Yeast

Yeast

Yeast
Yeast of the species Saccharomyces cerevisiae
Scientific classification
Domain: Eukaryota
Kingdom: Fungi
Phyla and Subphyla

Yeasts are multicellular through the formation of strings of connected budding cells known as pseudohyphae, or false hyphae, as seen in most molds.[3] Yeast size can vary greatly depending on the species, typically measuring 3–4 µm in diameter, although some yeasts can reach over 40 µm.[4] Most yeasts reproduce asexually by mitosis, and many do so by an asymmetric division process called budding.

By

  • genome databaseSaccharomyces
  • Yeast growth and the cell cycle
  • Yeast virtual library

External links

  • Alexopoulos CJ, Mims CW, Blackwell M. (1996). Introductory Mycology. New York, New York: Wiley.  
  • Kirk PM, Cannon PF, Minter DW, Stalpers JA. (2008). Dictionary of the Fungi (10th ed.). Wallingford, UK: CAB International.  
  • Kurtzman CP, Fell JW, Boekhout T., ed. (2011). The Yeasts: A Taxonomic Study 1 (5th ed.). Amsterdam, etc.: Elsevier.  
  • Moore-Landecker E. (1996). Fundamentals of the Fungi. Englewood Cliffs, New Jersey: Prentice Hall.  
  • Priest FG, Stewart GG. (2006). Handbook of Brewing (2nd ed.). CRC Press. p. 691.  

