Difference between revisions of "Yeast" - New World Encyclopedia
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| − | Yeasts are a group of single-celled [[eukaryote]]s ([[organism]]s with a distinct, membrane-bound [[nucleus]]) classified in the [[ | + | Yeasts are a group of single-celled [[eukaryote]]s ([[organism]]s with a distinct, membrane-bound [[nucleus]]) that are classified in the [[Kingdom]] [[Fungi]]. |
| − | + | Yeast [[species]] demonstrate an incredible diversity of forms. Although approximately 1,500 species have been described<ref name="YeastRef1">Kurtzman, C.P., Fell, J.W. 2006. [http://www.ars.usda.gov/research/publications/publications.htm?SEQ_NO_115=176765 "Yeast Systematics and Phylogeny — Implications of Molecular Identification Methods for Studies in Ecology."], Biodiversity and Ecophysiology of Yeasts, The Yeast Handbook, Springer. Retrieved [[January 7]] [[2007]].</ref>, it is estimated that this number represents only 1% of all yeast species.<ref name=Kurtzman2>Kurtzman, C.P., Piskur, J. 2006. [http://www.ars.usda.gov/research/publications/publications.htm?SEQ_NO_115=168320 Taxonomy and phylogenetic diversity among the yeasts]. In: Sunnerhagen, P. and Piskur, J., editors. Comparative Genomics: Using Fungi as Models. Vol. 15. Berlin: [[Springer Science+Business Media|Springer-Verlag]], Berlin. p. 29-46.</ref>The size of these [[microorganism]]s varies greatly depending on the [[species]]; yeast cells typically measure 3 to 7 micrometers in [[diameter]], although some can reach over 40 µm.<ref name=Walker>Walker K, Skelton H, Smith K., http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=12453301 Cutaneous lesions showing giant yeast forms of Blastomyces dermatitidis.], J Cutan Pathol. 2002 Nov;29(10):616-8.</ref> | |
| − | Most | + | Most yeasts reproduce by [[budding]], a process of [[asexual reproduction]] during which a small protrusion grows, matures, and detaches from the parent cell. In a few cases, the method of reproduction is [[binary fission]] (the division of the parent cell into two cells of the same size). |
| − | + | Since they derive energy from [[fermentation]] (a chemical conversion of [[carbohydrate]]s to [[ethanol]] and [[carbon dioxide]]), yeasts are found primarily in [[sugar]]-rich environments, such as fruit surfaces or flower nectars. Their ability to ferment sugars has made them an integral component of [[human]] [[civilization]]: the yeast species ''[[Saccharomyces cerevisiae]]'', in particular, has been used in the [[baking]] of [[leavened bread]] and the production of [[alcoholic beverages]] for thousands of years. Recently, yeasts have been used to generate electricity in [[microbial fuel cell]]s<ref name="YeastRef3"> [http://www.automation.hut.fi/research/bio/biofuel.htm "Biofuelcell"]. ''Helsinki University of Technology''. Retrieved [[December 24]] [[2006]].</ref> and to produce ethanol for the [[biofuel]] industry. | |
| − | + | Yeasts are also important as [[model organism]]s in modern [[cell biology]] research. ''S. cerevisiae'' was the first eukaryote whose [[genome]] (the complete [[DNA]] sequence of a set of [[chromosome]]s) was fully mapped. Given the extensive similarities between yeast cells and human cells, yeast homologues (or counterparts) of human genes, particularly those associated with specific diseases, may provide insights that aid in medical diagnosis and treatment. | |
Some species of yeast, such as ''Candida albicans'', are [[opportunistic pathogen]]s that can cause [[yeast infection|infection]] in humans, particularly those with compromised [[immune system]]s. | Some species of yeast, such as ''Candida albicans'', are [[opportunistic pathogen]]s that can cause [[yeast infection|infection]] in humans, particularly those with compromised [[immune system]]s. | ||
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==Growth and nutrition== | ==Growth and nutrition== | ||
| − | Yeasts are [[chemoorganotroph]]s: they use [[organic compound]]s as a source of energy and do not require light to grow. They obtain their main source of carbon from [[hexose]] (six-carbon) sugars such as [[glucose]], or disaccharides such as [[sucrose]] and [[maltose]]. Some species can metabolize five-carbon ([[pentose]]) sugars such as [[fructose]], as well as [[alcohol]]s and [[organic acid]]s. | + | Yeasts are [[chemoorganotroph]]s: they use [[organic compound]]s as a source of energy and do not require light to grow. They obtain their main source of carbon from [[hexose]] (six-carbon) sugars such as [[glucose]], or disaccharides such as [[sucrose]] and [[maltose]]. Some species can metabolize five-carbon ([[pentose]]) sugars such as [[fructose]], as well as [[alcohol]]s and [[organic acid]]s. Species that require oxygen for [[cellular respiration]] are an example of [[obligate aerobe]]s; others that are anaerobic but also have aerobic methods of energy production are called [[facultative anaerobe]]s. Unlike [[bacteria]], there are no known yeast species that grow only in the absence of oxygen (anaerobically). |
| − | Yeasts are ubiquitous in the environment, but are most frequently isolated from sugar-rich media, such as fruits and berries (e.g., [[grape]]s, [[apple]]s, or [[peach]]es) | + | Yeasts are ubiquitous in the environment, but are most frequently isolated from sugar-rich media, such as fruits and berries (e.g., [[grape]]s, [[apple]]s, or [[peach]]es) and exudates from plants (such as plant saps or cacti). Some yeasts are found in soils and in association with insects.<ref>{{cite journal | author = Suh S, McHugh J, Pollock D, Blackwell M | title = The beetle gut: a hyperdiverse source of novel yeasts | journal = Mycol Res | volume = 109 | issue = Pt 3 | pages = 261-5 | year = 2005 | id = PMID 15912941}}</ref><ref>{{cite journal | author = Sláviková E, Vadkertiová R | title = The diversity of yeasts in the agricultural soil | journal = J Basic Microbiol | volume = 43 | issue = 5 | pages = 430-6 | year = 2003 | id = PMID 12964187}}</ref> Yeast are generally grown in the laboratory on solid [[growth medium|growth media]] or liquid [[broth]]s. |
==Reproduction== | ==Reproduction== | ||
