Yeast
34385
226121455
2008-07-16T22:23:29Z
Narayanese
1324750
/* Pathogenic yeasts */ that conclusion is not found in the source, and is contradicted by e.g. Schaffer et al 2007 (Inflamm Bowel Dis.)
{{Taxobox
| color = red
| name = ''Yeasts''
| image = S cerevisiae under DIC microscopy.jpg
| image_width = 200px
| image_caption = Yeast of the species ''Saccharomyces cerevisiae''.
| domain = [[Eukaryote|Eukaryota]]
| regnum = [[Fungus|Fungi]]
| subdivision_ranks = Typical divisions
| subdivision = <div>
'''[[Ascomycota]]''' <small>(sac fungi)</small>
*[[Saccharomycotina]] <small>(true yeasts)</small>
*[[Taphrinomycotina]]
**[[Schizosaccharomycetes]] <small>(fission yeasts)</small>
'''[[Basidiomycota]]''' <small>(club fungi)</small>
*[[Urediniomycetes]]
**[[Sporidiales]]
</div>
}}
'''Yeasts''' are a growth form of [[eukaryote|eukaryotic]] [[microorganism]]s classified in the [[Kingdom (biology)|kingdom]] [[fungus|Fungi]], with about 1,500 [[species]] currently 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> they dominate fungal diversity in the oceans.<ref>{{Cite journal
| issn = 0962-8452
| last = Bass
| first = D.
| coauthors = Howe, A.; Brown, N.; Barton, H.; Demidova, M.; Michelle, H.; Li, L.; Sanders, H.; Watkinson, S.C.; Willcock, S.; Richards, T.A.
| title = Yeast forms dominate fungal diversity in the deep oceans
| journal = Proc Biol Sci
| date = 2007-10-16
}}</ref> Most reproduce [[asexual reproduction|asexual]]ly by [[budding]], although a few do by [[binary fission]]. Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation of a string of connected budding cells known as ''[[hypha|pseudohyphae]]'', or ''[[hypha|false hyphae]]'' as seen in most [[mold]]s.<ref name=Kurtzman1>Kurtzman, C.P., Fell, J.W. 2006. Yeast systematics and phylogeny - implications of molecular identification methods for studies in ecology. In: Rosa, C.A. and Peter, G., editors. The Yeast Handbook. Germany:Springer-Verlag Berlin Herdelberg. p. 11-30.</ref> Yeast size can vary greatly depending on the species, typically measuring 3–4 [[micrometre|µm]] in [[diameter]], although some yeasts 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>
The yeast species ''[[Saccharomyces cerevisiae]]'' has been used in [[baking]] and [[fermentation (food)|fermenting]] [[alcoholic beverages]] for thousands of years. It is also extremely important as a [[model organism]] in modern [[cell biology]] research, and is the most thoroughly researched eukaryotic microorganism. Researchers have used it to gather information into the biology of the eukaryotic cell and ultimately human biology.<ref name=Ostergaard> Ostergaard, S., Olsson, L., Nielsen, J., [http://mmbr.asm.org/cgi/content/full/64/1/34 Metabolic Engineering of Saccharomyces cerevisiae], Microbiol. Mol. Biol. Rev. 2000 64: 34-50</ref> Other species of yeast, such as ''[[Candida albicans]]'', are [[opportunistic pathogen]]s and can cause [[yeast infection|infection]] in humans. Yeasts have recently 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 produce ethanol for the [[biofuel]] industry.
