View source for Yeast

{{short description|Informal group of fungi}}
{{Other uses}}
{{pp-semi-indef}}
{{Use dmy dates|date=May 2018}}
{{Paraphyletic group
| auto = yes
| image = S cerevisiae under DIC microscopy.jpg
| image_caption = ''Saccharomyces cerevisiae'', a species of yeast
| image2 = Simple diagram of yeast cell (en).svg
| image2_alt = Cross-sectional 2D diagram of a yeast cell
| image2_caption = Cross-sectional labelled diagram of a typical yeast cell
| display_parents = 0
| parent = Fungi
| includes_text = Phyla and subphyla with yeast species
| includes = <div>
'''[[Ascomycota]]''' [[pro parte|p. p.]]
* [[Saccharomycotina]] <small>(true yeasts)</small>
* [[Taphrinomycotina]] p. p.
** [[Schizosaccharomycetes]] <small>(fission yeasts)</small>
'''[[Basidiomycota]]''' p. p.
* [[Agaricomycotina]] p. p.
** [[Tremellomycetes]]
* [[Pucciniomycotina]] p. p.
** [[Microbotryomycetes]]
</div>
}}
'''Yeasts''' are [[eukaryotic]], single-celled [[microorganism]]s classified as members of the [[fungus]] [[kingdom (biology)|kingdom]]. The first yeast originated hundreds of millions of years ago, and at least 1,500 [[species]] are currently recognized.<ref>{{Cite book|last1=Piškur|first1=Jure|url=https://books.google.com/books?id=7NclBAAAQBAJ&pg=PA98|title=Molecular Mechanisms in Yeast Carbon Metabolism|last2=Compagno|first2=Concetta|date=2014|publisher=Springer|isbn=978-3-642-55013-3|page=98| quote=The second completely sequenced yeast genome came 6 years later from the fission yeast ''Schizosaccharomyces pombe'', which diverged from ''S. cerevisiae'' probably more than 300 million years ago.
}}</ref><ref name="Kurtzman-2006b"/><ref name="Hoffman-2015">{{cite journal |vauthors=Hoffman CS, Wood V, Fantes PA |title=An Ancient Yeast for Young Geneticists: A Primer on the ''Schizosaccharomyces pombe'' Model System |journal=[[Genetics (journal)|Genetics]] |volume=201 |issue=2 |pages=403–23 |date=October 2015 |pmid=26447128 |doi=10.1534/genetics.115.181503 |pmc=4596657}}</ref> They are estimated to constitute 1% of all described fungal species.<ref name="Kurtzman-2006a"/>
Some yeast species have the ability to develop multicellular characteristics by forming strings of connected budding cells known as [[pseudohyphae]] or false [[hyphae]], or quickly evolve into a [[Multicellular organism|multicellular]] cluster with specialised [[Organelle|cell organelles]] function.<ref name="Kurtzman-2005"/><ref name="Yong-2012"/> Yeast sizes vary greatly, depending on species and environment, typically measuring 3–4&nbsp;[[micrometre|μm]] in [[diameter]], although some yeasts can grow to 40&nbsp;μm in size.<ref name="Walker-2002"/> Most yeasts reproduce [[asexual reproduction|asexually]] by [[mitosis]], and many do so by the asymmetric division process known as [[budding]]. With their single-celled growth habit, yeasts can be contrasted with [[Mold (fungus)|mold]]s, which grow [[hypha]]e. Fungal species that can take both forms (depending on temperature or other conditions) are called [[dimorphic fungi]].
The yeast species ''[[Saccharomyces cerevisiae]]'' converts [[carbohydrate]]s to [[carbon dioxide]] and [[Alcohol (chemistry)|alcohol]]s through the process of [[fermentation (food)|fermentation]]. The products of this reaction have been used in [[baking]] and the production of alcoholic beverages for thousands of years.<ref name="Legras-2007"/> ''S. cerevisiae'' is also an important [[model organism]] in modern [[cell biology]] research, and is one of the most thoroughly studied eukaryotic microorganisms. Researchers have cultured it in order to understand the biology of the eukaryotic cell and ultimately human biology in great detail.<ref name="Ostergaard-2000"/> Other species of yeasts, such as ''[[Candida albicans]]'', are [[opportunistic pathogen]]s and can cause [[yeast infection|infections]] in humans. Yeasts have recently been used to generate electricity in [[microbial fuel cell]]s<ref name="HelsinkiUniTech-2007"/> and to produce [[ethanol]] for the [[biofuel]] industry.
Yeasts do not form a single [[Taxonomy (biology)|taxonomic]] or [[phylogenetics|phylogenetic]] grouping. The term "yeast" is often taken as a [[synonym]] for ''Saccharomyces cerevisiae'',<ref name="Kurtzman-1994"/> but the phylogenetic diversity of yeasts is shown by their placement in two separate [[phylum|phyla]]: the [[Ascomycota]] and the [[Basidiomycota]]. The budding yeasts or "true yeasts" are classified in the [[order (biology)|order]] [[Saccharomycetales]],<ref name="YeastVirtualLibrary-2009"/> within the phylum Ascomycota.
== History ==
{{See also|History of bread|History of wine|History of beer}}
The word "yeast" comes from [[Old English language|Old English]] ''gist'', ''gyst'', and from the [[Proto Indo-European language|Indo-European]] root ''yes-'', meaning "boil", "foam", or "bubble".<ref name="Dictionary-2000"/> Yeast microbes are probably one of the earliest [[Domestication|domesticated]] organisms. Archaeologists digging in [[Ancient Egypt|Egyptian]] ruins found early [[millstone|grinding stones]] and baking chambers for yeast-raised bread, as well as drawings of 4,000-year-old bakeries and [[brewery|breweries]].<ref name="Phillips"/> Vessels studied from several archaeological sites in [[Israel]] (dating to around 5,000, 3,000 and 2,500 years ago), which were believed to have contained alcoholic beverages ([[beer]] and [[mead]]), were found to contain yeast colonies that had survived over the millennia, providing the first direct biological evidence of yeast use in early cultures.<ref>{{cite journal |url=https://mbio.asm.org/content/10/2/e00388-19/article-info |title=Isolation and Characterization of Live Yeast Cells from Ancient Vessels as a Tool in Bio-Archaeology |first1=Tzemach |last1=Aouizerat |first2=Itai |last2=Gutman |first3=Yitzhak |last3=Paz |first4=Aren M. |last4=Maeir |first5=Yuval |last5=Gadot |first6=Daniel |last6=Gelman |first7=Amir |last7=Szitenberg |first8=Elyashiv |last8=Drori |first9=Ania |last9=Pinkus |first10=Miriam |last10=Schoemann |first11=Rachel |last11=Kaplan |first12=Tziona |last12=Ben-Gedalya |first13=Shunit |last13=Coppenhagen-Glazer |first14=Eli |last14=Reich |first15=Amijai |last15=Saragovi |first16=Oded |last16=Lipschits |first17=Michael |last17=Klutstein |first18=Ronen |last18=Hazan |journal=mBio |volume=10 |number=2 |date=2019 |doi=10.1128/mBio.00388-19|pmid=31040238 |pmc=6495373 }}</ref> In 1680, [[Dutch (ethnic group)|Dutch]] naturalist [[Anton van Leeuwenhoek]] first [[microscopy|microscopically]] observed yeast, but at the time did not consider them to be [[organism|living organisms]], but rather globular structures<ref name="Huxley-1871"/> as researchers were doubtful whether yeasts were [[algae]] or fungi.<ref name="Ainsworth-1976"/>  [[Theodor Schwann]] recognized them as fungi in 1837.<ref name="Schwann T-1837"/><ref name="Barnett-2004"/>
In 1857, French microbiologist [[Louis Pasteur]] showed that by bubbling oxygen into the yeast broth, [[cell growth]] could be increased, but fermentation was inhibited&nbsp;– an observation later called the "[[Pasteur effect]]". In the paper "''Mémoire sur la fermentation alcoolique,''" Pasteur proved that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst.<ref name="Phillips" /><ref name="Barnett-2003" />