Cited texts

  1. ^ Kurtzman CP, Fell JW. (2006). "Yeast Systematics and Phylogeny—Implications of Molecular Identification Methods for Studies in Ecology". Biodiversity and Ecophysiology of Yeasts, The Yeast Handbook,. Springer. 
  2. ^ Kurtzman CP, Piškur J. (2006). Taxonomy and phylogenetic diversity among the yeasts (in Comparative Genomics: Using Fungi as Models. Sunnerhagen P, Piskur J, eds.). Berlin: Springer. pp. 29–46.  
  3. ^ Kurtzman CP, Fell JW (2005). Biodiversity and Ecophysiology of Yeasts (in: The Yeast Handbook, Gábor P, de la Rosa CL, eds.). Berlin: Springer. pp. 11–30.  
  4. ^ Walker K, Skelton H, Smith K. (2002). "Cutaneous lesions showing giant yeast forms of Blastomyces dermatitidis". Journal of Cutaneous Pathology 29 (10): 616–618.  
  5. ^ a b Legras JL, Merdinoglu D, Cornuet J-M, Karst F. (2007). "Bread, beer and wine: Saccharomyces cerevisiae diversity reflects human history". Molecular Ecology 16 (10): 2091–2102.  
  6. ^ Ostergaard S, Olsson L, Nielsen J. (2000). "Saccharomyces cerevisiae"Metabolic Engineering of . Microbiology and Molecular Biology Reviews 64 (1): 34–50.  
  7. ^ "Bioprocess automation". Helsinki University of Technology. 2007. Retrieved 15 January 2012. 
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  9. ^ "What are yeasts?". Yeast Virtual Library. 13 September 2009. Retrieved 28 November 2009. 
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  18. ^ Arthur H, Watson K. (1976). "Thermal adaptation in yeast: growth temperatures, membrane lipid, and cytochrome composition of psychrophilic, mesophilic, and thermophilic yeasts". Journal of Bacteriology 128 (1): 56–68.  
  19. ^ Kaufmann, Klaus; Annelies Schoneck (2002). Making Sauerkraut and Pickled Vegetables at Home: Creative Recipes for Lactic Fermented Food to Improve Your Health. Google books: Book Publishing Company.  
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  29. ^ Little AEF, Currie CR. (2008). "Black yeast symbionts compromise the efficiency of antibiotic defenses in fungus-growing ants". Ecology 89 (5): 1216–1222.  
  30. ^ a b Balasubramanian MK, Bi E, Glotzer M. (2004). "Comparative analysis of cytokinesis in budding yeast, fission yeast and animal cells". Current Biology 14 (18): R806–818.  
  31. ^ Yeong FM. (2005). "Severing all ties between mother and daughter: cell separation in budding yeast". Molecular Microbiology 55 (5): 1325–1331.  
  32. ^ Neiman AM. (2005). "Ascospore Formation in the Yeast Saccharomyces cerevisiae". Microbiology and Molecular Biology Reviews 69 (4): 565–584.  
  33. ^ Davey J (December 1998). "Fusion of a fission yeast". Yeast 14 (16): 1529–66.  
  34. ^ Bernstein C, Johns V (April 1989). "Sexual reproduction as a response to H2O2 damage in Schizosaccharomyces pombe". J. Bacteriol. 171 (4): 1893–7.  
  35. ^ Herskowitz I (December 1988). "Life cycle of the budding yeast Saccharomyces cerevisiae". Microbiol. Rev. 52 (4): 536–53.  
  36. ^ Katz Ezov T, Chang SL, Frenkel Z, Segrè AV, Bahalul M, Murray AW, Leu JY, Korol A, Kashi Y (January 2010). "Heterothallism in Saccharomyces cerevisiae isolates from nature: effect of HO locus on the mode of reproduction". Mol. Ecol. 19 (1): 121–31.  
  37. ^ a b Ruderfer DM, Pratt SC, Seidel HS, Kruglyak L (September 2006). "Population genomic analysis of outcrossing and recombination in yeast". Nat. Genet. 38 (9): 1077–81.  
  38. ^ Bernstein H and Bernstein C (2013). Evolutionary Origin and Adaptive Function of Meiosis. In Meiosis: Bernstein C and Bernstein H, editors. ISBN 978-953-51-1197-9, InTech, http://www.intechopen.com/books/meiosis/evolutionary-origin-and-adaptive-function-of-meiosis
  39. ^ Birdsell JA, Wills C (2003). The evolutionary origin and maintenance of sexual recombination: A review of contemporary models. Evolutionary Biology Series >> Evolutionary Biology, Vol. 33 pp. 27-137. MacIntyre, Ross J.; Clegg, Michael, T (Eds.), Springer. Hardcover ISBN 978-0306472619, ISBN 0306472619 Softcover ISBN 978-1-4419-3385-0.
  40. ^ Elvira Hörandl (2013). Meiosis and the Paradox of Sex in Nature, Meiosis, ISBN 978-953-51-1197-9, InTech, DOI: 10.5772/56542. Available from: http://www.intechopen.com/books/meiosis/meiosis-and-the-paradox-of-sex-in-nature