| − | [[Image:Budding yeast tomography.jpg|thumb|right|250px|A high magnification view of a budding yeast. Budding, an asexual form of | + | [[Image:Budding yeast tomography.jpg|thumb|right|250px|A high magnification view of a budding yeast. Budding, an asexual form of reproduction common among yeast species, entails the formation of a new organism from a protrusion on its parent cell.]] |
| − | The most common mode of vegetative growth in yeast is [[asexual reproduction]] by [[budding]] or [[binary fission|fission]].<ref name=Balasubramanian>{{cite journal | author = Balasubramanian M, Bi E, Glotzer M | title = Comparative analysis of cytokinesis in budding yeast, fission yeast and animal cells | journal = Curr Biol | volume = 14 | issue = 18 | pages = R806-18 | year = 2004 | id = PMID 15380095}}</ref> In the latter type of reproduction, the parent cell divides into equal cells. In the former, more common, method, a small bud, or daughter cell, is formed on the parent cell. The [[cell nucleus|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.<ref>{{cite journal | author = Yeong F | title = Severing all ties between mother and daughter: cell separation in budding yeast | journal = Mol Microbiol | volume = 55 | issue = 5 | pages = 1325-31 | year = 2005 | id = PMID 15720543}}</ref> The bud can develop on different areas of the parent cell depending on the [[genus]] of the yeast. | + | The most common mode of vegetative growth in yeast is [[asexual reproduction]] by [[budding]] or [[binary fission|fission]].<ref name=Balasubramanian>{{cite journal | author = Balasubramanian M, Bi E, Glotzer M | title = Comparative analysis of cytokinesis in budding yeast, fission yeast and animal cells | journal = Curr Biol | volume = 14 | issue = 18 | pages = R806-18 | year = 2004 | id = PMID 15380095}}</ref> In the latter type of reproduction, the parent cell divides into two equal cells. In the former, more common, method, a small bud, or daughter cell, is formed on the parent cell. The [[cell nucleus|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.<ref>{{cite journal | author = Yeong F | title = Severing all ties between mother and daughter: cell separation in budding yeast | journal = Mol Microbiol | volume = 55 | issue = 5 | pages = 1325-31 | year = 2005 | id = PMID 15720543}}</ref> The bud can develop on different areas of the parent cell depending on the [[genus]] of the yeast. |
Although rare, some species of yeast reproduce via sexual reproductive cycles. Yeast cells can exist as either [[diploid]]s (which contain two homologous sets of chromosomes) or [[haploid]]s (cells containing only one set of chromosomes). The mating (or conjugation) of yeast occurs only between haploids, which can be either the '''a''' or α (alpha) mating type (thus displaying simple [[sexual differentiation]]). Under high stress conditions, such as nutrient depletion, haploid yeast cells will generally die. However, under the same conditions, diploid cells can undergo ''sporulation'', producing a variety of haploid [[spores]]. These spores can go on to reproduce sexually, reforming the [[diploid]] cell.<ref>{{cite journal | author = Neiman A | title = Ascospore formation in the yeast Saccharomyces cerevisiae | url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1306807&blobtype=pdf | journal = Microbiol Mol Biol Rev | volume = 69 | issue = 4 | pages = 565-84 | year = 2005 | id = PMID 16339736}}</ref> | Although rare, some species of yeast reproduce via sexual reproductive cycles. Yeast cells can exist as either [[diploid]]s (which contain two homologous sets of chromosomes) or [[haploid]]s (cells containing only one set of chromosomes). The mating (or conjugation) of yeast occurs only between haploids, which can be either the '''a''' or α (alpha) mating type (thus displaying simple [[sexual differentiation]]). Under high stress conditions, such as nutrient depletion, haploid yeast cells will generally die. However, under the same conditions, diploid cells can undergo ''sporulation'', producing a variety of haploid [[spores]]. These spores can go on to reproduce sexually, reforming the [[diploid]] cell.<ref>{{cite journal | author = Neiman A | title = Ascospore formation in the yeast Saccharomyces cerevisiae | url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1306807&blobtype=pdf | journal = Microbiol Mol Biol Rev | volume = 69 | issue = 4 | pages = 565-84 | year = 2005 | id = PMID 16339736}}</ref> | ||
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===Food and beverage production=== | ===Food and beverage production=== | ||
| − | + | Yeasts feed on sugars, generating the end-products ethanol (essential to beer and wine production) and carbon dioxide (essential to the baking of leavened bread). | |
====Beer==== | ====Beer==== | ||
[[Image:Samadams2.jpg|thumb|250px|right|Fermenting tanks with yeast being used to brew beer.]] | [[Image:Samadams2.jpg|thumb|250px|right|Fermenting tanks with yeast being used to brew beer.]] | ||
Beer [[brewing|brewers]] classify yeasts as ''top-fermenting'' and ''bottom-fermenting''. | Beer [[brewing|brewers]] classify yeasts as ''top-fermenting'' and ''bottom-fermenting''. | ||
| − | Top-fermenting yeasts are so-called because they form a foam at the top of the [[wort]] ( | + | Top-fermenting yeasts are so-called because they form a foam at the top of the [[wort]] (the liquid produced from crushed malted grain and water). Top-fermenting yeasts can produce high ethanol concentrations and prefer high temperatures, yielding fruitier, sweeter, [[ale]]-type beers than bottom-fermenting yeasts, which are used to produce [[lager]]-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low temperatures. |
To ensure purity of strain in industrial brewing, a “clean” sample of the yeast is stored and refrigerated in a laboratory. After a certain number of fermentation cycles, a full scale [[biological reproduction|propagation]] is produced from this laboratory sample. Typically, it is grown in about three or four stages using sterile brewing wort and [[oxygen]]. | To ensure purity of strain in industrial brewing, a “clean” sample of the yeast is stored and refrigerated in a laboratory. After a certain number of fermentation cycles, a full scale [[biological reproduction|propagation]] is produced from this laboratory sample. Typically, it is grown in about three or four stages using sterile brewing wort and [[oxygen]]. | ||