By the late 18th century two yeast strains used in brewing had been identified: ''[[Saccharomyces cerevisiae]]'' (top-fermenting yeast) and ''[[Saccharomyces carlsbergensis|S. pastorianus]]'' (bottom-fermenting yeast). ''S.&nbsp;cerevisiae'' has been sold commercially by the Dutch for bread-making since 1780; while, around 1800, the Germans started producing ''S.&nbsp;cerevisiae'' in the form of cream. In 1825, a method was developed to remove the liquid so the yeast could be prepared as solid blocks.<ref name="Klieger-2004"/> The industrial production of yeast blocks was enhanced by the introduction of the [[filter press]] in 1867. In 1872, Baron [[Max de Springer]] developed a manufacturing process to create granulated yeast from [[beetroot]] [[molasses]],<ref name="Letourneau-1987">{{cite journal |doi=10.1007/BF01043394 |title=Saccharomyces yeast growth on beet molasses effects of substrate concentration on alcohol toxicity |date=1987 |last1=Letourneau |first1=F. |last2=Villa |first2=P. |journal=Biotechnology Letters |volume=9 |pages=53–58 }}</ref><ref name="Miniac-1988">Miniac (de) M., 1988. Conduite des ateliers de fermentation alcoolique de produits sucriers (mélasses et égouts). Industries alimentaires et Agricoles 105, 675–688.</ref><ref name="Visitez la fabrique de levure-2009">{{cite news |url=https://www.leparisien.fr/val-de-marne-94/maisons-alfort-94700/visitez-la-fabrique-de-levure-21-10-2009-682273.php |title=Visitez la fabrique de levure |date=21 October 2009 }}</ref> a technique that was used until the first World War.<ref name="urlLe Comité des Fabricants de levure&nbsp;— COFALEC&nbsp;— représente les fabricants"/> In the United States, naturally occurring airborne yeasts were used almost exclusively until commercial yeast was marketed at the [[Centennial Exposition]] in 1876 in Philadelphia, where [[Charles L. Fleischmann]] exhibited the product and a process to use it, as well as serving the resultant baked bread.<ref name="Snodgrass-2004"/>
The [[refrigeration|mechanical refrigerator]] (first patented in the 1850s in Europe) liberated [[Brewing|brewers]] and [[winemakers]] from seasonal constraints for the first time and allowed them to exit cellars and other earthen environments. For [[John Molson]], who made his livelihood in [[Montreal]] prior to the development of the fridge, the brewing season lasted from September through to May. The same seasonal restrictions formerly governed the [[distiller]]'s art.<ref name="Denison-1955"/>
== Nutrition and growth ==
Yeasts are [[chemoorganotroph]]s, as they use [[organic compound]]s as a source of energy and do not require sunlight to grow. Carbon is obtained mostly from [[hexose]] sugars, such as [[glucose]] and [[fructose]], or disaccharides such as [[sucrose]] and [[maltose]]. Some species can metabolize [[pentose]] sugars such as ribose,<ref name="Barnett-1975"/> alcohols, 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]], no known yeast species grow only anaerobically ([[obligate anaerobe]]s). Most yeasts grow best in a neutral or slightly acidic pH environment.
Yeasts vary in regard to the temperature range in which they grow best. For example, ''[[Leucosporidium frigidum]]'' grows at {{convert|-2|to|20|C|F}}, ''[[Saccharomyces telluris]]'' at {{convert|5|to|35|C|F}}, and ''[[Candida slooffi]]'' at {{convert|28|to|45|C|F}}.<ref name="Arthur-1976"/> The cells can survive freezing under certain conditions, with viability decreasing over time.
In general, yeasts are grown in the laboratory on solid [[growth medium|growth media]] or in liquid [[broth]]s. Common media used for the cultivation of yeasts include [[potato dextrose agar]] or [[potato dextrose broth]], Wallerstein Laboratories nutrient [[agar]], yeast [[peptone]] [[dextrose]] agar, and yeast mould agar or broth. Home brewers who cultivate yeast frequently use dried [[malt extract]] and agar as a solid growth medium. The [[fungicide]] [[cycloheximide]] is sometimes added to yeast growth media to inhibit the growth of ''[[Saccharomyces]]'' yeasts and select for wild/indigenous yeast species. This will change the yeast process.
The appearance of a white, thready yeast, commonly known as [[kahm]] yeast, is often a byproduct of the lactofermentation (or pickling) of certain vegetables. It is usually the result of exposure to air. Although harmless, it can give pickled vegetables a bad flavor and must be removed regularly during fermentation.<ref name="Kaufmann-2002"/>
== Ecology ==
Yeasts are very common in the environment, and are often isolated from sugar-rich materials. Examples include naturally occurring yeasts on the skins of fruits and berries (such as grapes, apples, or [[peach]]es), and [[Exudate|exudates]] from plants (such as plant saps or cacti). Some yeasts are found in association with soil and insects.<ref name="Suh-2005"/><ref name="Sláviková-2003"/> Yeasts from the soil and from the skins of fruits and berries have been shown to dominate fungal [[Ecological succession|succession]] during fruit decay.<ref>{{Cite journal|last1=Martin|first1=Phillip L.|last2=King|first2=William|last3=Bell|first3=Terrence H|last4=Peter|first4=Kari|date=2021|title=The decay and fungal succession of apples with bitter rot across a vegetation diversity gradient|journal=Phytobiomes Journal|volume=6|pages=26–34|doi=10.1094/pbiomes-06-21-0039-r|s2cid=239658496|issn=2471-2906|doi-access=free}}</ref> The ecological function and [[biodiversity]] of yeasts are relatively unknown compared to those of other [[microorganism]]s.<ref name="Herrera-2010"/> Yeasts, including ''[[Candida albicans]]'', ''[[Rhodotorula]] rubra'', ''[[Torulopsis]]'' and ''[[Trichosporon cutaneum]]'', have been found living in between people's toes as part of their [[skin flora]].<ref name="Oyeka-2002"/> Yeasts are also present in the [[gut flora]] of mammals and some insects<ref name="Martini-1992"/> and even deep-sea environments host an array of yeasts.<ref name="Bass-2007"/><ref>{{Cite journal|vauthors=Kutty SN, Philip R | url = http://dyuthi.cusat.ac.in/xmlui/bitstream/handle/purl/2035/Marine%20yeasts-a%20rewiew.pdf| doi = 10.1002/yea.1599 | title = Marine yeasts—a review | journal = Yeast | volume = 25 | issue = 7 | pages = 465–483| year = 2008 | pmid = 18615863| s2cid = 26625932}}</ref>
An Indian study of seven [[honey bee|bee]] species and nine plant species found 45 species from 16 genera colonize the [[nectaries]] of flowers and honey stomachs of bees. Most were members of the genus ''[[Candida (genus)|Candida]]''; the most common species in honey stomachs was ''[[Dekkera intermedia]]'' and in flower nectaries, ''[[Candida blankii]]''.<ref name="Sandhu-1985"/> Yeast colonising nectaries of the [[stinking hellebore]] have been found to raise the temperature of the flower, which may aid in attracting pollinators by increasing the evaporation of [[volatile organic compound]]s.<ref name="Herrera-2010"/><ref name="Barley-2010"/> A [[black yeast]] has been recorded as a partner in a complex relationship between [[ant]]s, their [[ant-fungus mutualism|mutualistic fungus]], a fungal [[parasite]] of the fungus and a bacterium that kills the parasite. The yeast has a negative effect on the bacteria that normally produce antibiotics to kill the parasite, so may affect the ants' health by allowing the parasite to spread.<ref name="Little-2008"/>