  41. ^ Bai FY, Zhao JH, Takashima M, Jia JH, Boekhout T, Nakase T. (2002). "Reclassification of the Sporobolomyces roseus and Sporidiobolus pararoseus complexes, with the description of Sporobolomyces phaffii sp. nov". International Journal of Systematic and Evolutionary Microbiology 52 (6): 2309–2314.  
  42. ^ Rao RS, Prakasham RS, Prasad KK, Rajesham S, Sarma PN, Rao L. (2004). "Xylitol production by Candida sp.: parameter optimization using Taguchi approach". Process Biochemistry 39 (8): 951–956.  
  43. ^ Botstein D, Fink GR. (2011). "Yeast: an experimental organism for 21st Century biology". Genetics 189 (3): 695–704.  
  44. ^ Priest and Stewart (2006), p. 84.
  45. ^ Gibson M. (2010). The Sommelier Prep Course: An Introduction to the Wines, Beers, and Spirits of the World. John Wiley and Sons. p. 361.  
  46. ^ For more on the taxonomical differences, see Terrance M. Dowhanick, B.SC., PH. D "Yeast – Strains and Handling Techiques" in The Practical Brewer, a publication of the Master Brewers Association of the Americas.
  47. ^ Amendola J, Rees N. (2002). Understanding Baking: The Art and Science of Baking. John Wiley and Sons. p. 36.  
  48. ^ a b "Brewer's yeast". University of Maryland Medical Center. Retrieved 15 January 2012. 
  49. ^ Vanderhaegen B, Neven H, Cogne S, Vertrepin KJ, Derdelinckx C, Verachtert H.; Neven; Coghe; Verstrepen; Derdelinckx; Verachtert (2003). "Bioflavoring and Beer Refermentation". Applied Microbiology and Biotechnology 62 (2–3): 140–150.  
  50. ^ Custers MTJ. (1940). Onderzoekingen over het gistgeslacht Brettanomyces (PhD thesis) (in Dutch). Delft, the Netherlands: Delft University. 
  51. ^ Van der Walt JP. (1984). "The Yeasts: A Taxonomic Study". Elsevier Science, Amsterdam (3rd ed.): 146–150. 
  52. ^ Oelofse A, Pretorius IS, du Toit M. (2008). during winemaking: a synoptic review"Dekkera and Brettanomyces"Significance of (PDF). South African Journal of Enology and Viticulture 29 (2): 128–144. 
  53. ^ Yakobson CM. (2010). yeast species and their use in the brewing industry"Brettanomyces"Pure culture fermentation characteristics of . 
  54. ^ De Graaf, Mia (19 September 2013). "'"The human brewery: Tee-total Texan whose family feared he was an alcoholic cannot stay sober as his stomach turns food into 'beer. Daily Mail (London). 
  55. ^ Barbara Cordell; Justin McCarthy (July 2013). "A Case Study of Gut Fermentation Syndrome (Auto-Brewery) withSaccharomyces cerevisiae as the Causative Organism". International Journal of Clinical Medicine 4: 309–312.  
  56. ^ Ross JP. (September 1997). "Going wild: wild yeast in winemaking". Wines & Vines. Retrieved 15 January 2012. 
  57. ^ a b González Techera A, Jubany S, Carrau FM, Gaggero C. (2001). "Differentiation of industrial wine yeast strains using microsatellite markers". Letters in Applied Microbiology 33 (1): 71–75.  
  58. ^ Dunn B, Levine RP, Sherlock G. (2005). "Microarray karyotyping of commercial wine yeast strains reveals shared, as well as unique, genomic signatures". BMC Genomics 6 (1): 53.  
  59. ^ Research enables yeast suppliers to expand options at the Wayback Machine (archived September 21, 2006). Retrieved 10 January 2007.
  60. ^ McBryde C, Gardner JM, de Barros Lopes M, Jiranek V. (2006). "Generation of novel wine yeast strains by adaptive evolution". American Journal of Enology and Viticulture 57 (4): 423–430. 
  61. ^ a b Loureiro V, Malfeito-Ferreira M. (2003). "Spoilage yeasts in the wine industry". International Journal of Food Microbiology 86 (1–2): 23–50.  
  62. ^ Lamar J. (Dekkera)"Brettanomyces". Vincyclopedia. Retrieved 28 November 2009. 
  63. ^ Shore, Randy (15 February 2011). "Eureka! Vancouver scientists take the headache out of red wine".  
  64. ^ Moore-Landecker, pp. 533–534.
  65. ^ Oswal N, Sarma PM, Zinjarde SS, Pant A. (2002). "Palm oil mill effluent treatment by a tropical marine yeast". Bioresource Technology 85 (1): 35–37.  
  66. ^ Jain MR, Zinjarde SS, Deobagkar DD, Deobagkar DN. (2004). "2,4,6-trinitrotoluene transformation by a tropical marine yeast, Yarrowia lipolytica NCIM 3589". Marine Pollution Bulletin 49 (9–10): 783–788.  
