugar; this is referred to as proofing the sponge.

When yeast is used for making bread, it is mixed with flour, salt, and warm water or milk. The dough is kneaded until it is smooth, and then left to rise, sometimes until it has doubled in size. The dough is then shaped into loaves. Some bread doughs are knocked back after one rising and left to rise again (this is called dough proofing) and then baked. A longer rising time gives a better flavour, but the yeast can fail to raise the bread in the final stages if it is left for too long initially.

Bioremediation

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Some yeasts can find potential application in the field of bioremediation. One such yeast, Yarrowia lipolytica, is known to degrade palm oil mill effluent, TNT (an explosive material), and other hydrocarbons, such as alkanes, fatty acids, fats and oils.[66] It can also tolerate high concentrations of salt and heavy metals,[67] and is being investigated for its potential as a heavy metal biosorbent.[68] Saccharomyces cerevisiae has potential to bioremediate toxic pollutants like arsenic from industrial effluent.[69] Bronze statues are known to be degraded by certain species of yeast.[70] Different yeasts from Brazilian gold mines bioaccumulate free and complexed silver ions.[71]

Industrial ethanol production

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Δείτε επίσης: Bioethanol

The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry to produce ethanol fuel. The process starts by milling a feedstock, such as sugar cane, field corn, or other cereal grains, and then adding dilute sulfuric acid, or fungal alpha amylase enzymes, to break down the starches into complex sugars. A glucoamylase is then added to break the complex sugars down into simple sugars. After this, yeasts are added to convert the simple sugars to ethanol, which is then distilled off to obtain ethanol up to 96% in purity.[72]

Saccharomyces yeasts have been genetically engineered to ferment xylose, one of the major fermentable sugars present in cellulosic biomasses, such as agriculture residues, paper wastes, and wood chips.[73][74] Such a development means ethanol can be efficiently produced from more inexpensive feedstocks, making cellulosic ethanol fuel a more competitively priced alternative to gasoline fuels.[75]

Nonalcoholic beverages

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A kombucha culture fermenting in a jar
Yeast and bacteria in kombucha at 400X

A number of sweet carbonated beverages can be produced by the same methods as beer, except the fermentation is stopped sooner, producing carbon dioxide, but only trace amounts of alcohol, leaving a significant amount of residual sugar in the drink.

Nutritional supplements

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Δείτε επίσης: Tibicos

Yeast is used in nutritional supplements popular with health-conscious individuals and those following vegan diets. It is often referred to as "nutritional yeast" when sold as a dietary supplement. Nutritional yeast is a deactivated yeast, usually S.  cerevisiae. It is an excellent source of protein and vitamins, especially the B-complex vitamins,[79] as well as other minerals and cofactors required for growth. It is also naturally low in fat and sodium. Contrary to some claims, it contains little or no vitamin B12.[51] Some brands of nutritional yeast, though not all, are fortified with vitamin B12, which is produced separately by bacteria.[80]

In 1920, the Fleischmann Yeast Company began to promote yeast cakes in a successful "Yeast for Health" campaign. They initially emphasized yeast's importance as a source of vitamins, good for skin and digestion. Their later advertising claimed a much broader range of health benefits, and was censured as misleading by the Federal Trade Commission. The fad for yeast cakes lasted until the late 1930s.[81]

Nutritional yeast has a nutty, cheesy flavor that makes it popular as an ingredient in cheese substitutes. It is often used by vegans in place of Parmesan cheese. Another popular use is as a topping for popcorn. It can also be used in mashed and fried potatoes, as well as in scrambled eggs. It comes in the form of flakes, or as a yellow powder similar in texture to cornmeal, and can be found in the bulk aisle of most natural food stores. In Australia, it is sometimes sold as "savory yeast flakes". Though "nutritional yeast" usually refers to commercial products, inadequately fed prisoners have used "home-grown" yeast to prevent vitamin deficiency.[82]

Probiotics

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Some probiotic supplements use the yeast S. boulardii to maintain and restore the natural flora in the gastrointestinal tract. S. boulardii has been shown to reduce the symptoms of acute diarrhea,[83] reduce the chance of infection by Clostridium difficile,[84] reduce bowel movements in diarrhea-predominant IBS patients,[85] and reduce the incidence of antibiotic-, traveler's-, and HIV/AIDS-associated diarrheas.[86]

Aquarium hobby

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Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO2) to nourish plants in planted aquaria.[87] CO2 levels from yeast are more difficult to regulate than those from pressurized CO2 systems. However, the low cost of yeast makes it a widely used alternative.[87]

Several recipes for homemade CO2 are available, but they are variations of the basic recipe: Baker's yeast, with sugar, baking soda, and water, are added to a plastic bottle. A few drops of vegetable oil at the start reduces surface tension and speeds the release of CO2. This will produce CO2 for about two or three weeks; the use of a bubble counter determines production. The CO2 is injected in the aquarium through a narrow hose and released through a diffuser that helps dissolve the gas in the water. The CO2 is used by plants in the photosynthesis process.