Certain strains of some species of yeasts produce proteins called yeast killer toxins that allow them to eliminate competing strains. (See main article on [[killer yeast]].) This can cause problems for winemaking but could potentially also be used to advantage by using killer toxin-producing strains to make the wine. Yeast killer toxins may also have medical applications in treating yeast infections (see "Pathogenic yeasts" section below).<ref name="Magliani-2006"/>
Marine yeasts, defined as the yeasts that are isolated from marine environments, are able to grow better on a medium prepared using seawater rather than freshwater.<ref name="Zaky-2014"/> The first marine yeasts were isolated by Bernhard Fischer in 1894 from the [[Atlantic Ocean]], and those were identified as ''Torula'' sp. and ''Mycoderma'' sp.<ref name="Kutty-2008">{{cite journal |last1=Kutty |first1=Sreedevi N. |last2=Philip |first2=Rosamma |title=Marine yeasts—a review |journal=Yeast |date=July 2008 |volume=25 |issue=7 |pages=465–483 |doi=10.1002/yea.1599|pmid=18615863 |s2cid=26625932 |doi-access= }}</ref> Following this discovery, various other marine yeasts have been isolated from around the world from different sources, including seawater, seaweeds, marine fish and mammals.<ref>{{cite journal |last1=Zaky |first1=Abdelrahman Saleh |last2=Greetham |first2=Darren |last3=Louis |first3=Edward J. |last4=Tucker |first4=Greg A. |last5=Du |first5=Chenyu |title=A New Isolation and Evaluation Method for Marine-Derived Yeast spp. with Potential Applications in Industrial Biotechnology |journal=Journal of Microbiology and Biotechnology |date=28 November 2016 |volume=26 |issue=11 |pages=1891–1907 |doi=10.4014/jmb.1605.05074|pmid=27435537 |s2cid=40476719 |url=http://www.jmb.or.kr/journal/view.html?doi=10.4014/jmb.1605.05074 }}</ref> Among these isolates, some marine yeasts originated from terrestrial habitats (grouped as facultative marine yeast), which were brought to and survived in marine environments. The other marine yeasts were grouped as obligate or indigenous marine yeasts, which are confined to marine habitats.<ref name="Kutty-2008"/> However, no sufficient evidence has been found to explain the indispensability of seawater for obligate marine yeasts.<ref name="Zaky-2014">{{cite journal|last1=Zaky|first1=Abdelrahman Saleh|last2=Tucker|first2=Gregory A.|last3=Daw|first3=Zakaria Yehia|last4=Du|first4=Chenyu|date=September 2014|title=Marine yeast isolation and industrial application|journal=FEMS Yeast Research|volume=14|issue=6|pages=813–825|doi=10.1111/1567-1364.12158|pmc=4262001|pmid=24738708}} [[File:CC-BY icon.svg|50x50px|class=noviewer]] This article contains quotations from this source, which is available under a Creative Commons Attribution license.</ref> It has been reported that marine yeasts are able to produce many bioactive substances, such as amino acids, glucans, glutathione, toxins, enzymes, phytase, and vitamins with potential applications in the food, pharmaceutical, cosmetic, and chemical industries as well as for marine culture and environmental protection.<ref name="Zaky-2014"/> Marine yeast was successfully used to produce bioethanol using seawater-based media which will potentially reduce the [[water footprint]] of bioethanol.<ref>{{cite journal |last1=Zaky |first1=Abdelrahman Saleh |last2=Greetham |first2=Darren |last3=Tucker |first3=Gregory A. |last4=Du |first4=Chenyu |title=The establishment of a marine focused biorefinery for bioethanol production using seawater and a novel marine yeast strain |journal=Scientific Reports |date=14 August 2018 |volume=8 |issue=1 |pages=12127 |doi=10.1038/s41598-018-30660-x |pmid=30108287 |pmc=6092365 |issn=2045-2322|bibcode=2018NatSR...812127Z }}</ref>
== Reproduction ==
[[File:Yeast lifecycle.svg|thumb|right|250px|The yeast cell's life cycle: {{ordered list |Budding |Conjugation |Spore}}]]
{{See also|Mating of yeast}}
Yeasts, like all fungi, may have [[asexual reproduction|asexual]] and [[sexual reproduction|sexual]] reproductive cycles. The most common mode of vegetative growth in yeast is asexual reproduction by [[budding]],<ref name="Balasubramanian-2004"/> where a small bud (also known as a [[Bleb (cell biology)|bleb]] 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 then continues to grow until it separates from the parent cell, forming a new cell.<ref name="Yeong-2005"/> The daughter cell produced during the budding process is generally smaller than the mother cell. Some yeasts, including ''[[Schizosaccharomyces pombe]]'', reproduce by [[Fission (biology)|fission]] instead of budding,<ref name="Balasubramanian-2004"/> and thereby creating two identically sized daughter cells.
In general, under high-stress conditions such as [[nutrient]] starvation, [[haploid]] cells will die; under the same conditions, however, [[diploid]] cells can undergo sporulation, entering sexual reproduction ([[meiosis]]) and producing a variety of haploid [[spore]]s, which can go on to [[mating of yeast|mate]] (conjugate), reforming the diploid.<ref name="Neiman-2005"/>
The haploid fission yeast ''Schizosaccharomyces pombe'' is a facultative sexual microorganism that can undergo mating when nutrients are limited.<ref name="Hoffman-2015"/><ref name="Davey-1998"/> Exposure of ''S. pombe'' to hydrogen peroxide, an agent that causes oxidative stress leading to oxidative DNA damage, strongly induces mating and the formation of meiotic spores.<ref name="Bernstein-1989"/> The budding yeast ''Saccharomyces cerevisiae'' reproduces by mitosis as diploid cells when nutrients are abundant, but when starved, this yeast undergoes meiosis to form haploid spores.<ref name="Herskowitz-1988"/> Haploid cells may then reproduce asexually by mitosis. Katz Ezov et al.<ref name="Katz-2010"/> presented evidence that in natural ''S. cerevisiae'' populations clonal reproduction and selfing (in the form of intratetrad mating) predominate. In nature, the mating of haploid cells to form diploid cells is most often between members of the same clonal population and [[Outcrossing|out-crossing]] is uncommon.<ref name="Ruderfer-2006"/> Analysis of the ancestry of natural ''S. cerevisiae'' strains led to the conclusion that out-crossing occurs only about once every 50,000 cell divisions.<ref name="Ruderfer-2006"/> These observations suggest that the possible long-term benefits of outcrossing (e.g. generation of diversity) are likely to be insufficient for generally maintaining sex from one generation to the next.{{Citation needed|date=December 2019|reason=removal of citation referring to predatory publisher content}}  Rather, a short-term benefit, such as recombinational repair during meiosis,<ref name="Birdsell-2003">{{cite book|vauthors=Birdsell JA, Wills C |date=2003 |title=The evolutionary origin and maintenance of sexual recombination: A review of contemporary models |series=Evolutionary Biology Series >> Evolutionary Biology |volume=33 |pages=27–137 |veditors=MacIntyre RJ, Clegg MT |publisher=Springer |isbn=978-0306472619}}</ref> may be the key to the maintenance of sex in ''S. cerevisiae''.