Yeasts do not form a specific [[taxonomy|taxonomic]] or [[phylogenetics|phylogenetic]] grouping. At present it is estimated that only 1% of all yeast species have been described.<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 diversidamuladyty 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 term "''yeast''" is often taken as a [[synonym]] for ''S. cerevisiae'',<ref name=Kurtzman>Kurtzman C.P., [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=7747515&dopt=Abstract Molecular taxonomy of the yeasts.], Yeast. 1994 Dec;10(13):1727-40</ref> but the phylogenetic diversity of yeasts is shown by their placement in both [[division (biology)|division]]s [[Ascomycota]] and [[Basidiomycota]]. The budding yeasts ("true yeasts") are classified in the [[order (biology)|order]] [[Saccharomycetales]].<ref name="YeastRef2"> [http://www.yeastgenome.org/VL-what_are_yeast.html "What are yeasts?"]. ''Saccharomyces Genome Database''. Retrieved [[December 24]] [[2006]].</ref>
==History==
{{see also|History of wine|History of beer}}
The word "''yeast ''" comes from the [[Old English language]] "''gist''", "''gyst''", and ultimately 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. People have used yeast for fermentation and baking throughout history. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted 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> In 1680 the [[Dutch (ethnic group)|Dutch]] naturalist [[Antoine van Leeuwenhoek]] first [[microscopy|microscopically]] observed yeast, but at the time did not consider them to be living organisms but rather globular structures.<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.<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, but the fermentation inhibited - an observation later called the ''[[Pasteur effect]]''.
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]] exibited the product and a process to use it, as well as serving the resultant baked bread.
== Growth and nutrition ==
Yeasts are [[chemoorganotroph]]s as they use [[organic compound]]s as a source of energy and do not require sunlight to grow. The main source of carbon is obtained by [[hexose]] sugars such as [[glucose]] and [[fructose]], or disaccharides such as [[sucrose]] and [[maltose]]. Some species can metabolize [[pentose]] sugars, [[alcohol]]s, and [[organic acid]]s. Yeast species either require oxygen for aerobic [[cellular respiration]] ([[obligate aerobe]]s), or are anaerobic but also have aerobic methods of energy production ([[facultative anaerobe]]s). Unlike [[bacteria]], there are no known yeast species that grow only anaerobically ([[obligate anaerobe]]s). Yeasts grow best in a neutral or slightly acidic pH environment.
Yeasts will grow over a temperature range of 10°-37°C (50°-98.6°F), with an optimal temperature range of 30°-37°C (86°-98.6°F), depending on the type of species. ''S. cerevisiae'' works best at about 30°C. There is little activity in the range of 0°-10°C. Above 37°C yeast cells become stressed and will not divide properly. Most yeast cells die above 50°C (122°F). The cells can survive freezing under certain conditions, with viability decreasing over time.
Yeasts are ubiquitous in the environment, but are most frequently isolated from sugar-rich samples. Some good examples include fruits and berries (such as [[grape]]s, [[apple]]s or [[peach]]es), and exudates from plants (such as plant saps or cacti). Some yeasts are found in association with soil and 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 | pmid = 15912941 | doi = 10.1017/S0953756205002388 <!--Retrieved from CrossRef by DOI bot-->}}</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 | pmid = 12964187 | doi = 10.1002/jobm.200310277 <!--Retrieved from CrossRef by DOI bot-->}}</ref> Yeast are generally grown in the laboratory on solid [[growth medium|growth media]] or liquid [[broth]]s. Common media used for the cultivation of yeasts include; potato dextrose agar (PDA) or [[potato dextrose broth]], Wallerstien Laboratories Nutrient agar (WLN), Yeast [[Peptone]] [[Dextrose]] agar (YPD), and Yeast Mould agar or broth (YM). The [[antibiotic]] [[cycloheximide]] is sometimes added to yeast growth media to inhibit the growth of ''Saccharomyces'' yeasts and select for wild/indigenous yeast species.