  67. ^ Fickers P, Benetti PH, Wache Y, Marty A, Mauersberger S, Smit MS, Nicaud JM. (2005). "Hydrophobic substrate utilisation by the yeast Yarrowia lipolytica, and its potential applications". FEMS Yeast Research 5 (6–7): 527–543.  
  68. ^ Bankar AV, Kumar AR, Zinjarde SS. (2009). "Environmental and industrial applications of Yarrowia lipolytica". Applied Microbiology and Biotechnology 84 (5): 847–865.  
  69. ^ Bankar AV, Kumar AR, Zinjarde SS. (2009). "Removal of chromium (VI) ions from aqueous solution by adsorption onto two marine isolates of Yarrowia lipolytica". Journal of Hazardous Materials 170 (1): 487–494.  
  70. ^ Soares EV, Soares HMVM. (2012). "Bioremediation of industrial effluents containing heavy metals using brewing cells of Saccharomyces cerevisiae as a green technology: A review". Environmental Science and Pollution Research 19 (4): 1066–83.  
  71. ^ Francesca Cappitelli; Claudia Sorlini (2008). "Microorganisms Attack Synthetic Polymers in Items Representing Our Cultural Heritage". Applied Environmental Microbiology 74.  
  72. ^ Harbhajan Singh (2006-11-17). Mycoremediation: Fungal Bioremediation. p. 507.  
  73. ^ "Fuel Ethanol Production: GSP Systems Biology Research". Genomic Science Program. U.S. Department of Energy Office of Science. Retrieved 28 November 2009. 
  74. ^ Brat D, Boles E, Wiedemann B. (2009). "Saccharomyces cerevisiae"Functional expression of a bacterial xylose isomerase in . Applied and Environmental Microbiology 75 (8): 2304–2311.  
  75. ^ Ho NW, Chen Z, Brainard AP. (1998). yeast capable of effective cofermentation of glucose and xylose"Saccharomyces"Genetically engineered . Applied and Environmental Microbiology 64 (5): 1852–1859.  
  76. ^ Madhavan A, Srivastava A, Kondo A, Bisaria VS. (2012). "Bioconversion of lignocellulose-derived sugars to ethanol by engineered Saccharomyces cerevisiae". Critical Reviews in Biotechnology 32 (1): 22–48.  
  77. ^ Smith A, Kraig B. (2013). The Oxford Encyclopedia of Food and Drink in America. Oxford University Press. p. 440.  
  78. ^ Teoh AL, Heard G, Cox J. (2004). "Yeast ecology of Kombucha fermentation". International Journal of Food Microbiology 95 (2): 119–126.  
  79. ^ de Oliveira Leite AM, Miguel MA, Peixoto RS, Rosado AS, Silva JT, Paschoalin VM. (2013). "Microbiological, technological and therapeutic properties of kefir: A natural probiotic beverage". Brazilian Journal of Microbiology 44 (2): 341–9.  
  80. ^ Lee JG (ed.). "South East Asia Under Japanese Occupation – Harukoe (Haruku)". Children (& Families) of the Far East Prisoners of War. Retrieved 28 November 2009. 
  81. ^ Dinleyici EC, Eren M, Ozen M, Yargic ZA, Vandenplas Y. (2012). "Effectiveness and safety of Saccharomyces boulardii for acute infectious diarrhea". Expert Opinion on Biological Therapy 12 (4): 395–410.  
  82. ^ Johnson S, Maziade PJ, McFarland LV, Trick W, Donskey C, Currie B, Low DE, Goldstein EJ. (2012). "Is primary prevention of Clostridium difficile infection possible with specific probiotics?". International Journal of Infectious Diseases 16 (11): e786–92.  
  83. ^ Dai C, Zheng CQ, Jiang M, Ma XY, Jiang LJ. (2013). "Probiotics and irritable bowel syndrome". World Journal of Gastroenterology 19 (36): 5973–80.  
  84. ^ McFarland LV. (2010). in adult patients"Saccharomyces boulardii"Systematic review and meta-analysis of . World Journal of Gastroenterology 16 (18): 2202–22.  
  85. ^ Pedersen O, Andersen T, Christensen C. (2007). "CO2 in planted aquaria". The Aquatic Gardener 20 (3): 24–33. 
  86. ^ Williams N. (1996). "Genome Projects: Yeast genome sequence ferments new research". Science 272 (5261): 481.  
  87. ^ Henahan, Sean (24 April 1996). "Complete DNA Sequence Of Yeast". Science Updates. Retrieved 15 January 2012. 
  88. ^ Wood V, Gwilliam R, Rajandream MA, et al. (2002). "The genome sequence of Schizosaccharomyces pombe".  
  89. ^ Reinert B. (1 March 2002). : Second yeast genome sequenced"Schizosaccharomyces pombe". Genome News Network. Retrieved 15 January 2012. 
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References