| − | Root beer and sodas can be produced using | + | Root beer and sodas can be produced using similar methods. The carbonation process created by the active yeast is halted at an earlier stage, generating only trace amounts of alcohol and leaving a significant amount of sugar in the drink. |
====Wine==== | ====Wine==== | ||
[[Image:Yeast on grapes.jpg|thumb|left|250px|Grapes covered in yeast growth observable as a white film, also known as the ''blush''.]] | [[Image:Yeast on grapes.jpg|thumb|left|250px|Grapes covered in yeast growth observable as a white film, also known as the ''blush''.]] | ||
| − | In [[winemaking]], yeast converts the sugars present in [[grape juice]] or [[must]] into ethanol. | + | In [[winemaking]], yeast converts the sugars present in [[grape juice]] or [[must]] into ethanol. Typically, yeast is already present on the grapes, often visible as a powdery film (also known as the ''bloom'' or ''blush'') on their exterior. The fermentation can be done with this indigenous (or ''wild'') yeast;<ref name =Ross>Jordan P. Ross, [http://www.findarticles.com/p/articles/mi_m3488/is_n9_v78/ai_19900987 Going wild: wild yeast in winemaking] Wines & Vines, Sept, 1997. Retrieved [[10 January]] [[2007]].</ref> however, this may yield unpredictable results depending on the yeast species that are present. For this reason, a pure yeast culture is generally added to the must, which rapidly predominates the fermentation as it proceeds, ensuring a reliable fermentation.<ref name=Gonzalez>A. González Techera, S. Jubany, F.M. Carrau, C. Gaggero, '''[http://www.blackwell-synergy.com/links/doi/10.1046/j.1472-765X.2001.00946.x/full/?cookieSet=1 Differentiation of industrial wine yeast strains using microsatellite markers]''', Letters in Applied Microbiology 2001 33:1 71.</ref> |
====Distilled beverages==== | ====Distilled beverages==== | ||
| − | A [[distilled beverage]] | + | A [[distilled beverage]] contains 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, [[ester]]s, and other alcohols that account for the [[flavor]] of the beverage. |
====Baking==== | ====Baking==== | ||
[[Image:Bread rise.jpg|right|thumb|250px|Bread showing pockets left by carbon dioxide.]] | [[Image:Bread rise.jpg|right|thumb|250px|Bread showing pockets left by carbon dioxide.]] | ||
| − | Yeast | + | Yeast is used in [[baking]] as a [[leavening agent]], converting the fermentable sugars present in the [[dough]] into [[carbon dioxide]]. This process causes the dough to rise as the carbon dioxide forms pockets (or [[liquid bubble|bubbles]]). When the dough is baked, it "sets" and the pockets remain, giving the baked product a soft and spongy texture. The use of [[potato]]es, water from potato boiling, [[Egg (food)|eggs]], or [[sugar]] in a bread dough accelerates the growth of yeasts. [[Sodium chloride|Salt]] and [[fat]]s such as [[butter]] slow down yeast growth. The majority of yeast used in baking is of the same species (''S. cerevisiae'') common in alcohol fermentation. |
====Kombucha==== | ====Kombucha==== | ||
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====Yeast extract==== | ====Yeast extract==== | ||
| − | Yeast extract is the common name for various forms of processed yeast products that are used as [[food additive]]s or flavorings. 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 the cells to shrivel up. This contraction triggers [[autolysis (biology)|autolysis]], during which the yeast's [[digestion|digestive]] enzymes break down their own [[protein]]s | + | Yeast extract is the common name for various forms of processed yeast products that are used as [[food additive]]s or flavorings. 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 the cells to shrivel up. This contraction triggers [[autolysis (biology)|autolysis]], during which the yeast's [[digestion|digestive]] enzymes break down their own [[protein]]s into simpler compounds, a self-destructive process. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeasts with thick cell walls) are separated. Yeast autolysates are used in [[Vegemite]] and [[Promite]] ([[Australia]]), [[Marmite]] and [[Bovril]] (the [[United Kingdom]] and [[Republic of Ireland]]), [[Oxo (food)|Oxo]] ([[South Africa]], [[United Kingdom]], and [[Republic of Ireland]]), and [[Cenovis]] ([[Switzerland]]). |
[[Image:Vegemite and Marmite.jpg|thumb|right|150px|[[Vegemite]] and [[Marmite]], products made from yeast extract.]] | [[Image:Vegemite and Marmite.jpg|thumb|right|150px|[[Vegemite]] and [[Marmite]], products made from yeast extract.]] | ||
===Nutritional supplements=== | ===Nutritional supplements=== | ||
| − | Yeast is used in nutritional supplements popular with [[vegan]]s and the health conscious; in this context, it is often referred to as ''nutritional yeast''. It is a deactivated yeast, usually of the species '' | + | Yeast is used in nutritional supplements popular with [[vegan]]s and the health conscious; in this context, it is often referred to as ''nutritional yeast''. It is a deactivated yeast, usually of the species ''S. cerevisiae''. Nutritional yeast is an excellent source of protein and vitamins, especially the [[Vitamin B|B-complex]] vitamins, whose functions are related to metabolism, as well as other [[mineral]]s and [[cofactor]]s required for growth. It is also naturally low in [[fat]] and [[sodium]]. Nutritional yeast has a nutty, cheesy, creamy flavor that makes it popular as an ingredient in [[cheese]] substitutes; it is often used in place of [[Parmigiano Reggiano|parmesan cheese]] and as a topping for [[popcorn]]. |
| − | Some [[probiotic]] supplements use the yeast ''[[Saccharomyces boulardii]]'' to maintain and restore the natural flora in the large and small gastrointestinal tract. | + | Some [[probiotic]] supplements use the yeast ''[[Saccharomyces boulardii]]'' to maintain and restore the natural flora in the large and small gastrointestinal tract. |