==Reproduction==
[[Image:Yeast lifecycle.svg|thumb|right|250px|The yeast cell's life cycle.<br>
1. Budding<br>
2. Conjugation<br>
3. Spore]]
{{see also|Mating of yeast}}
Yeasts have [[asexual reproduction|asexual]] and [[sexual reproduction|sexual]] reproductive cycles; however 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 | pmid = 15380095 | doi = 10.1016/j.cub.2004.09.022 <!--Retrieved from CrossRef by DOI bot-->}}</ref> Here 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 | pmid = 15720543 | doi = 10.1111/j.1365-2958.2005.04507.x <!--Retrieved from CrossRef by DOI bot-->}}</ref> The bud can develop on different parts of the parent cell depending on the [[genus]] of the yeast.Yeast needs the exact chemical form of sugar and cannot reproduce with sugar substitutes. However if the sugar substitute's chemical form is similar to sugar, yeast will reproduce a bit compared with many sugar substitutes where yeast will not reproduce at all.
Under high stress conditions [[haploid]] cells will generally die, however under the same conditions [[diploid]] cells can undergo sporulation, entering sexual reproduction ([[meiosis]]) and producing a variety of haploid [[spores]], which can go on to [[Mating of yeast|mate]] (conjugate), reforming the [[diploid]].<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 | pmid = 16339736 | doi = 10.1128/MMBR.69.4.565-584.2005 <!--Retrieved from CrossRef by DOI bot-->}}</ref>
Yeast of the species [[Schizosaccharomyces pombe]] reproduce by [[binary fission]] instead of budding.<ref name=Balasubramanian/>
==Uses==
The useful physiological properties of yeast have led to their use in the field of [[biotechnology]]. [[fermentation (biochemistry)|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, yeast in [[wine]] fermentation and for [[xylitol]]<ref> R. Sreenivas Rao, R.S. Prakasham, K. [[Krishna Prasad]], S. Rajesham,P.N. Sarma, L. Venkateswar Rao (2004) Xylitol production by Candida sp.: parameter optimization using Taguchi approach, Process Biochemistry 39:951-956 </ref> production. Yeasts are also one of the most widely used [[model organisms]] for [[genetics]] and [[cell biology]].
===Alcoholic beverages===
[[Alcoholic beverage]]s are loosely defined as a [[drink|beverage]] that contains [[ethanol]] (C<sub>2</sub>H<sub>5</sub>OH). This ethanol is almost always produced by [[fermentation (food)|fermentation]] - the [[metabolism]] of [[carbohydrate]]s by certain species of yeast. Beverages such as [[wine]], [[beer]], or [[distilled beverage|distilled spirits]] all use yeast at some stage of their production.
====Beer====<!-- This section is linked from [[Swedish beer]] -->
[[Image:DE and yeast.JPG|right|thumb|250px|A mixture of [[diatomaceous earth]] and yeast after filtering [[beer]].]]
Beer [[brewing|brewers]] classify yeasts as top-fermenting and bottom-fermenting. This distinction was introduced by the [[Danish people|Dane]] [[Emil Christian Hansen]]. ''Top-fermenting yeasts'' are so called because they form a foam at the top of the [[wort]] during fermentation. They can produce higher [[ethanol|alcohol]] concentrations and prefer higher temperatures, producing fruitier [[ale]]-type beers. An example of a top-fermenting yeast is ''[[Saccharomyces cerevisiae]]'', known to brewers as [[ale]] yeast. ''Bottom-fermenting yeasts'' are used to produce [[lager]]-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low temperatures. An example of a bottom-fermenting yeast is ''[[Saccharomyces pastorianus]]''.
For both types, yeast is fully distributed through the beer while it is fermenting, and both equally [[flocculation|flocculate]] (clump together and precipitate to the bottom of the vessel) when it is finished. By no means do all top-fermenting yeasts demonstrate this behaviour, but it features strongly in many English ale yeasts which may also exhibit chain forming (the failure of budded cells to break from the mother cell) which is technically different from true flocculation.
[[Lambic]], a style of [[Belgium|Belgian]] beer, is fermented spontaneously by wild yeasts primarily of the genus ''[[Brettanomyces]]''.
[[Image:Samadams2.jpg|thumb|250px|right|Fermenting tanks with yeast being used to [[Brewing|brew]] [[beer]].]]