See also

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.[97] 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.[98] The growth of yeast within food products is often seen on their surfaces, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semiliquid products, such as syrups and jams.[97] The yeast of the Zygosaccharomyces genus have had a long history as spoilage yeasts within the food industry. This is mainly because these species can grow in the presence of high sucrose,ethanol, acetic acid, sorbic acid, benzoic acid, and sulphur dioxide concentrations,[61] representing some of the commonly used food preservation methods. Methylene blue is used to test for the presence of live yeast cells.[99] In oenology, the major spoilage yeast is Dekkera bruxellensis .

Food spoilage

Yeasts of the Candida genus, another group of opportunistic pathogens, cause oral and vaginal infections in humans, known as candidiasis. Candida is commonly found as a commensal yeast in the mucous membranes of humans and other warm-blooded animals. However, sometimes these same strains can become pathogenic. The yeast cells sprout a hyphal outgrowth, which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues.[94] The pathogenic yeasts of candidiasis in probable descending order of virulence for humans are: C. albicans, C. tropicalis, C. stellatoidea, C. glabrata, C. krusei, C. parapsilosis, C. guilliermondii, C. viswanathii, C. lusitaniae, and Rhodotorula mucilaginosa.[95] Candida glabrata is the second most common Candida pathogen after C. albicans, causing infections of the urogenital tract, and of the bloodstream (candidemia).[96]

Some species of yeast are opportunistic pathogens that can cause infection in people with compromised immune systems. Cryptococcus neoformans and Cryptococcus gattii are significant pathogens of immunocompromised people. They are the species primarily responsible for cryptococcosis, a fungal disease that occurs in about one million HIV/AIDS patients, causing over 600,000 deaths annually.[92] The cells of these yeast are surrounded by a rigid polysaccharide capsule, which helps to prevent them from being recognised and engulfed by white blood cells in the human body.[93]

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

Pathogenic yeasts

Yeast extract is the common name for various forms of processed yeast products that are used as food additives or flavours. They are often used in the same way that monosodium glutamate (MSG) is used and, like MSG, often contain free glutamic acid.[91] The general method for making yeast extract for food products such as Vegemite and Marmite on a commercial scale is to add salt to a suspension of yeast, making the solution hypertonic, which leads to the cells' shrivelling up. This triggers autolysis, wherein the yeast's digestive enzymes break their own proteins down into simpler compounds, a process of self-destruction. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeast with thick cell walls that would give poor texture) are separated. Yeast autolysates are used in Vegemite and Promite (Australia); Marmite, Bovril and Oxo (the United Kingdom, Republic of Ireland and South Africa); and Cenovis (Switzerland).

Marmite and Vegemite, products made from yeast extract

Yeast extract

[90] It was the sixth eukaryotic genome sequenced and consists of 13.8 million base pairs. As of 2012, over 30 yeast species have had their genomes sequenced and published.[89][88], which was completed in 2002.Schizosaccharomyces pombe The second yeast species to have its genome sequenced was [87] was announced to be the first eukaryote to have its S. cerevisiae On 24 April 1996,

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

Diagram showing a yeast cell

Science

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

Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO2) to nourish plants in planted aquaria.[85] A homemade setup is widely used as a cheap and simple alternative to pressurized CO2 systems. While not as effective as these, the homemade setup is considerably cheaper for less-demanding hobbyists.

Aquarium hobby

Some probiotic supplements use the yeast S. boulardii to maintain and restore the natural flora in the gastrointestinal tract. S. boulardii has been shown to reduce the symptoms of acute diarrhea,[81] reduce the chance of infection by Clostridium difficile,[82] reduce bowel movements in diarrhea-predominant IBS patients,[83] and reduce the incidence of antibiotic-, traveler's-, and HIV/AIDS-associated diarrheas.[84]

Probiotics

Nutritional yeast has a nutty, cheesy flavor that makes it popular as an ingredient in cheese substitutes. It is often used by vegans in place of Parmesan cheese. Another popular use is as a topping for popcorn. It can also be used in mashed and fried potatoes, as well as in scrambled eggs. It comes in the form of flakes, or as a yellow powder similar in texture to cornmeal, and can be found in the bulk aisle of most natural food stores. In Australia, it is sometimes sold as "savory yeast flakes". Though "nutritional yeast" usually refers to commercial products, inadequately fed prisoners have used "home-grown" yeast to prevent vitamin deficiency.[80]

Yeast is used in nutritional supplements popular with health-conscious individuals and those following vegan diets. It is often referred to as "nutritional yeast" when sold as a dietary supplement. Nutritional yeast is a deactivated yeast, usually S. cerevisiae. It is an excellent source of protein and vitamins, especially the B-complex vitamins, as well as other minerals and cofactors required for growth. It is also naturally low in fat and sodium. Contrary to some claims, it contains little or no vitamin B12.[48] Some brands of nutritional yeast, though not all, are fortified with vitamin B12, which is produced separately by bacteria.