===Other applications=== | ===Other applications=== | ||
====Bioremediation==== | ====Bioremediation==== | ||
| − | Some yeasts can find potential application in the field of [[bioremediation]], which can be defined as any process that uses [[microorganism]]s, [[fungi]], [[phytoremediation|green plants]] or their [[enzyme]]s to return | + | Some yeasts can find potential application in the field of [[bioremediation]], which can be defined as any process that uses [[microorganism]]s, [[fungi]], [[phytoremediation|green plants]] or their [[enzyme]]s to return an [[Natural environment|environment]] altered by [[contaminant]]s to its original condition. The species ''[[Yarrowia lipolytica]]'', for example, is known to degrade [[palm oil]] mill [[effluent]],<ref name=Oswal>{{cite journal |quotes= |last=Oswal |first=N |authorlink= |coauthors=Sarma PM, Zinjarde SS, Pant A. |year=2002 |month=Oct |title=Palm oil mill effluent treatment by a tropical marine yeast. |journal=Bioresour Technol. |volume=85 |issue=1 |pages= |id=PMID 12146640 |url= |accessdate=2007-01-21 }}</ref> [[Trinitrotoluene|TNT]] (an explosive material),<ref name=Jain>{{cite journal |quotes= |last=Jain |first=MR |authorlink= |coauthors=Zinjarde SS, Deobagkar DD, Deobagkar DN |year=2004 |month=Nov |title=2,4,6-trinitrotoluene transformation by a tropical marine yeast, Yarrowia lipolytica NCIM 3589. |journal=Mar Pollut Bull. |volume=49 |issue=9-10 |pages=783-8 |id= PMID 15530522 |url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=15530522&dopt=Abstract |accessdate=2007-01-21 }}</ref> and other [[hydrocarbon]]s such as [[alkane]]s, [[fatty acid]]s, [[fat]]s and [[oil]]s.<ref name=Fickers>{{cite journal |quotes= |last=Fickers |first=P |authorlink= |coauthors=Benetti PH, Wache Y, Marty A, Mauersberger S, Smit MS, Nicaud JM |year=2005 |month=April |title=Hydrophobic substrate utilisation by the yeast Yarrowia lipolytica, and its potential applications. |journal=FEMS Yeast Res. |volume=5 |issue=6-7 |pages=527-543 |id=PMID 15780653 |url= |accessdate=2007-01-21 }}</ref> |
====Industrial ethanol production==== | ====Industrial ethanol production==== | ||
| − | The ability of yeast to convert sugar into ethanol has been harnessed by the [[biotechnology]] industry to generate [[ethanol fuel]]. The process starts by milling a [[feedstock]], such as [[sugar cane]], [[sweetcorn]], or cheap [[cereal grain]]s, then adding dilute [[sulfuric acid]], or fungal alpha [[amylase]] enzymes, to break down the starches | + | The ability of yeast to convert sugar into ethanol has been harnessed by the [[biotechnology]] industry to generate [[ethanol fuel]]. The process starts by milling a [[feedstock]], such as [[sugar cane]], [[sweetcorn]], or cheap [[cereal grain]]s, then adding dilute [[sulfuric acid]], or fungal alpha [[amylase]] enzymes, to break down the starches into complex sugars. A gluco amylase is then added to degrade the complex sugars further into simple sugars. Yeasts are then added to convert the simple sugars to ethanol, which is [[distillation|distilled]] off to obtain ethanol up to 96% in concentration.<ref name="YeastRef4">[http://genomicsgtl.energy.gov/biofuels/ethanolproduction.shtml "Fuel Ethanol Production"]. ''Genomics:GTL''. Retrieved [[December 24]] [[2006]].</ref> |
''Saccharomyces'' yeasts have been [[genetically engineered]] to ferment [[xylose]], one of the major fermentable sugars present in [[cellulose|cellulosic]] biomasses, such as agriculture residues, paper wastes, and wood chips.<ref name="YeastRef5">[http://aem.asm.org/cgi/content/full/64/5/1852 "Genetically Engineered Saccharomyces Yeast Capable of Effective Cofermentation of Glucose and Xylose"]. ''American Society for Microbiology''. Retrieved [[December 24]] [[2006]].</ref> Such a development means that ethanol can be efficiently produced from more inexpensive feedstocks, making [[cellulosic ethanol]] fuel a more competitively priced alternative to [[gasoline]] fuels.<ref name="YeastRef6">[http://www.agriculture.purdue.edu/agcomm/AgCom/news/backgrd/9808.Ho.yeast.html "Yeast rises to a new occasion"]. ''American Society for Microbiology''. Retrieved [[December 24]] [[2006]].</ref> | ''Saccharomyces'' yeasts have been [[genetically engineered]] to ferment [[xylose]], one of the major fermentable sugars present in [[cellulose|cellulosic]] biomasses, such as agriculture residues, paper wastes, and wood chips.<ref name="YeastRef5">[http://aem.asm.org/cgi/content/full/64/5/1852 "Genetically Engineered Saccharomyces Yeast Capable of Effective Cofermentation of Glucose and Xylose"]. ''American Society for Microbiology''. Retrieved [[December 24]] [[2006]].</ref> Such a development means that ethanol can be efficiently produced from more inexpensive feedstocks, making [[cellulosic ethanol]] fuel a more competitively priced alternative to [[gasoline]] fuels.<ref name="YeastRef6">[http://www.agriculture.purdue.edu/agcomm/AgCom/news/backgrd/9808.Ho.yeast.html "Yeast rises to a new occasion"]. ''American Society for Microbiology''. Retrieved [[December 24]] [[2006]].</ref> | ||
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[[Image:yeast cell english.svg|thumb|left|Diagram showing a yeast cell.]] | [[Image:yeast cell english.svg|thumb|left|Diagram showing a yeast cell.]] | ||
| − | Several yeasts, particularly ''Saccharomyces cerevisiae'', have been widely studied in [[genetics]] and [[cell biology]] | + | Several yeasts, particularly ''Saccharomyces cerevisiae'', have been widely studied as model organisms in [[genetics]] and [[cell biology]]. The [[cell cycle]] in a yeast cell is very similar to the cell cycle in [[human]]s; therefore, the basic cellular mechanics of [[DNA replication]], [[recombination]], [[cell division]] and [[metabolism]] are comparable.<ref name=YeastRef2/> Many proteins important in human biology were first discovered by studying their [[homology (biology)|homologue]]s in yeast; these proteins include [[cell cycle protein]]s, [[Cell signaling|signaling proteins]], and protein-processing enzymes. |