In industrial brewing, to ensure purity of strain, a 'clean' sample of the yeast is stored refrigerated in a laboratory. After a certain number of [[Fermentation (food)|fermentation]] cycles, a full scale [[biological reproduction|propagation]] is produced from this laboratory sample. Typically, it is grown up in about three or four stages using sterile brewing wort and [[oxygen]].
====Root Beer and Sodas====
Root beer and other sweet carbonated beverages can be produced using the same methods as beer, except that fermentation is stopped sooner, producing carbon dioxide, but only trace amounts of alcohol, and a significant amount of sugar is left in the drink.
====Distilled beverages====
A [[distilled beverage]] is a beverage that 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 [[alcohol]]s which account for the [[flavor]] of the beverage.
====Wine====
[[Image:Yeast on grapes.jpg|thumb|right|250px|Fresh grapes]]
{{main|Fermentation (wine)}}
Yeast is used in [[winemaking]] where it converts the sugars present in [[grape juice]] or [[must]] into [[ethanol|alcohol]]. Yeast is normally already invisibly present on the grapes. The fermentation can be done with this endogenous (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 give unpredictable results depending on the exact types of 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. This represses the wild yeasts and ensures a reliable and predictable 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> Most added wine yeasts are strains of ''[[Saccharomyces cerevisiae]]'', however not all strains of the species are suitable.<ref name=Gonzalez/> 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.<ref name=Dunn>Dunn, B., Levine, R.P., Sherlock, G., [http://www.biomedcentral.com/1471-2164/6/53 Microarray karyotyping of commercial wine yeast strains reveals shared, as well as unique, genomic signatures], BMC Genomics. 2005 Apr 16;6(1):53.</ref> Significant research has been undertaken into the development of ''novel'' wine yeast strains that produce atypical flavour profiles or increased complexity in wines.<ref name=YeastRef14>[http://www.gwrdc.com.au/downloads/0506docs/researcher.pdf Research enables yeast suppliers to expand options]. Retrieved [[10 January]] [[2007]].</ref><ref name=McBryde>McBryde, Colin, Gardner, Jennifer M., de Barros Lopes, Miguel, Jiranek, Vladimir, [Generation of Novel Wine Yeast Strains by Adaptive Evolution], Am. J. Enol. Vitic. 2006 57: 423-430</ref>
The growth of some yeasts such as ''[[Zygosaccharomyces]]'' and ''[[Brettanomyces]]'' in wine can result in [[wine fault]]s and subsequent spoilage.<ref>{{cite journal | author = Loureiro V, Malfeito-Ferreira M | title = Spoilage yeasts in the wine industry | journal = Int J Food Microbiol | volume = 86 | issue = 1-2 | pages = 23–50 | year = 2003 | pmid = 12892920 | doi = 10.1016/S0168-1605(03)00246-0 <!--Retrieved from CrossRef by DOI bot-->}}</ref> ''[[Brettanomyces]]'' produces an array of [[metabolite]]s when growing in wine, some of which are volatile [[phenol]]ic 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 fault]]s within the wine industry.<ref name=YeastRef15>[http://www.winepros.org/wine101/vincyc-bret.htm BRETTANOMYCES]. Retrieved [[10 January]] [[2007]].</ref>
===Baking===
{{main|Baker's yeast}}
Yeast, most commonly ''[[Saccharomyces cerevisiae]]'', is used in [[baking]] as a [[leavening agent]], where it converts the [[fermentation (food)|fermentable]] [[sugar]]s present in the [[dough]] into [[carbon dioxide]]. This causes the dough to expand or 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 the yeast used in baking is of the same species common in alcoholic [[Fermentation (food)|fermentation]]. Additionally, ''[[Saccharomyces exiguus]]'' (also known as ''S. minor'') is a wild yeast found on plants, fruits, and grains that is occasionally used for baking. Sugar and vinegar are the best conditions for yeast to ferment. In bread making the yeast respires aerobically at first producing carbon dioxide and water. When the oxygen is used up anaerobic respiration is used producing ethanol as a waste product however this is evaporated off during the baking process.