Some [[Pucciniomycotina|pucciniomycete]] yeasts, in particular species of ''[[Sporidiobolus]]'' and ''[[Sporobolomyces]]'', produce aerially dispersed, asexual [[ballistospore|ballistoconidia]].<ref name="Bai-2002"/>
== 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]], and yeast in wine fermentation and for [[xylitol]] production.<ref name="Chen-2010"/> So-called [[red rice yeast]] is actually a [[Mold (fungus)|mold]], ''[[Monascus purpureus]]''. Yeasts include some of the most widely used [[model organism]]s for [[genetics]] and [[cell biology]].<ref name="Botstein-2011"/><!--cites previous sentence-->
=== Alcoholic beverages ===
Alcoholic beverages are defined as [[drink|beverages]] that contain [[ethanol]] (C<sub>2</sub>H<sub>5</sub>OH). This ethanol is almost always produced by [[fermentation (food)|fermentation]]&nbsp;– the [[metabolism]] of [[carbohydrate]]s by certain species of yeasts under anaerobic or low-oxygen conditions. Beverages such as mead, wine, beer, or [[distilled beverage|distilled spirits]] all use yeast at some stage of their production. A distilled beverage is a beverage containing ethanol that has been purified by [[distillation]]. Carbohydrate-containing plant material is fermented by yeast, producing a dilute solution of ethanol in the process. Spirits such as [[whiskey]] and [[rum]] are prepared by distilling these dilute solutions of ethanol. Components other than ethanol are collected in the condensate, including water, [[ester]]s, and other alcohols, which (in addition to that provided by the oak in which it may be aged) account for the [[Flavour (taste)|flavour]] of the beverage.
{{anchor|top-fermenting|bottom-fermenting}}
===={{anchor|Beer}}Beer ====
{{Main|Brewing}}
{{see also|Barm}}
[[File:NM.0019545 Jästkrans.jpg|thumb|[[Yeast ring]] used by Swedish farmhouse brewers in the 19th century to preserve yeast between brewing sessions.]]<!-- This section is linked from [[Swedish beer]], [[Brewer's yeast]], [[Brewers yeast]], [[Brewing yeast]], [[Brewing Yeast]], [[Brewer's Yeast]], and [[Brewers' yeast]] -->
[[File:2009-03-21 Beer brewing bubbles.jpg|right|thumb|Bubbles of [[carbon dioxide]] forming during beer-brewing<ref name="Ostergaard-2000"/>]]
Brewing yeasts may be classed as "top-cropping" (or "top-fermenting") and "bottom-cropping" (or "bottom-fermenting").<ref>{{cite book|vauthors=Priest FG, Stewart GG |date=2006 |url=https://books.google.com/books?id=TIYbNdrIsPEC&pg=PA84 |page=84 |publisher=CRC Press |title=Handbook of Brewing|isbn=9781420015171 }}</ref> Top-cropping yeasts are so called because they form a foam at the top of the [[wort]] during fermentation. An example of a top-cropping yeast is ''[[Saccharomyces cerevisiae]]'', sometimes called an "ale yeast".<ref name="Gibson-2010"/> Bottom-cropping yeasts are typically used to produce [[lager]]-type beers, though they can also produce [[ale]]-type beers. These yeasts ferment well at low temperatures. An example of bottom-cropping yeast is ''[[Saccharomyces pastorianus]]'', formerly known as ''S.&nbsp;carlsbergensis''.
Decades ago,{{vague|date=March 2018}} taxonomists reclassified ''S.&nbsp;carlsbergensis'' (uvarum) as a member of ''S.&nbsp;cerevisiae'', noting that the only distinct difference between the two is metabolic. {{dubious |reason=In conflict with newer sources cited by “Saccharomyces uvarum” article. Could be a lumper/splitter thing, but the names ain’t invalid yet.|date=January 2022}} Lager strains of ''S.&nbsp;cerevisiae'' secrete an enzyme called melibiase, allowing them to hydrolyse [[melibiose]], a [[disaccharide]], into more fermentable [[monosaccharide]]s. Top- and bottom-cropping and cold- and warm-fermenting distinctions are largely generalizations used by laypersons to communicate to the general public.<ref>For more on the taxonomical differences, see {{cite book |author=Dowhanick TM |chapter=Yeast&nbsp;– Strains and Handling Techniques |title=The Practical Brewer |editor=McCabe JT |publisher=Master Brewers Association of the Americas |year=1999}}</ref>
The most common top-cropping brewer's yeast, ''S.&nbsp;cerevisiae'', is the same species as the common baking yeast.<ref name="Amendola-2002"/> Brewer's yeast is also very rich in [[essential mineral]]s and the [[B vitamin]]s (except B<sub>12</sub>), a feature exploited in food products made from leftover ([[by-product]]) yeast from brewing.<ref name="University of Maryland Medical Center"/> However, baking and brewing yeasts typically belong to different strains, cultivated to favour different characteristics: baking yeast strains are more aggressive, to carbonate [[dough]] in the shortest amount of time possible; brewing yeast strains act more slowly but tend to produce fewer off-flavours and tolerate higher alcohol concentrations (with some strains, up to 22%).
''[[Dekkera/Brettanomyces]]'' is a genus of yeast known for its important role in the production of '[[lambic]]' and specialty [[sour ale]]s, along with the secondary conditioning of a particular Belgian [[Trappist beer]].<ref name="Vanderhaegen-2003"/> The taxonomy of the genus ''Brettanomyces'' has been debated since its early discovery and has seen many reclassifications over the years. Early classification was based on a few species that reproduced asexually (anamorph form) through multipolar budding.<ref name="Custers-1940"/> Shortly after, the formation of ascospores was observed and the genus ''Dekkera'', which reproduces sexually (teleomorph form), was introduced as part of the taxonomy.<ref name="VanderWalt-1984"/> The current taxonomy includes five species within the genera of ''Dekkera/Brettanomyces''. Those are the anamorphs ''[[Brettanomyces bruxellensis]]'', ''[[Brettanomyces anomalus]]'', ''[[Brettanomyces custersianus]]'', ''[[Brettanomyces naardenensis]]'', and ''[[Brettanomyces nanus]]'', with teleomorphs existing for the first two species, ''[[Dekkera bruxellensis]]'' and ''[[Dekkera anomala]]''.<ref name="Oelofse-2008"/> The distinction between ''Dekkera'' and ''Brettanomyces'' is arguable, with Oelofse et al. (2008) citing Loureiro and Malfeito-Ferreira from 2006 when they affirmed that current molecular DNA detection techniques have uncovered no variance between the anamorph and teleomorph states. Over the past decade, ''Brettanomyces'' spp. have seen an increasing use in the craft-brewing sector of the industry, with a handful of breweries having produced beers that were primarily fermented with pure cultures of ''Brettanomyces'' spp. This has occurred out of experimentation, as very little information exists regarding pure culture fermentative capabilities and the aromatic compounds produced by various strains. ''Dekkera''/''Brettanomyces'' spp. have been the subjects of numerous studies conducted over the past century, although a majority of the recent research has focused on enhancing the knowledge of the wine industry. Recent research on eight ''Brettanomyces'' strains available in the brewing industry focused on strain-specific fermentations and identified the major compounds produced during pure culture anaerobic fermentation in wort.<ref name="Yakobson-2010"/>
==== Wine ====
{{Main|Yeast in winemaking}}
[[File:Schramsberg Vineyards, July 2019-7609.jpg|thumb|Yeast in a bottle during sparkling wine production at [[Schramsberg Vineyards]], Napa]]
Yeast is used in [[winemaking]], where it converts the sugars present ([[glucose]] and [[fructose]]) in [[grape juice]] ([[must]]) into ethanol. Yeast is normally already present on grape skins. [[Fermentation (wine)|Fermentation]] can be done with this endogenous "wild yeast",<ref name="Ross-1997"/> but this procedure gives unpredictable results, which depend upon the exact types of yeast species present. For this reason, a pure yeast culture is usually added to the must; this yeast quickly dominates the fermentation. The wild yeasts are repressed, which ensures a reliable and predictable fermentation.<ref name="González-2001"/>