| − | On April 24, 1996, ''S. cerevisiae'' was announced to be the first eukaryote to have its [[genome]], consisting of 12 million [[base pair]]s, fully sequenced as part of the [[Genome project]].<ref name="Williams1996">{{cite journal |last=Williams |first=N |date=[[April 26]], [[1996]] |title=Genome Projects: Yeast Genome Sequence Ferments New Research |journal=Science |volume=272 | issue=5261 |pages=481–0 |doi=10.1126/science.272.5261.481 }}</ref> At the time, it was the most complex organism to have its full genome sequenced; the project took | + | On April 24, 1996, ''S. cerevisiae'' was announced to be the first eukaryote to have its [[genome]], consisting of 12 million [[base pair]]s, fully sequenced as part of the [[Genome project]].<ref name="Williams1996">{{cite journal |last=Williams |first=N |date=[[April 26]], [[1996]] |title=Genome Projects: Yeast Genome Sequence Ferments New Research |journal=Science |volume=272 | issue=5261 |pages=481–0 |doi=10.1126/science.272.5261.481 }}</ref> At the time, it was the most complex organism to have its full genome sequenced; the project took seven years and the involvement of over 100 laboratories to accomplish.<ref>[http://www.accessexcellence.org/WN/SUA07/yeast496.html COMPLETE DNA SEQUENCE OF YEAST]. Retrieved on [[31 January]] 2007.</ref> The second yeast species to have its genome mapped was [[Schizosaccharomyces pombe]]; the sequencing was completed in 2002.<ref>[http://www.genomenewsnetwork.org/articles/03_02/s_pombe.shtml Schizosaccharomyces pombe: Second yeast genome sequenced]. Retrieved on [[31 January]] 2007.</ref> ''S. pombe'', whose genome consists of 13.8 million base pairs, was the sixth eukaryotic genome fully sequenced. |
==Pathogenic yeasts== | ==Pathogenic yeasts== | ||
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''[[Cryptococcus neoformans]]'' is a significant pathogen of immunocompromised people, causing the disease [[cryptococcosis]], which occurs in about 7-8% of [[AIDS]] patients in the USA, and in a slightly smaller percentage (3-6%) in western Europe.<ref name="YeastRef9">[http://helios.bto.ed.ac.uk/bto/microbes/yeast.htm "The Microbial World: Yeasts and yeast-like fungi"]. ''Institute of Cell and Molecular Biology''. Retrieved [[December 24]] [[2006]].</ref> The cells of the yeast are surrounded by a rigid [[polysaccharide]] capsule, which helps to prevent them from being recognized and engulfed by [[white blood cells]] in the human body. | ''[[Cryptococcus neoformans]]'' is a significant pathogen of immunocompromised people, causing the disease [[cryptococcosis]], which occurs in about 7-8% of [[AIDS]] patients in the USA, and in a slightly smaller percentage (3-6%) in western Europe.<ref name="YeastRef9">[http://helios.bto.ed.ac.uk/bto/microbes/yeast.htm "The Microbial World: Yeasts and yeast-like fungi"]. ''Institute of Cell and Molecular Biology''. Retrieved [[December 24]] [[2006]].</ref> The cells of the yeast are surrounded by a rigid [[polysaccharide]] capsule, which helps to prevent them from being recognized and engulfed by [[white blood cells]] in the human body. | ||
| − | Yeasts of the ''[[Candida (genus)|Candida]]'' genus are another group of opportunistic pathogens, | + | Yeasts of the ''[[Candida (genus)|Candida]]'' genus are another group of opportunistic pathogens, causing oral and [[vagina]]l [[infection]]s in humans. ''Candida'' is commonly found as a [[commensal]] yeast in the [[mucus membranes]] of humans and other warm-blooded animals. However, sometimes these strains, which typically do not harm their host, can become pathogenic. In the figure above, the yeast cells sprout a [[hypha]]l outgrowth, which locally penetrates the [[mucous membrane|mucosal membrane]], causing irritation and shedding of the tissues. ''[[Candida glabrata]]'' is the second most common ''[[Candida]]'' pathogen after ''C. albicans'', causing infections of the [[urogenital tract]] and of the [[bloodstream]].<ref name="YeastRef10">{{cite journal | last = Stoyan | first = Tanja | coauthors = John Carbon | title = Inner Kinetochore of the Pathogenic Yeast Candida glabrata | journal = Eukaryotic Cell | volume = 3 | issue = 5 | pages = 1154-1163 | doi = 10.1128/EC.3.5.1154-1163.2004 | id = {{PMID|15470243}} | url = http://ec.asm.org/cgi/content/full/3/5/1154 | accessdate = 2006-12-24}}</ref> |
==History== | ==History== | ||
[[Image:Antoni van Leeuwenhoek.png|thumb|250px|Antony van Leeuwenhoek was the first scientist to observe yeast cells under the microscope.]] | [[Image:Antoni van Leeuwenhoek.png|thumb|250px|Antony van Leeuwenhoek was the first scientist to observe yeast cells under the microscope.]] | ||
| − | The word ''yeast'' comes from the [[Old English]] ''gist'' (or ''gyst''), which in turn | + | The word ''yeast'' comes from the [[Old English]] ''gist'' (or ''gyst''), which in turn derives from the [[Proto Indo-European language|Indo-European]] root ''yes-'', meaning ''boil'', ''foam'', or ''bubble''.<ref>American Heritage Dictionary.[http://www.bartleby.com/61/roots/IE598.html "yes-"]. Retrieved January 22, 2007.</ref> Yeast microbes are probably one of the earliest domesticated organisms, used for alcoholic fermentation and baking throughout history. Archaeologists digging in Egyptian ruins have found early grinding stones and baking chambers for leavened bread, as well as drawings of 4,000-year-old bakeries and breweries.<ref name=NASA>[http://science.nasa.gov/newhome/headlines/msad16mar99_1b.htm Planets in a Bottle, More about Yeast], Science@NASA, Retrieved [[6 January]] [[2007]].</ref> |