Nutritional supplements

Root beer and other sweet carbonated beverages can be produced using the same methods as beer, except the fermentation is stopped sooner, producing carbon dioxide, but only trace amounts of alcohol, and a significant amount of sugar is left in the drink. Kvass, a fermented drink made from rye, is popular in Eastern Europe; it has a recognizable, but low alcoholic content.[77] Yeast in symbiosis with acetic acid bacteria is used in the preparation of kombucha, a fermented sweetened tea. Species of yeasts found in the tea can vary, and may include: Brettanomyces bruxellensis, Candida stellata, Schizosaccharomyces pombe, Torulaspora delbrueckii and Zygosaccharomyces bailii.[78] Kombucha is a popular beverage in Eastern Europe and some former Soviet republics under the name chajnyj grib (Чайный гриб), which means "tea mushroom". Kefir and kumis are made by fermenting milk with yeast and bacteria.[79]

A kombucha culture fermenting in a jar

Nonalcoholic beverages

Saccharomyces yeasts have been genetically engineered to ferment xylose, one of the major fermentable sugars present in cellulosic biomasses, such as agriculture residues, paper wastes, and wood chips.[74][75] 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.[76]

The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry to produce ethanol fuel. The process starts by milling a feedstock, such as sugar cane, field corn, or other cereal grains, and then adding dilute sulfuric acid, or fungal alpha amylase enzymes, to break down the starches into complex sugars. A glucoamylase is then added to break the complex sugars down into simple sugars. After this, yeasts are added to convert the simple sugars to ethanol, which is then distilled off to obtain ethanol up to 96% in purity.[73]

Industrial ethanol production

Some yeasts can find potential application in the field of bioremediation. One such yeast, Yarrowia lipolytica, is known to degrade palm oil mill effluent,[65] TNT (an explosive material),[66] and other hydrocarbons, such as alkanes, fatty acids, fats and oils.[67] It can also tolerate high concentrations of salt and heavy metals,[68] and is being investigated for its potential as a heavy metal biosorbent.[69] Saccharomyces cerevisiae has potential to bioremediate toxic pollutants like arsenic from industrial effluent.[70] Bronze statues are known to be degraded by certain species of yeast.[71] Different yeasts from Brazilian gold mines bioaccumulate free and complexed silver ions.[72]

Bioremediation

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. Bread machine recipes usually call for dried yeast; however, a (wet) sourdough starter can also work.

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

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

It is not known when yeast was first used to bake bread. The first records that show this use came from Ancient Egypt.[5] 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.

A block of compressed fresh yeast

Yeast, the most common one being S. cerevisiae, is used in baking as a leavening agent, where it converts the food/fermentable sugars present in dough into the gas carbon dioxide. This causes the dough to expand or rise as gas forms pockets or bubbles. When the dough is baked, the yeast dies and the air pockets "set", giving the baked product a soft and spongy texture. The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Most yeasts used in baking are of the same species common in alcoholic fermentation. In addition, Saccharomyces exiguus (also known as S. minor), a wild yeast found on plants, fruits, and grains, is occasionally used for baking. In breadmaking, the yeast initially respires aerobically, producing carbon dioxide and water. When the oxygen is depleted, fermentation begins, producing ethanol as a waste product; however, this evaporates during baking.[64]

Baking

Researchers from University of British Columbia, Canada, have found a new strain of yeast that has reduced amines. The amines in red wine and Chardonnay produce off-flavors and cause headaches and hypertension in some people. About 30% of people are sensitive to biogenic amines, such as histamines.[63]