[[Image:Compressed fresh yeast - 1.jpg|right|thumb|250px|A block of 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.<ref name="Egypt">[http://www.bbc.co.uk/dna/h2g2/A2791820 "The History of Bread Yeast"]. [[British Broadcasting Company]]. Retrieved [[December 24]] [[2006]].</ref> Researchers speculate that 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 more tasty than the normal flat, hard cake.
[[Image:Dry yeast.jpg|thumb|right|250px|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 is [[Fleischmann’s Yeast]], which was developed in 1868. During [[World War II]] Fleischmann's developed a [[Wiktionary:granulate|granulated]] active dry yeast, which did not require refrigeration and had a longer shelf life than fresh yeast. The company created yeast that would rise twice as fast, cutting down on baking time. Baker's yeast is also sold as a fresh yeast compressed into a square "cake". This form perishes quickly, and must be used soon after production in order to maintain [[viability]]. A weak solution of [[Water (molecule)|water]] and sugar can be used to determine if yeast is expired. When dissolved 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 gives proof of the viability of the yeast before the other ingredients are added. When using a sourdough starter, flour and water are added instead of sugar and 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. Some bread doughs are knocked back after one rising and left to rise again. 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. The dough is then shaped into loaves, left to rise until it is the correct size, and then baked. Dried yeast is usually specified for use in a [[bread machine]], however a (wet) sourdough starter can also work.
===Bioremediation===
Some yeasts can find potential application in the field of [[bioremediation]]. One such yeast ''[[Yarrowia lipolytica]]'' 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= 35|pmid=12146640 |url= |accessdate=2007-01-21 |doi=10.1016/S0960-8524(02)00063-9 }}</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 |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 |doi=10.1016/j.marpolbul.2004.06.007 }}</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 |pmid=15780653 |url= |accessdate=2007-01-21 |doi=10.1016/j.femsyr.2004.09.004 }}</ref>
===Industrial ethanol production===
The ability of yeast to convert sugar into [[ethanol]] has been harnessed by the [[biotechnology]] industry, which has various uses including [[ethanol fuel]]. The process starts by milling a feedstock, such as [[sugar cane]], [[sweetcorn]], or cheap [[cereal grain]]s, and 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 break the complex sugars down into simple sugars. After this, yeasts are added to convert the simple sugars to ethanol, which is then [[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>
===Kombucha===
[[Image:Kombucha jar.jpg|thumb|right|185px|A [[Kombucha]] culture fermenting in a jar]]
Yeast in [[symbiosis]] with [[acetic acid bacteria]] is used in the preparation of [[Kombucha]], a fermented sweetened [[tea]]. Species of yeast found in the tea can vary, and may include: ''[[Brettanomyces bruxellensis]]'', ''[[Candida stellata]]'', ''[[Schizosaccharomyces pombe]]'', ''[[Torulaspora delbrueckii]]'' and ''[[Zygosaccharomyces bailii]]''.<ref name="YeastRef7">{{cite journal | first = Ai Leng | last = Teoh | coauthors = Gillian Heard and Julian Cox | date = [[September 1]], [[2004]] | title = Yeast ecology of Kombucha fermentation | journal = International Journal of Food Microbiology | volume = 95 | issue = 2 | pages = 119–126 | doi = 10.1016/j.ijfoodmicro.2003.12.020 | url = http://www.sciencedirect.com/science/article/B6T7K-4C76F1S-4/2/c9a198f08bec6a0a63fc884b5ff2693e | accessdate = 2006-12-24}}</ref>
===Nutritional supplements===
Yeast is used in nutritional supplements popular with [[vegan]]s and the health conscious, where it is often referred to as "nutritional yeast". It is a deactivated yeast, usually ''[[Saccharomyces cerevisiae]]''. It 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]]. Some brands of [[nutritional yeast]], though not all, are fortified with [[Cyanocobalamin|vitamin B12]], which is produced separately from [[bacteria]]. Nutritional yeast, though it has a similar appearance to brewer's yeast, is very different and has a very different taste.