| − | Yeast were first observed under the [[microscope]] in 1680 by the [[Dutch (ethnic group)|Dutch]] naturalist [[Anton van Leeuwenhoek]], who at the time considered them to be globular structures rather than living organisms.<ref name=YeastRef11>[http://aleph0.clarku.edu/huxley/CE8/Yeast.html Yeast, The Contemporary Review (1871), Collected Essays VIII.]. Retrieved [[6 January]] [[2007]].</ref> In 1857, [[French people|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, as previously thought.<ref name=NASA/><ref name=Barnett>Barnett, James A., [http://mic.sgmjournals.org/cgi/content/full/149/3/557#R53 Beginnings of microbiology and biochemistry: the contribution of yeast research], Microbiology '''149''' (2003), 557-567</ref> Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, while fermentation was inhibited – a phenomenon later called the ''Pasteur effect''. | + | Yeast cells were first observed under the [[microscope]] in 1680 by the [[Dutch (ethnic group)|Dutch]] naturalist [[Anton van Leeuwenhoek]], who at the time considered them to be globular structures rather than living organisms.<ref name=YeastRef11>[http://aleph0.clarku.edu/huxley/CE8/Yeast.html Yeast, The Contemporary Review (1871), Collected Essays VIII.]. Retrieved [[6 January]] [[2007]].</ref> In 1857, [[French people|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, as previously thought.<ref name=NASA/><ref name=Barnett>Barnett, James A., [http://mic.sgmjournals.org/cgi/content/full/149/3/557#R53 Beginnings of microbiology and biochemistry: the contribution of yeast research], Microbiology '''149''' (2003), 557-567</ref> Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, while fermentation was inhibited – a phenomenon later called the ''Pasteur effect''. |
==References== | ==References== | ||
Revision as of 18:56, 9 June 2007
| Yeasts | ||||
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 Yeast of the species Saccharomyces cerevisiae.
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| Scientific classification | ||||
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Ascomycota (sac fungi)
Basidiomycota (club fungi)
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Yeasts are a group of single-celled eukaryotes (organisms with a distinct, membrane-bound nucleus) that are classified in the Kingdom Fungi.
Yeast species demonstrate an incredible diversity of forms. Although approximately 1,500 species have been described[1], it is estimated that this number represents only 1% of all yeast species.[2]The size of these microorganisms varies greatly depending on the species; yeast cells typically measure 3 to 7 micrometers in diameter, although some can reach over 40 µm.[3]
Most yeasts reproduce by budding, a process of asexual reproduction during which a small protrusion grows, matures, and detaches from the parent cell. In a few cases, the method of reproduction is binary fission (the division of the parent cell into two cells of the same size).
Since they derive energy from fermentation (a chemical conversion of carbohydrates to ethanol and carbon dioxide), yeasts are found primarily in sugar-rich environments, such as fruit surfaces or flower nectars. Their ability to ferment sugars has made them an integral component of human civilization: the yeast species Saccharomyces cerevisiae, in particular, has been used in the baking of leavened bread and the production of alcoholic beverages for thousands of years. Recently, yeasts have been used to generate electricity in microbial fuel cells[4] and to produce ethanol for the biofuel industry.
Yeasts are also important as model organisms in modern cell biology research. S. cerevisiae was the first eukaryote whose genome (the complete DNA sequence of a set of chromosomes) was fully mapped. Given the extensive similarities between yeast cells and human cells, yeast homologues (or counterparts) of human genes, particularly those associated with specific diseases, may provide insights that aid in medical diagnosis and treatment.
Some species of yeast, such as Candida albicans, are opportunistic pathogens that can cause infection in humans, particularly those with compromised immune systems.
Classification
Yeasts do not form a specific taxonomic or phylogenetic grouping. The term yeast is often taken as a synonym for S. cerevisiae;[5] however, the phylogenetic diversity of yeasts is shown by their placement in both divisions Ascomycota and Basidiomycota. Yeasts that reproduce via budding ("true yeasts") are classified in the order Saccharomycetales.[6]
Growth and nutrition
Yeasts are chemoorganotrophs: they use organic compounds as a source of energy and do not require light to grow. They obtain their main source of carbon from hexose (six-carbon) sugars such as glucose, or disaccharides such as sucrose and maltose. Some species can metabolize five-carbon (pentose) sugars such as fructose, as well as alcohols and organic acids. Species that require oxygen for cellular respiration are an example of obligate aerobes; others that are anaerobic but also have aerobic methods of energy production are called facultative anaerobes. Unlike bacteria, there are no known yeast species that grow only in the absence of oxygen (anaerobically).
Yeasts are ubiquitous in the environment, but are most frequently isolated from sugar-rich media, such as fruits and berries (e.g., grapes, apples, or peaches) and exudates from plants (such as plant saps or cacti). Some yeasts are found in soils and in association with insects.[7][8] Yeast are generally grown in the laboratory on solid growth media or liquid broths.
Reproduction
The most common mode of vegetative growth in yeast is asexual reproduction by budding or fission.[9] In the latter type of reproduction, the parent cell divides into two equal cells. In the former, more common, method, a small bud, 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.[10] The bud can develop on different areas of the parent cell depending on the genus of the yeast.
Although rare, some species of yeast reproduce via sexual reproductive cycles. Yeast cells can exist as either diploids (which contain two homologous sets of chromosomes) or haploids (cells containing only one set of chromosomes). The mating (or conjugation) of yeast occurs only between haploids, which can be either the a or α (alpha) mating type (thus displaying simple sexual differentiation). Under high stress conditions, such as nutrient depletion, haploid yeast cells will generally die. However, under the same conditions, diploid cells can undergo sporulation, producing a variety of haploid spores. These spores can go on to reproduce sexually, reforming the diploid cell.[11]
Uses
Food and beverage production
Yeasts feed on sugars, generating the end-products ethanol (essential to beer and wine production) and carbon dioxide (essential to the baking of leavened bread).