Yeast extract

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 Η ενότητα αυτή δεν τεκμηριώνεται επαρκώς με παραπομπές. Παρακαλούμε βοηθήστε προσθέτοντας την κατάλληλη τεκμηρίωση. Ατεκμηρίωτο υλικό μπορεί να αμφισβητηθεί και να αφαιρεθεί.
Προσοχή, το πρότυπο τοποθετήθηκε χωρίς ημερομηνία. Για την σημερινή ημερομηνία χρησιμοποιήστε: {{πηγές ενότητας|20|11|2024}}
Κύριο λήμμα: Yeast extract
Marmite and Vegemite, products made from yeast extract
Marmite and Vegemite have a distinctive dark colour

Yeast extract is the common name for various forms of processed yeast products that are used as food additives or flavours. They are often used in the same way that monosodium glutamate (MSG) is used and, like MSG, often contain free glutamic acid.[88] The general method for making yeast extract for food products such as Vegemite and Marmite on a commercial scale is to add salt to a suspension of yeast, making the solution hypertonic, which leads to the cells' shrivelling up. This triggers autolysis, wherein the yeast's digestive enzymes break their own proteins down into simpler compounds, a process of self-destruction. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeast with thick cell walls that would give poor texture) are separated. Yeast autolysates are used in Vegemite and Promite (Australia); Marmite, Bovril and Oxo (the United Kingdom, Republic of Ireland and South Africa); and Cenovis (Switzerland).

Scientific research

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Diagram showing a yeast cell

Several yeasts, in particular S. cerevisiae, have been widely used in genetics and cell biology, largely because S. cerevisiae is a simple eukaryotic cell, serving as a model for all eukaryotes, including humans, for the study of fundamental cellular processes such as the cell cycle, DNA replication, recombination, cell division, and metabolism. Also, yeasts are easily manipulated and cultured in the laboratory, which has allowed for the development of powerful standard techniques, such as yeast two-hybrid,[89] synthetic genetic array analysis,[90] and tetrad analysis. Many proteins important in human biology were first discovered by studying their homologues in yeast; these proteins include cell cycle proteins, signaling proteins, and protein-processing enzymes.[91]

On 24 April 1996, S. cerevisiae was announced to be the first eukaryote to have its genome, consisting of 12 million base pairs, fully sequenced as part of the Genome Project.[92] At the time, it was the most complex organism to have its full genome sequenced, and the work seven years and the involvement of more than 100 laboratories to accomplish.[93] The second yeast species to have its genome sequenced was Schizosaccharomyces pombe, which was completed in 2002.[94][95] It was the sixth eukaryotic genome sequenced and consists of 13.8 million base pairs. As of 2014, over 50 yeast species have had their genomes sequenced and published.[96]

Genetically engineered biofactories

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Various yeast species have been genetically engineered to efficiently produce various drugs, a technique called metabolic engineering. S. cerevisiae is easy to genetically engineer; its physiology, metabolism and genetics are well known, and it is amenable for use in harsh industrial conditions. A wide variety of chemical in different classes can be produced by engineered yeast, including phenolics, isoprenoids, alkaloids, and polyketides.[97] About 20% of biopharmaceuticals are produced in S. cerevisiae, including insulin, vaccines for hepatitis, and human serum albumin.[98]

Pathogenic yeasts

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A photomicrograph of Candida albicans showing hyphal outgrowth and other morphological characteristics

Some species of yeast are opportunistic pathogens that can cause infection in people with compromised immune systems. Cryptococcus neoformans and Cryptococcus gattii are significant pathogens of immunocompromised people. They are the species primarily responsible for cryptococcosis, a fungal disease that occurs in about one million HIV/AIDS patients, causing over 600,000 deaths annually.[99] The cells of these yeast are surrounded by a rigid polysaccharide capsule, which helps to prevent them from being recognised and engulfed by white blood cells in the human body.[100]

Yeasts of the Candida genus, another group of opportunistic pathogens, cause oral and vaginal infections in humans, known as candidiasis. Candida is commonly found as a commensal yeast in the mucous membranes of humans and other warm-blooded animals. However, sometimes these same strains can become pathogenic. The yeast cells sprout a hyphal outgrowth, which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues.[101] The pathogenic yeasts of candidiasis in probable descending order of virulence for humans are: C. albicans, C. tropicalis, C. stellatoidea, C. glabrata, C. krusei, C. parapsilosis, C. guilliermondii, C. viswanathii, C. lusitaniae, and Rhodotorula mucilaginosa.[102] Candida glabrata is the second most common Candida pathogen after C. albicans, causing infections of the urogenital tract, and of the bloodstream (candidemia).[103]