Nutritional yeast has a nutty, cheesy, creamy flavor which makes it popular as an ingredient in [[cheese]] substitutes. It is often used by vegans in place of [[Parmigiano Reggiano|parmesan cheese]]. Another popular use is as a topping for [[popcorn]]. Some movie theaters are beginning to offer it along with salt or cayenne pepper as a popcorn condiment. 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.<ref name="YeastRef8">[http://www.cofepow.org.uk/pages/asia_haruku2.htm "Harukoe (Haruku)"]. ''Children of Far East Prisoners of War''. Retrieved [[December 24]] [[2006]].</ref>
===Probiotics===
Some [[probiotic]] supplements use the yeast ''[[Saccharomyces boulardii]]'' to maintain and restore the natural flora in the large and small gastrointestinal tract. ''S. boulardii'' has been shown to reduce the symptoms of acute [[diarrhea]] in children,<ref>{{ cite journal | title=Therapeutic evaluation of ''Saccharomyces boulardii'' in children with acute diarrhea | author=Centina-Sauri G, Sierra Basto G | journal=Ann Pediatr | year=1994 | volume=41 | pages=397–400}}</ref><ref>{{ cite journal | journal=Acta Paediatrica | year=2005 Jan | volume=94 | pages=44–47 | author=Kurugol Z, Koturoglu G | title=Effects of ''Saccharomyces boulardii'' in children with acute diarrhea | doi=10.1080/08035250410022521}}</ref> prevent reinfection of ''[[Clostridium difficile]]'',<ref>{{ cite journal | journal=J Am Med Assoc | title=A randomised placebo-controlled trial of ''Saccharomyces boulardii'' in combination with standard antibiotics for ''Clostridium difficile'' disease | author=McFarland L, Surawicz C, Greenberg R | volume=271 | year=1994 | pages=1913–8 | doi = 10.1001/jama.271.24.1913 <!--Retrieved from CrossRef by DOI bot-->}}</ref> reduce bowel movements in diarrhea predominant [[irritable bowel syndrome|IBS]] patients,<ref>{{ cite journal | title=Treatment of irritable bowel syndrome with ''Saccharomyces boulardii'': a double blind, placebo controlled study | year=1983 | journal=Medicine Chirurgie Digestives | author=Maupas J, Champemont P, Delforge M | volume=12(1) | pages=77–9}}</ref> and reduce the incidence of [[antibiotic-associated diarrhea|antibiotic]],<ref>{{ cite journal | title=Prevention of β-lactam associated diarrhea by ''Saccharomyces boulardii'' compared with placebo | journal=Am J Gastroenterol | year=1995 | author=McFarland L, Surawicz C, Greenberg R | volume=90 | pages=439–48}}</ref> [[travelers' diarrhea|traveler's]],<ref>{{ cite journal | journal=Travel Med Int | title=Prevention of traveller's diarrhea. Comparison of different non-antibiotic preparations | author=Kollaritsch H, Kemsner P, Wiedermann G, Scheiner O | year=1989 | pages=9–17}}</ref> and [[HIV/AIDS]]<ref>{{ cite journal | title=AIDS related diarrhea: a double-blind trial of ''Saccharomyces boulardii'' | journal=Sem Hôsp Paris | year=1995 | volume=71 | pages=735–41 | author=Saint-Marc T, Blehaut H, Musial C, Touraine J}}</ref> associated diarrheas.