Beer
Beer brewers classify yeasts as top-fermenting and bottom-fermenting. Top-fermenting yeasts are so-called because they form a foam at the top of the wort (the liquid produced from crushed malted grain and water). Top-fermenting yeasts can produce high ethanol concentrations and prefer high temperatures, yielding fruitier, sweeter, ale-type beers than bottom-fermenting yeasts, which are used to produce lager-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low temperatures.
To ensure purity of strain in industrial brewing, a “clean” sample of the yeast is stored and refrigerated in a laboratory. After a certain number of fermentation cycles, a full scale propagation is produced from this laboratory sample. Typically, it is grown in about three or four stages using sterile brewing wort and oxygen.
Root beer and sodas can be produced using similar methods. The carbonation process created by the active yeast is halted at an earlier stage, generating only trace amounts of alcohol and leaving a significant amount of sugar in the drink.
Wine
In winemaking, yeast converts the sugars present in grape juice or must into ethanol. Typically, yeast is already present on the grapes, often visible as a powdery film (also known as the bloom or blush) on their exterior. The fermentation can be done with this indigenous (or wild) yeast;[12] however, this may yield unpredictable results depending on the yeast species that are present. For this reason, a pure yeast culture is generally added to the must, which rapidly predominates the fermentation as it proceeds, ensuring a reliable fermentation.[13]
Distilled beverages
A distilled beverage contains 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 that account for the flavor of the beverage.
Baking
Yeast is used in baking as a leavening agent, converting the fermentable sugars present in the dough into carbon dioxide. This process causes the dough to rise as the carbon dioxide forms pockets (or bubbles). When the dough is baked, it "sets" and the pockets remain, 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. Salt and fats such as butter slow down yeast growth. The majority of yeast used in baking is of the same species (S. cerevisiae) common in alcohol fermentation.
Kombucha
Various species of yeast are used in symbiosis with acetic acid bacteria in the preparation of Kombucha, a fermented sweetened tea.
Yeast extract
Yeast extract is the common name for various forms of processed yeast products that are used as food additives or flavorings. 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 the cells to shrivel up. This contraction triggers autolysis, during which the yeast's digestive enzymes break down their own proteins into simpler compounds, a self-destructive process. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeasts with thick cell walls) are separated. Yeast autolysates are used in Vegemite and Promite (Australia), Marmite and Bovril (the United Kingdom and Republic of Ireland), Oxo (South Africa, United Kingdom, and Republic of Ireland), and Cenovis (Switzerland).
Nutritional supplements
Yeast is used in nutritional supplements popular with vegans and the health conscious; in this context, it is often referred to as nutritional yeast. It is a deactivated yeast, usually of the species S. cerevisiae. Nutritional yeast is an excellent source of protein and vitamins, especially the B-complex vitamins, whose functions are related to metabolism, as well as other minerals and cofactors required for growth. It is also naturally low in fat and sodium. Nutritional yeast has a nutty, cheesy, creamy flavor that makes it popular as an ingredient in cheese substitutes; it is often used in place of parmesan cheese and as a topping for popcorn.
Some probiotic supplements use the yeast Saccharomyces boulardii to maintain and restore the natural flora in the large and small gastrointestinal tract.
Other applications
Bioremediation
Some yeasts can find potential application in the field of bioremediation, which can be defined as any process that uses microorganisms, fungi, green plants or their enzymes to return an environment altered by contaminants to its original condition. The species Yarrowia lipolytica, for example, is known to degrade palm oil mill effluent,[14] TNT (an explosive material),[15] and other hydrocarbons such as alkanes, fatty acids, fats and oils.[16]
Industrial ethanol production
The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry to generate ethanol fuel. The process starts by milling a feedstock, such as sugar cane, sweetcorn, or cheap cereal grains, then adding dilute sulfuric acid, or fungal alpha amylase enzymes, to break down the starches into complex sugars. A gluco amylase is then added to degrade the complex sugars further into simple sugars. Yeasts are then added to convert the simple sugars to ethanol, which is distilled off to obtain ethanol up to 96% in concentration.[17]
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.[18] Such a development means that ethanol can be efficiently produced from more inexpensive feedstocks, making cellulosic ethanol fuel a more competitively priced alternative to gasoline fuels.[19]
Yeast as a model organism
Several yeasts, particularly Saccharomyces cerevisiae, have been widely studied as model organisms in genetics and cell biology. The cell cycle in a yeast cell is very similar to the cell cycle in humans; therefore, the basic cellular mechanics of DNA replication, recombination, cell division and metabolism are comparable.[6] 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.
On April 24, 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.[20] At the time, it was the most complex organism to have its full genome sequenced; the project took seven years and the involvement of over 100 laboratories to accomplish.[21] The second yeast species to have its genome mapped was Schizosaccharomyces pombe; the sequencing was completed in 2002.[22] S. pombe, whose genome consists of 13.8 million base pairs, was the sixth eukaryotic genome fully sequenced.
Pathogenic yeasts
Some species of yeast are opportunistic pathogens; that is, they can cause infection in people with compromised immune systems.
Cryptococcus neoformans is a significant pathogen of immunocompromised people, causing the disease cryptococcosis, which occurs in about 7-8% of AIDS patients in the USA, and in a slightly smaller percentage (3-6%) in western Europe.[23] The cells of the yeast are surrounded by a rigid polysaccharide capsule, which helps to prevent them from being recognized and engulfed by white blood cells in the human body.