Food spoilage

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Yeasts are able to grow in foods with a low pH (5.0 or lower) and in the presence of sugars, organic acids, and other easily metabolized carbon sources.[104] During their growth, yeasts metabolize some food components and produce metabolic end products. This causes the physical, chemical, and sensible properties of a food to change, and the food is spoiled.[105] The growth of yeast within food products is often seen on their surfaces, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semiliquid products, such as syrups and jams.[104] The yeast of the Zygosaccharomyces genus have had a long history as spoilage yeasts within the food industry. This is mainly because these species can grow in the presence of high sucrose, ethanol, acetic acid, sorbic acid, benzoic acid, and sulphur dioxide concentrations,[62] representing some of the commonly used food preservation methods. Methylene blue is used to test for the presence of live yeast cells.[106] In oenology, the major spoilage yeast is Brettanomyces bruxellensis.

See also

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References

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  1. ↑ Molecular Mechanisms in Yeast Carbon Metabolism The second completely sequenced yeast genome came 6 years later from the fission yeast Schizosaccharomyces pombe, which diverged from S. cerevisiae probably more than 300 million years ago
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  100. ↑ O'Meara TR, Alspaugh JA. (2012). «The Cryptococcus neoformans capsule: A sword and a shield». Clinical Microbiology Reviews 25 (3): 387–408. doi:10.1128/CMR.00001-12. PMID 22763631.  open access
  101. ↑ "The Microbial World: Yeasts and yeast-like fungi" στη Wayback Machine του Internet Archive (αρχειοθετήθηκε Σεπτέμβριος 25, 2006). Institute of Cell and Molecular Biology. Retrieved 24 December 2006.
  102. ↑ Hurley R, de Louvois J, Mulhall A. (1987). «Yeast as human and animal pathogens». Στο: Rose AH· Harrison JS. The Yeasts. Volume 1: Biology of Yeasts (2nd έκδοση). New York, New York: Academic Press. σελίδες 207–281. CS1 maint: Uses authors parameter (link)
  103. ↑ Brunke S, Hube B. (2013). «Two unlike cousins: Candida albicans and C. glabrata infection strategies». Cellular Microbiology 15 (5): 701–708. doi:10.1111/cmi.12091. PMID 23253282.  open access
  104. ↑ 104,0 104,1 Kurtzman CP. (2006). «Detection, identification and enumeration methods for spoilage yeasts». Στο: Blackburn CDW. Food spoilage microorganisms. Cambridge, England: Woodhead Publishing. σελίδες 28–54. ISBN 978-1-85573-966-6. 
  105. ↑ Fleet GH, Praphailong W. (2001). «Yeasts». Στο: Moir CJ. Spoilage of Processed Foods: Causes and Diagnosis. Food Microbiology Group of the Australian Institute of Food Science and Technology (AIFST). σελίδες 383–397. ISBN 978-0-9578907-0-1. CS1 maint: Uses authors parameter (link)
  106. ↑ Downes FP, Ito K. (2001). Compendium of Methods for the Microbiological Examination of Foods. Washington, DC: American Public Health Association. σελ. 211. ISBN 978-0-87553-175-5. CS1 maint: Uses authors parameter (link)

Further reading

[Επεξεργασία | επεξεργασία κώδικα]
  • Alexopoulos CJ, Mims CW, Blackwell M. (1996). Introductory Mycology. New York, New York: Wiley. ISBN 978-0-471-52229-4. CS1 maint: Uses authors parameter (link)
  • Kirk PM, Cannon PF, Minter DW, Stalpers JA. (2008). Dictionary of the Fungi (10th έκδοση). Wallingford, UK: CAB International. ISBN 978-0-85199-826-8. CS1 maint: Uses authors parameter (link)
  • Kurtzman CP· Fell JW· Boekhout T, επιμ. (2011). The Yeasts: A Taxonomic Study. 1 (5th έκδοση). Amsterdam, etc.: Elsevier. ISBN 978-0-12-384708-9. 
  • Moore-Landecker E. (1996). Fundamentals of the Fungi. Englewood Cliffs, New Jersey: Prentice Hall. ISBN 978-0-13-376864-0. 
  • Priest FG, Stewart GG. (2006). Handbook of Brewing (2nd έκδοση). CRC Press. σελ. 691. ISBN 978-1-4200-1517-1. CS1 maint: Uses authors parameter (link)
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