Most added wine yeasts are strains of ''S. cerevisiae'', though not all strains of the species are suitable.<ref name="González-2001"/> Different ''S.&nbsp;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-2005"/> 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/><ref name="McBryde-2006"/>
The growth of some yeasts, such as ''[[Zygosaccharomyces]]'' and ''[[Brettanomyces]]'', in wine can result in [[wine fault]]s and subsequent spoilage.<ref name="Loureiro-2003"/> ''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 faults within the wine industry.<ref name="Lamar"/>
Researchers from the [[University of British Columbia]], Canada, have found a new strain of yeast that has reduced [[amine]]s. The amines in [[Wine color|red wine]] and [[Chardonnay]] produce off-flavors and cause headaches and hypertension in some people. About 30% of people are sensitive to biogenic amines, such as [[histamine]]s.<ref>{{Cite news|title=Eureka! Vancouver scientists take the headache out of red wine |url=https://vancouversun.com/health/Eureka+Vancouver+scientists+take+headache+wine/4281742/story.html |author=Shore R |date=15 February 2011 |work=[[The Vancouver Sun]] |archive-url=https://web.archive.org/web/20110217024652/http://www.vancouversun.com/health/Eureka%2BVancouver%2Bscientists%2Btake%2Bheadache%2Bwine/4281742/story.html |archive-date=17 February 2011 |url-status=dead }}</ref>
=== Baking ===
{{Main|Baker's yeast}}
{{More citations needed section|date=April 2013}}
Yeast, most commonly ''S.&nbsp;cerevisiae'', is used in baking as a [[leavening agent]], converting the [[fermentation (food)|fermentable]] sugars present in dough into [[carbon dioxide]]. This causes the dough to expand or rise as gas forms pockets or bubbles. When the dough is baked, the yeast dies and the air pockets "set", giving the baked product a soft and spongy texture. The use of potatoes, water from potato boiling, [[egg (food)|eggs]], or sugar in a bread dough accelerates the growth of yeast. Most yeasts used in baking are of the same species common in alcoholic fermentation. In addition, ''[[Saccharomyces exiguus]]'' (also known as ''S.&nbsp;minor''), a wild yeast found on plants, fruits, and grains, is occasionally used for baking. In breadmaking, the yeast initially respires aerobically, producing carbon dioxide and water. When the oxygen is depleted, [[fermentation (biochemistry)|fermentation]] begins, producing ethanol as a waste product; however, this evaporates during baking.<ref name="Moore-Landecker-1996"/>
[[File:Compressed fresh yeast - 1.jpg|right|thumb|A block of compressed fresh yeast]]
It is not known when yeast was first used to bake bread. The first records that show this use came from [[Ancient Egypt]].<ref name="Legras-2007"/> Researchers speculate a mixture of flour meal and water was left longer than usual on a warm day and the yeasts that occur in natural contaminants of the [[flour]] caused it to ferment before baking. The resulting bread would have been lighter and tastier than the normal flat, hard cake.
[[File:Dry yeast.jpg|thumb|right|Active dried yeast, a granulated form in which yeast is commercially sold]]
Today, there are several retailers of baker's yeast; one of the earlier developments in North America is [[Fleischmann's Yeast]], in 1868. During World War II, Fleischmann's developed a [[wikt:granulate|granulated]] active dry yeast which did not require refrigeration, had a longer [[shelf life]] than fresh yeast, and rose twice as fast. Baker's yeast is also sold as a fresh yeast compressed into a square "cake". This form perishes quickly, so must be used soon after production. A weak solution of water and sugar can be used to determine whether yeast is expired.<ref>{{Cite web |last=John |date=2023-08-24 |title=Does Yeast Expire? [Active Dry vs Instant Yeast] |url=https://pizzaovenshub.com/does-yeast-expire/ |access-date=2023-09-27 |website=PizzaOvensHub |language=en-US}}</ref> 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 (baking technique)|proofing]] the yeast, as it "proves" (tests) the viability of the yeast before the other ingredients are added. When a [[sourdough]] starter is used, flour and water are added instead of sugar; this is referred to as proofing the [[Sponge and dough|sponge]].{{Citation needed|reason='Reliable sources needed for the whole paragraph, also maybe some irrelevant information for an WP article'|date=October 2016}}
When yeast is used for making bread, it is mixed with [[flour]], salt, and warm water or milk. The dough is [[Kneading|kneaded]] until it is smooth, and then left to rise, sometimes until it has doubled in size. The dough is then shaped into loaves. Some bread doughs are knocked back after one rising and left to rise again (this is called [[proofing (baking technique)|dough proofing]]) and then baked. A longer rising time gives a better flavor, but the yeast can fail to raise the bread in the final stages if it is left for too long initially.{{cn|date=August 2024}}
=== 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]], [[Trinitrotoluene|TNT]] (an explosive material), and other [[hydrocarbon]]s, such as [[alkane]]s, [[fatty acid]]s, fats and oils.<ref name="Zinjarde-2014"/> It can also tolerate high concentrations of salt and [[heavy metal (chemistry)|heavy metals]],<ref name="Bankar-2009b"/> and is being investigated for its potential as a heavy metal [[Biosorption|biosorbent]].<ref name="Bankar-2009a"/> ''Saccharomyces cerevisiae'' has potential to bioremediate toxic pollutants like [[arsenic]] from industrial effluent.<ref name="Soares-2012"/> Bronze statues are known to be degraded by certain species of yeast.<ref name="Cappitelli-2008"/> Different yeasts from Brazilian gold mines [[bioaccumulation|bioaccumulate]] free and [[Coordination complex|complexed]] silver ions.<ref name="Singh-2006"/>
=== Industrial ethanol production ===
{{See also|Biofuel#Bioalcohols|label 1=Bioethanol}}
The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry to produce [[ethanol fuel]]. The process starts by milling a feedstock, such as [[sugar cane]], [[field corn]], or other [[cereal grain]]s, and then adding dilute [[sulfuric acid]], or fungal alpha [[amylase]] enzymes, to break down the starches into complex sugars. A glucoamylase is then added to break the complex sugars down into simple sugars. After this, yeasts are added to convert the simple sugars to ethanol, which is then distilled off to obtain ethanol up to 96% in purity.<ref name="USDE-2009"/>
''Saccharomyces'' yeasts have been [[genetically engineered]] to ferment [[xylose]], one of the major fermentable sugars present in [[Lignocellulosic biomass|cellulosic biomasses]], such as agriculture residues, paper wastes, and wood chips.<ref name="Brat-2009"/><ref name="Ho-1998"/> Such a development means ethanol can be efficiently produced from more inexpensive feedstocks, making [[cellulosic ethanol]] fuel a more competitively priced alternative to gasoline fuels.<ref name="Madhavan-2012"/>
=== Nonalcoholic beverages ===
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A number of sweet [[soft drink|carbonated beverages]] can be produced by the same methods as beer, except the fermentation is stopped sooner, producing carbon dioxide, but only trace amounts of alcohol, leaving a significant amount of residual sugar in the drink.