The growth of some yeasts, such as Zygosaccharomyces and Brettanomyces, in wine can result in wine faults and subsequent spoilage.[61] Brettanomyces produces an array of metabolites when growing in wine, some of which are volatile phenolic compounds. Together, these compounds are often referred to as "Brettanomyces character", and are often described as "antiseptic" or "barnyard" type aromas. Brettanomyces is a significant contributor to wine faults within the wine industry.[62]

Most added wine yeasts are strains of S. cerevisiae, though not all strains of the species are suitable.[57] 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.[58] Significant research has been undertaken into the development of novel wine yeast strains that produce atypical flavour profiles or increased complexity in wines.[59][60]

Yeast is used in winemaking, where it converts the sugars present in grape juice (must) into ethanol. Yeast is normally already present on grape skins. Fermentation can be done with this endogenous "wild yeast,"[56] 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.[57]

Wine

Auto-brewery syndrome is a rare medical condition where the stomach produces brewer's yeast that breaks down starches into ethanol, which enters the blood stream.[54][55]

Dekkera/Brettanomyces is a genus of yeast known for its important role in the production of 'Lambic' and specialty sour ales, along with the secondary conditioning of a particular Belgian Trappist beer.[49] 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.[50] Shortly after, the formation of ascospores was observed and the genus Dekkera, which reproduces sexually (teleomorph form), was introduced as part of the taxonomy.[51] 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.[52] 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.[53]

The most common top-cropping brewer's yeast, S. cerevisiae, is the same species as the common baking yeast.[47] Brewer's yeast is also very rich in essential minerals and the B vitamins (except B12).[48] However, baking and brewing yeasts typically belong to different strains, cultivated to favour different characteristics: baking yeast strains are more aggressive, to carbonate dough in the shortest amount of time possible; brewing yeast strains act slower but tend to produce fewer off-flavours and tolerate higher alcohol concentrations (with some strains, up to 22%).

Decades ago, taxonomists reclassified S. carlsbergensis (uvarum) as a member of S. cerevisiae, noting that the only distinct difference between the two is metabolic. Lager strains of S. cerevisiae secrete an enzyme called melibiase, allowing them to hydrolyse melibiose, a disaccharide, into more fermentable monosaccharides. Top- and bottom-cropping and cold- and warm-fermenting distinctions are largely generalizations used by laypersons to communicate to the general public.[46]

Brewing yeasts may be classed as "top-cropping" (or "top-fermenting") and "bottom-cropping" (or "bottom-fermenting").[44] 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".[45] Bottom-cropping yeasts are typically used to produce lager-type beers, though they can also produce ale-type beers. These yeasts ferment well at low temperatures. An example of bottom-cropping yeast is Saccharomyces pastorianus, formerly known as S. carlsbergensis.

Beer being fermented by brewer's yeast

Beer

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

Alcoholic beverages

The useful physiological properties of yeast have led to their use in the field of genetics and cell biology.[43]

Uses

Some pucciniomycete yeasts, in particular species of Sporidiobolus and Sporobolomyces, produce aerially dispersed, asexual ballistoconidia.[41]

The haploid fission yeast Schizosaccharomyces pombe is a facultative sexual microorganism that can undergo mating when nutrients are limiting.[33] 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.[34] 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.[35] Haploid cells may then reproduce asexually by mitosis. Katz Ezov et al.[36] presented evidence that in natural S. cerevisiae populations clonal reproduction and selfing (in the form of intratetrad mating) predominate. In nature, mating of haploid cells to form diploid cells is most often between members of the same clonal population and out-crossing is uncommon.[37] 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.[37] 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.[38] Rather, a short term benefit, such as recombinational repair during meiosis,[39][40] may be the key to the maintenance of sex in S. cerevisiae.