Yeasts of the Candida genus are another group of opportunistic pathogens, causing oral and vaginal infections in humans. Candida is commonly found as a commensal yeast in the mucus membranes of humans and other warm-blooded animals. However, sometimes these strains, which typically do not harm their host, can become pathogenic. In the figure above, the yeast cells sprout a hyphal outgrowth, which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues. Candida glabrata is the second most common Candida pathogen after C. albicans, causing infections of the urogenital tract and of the bloodstream.[24]
History
The word yeast comes from the Old English gist (or gyst), which in turn derives from the Indo-European root yes-, meaning boil, foam, or bubble.[25] Yeast microbes are probably one of the earliest domesticated organisms, used for alcoholic fermentation and baking throughout history. Archaeologists digging in Egyptian ruins have found early grinding stones and baking chambers for leavened bread, as well as drawings of 4,000-year-old bakeries and breweries.[26]
Yeast cells were first observed under the microscope in 1680 by the Dutch naturalist Anton van Leeuwenhoek, who at the time considered them to be globular structures rather than living organisms.[27] 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, as previously thought.[26][28] Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, while fermentation was inhibited – a phenomenon later called the Pasteur effect.
ReferencesISBN links support NWE through referral fees
- ↑ Kurtzman, C.P., Fell, J.W. 2006. "Yeast Systematics and Phylogeny — Implications of Molecular Identification Methods for Studies in Ecology.", Biodiversity and Ecophysiology of Yeasts, The Yeast Handbook, Springer. Retrieved January 7 2007.
- ↑ Kurtzman, C.P., Piskur, J. 2006. Taxonomy and phylogenetic diversity among the yeasts. In: Sunnerhagen, P. and Piskur, J., editors. Comparative Genomics: Using Fungi as Models. Vol. 15. Berlin: Springer-Verlag, Berlin. p. 29-46.
- ↑ Walker K, Skelton H, Smith K., http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&dopt=Abstract&list_uids=12453301 Cutaneous lesions showing giant yeast forms of Blastomyces dermatitidis.], J Cutan Pathol. 2002 Nov;29(10):616-8.
- ↑ "Biofuelcell". Helsinki University of Technology. Retrieved December 24 2006.
- ↑ Kurtzman C.P., Molecular taxonomy of the yeasts., Yeast. 1994 Dec;10(13):1727-40
- ↑ 6.0 6.1 "What are yeasts?". Saccharomyces Genome Database. Retrieved December 24 2006.
- ↑ Suh S, McHugh J, Pollock D, Blackwell M (2005). The beetle gut: a hyperdiverse source of novel yeasts. Mycol Res 109 (Pt 3): 261-5. PMID 15912941.
- ↑ Sláviková E, Vadkertiová R (2003). The diversity of yeasts in the agricultural soil. J Basic Microbiol 43 (5): 430-6. PMID 12964187.
- ↑ Balasubramanian M, Bi E, Glotzer M (2004). Comparative analysis of cytokinesis in budding yeast, fission yeast and animal cells. Curr Biol 14 (18): R806-18. PMID 15380095.
- ↑ Yeong F (2005). Severing all ties between mother and daughter: cell separation in budding yeast. Mol Microbiol 55 (5): 1325-31. PMID 15720543.
- ↑ Neiman A (2005). Ascospore formation in the yeast Saccharomyces cerevisiae. Microbiol Mol Biol Rev 69 (4): 565-84. PMID 16339736.
- ↑ Jordan P. Ross, Going wild: wild yeast in winemaking Wines & Vines, Sept, 1997. Retrieved 10 January 2007.
- ↑ A. González Techera, S. Jubany, F.M. Carrau, C. Gaggero, Differentiation of industrial wine yeast strains using microsatellite markers, Letters in Applied Microbiology 2001 33:1 71.
- ↑ Oswal, N and Sarma PM, Zinjarde SS, Pant A. (Oct 2002). Palm oil mill effluent treatment by a tropical marine yeast.. Bioresour Technol. 85 (1). PMID 12146640.
- ↑ Jain, MR and Zinjarde SS, Deobagkar DD, Deobagkar DN (Nov 2004). 2,4,6-trinitrotoluene transformation by a tropical marine yeast, Yarrowia lipolytica NCIM 3589.. Mar Pollut Bull. 49 (9-10): 783-8. PMID 15530522.
- ↑ Fickers, P and Benetti PH, Wache Y, Marty A, Mauersberger S, Smit MS, Nicaud JM (April 2005). Hydrophobic substrate utilisation by the yeast Yarrowia lipolytica, and its potential applications.. FEMS Yeast Res. 5 (6-7): 527-543. PMID 15780653.
- ↑ "Fuel Ethanol Production". Genomics:GTL. Retrieved December 24 2006.
- ↑ "Genetically Engineered Saccharomyces Yeast Capable of Effective Cofermentation of Glucose and Xylose". American Society for Microbiology. Retrieved December 24 2006.
- ↑ "Yeast rises to a new occasion". American Society for Microbiology. Retrieved December 24 2006.
- ↑ Williams, N (April 26, 1996). Genome Projects: Yeast Genome Sequence Ferments New Research. Science 272 (5261): 481–0.
- ↑ COMPLETE DNA SEQUENCE OF YEAST. Retrieved on 31 January 2007.
- ↑ Schizosaccharomyces pombe: Second yeast genome sequenced. Retrieved on 31 January 2007.
- ↑ "The Microbial World: Yeasts and yeast-like fungi". Institute of Cell and Molecular Biology. Retrieved December 24 2006.
- ↑ Stoyan, Tanja and John Carbon. Inner Kinetochore of the Pathogenic Yeast Candida glabrata. Eukaryotic Cell 3 (5): 1154-1163. PubMed.
- ↑ American Heritage Dictionary."yes-". Retrieved January 22, 2007.
- ↑ 26.0 26.1 Planets in a Bottle, More about Yeast, Science@NASA, Retrieved 6 January 2007.
- ↑ Yeast, The Contemporary Review (1871), Collected Essays VIII.. Retrieved 6 January 2007.
- ↑ Barnett, James A., Beginnings of microbiology and biochemistry: the contribution of yeast research, Microbiology 149 (2003), 557-567
External links
- Yeast Resource Center
- Yeast growth and the cell cycle
- About Nutritional Yeast, Recipes Using Nutritional Yeast
- Yeast virtual library
- Ancient Egyptian Bread Making
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