===Science===
[[Image:yeast cell english.svg|thumb|right|Diagram showing a yeast cell]]
Several yeasts, particularly ''[[Saccharomyces cerevisiae]]'', have been widely used in [[genetics]] and [[cell biology]]. This is largely because the [[cell cycle]] in a yeast cell is very similar to the cell cycle in [[human]]s, and therefore the basic cellular mechanics of [[DNA replication]], [[recombination]], [[cell division]] and [[metabolism]] are comparable.<ref name=YeastRef2/> Also many proteins important in human biology were first discovered by studying their [[homology (biology)|homolog]]s in yeast; these proteins include [[cell cycle protein]]s, [[Cell signaling|signaling proteins]], and protein-processing enzymes.
On [[24 April]] [[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 |pmid=8614793 }}</ref> At the time it was the most complex organism to have its full genome sequenced and took 7 years and the involvement of more than 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 sequenced was ''[[Schizosaccharomyces pombe]]'', which 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> It was the 6th eukaryotic genome sequenced and consists of 13.8 million base pairs.
===Yeast extract===
{{main|Yeast extract}}
{|align="right"
|[[Image:Marmite.jpg|thumb|right|124px| Marmite and Vegemite have a distinctive dark colour]]
|}
{|align="right"
|[[Image:Vegemite and Marmite.jpg|thumb|right|150px|[[Vegemite]] and [[Marmite]], products made from [[yeast extract]]]]
|}
Yeast extract is the common name for various forms of processed yeast products that are used as [[food additive]]s or [[flavour]]s. They are often used in the same way that [[monosodium glutamate]] (MSG) is used, and like MSG, often contain free [[glutamic acid]]s. 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 (biology)|autolysis]]'', where the yeast's [[digestion|digestive]] enzymes break their own [[protein]]s 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 which would give poor texture) are separated. Yeast autolysates are used in [[Vegemite]] and [[Promite]] ([[Australia]]); [[Marmite]], [[Bovril]] and [[Oxo (food)|Oxo]] (the [[United Kingdom]], [[Republic of Ireland]] and [[South Africa]]); and [[Cenovis]] ([[Switzerland]]).
==Pathogenic yeasts==
[[Image:C albicans en.jpg|thumb|right|250px|A [[photomicrograph]] of ''[[Candida albicans]]'' showing hyphal outgrowth and other morphological characteristics.]]
Some species of yeast are [[opportunistic pathogen]]s where they can cause infection in people with compromised [[immune systems]].
''[[Cryptococcus neoformans]]'' is a significant pathogen of immunocompromised people causing the disease termed [[Cryptococcosis]]. This disease occurs in about 7–9% of [[AIDS]] patients in the USA, and 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 recognised and engulfed by [[white blood cells]] in the human body.
Yeasts of the ''[[Candida (genus)|Candida]]'' genus are another group of opportunistic pathogens which causes oral and [[vagina]]l [[infection]]s in humans, known as [[Candidiasis]]. ''Candida'' is commonly found as a [[commensal]] yeast in the [[mucus membranes]] of humans and other warm-blooded animals. However, sometimes these same strains can become pathogenic. Here the yeast cells sprout a [[hypha]]l outgrowth, which locally penetrates the [[mucous membrane|mucosal membrane]], causing irritation and shedding of the tissues.