* '''[[Root beer]]''', originally made by Native Americans, commercialized in the United States by [[Charles Elmer Hires]] and especially popular during [[Prohibition in the United States|Prohibition]]
* '''[[Kvass]]''', a [[fermented drink]] made from [[rye]], popular in Eastern Europe. It has a recognizable, but low alcoholic content.<ref name="Smith-2013"/>
* '''[[Kombucha]]''', a fermented sweetened tea. Yeast in [[symbiosis]] with [[acetic acid bacteria]] is used in its preparation. Species of yeasts found in the tea can vary, and may include: ''[[Brettanomyces bruxellensis]]'', ''[[Candida stellata]]'', ''[[Schizosaccharomyces pombe]]'', ''[[Torulaspora delbrueckii]]'' and ''[[Zygosaccharomyces bailii]]''.<ref name="Teoh-2004"/> Also popular in Eastern Europe and some [[Post-Soviet states|former Soviet republics]] under the name ''chajnyj grib'' ({{langx|ru|Чайный гриб}}), which means "tea mushroom".
* '''[[Kefir]]''' and '''[[kumis]]''' are made by fermenting milk with yeast and bacteria.<ref name="de Oliveira-2013"/>
* '''[[Mauby]]''' ({{langx|es|mabí}}), made by fermenting sugar with the wild yeasts naturally present on the bark of the ''[[Colubrina elliptica]]'' tree, popular in the [[Caribbean]]
{{See also|Tibicos}}
=== Foods and {{vanchor|nutritional supplements}} ===
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Yeast is used as an ingredient in foods for its [[umami]] flavor, in much of the same way that [[monosodium glutamate]] (MSG) is used and, like MSG, yeast often contains free [[glutamic acid]]. Examples include:<ref>{{Cite book|last1=Stewart|first1=Graham G.|url=https://books.google.com/books?id=TIYbNdrIsPEC&pg=PA691|title=Handbook of Brewing, Second Edition|last2=Priest|first2=Fergus G.|date=2006-02-22|publisher=CRC Press|isbn=978-1-4200-1517-1|language=en|page=691}}</ref>
* {{anchor|Yeast extract}}[[Yeast extract]], made from the intracellular contents of yeast and used as [[food additive]]s or [[Flavoring|flavour]]s. The general method for making yeast extract for food products such as [[Vegemite]] and [[Marmite]] on a commercial scale is ''heat autolysis'', i.e. to add salt to a suspension of yeast, making the solution hypertonic, which leads to the cells' shrivelling up. This triggers [[autolysis (biology)|autolysis]], wherein 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 that would give poor texture) are removed. Yeast autolysates are used in [[Vegemite]] and [[Promite]] (Australia); [[Marmite]] (the United Kingdom); the unrelated [[Marmite (New Zealand)|Marmite]] (New Zealand); [[Vitam-R]] (Germany); and [[Cenovis]] ([[Switzerland]]).
* [[File:Hefeflocken Naturata.jpg|thumb|150px|Nutritional yeast flakes are yellow in colour]]<!--
-->[[Nutritional yeast]], which is whole dried, deactivated yeast cells, usually ''S.&nbsp;cerevisiae''. Usually in the form of yellow flake or powder, its nutty and umami flavor makes it a vegan substitute for cheese powder.<ref name="Thaler-2014"/> Another popular use is as a topping for popcorn. It can also be used in mashed and fried potatoes, as well as in [[scrambled egg]]s. It comes in the form of flakes, or as a yellow powder similar in texture to [[cornmeal]]. In Australia, it is sometimes sold as "savoury yeast flakes".<ref name="Lee"/>
Both types of yeast foods above are rich in [[Vitamin B|B-complex]] vitamins (besides vitamin B<sub>12</sub> unless fortified),<ref name="University of Maryland Medical Center"/> making them an attractive nutritional supplement to vegans.<ref name="Thaler-2014"/> The same vitamins are also found in some yeast-fermented products mentioned above, such as [[kvass]].<ref name="Līdums-2017">{{cite journal |last1=Līdums |first1=Ivo |last2=Kārkliņa |first2=Daina |last3=Ķirse |first3=Asnate |last4=Šabovics |first4=Mārtiņš |date=April 2017 |title=Nutritional value, vitamins, sugars and aroma volatiles in naturally fermented and dry kvass |url=http://llufb.llu.lv/conference/foodbalt/2017/FoodBalt_2017_Conference_Proceedings.pdf#page=61 |journal=Foodbalt |publisher=[[Faculty of Food Technology, Latvia University of Life Sciences and Technologies]] |pages=61–65 |doi=10.22616/foodbalt.2017.027 |issn=2501-0190}}</ref> Nutritional yeast in particular is naturally low in fat and [[sodium]] and a source of protein and vitamins as well as other minerals and [[cofactor (biochemistry)|cofactors]] required for growth. Many brands of nutritional yeast and yeast extract spreads, though not all, are fortified with [[Cyanocobalamin|vitamin B<sub>12</sub>]], which is produced separately by [[bacteria]].<ref name="Duyff-2012"/>
In 1920, the [[Fleischmann's Yeast|Fleischmann Yeast Company]] began to promote yeast cakes in a "Yeast for Health" campaign. They initially emphasized yeast as a source of vitamins, good for skin and digestion. Their later advertising claimed a much broader range of health benefits, and was censured as misleading by the [[Federal Trade Commission]]. The [[fad]] for yeast cakes lasted until the late 1930s.<ref name="Price-2015">{{cite journal|author1=Price C|title=The healing power of compressed yeast|journal=Distillations Magazine|date=Fall 2015|volume=1|issue=3|pages=17–23|url=https://www.sciencehistory.org/distillations/magazine/the-healing-power-of-compressed-yeast|access-date=20 March 2018}}</ref>
=== Probiotics ===
Some [[probiotic]] supplements use the yeast ''[[Saccharomyces boulardii|S.&nbsp;boulardii]]'' to maintain and restore the natural flora in the [[gastrointestinal tract]]. ''S.&nbsp;boulardii'' has been shown to reduce the symptoms of acute [[diarrhea]],<ref name="Dinleyici-2012"/> reduce the chance of infection by ''[[Clostridium difficile (bacteria)|Clostridium difficile]]'' (often identified simply as C. difficile or C. diff),<ref name="Johnson-2012"/> reduce bowel movements in diarrhea-predominant [[irritable bowel syndrome|IBS]] patients,<ref name="Dai-2013"/> and reduce the incidence of [[antibiotic-associated diarrhea|antibiotic]]-, [[travelers' diarrhea|traveler's]]-, and [[HIV/AIDS]]-associated diarrheas.<ref name="McFarland LV-2010"/>
=== Aquarium hobby ===
Yeast is often used by [[aquarium]] hobbyists to generate carbon dioxide (CO<sub>2</sub>) to nourish plants in [[Aquascaping|planted aquaria]].<ref name="Pedersen-2007"/> CO<sub>2</sub> levels from yeast are more difficult to regulate than those from pressurized CO<sub>2</sub> systems. However, the low cost of yeast makes it a widely used alternative.<ref name="Pedersen-2007"/>
=== Scientific research ===
[[File:Yeast cell english.svg|thumb|right|Diagram showing a yeast cell]]