In general, under high-stress conditions such as nutrient starvation, haploid cells will die; under the same conditions, however, diploid cells can undergo sporulation, entering sexual reproduction (meiosis) and producing a variety of haploid spores, which can go on to mate (conjugate), reforming the diploid.[32]

Yeasts, like all fungi, may have asexual and sexual reproductive cycles. The most common mode of vegetative growth in yeast is asexual reproduction by budding.[30] Here, a small bud (also known as a bleb), or daughter cell, is formed on the parent cell. The nucleus of the parent cell splits into a daughter nucleus and migrates into the daughter cell. The bud continues to grow until it separates from the parent cell, forming a new cell.[31] The daughter cell produced during the budding process is generally smaller than the mother cell. Some yeasts, including Schizosaccharomyces pombe, reproduce by fission instead of budding,[30] thereby creating two identically sized daughter cells.

The yeast cell's life cycle:
1. Budding
2. Conjugation
3. Spore

Reproduction

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).

[29] 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.parasite, a fungal mutualistic fungus, their ants has been recorded as a partner in a complex relationship between black yeast A [28][22] An Indian study of seven

[26][25] and even deep-sea environments host an array of yeasts.[24] of mammals and some insectsgut flora Yeasts are also present in the [23].skin flora, have been found living in between people's toes as part of their Trichosporon cutaneum and Torulopsis, Rhodotorula rubra, Candida albicans Yeasts, including [22] 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

Ecology

The appearance of a white, thready yeast, commonly known as kahm yeast, is often a byproduct of the lactofermentation (or pickling) of certain vegetables, usually the result of exposure to air. Although harmless, it can give pickled vegetables a bad flavor and must be removed regularly during fermentation.[19]

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

Yeasts vary in what temperature range they grow best. For example, Leucosporidium frigidum grows at −2 to 20 °C (28 to 68 °F), Saccharomyces telluris at 5 to 35 °C (41 to 95 °F), and Candida slooffi at 28 to 45 °C (82 to 113 °F).[18] The cells can survive freezing under certain conditions, with viability decreasing over time.

Yeasts are cellular respiration (obligate aerobes) or are anaerobic, but also have aerobic methods of energy production (facultative anaerobes). Unlike bacteria, no known yeast species grow only anaerobically (obligate anaerobes). Yeasts grow best in a neutral or slightly acidic pH environment.

Nutrition and growth

By the late 18th century, two yeast strains used in brewing had been identified: Saccharomyces cerevisiae (top-fermenting yeast) and S. carlsbergensis (bottom-fermenting yeast). S. cerevisiae has been sold commercially by the Dutch for bread-making since 1780; while, around 1800, the Germans started producing S. cerevisiae in the form of cream. In 1825, a method was developed to remove the liquid so the yeast could be prepared as solid blocks.[14] The industrial production of yeast blocks was enhanced by the introduction of the filter press in 1867. In 1872, Baron Max de Springer developed a manufacturing process to create granulated yeast, a technique that was used until the first World War.[15] 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.[16]

". Pasteur effect could be increased, but fermentation was inhibited – an observation later called the "cell growth Pasteur showed that by bubbling oxygen into the yeast broth, [13][11]" that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst.Mémoire sur la fermentation alcoolique proved in the paper "Louis Pasteur In 1857, French microbiologist [12] The word "yeast" comes from

History

Contents

  • History 1
  • Nutrition and growth 2
  • Ecology 3
  • Reproduction 4
  • Uses 5
    • Alcoholic beverages 5.1
      • Beer 5.1.1
      • Wine 5.1.2
    • Baking 5.2
    • Bioremediation 5.3
    • Industrial ethanol production 5.4
    • Nonalcoholic beverages 5.5
    • Nutritional supplements 5.6
    • Probiotics 5.7
    • Aquarium hobby 5.8
    • Science 5.9
    • Yeast extract 5.10
  • Pathogenic yeasts 6
  • Food spoilage 7
  • See also 8
  • References 9
    • Cited texts 9.1
  • External links 10

Yeasts do not form a single taxonomic or phylogenetic grouping. The term "yeast" is often taken as a synonym for Saccharomyces cerevisiae,[8] but the phylogenetic diversity of yeasts is shown by their placement in two separate phyla: the Ascomycota and the Basidiomycota. The budding yeasts ("true yeasts") are classified in the order Saccharomycetales.[9]

industry. biofuel for the ethanol and produce [7],microbial fuel cells in humans. Yeasts have recently been used to generate electricity in infections and can cause opportunistic pathogens, are Candida albicans Other species of yeasts, such as [6]