<ref name=YeastRef9/> The pathogenic yeasts of candidiasis in probable descending order of virulence for humans are: ''[[Candida albicans|C. albicans]]'', ''[[Candida tropicalis|C. tropicalis]]'', ''[[Candida stellatoidea|C. stellatoidea]]'', ''[[Candida glabrata|C. glabrata]]'', ''[[Candida krusei|C. krusei]]'', ''[[Candida parapsilosis|C. parapsilosis]]'', ''[[Candida guilliermondii|C. guilliermondii]]'', ''[[Candida viswanathii|C. viswanathii]]'', ''[[Candida lusitaniae|C. lusitaniae]]'' and ''[[Rhodotorula|Rhodotorula mucilaginosa]]''.<ref>Hurley, R., J. de Louvois, and A. Mulhall. 1987. Yeast as human and animal pathogens, p. 207-281. In A. H. Rose and J. S. Harrison (ed.), The yeasts, vol. 1. Academic Press, Inc., New York, N.Y.</ref> ''[[Candida glabrata]]'' is the second most common ''Candida'' pathogen after ''C. albicans'', causing infections of the [[urogenital tract]], and of the [[bloodstream]] ([[Candidemia]]).<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 | pmid = 15470243 | url = http://ec.asm.org/cgi/content/full/3/5/1154 | accessdate = 2006-12-24 | year = 2004}}</ref>
==Food spoilage==
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.<ref name=Kurtzman3>Kurtzman, C.P. 2006. [http://www.ars.usda.gov/research/publications/publications.htm?SEQ_NO_115=179383 Detection, identification and enumeration methods for spoilage yeasts]. In: Blackburn, C. de. W, editor. Food spoilage microorganisms. Cambridge, England: Woodhead Publishing. p. 28-54.</ref> During their growth, yeasts metabolize some food components and produce metabolic end products. This causes the physical, chemical, and sensory properties of a food to change, and the food is spoiled.<ref name=Fleet>Fleet, G.H., and Praphailong, W., '''Yeasts''', In: Spoilage of Processed Foods: Causes and Diagnosis, AIFST (2001), Southwood Press. p 383-397</ref> The growth of yeast within food products is often seen on their surface, as in [[cheese]]s or [[meat]]s, or by the fermentation of sugars in beverages, such as [[juice]]s, and semi-liquid products, such as [[syrup]]s and [[jam]]s.<ref name=Kurtzman3/> The yeast of the ''[[Zygosaccharomyces]]'' genus have had a long history as a spoilage yeast within the food industry. This is mainly due to the fact that these species can grow in the presence of high [[sucrose]], [[ethanol]], [[acetic acid]], [[sorbic acid]], [[benzoic acid]], and [[sulfur dioxide]] concentrations,<ref name=Fugelsang>Fugelsang, K.C., [http://cati.csufresno.edu/verc/rese/96/960804/ Zygosaccharomyces, A Spoilage Yeast Isolated from Wine], California Agriculture Technology Institute. Retrieved [[10 January]] [[2007]].</ref> representing some of the commonly used [[food preservation]] methods. [[Methylene Blue]] is used to test for the presence of live yeast cells.
==See also==
{{wiktionary}}
{{Commonscat}}
*[[Bioaerosol]]
*[[Brewing]]
*[[Brewer's yeast]]
*[[Baker's yeast]]
*[[Ethanol fermentation]]
*[[Fermentation (food)]]
*[[Fungi]]
*''[[Saccharomyces cerevisiae]]''
*[[Winemaking]]
==References==
<div class="references-small" style="-moz-column-count:2; column-count:2;">
<references /></div>
==External links==
*[http://www.exploreyeast.com/ Yeast Development, Different Forms of Yeast...]
*[http://www.sceptrans.org/ Cell cycle and metabolic cycle regulated transcription in yeast]
*[http://www.yeastrc.org/ Yeast Resource Center]
*[http://biochemie.web.med.uni-muenchen.de/Yeast_Biol/10%20Yeast%20Growth%20and%20the%20Cell%20Cycle.pdf Yeast growth and the cell cycle]
*[http://www.yeastgenome.org/VL-yeast.html Yeast virtual library]
*[http://www.mnsu.edu/emuseum/prehistory/egypt/dailylife/breadmaking.htm Ancient Egyptian Bread Making]
[[Category:Brewing]]
[[Category:Food additives]]
[[Category:Leavening agents]]
[[Category:Medicinal fungi]]
[[Category:Vegan cuisine]]
[[Category:Yeasts|*]]
[[Category:Yeast extract spreads|*]]
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