Several yeasts, in particular ''[[S.&nbsp;cerevisiae]]'' and ''[[S. pombe]]'', have been widely used in genetics and cell biology, largely because they are simple [[eukaryote|eukaryotic]] cells, serving as a model for all eukaryotes, including humans, for the study of fundamental cellular processes such as the [[cell cycle]], [[DNA replication]], [[genetic recombination|recombination]], [[cell division]], and metabolism. Also, yeasts are easily manipulated and cultured in the laboratory, which has allowed for the development of powerful standard techniques, such as [[yeast two-hybrid]],<ref name="Brückner-2009"/> [[synthetic genetic array]] analysis,<ref name="Boone-2006"/> and [[tetrad (genetics)|tetrad analysis]]. Many proteins important in human biology were first discovered by studying their [[homology (biology)|homologues]] in yeast; these proteins include [[cell cycle protein]]s, [[Cell signaling|signaling proteins]], and protein-processing [[enzyme]]s.<ref name="Ishiwata-2007"/>
On 24 April 1996, ''S.&nbsp;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="Williams-1996"/> At the time, it was the most complex organism to have its full genome sequenced, and the work of seven years and the involvement of more than 100 laboratories to accomplish.<ref name="Henahan-1996"/> The second yeast species to have its genome sequenced was ''Schizosaccharomyces pombe'', which was completed in 2002.<ref name="Wood-2002"/><ref name="Reinert-2002"/> It was the sixth eukaryotic genome sequenced and consists of 13.8&nbsp;million base pairs. As of 2014, over 50 yeast species have had their genomes sequenced and published.<ref name="Lin-2014"/>
Genomic and functional gene annotation of the two major yeast models can be accessed via their respective [[model organism databases]]: SGD<ref>{{cite web |url=https://www.yeastgenome.org |title=About SGD |website=Saccharomyces Genome Database}}</ref><ref>{{cite journal |last1=Cherry |first1=JM |last2=Hong |first2=EL |last3=Amundsen |first3=C |last4=Balakrishnan |first4=R |last5=Binkley |first5=G |last6=Chan |first6=ET |last7=Christie |first7=KR |last8=Costanzo |first8=MC |last9=Dwight |first9=SS |last10=Engel |first10=SR |last11=Fisk |first11=DG |last12=Hirschman |first12=JE |last13=Hitz |first13=BC |last14=Karra |first14=K |last15=Krieger |first15=CJ |last16=Miyasato |first16=SR |last17=Nash |first17=RS |last18=Park |first18=J |last19=Skrzypek |first19=MS |last20=Simison |first20=M |last21=Weng |first21=S |last22=Wong |first22=ED |title=Saccharomyces Genome Database: the genomics resource of budding yeast. |journal=Nucleic Acids Research |date=January 2012 |volume=40 |issue=Database issue |pages=D700–5 |doi=10.1093/nar/gkr1029 |pmid=22110037|pmc=3245034 }}</ref> and PomBase.<ref>{{cite web |url=http://www.pombase.org |title=PomBase}}</ref><ref>{{cite journal |last1=Lock |first1=A |last2=Rutherford |first2=K |last3=Harris |first3=MA |last4=Hayles |first4=J |last5=Oliver |first5=SG |last6=Bähler |first6=J |last7=Wood |first7=V |title=PomBase 2018: user-driven reimplementation of the fission yeast database provides rapid and intuitive access to diverse, interconnected information. |doi-access=free |journal=Nucleic Acids Research |volume=47 |issue=D1 |pages=D821–D827 |date=13 October 2018 |doi=10.1093/nar/gky961 |pmid=30321395|pmc=6324063 }}</ref>
=== Genetically engineered biofactories ===
Various yeast species have been genetically engineered to efficiently produce various drugs, a technique called [[metabolic engineering]].<ref>{{Cite journal|first1=N. |last1=Milne|first2=P.  |last2=Thomsen|first3=N.  |last3=Mølgaard Knudsen|first4=P.  |last4=Rubaszka|first5=M.  |last5=Kristensen|first6=L.  |last6=Borodina|date=2020-07-01|title=Metabolic engineering of ''Saccharomyces cerevisiae'' for the ''de novo'' production of psilocybin and related tryptamine derivatives|journal=Metabolic Engineering|language=en|volume=60|pages=25–36|doi=10.1016/j.ymben.2019.12.007|pmid=32224264|pmc=7232020|issn=1096-7176|doi-access=free}}</ref> ''S.&nbsp;cerevisiae'' is easy to genetically engineer; its physiology, metabolism and genetics are well known, and it is amenable for use in harsh industrial conditions. A wide variety of chemical in different classes can be produced by engineered yeast, including [[phenols|phenolics]], [[isoprenoid]]s, [[alkaloid]]s, and [[polyketide]]s.<ref name="Siddiqui-2012"/> About 20% of [[biopharmaceutical]]s are produced in ''S.&nbsp;cerevisiae'', including [[insulin]], [[vaccine]]s for [[hepatitis]], and [[human serum albumin]].<ref name="Nilesen-2012"/>
== Pathogenic yeasts ==
{{Main|Fungal infection}}
[[File:Candida Gram stain.jpg|thumb|right|[[Gram stain]] of ''Candida albicans'' from a vaginal swab. The small oval chlamydospores are 2–4 [[micrometre|μm]] in diameter.]]
[[File:C albicans en.jpg|thumb|right|A [[photomicrograph]] of ''[[Candida albicans]]'' showing hyphal outgrowth and other morphological characteristics]]
Some species of yeast are [[opportunistic pathogen]]s that can cause infection in people with compromised [[immune system]]s. ''[[Cryptococcus neoformans]]'' and ''[[Cryptococcus gattii]]'' are significant pathogens of [[immunocompromised]] people. They are the species primarily responsible for [[cryptococcosis]], a [[fungal infection]] that occurs in about one million [[HIV/AIDS]] patients, causing over 600,000 deaths annually.<ref name="Cogliati-2013"/> The cells of these yeast are surrounded by a rigid [[polysaccharide]] capsule, which helps to prevent them from being recognised and engulfed by [[white blood cell]]s in the human body.<ref name="O'Meara-2012"/>
Yeasts of the genus ''[[Candida (genus)|Candida]]'', another group of opportunistic pathogens, cause [[Oral candidiasis|oral]] and [[vaginal infection]]s in humans, known as [[candidiasis]]. ''Candida'' is commonly found as a [[commensal]] yeast in the [[mucous membrane]]s of humans and other warm-blooded animals. However, sometimes these same strains can become pathogenic. 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="Deacon-2006"/> A book from the 1980s listed the pathogenic yeasts of candidiasis in probable descending order of [[virulence]] for humans as: ''[[Candida albicans|C.&nbsp;albicans]]'', ''[[Candida tropicalis|C.&nbsp;tropicalis]]'', ''[[Candida stellatoidea|C.&nbsp;stellatoidea]]'', ''[[Candida glabrata|C.&nbsp;glabrata]]'', ''[[Candida krusei|C.&nbsp;krusei]]'', ''[[Candida parapsilosis|C.&nbsp;parapsilosis]]'', ''[[Candida guilliermondii|C.&nbsp;guilliermondii]]'', ''[[Candida viswanathii|C.&nbsp;viswanathii]]'', ''[[Candida lusitaniae|C.&nbsp;lusitaniae]]'', and ''[[Rhodotorula|Rhodotorula mucilaginosa]]''.<ref name="Hurley-1987"/> ''Candida glabrata'' is the second most common ''Candida'' pathogen after ''C.&nbsp;albicans'', causing infections of the [[urogenital tract]], and of the [[bloodstream]] ([[candidemia]]).<ref name="Brunke-2013"/> ''[[Candida auris|C.&nbsp;auris]]'' has been more recently identified.
== 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="Kurtzman-2006"/> During their growth, yeasts metabolize some food components and produce metabolic end products. This causes the physical, chemical, and sensible properties of a food to change, and the food is spoiled.<ref name="Fleet-2001"/> The growth of yeast within food products is often seen on their surfaces, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semiliquid products, such as [[syrup]]s and [[jam]]s.<ref name="Kurtzman-2006"/> The yeast of the genus ''[[Zygosaccharomyces]]'' have had a long history as spoilage yeasts within the [[food industry]]. This is mainly because these species can grow in the presence of high sucrose, ethanol, [[acetic acid]], [[sorbic acid]], [[benzoic acid]], and [[sulfur dioxide]] concentrations,<ref name="Loureiro-2003"/> representing some of the commonly used [[food preservation]] methods. [[Methylene blue]] is used to test for the presence of live yeast cells.<ref name="Downes-2001"/> In [[oenology]], the major spoilage yeast is ''[[Brettanomyces bruxellensis]]''.
''[[Candida blankii]]'' has been detected in [[Iberian ham]] and meat.<ref>{{cite book |url=https://books.google.com/books?id=yIgeBQAAQBAJ&pg=PA140 |title=Handbook of Fermented Meat and Poultry|first1=Fidel |last1=Toldrá |page=140 |edition=2nd |editor-first1=Fidel |editor-last1= Toldrá |editor-first2= Y. H. |editor-last2=Hui |editor-first3=Iciar |editor-last3=Astiasaran |editor-first4=Joseph |editor-last4=Sebranek |editor-first5=Regine |editor-last5=Talon |isbn= 978-1-118-52267-7 |date=October 2014 |location=Chichester, West Sussex, UK|publisher=[[Wiley-Blackwell]]}}</ref>
==Symbiosis==
{{Main|Symbiosis}}
An Indian study of seven [[honey bee|bee]] species and nine plant species found 45 yeast species from 16 genera colonise the [[nectaries]] of flowers and honey stomachs of bees. Most were members of the genus ''[[Candida (genus)|Candida]]''; the most common species in honey bee stomachs was ''[[Dekkera intermedia]]'', while the most common species colonising flower nectaries was ''[[Candida blankii]]''.  Although the mechanism is not fully understood, it was found that ''A. indica'' flowers more if ''[[Candida blankii]]'' is present.<ref name="Sandhu-1985"/>
In another example, ''Spathaspora passalidarum'', found in the digestive tract of [[Passalidae|bess beetles]], aids the digestion of plant cells by fermenting [[xylose]].<ref>{{Cite journal |last1=Nguyen |first1=Nhu H. |last2=Suh |first2=Sung-Oui |last3=Marshall |first3=Christopher J. |last4=Blackwell |first4=Meredith |date=2006-10-01 |title=Morphological and ecological similarities: wood-boring beetles associated with novel xylose-fermenting yeasts, ''Spathaspora passalidarum'' gen. sp. nov. and ''Candida jeffriesii'' sp. nov. |journal=Mycological Research |volume=110 |issue=10 |pages=1232–1241 |doi=10.1016/j.mycres.2006.07.002 |pmid=17011177 |issn=0953-7562}}</ref>
Many [[fruit]]s produce different types of sugars that attract yeasts, which ferment the sugar and turns it into alcohol. Fruit eating mammals find the scent of alcohol attractive as it indicates a ripe, sugary fruit which provides more nutrition. In turn, the mammals helps disperse both the fruit's seeds and the yeast's spores.<ref>{{Cite web |last=Yirka |first=Bob |title=Wild fruits with higher alcohol content found to be more widely dispersed by mammals |url=https://phys.org/news/2023-07-wild-fruits-higher-alcohol-content.html |date=July 20, 2023  |website=Phys.org |language=en |url-status=live |archive-url=https://web.archive.org/web/20240308211512/https://phys.org/news/2023-07-wild-fruits-higher-alcohol-content.html |archive-date= Mar 8, 2024 }}</ref><ref>{{Cite web |last=Sagar |first=Soumya |date=19 July 2023 |title=Alcoholic fruit may help plants recruit mammals to spread their seeds |url=https://www.newscientist.com/article/2383191-alcoholic-fruit-may-help-plants-recruit-mammals-to-spread-their-seeds/ |url-status=live |archive-url=https://web.archive.org/web/20240308211535/https://www.newscientist.com/article/2383191-alcoholic-fruit-may-help-plants-recruit-mammals-to-spread-their-seeds/ |archive-date=Mar 8, 2024 |website=New Scientist}}</ref>
Yeast and [[small hive beetle]] have mutualistic relationship. While small hive beetle is attracted by the pheromone released by the host honeybee, yeast can produce a similar pheromone which have the same attractive effect to the small hive beetle. Therefore, yeast facilitates SHB's infestation if the beehive contains yeast inside.<ref>{{Cite journal |last1=Torto |first1=Baldwyn |last2=Boucias |first2=Drion G. |last3=Arbogast |first3=Richard T. |last4=Tumlinson |first4=James H. |last5=Teal |first5=Peter E. A. |date=2007-05-15 |title=Multitrophic interaction facilitates parasite–host relationship between an invasive beetle and the honey bee |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=104 |issue=20 |pages=8374–8378 |doi=10.1073/pnas.0702813104 |doi-access=free |issn=0027-8424 |pmc=1895957 |pmid=17483478|bibcode=2007PNAS..104.8374T }}</ref>
== See also ==
{{Portal|Fungi}}
* [[Bioaerosol]]
* [[Ethanol fermentation]]
* [[Evolution of aerobic fermentation]]
* ''[[Kazachstania yasuniensis]]'' – a yeast isolated in 2015
* [[Mycosis]] (fungal infection in animals)
* [[Start point (yeast)]]
* [[WHI3]]
* [[Plasmid#Yeast plasmids|Yeast plasmids]]
* [[Zymology]]
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}}
== Further reading ==
* {{Cite book |vauthors=Alexopoulos CJ, Mims CW, Blackwell M |year=1996 |title=Introductory Mycology |location=New York |publisher=Wiley |isbn=978-0-471-52229-4}}
* {{Cite book |vauthors=Kirk PM, Cannon PF, Minter DW, Stalpers JA |year=2008 |title=Dictionary of the Fungi |edition=10th |location=Wallingford, UK |publisher=CAB International |isbn=978-0-85199-826-8}}
* {{Cite book |editor1=Kurtzman CP |editor2=Fell JW |editor3=Boekhout T |year=2011 |title=The Yeasts: A Taxonomic Study |volume=1 |edition=5th |location=Amsterdam, etc. |publisher=Elsevier |isbn=978-0-12-384708-9}}
* {{Cite book |last=Money |first=Nicholas P. |year=2018 |title=The Rise of Yeast: How the Sugar Fungus Shaped Civilisation |publisher=Oxford University Press |isbn=978-0198749707}}
* {{Cite book |vauthors=Priest FG, Stewart GG |year=2006 |title=Handbook of Brewing |edition=2nd |url=https://books.google.com/books?id=TIYbNdrIsPEC&pg=PA691 |publisher=CRC Press |isbn=978-1-4200-1517-1 |page=691}}
== External links ==
{{Wiktionary}}
{{Commons category}}
* [http://www.yeastgenome.org ''Saccharomyces'' genome database]
* [https://web.archive.org/web/20070721115019/http://biochemie.web.med.uni-muenchen.de/Yeast_Biol/10%20Yeast%20Growth%20and%20the%20Cell%20Cycle.pdf Yeast growth and the cell cycle] (archived 21 July 2007)
* [http://wiki.yeastgenome.org/index.php/General_Topics Yeast virtual library]
{{Fungus}}
{{Bread}}
{{Alcoholic drinks}}
{{Authority control}}
[[Category:Yeasts| ]]
[[Category:Abortifacients]]
[[Category:Brewing ingredients]]
[[Category:Fermentation in food processing]]
[[Category:Food additives]]
[[Category:Fungus common names]]
[[Category:Leavening agents]]
[[Category:Medicinal fungi]]