Difference between revisions of "What are yeast?"

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[[File:Fungi.jpeg|thumb|left|'''Introduction to Fungi''' Credit: ''Kandis Elliot, Mo Fayyaz, University of Wisconsin, Madison'']]
 
[[File:Fungi.jpeg|thumb|left|'''Introduction to Fungi''' Credit: ''Kandis Elliot, Mo Fayyaz, University of Wisconsin, Madison'']]
  
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====General Information====
 
[http://en.wikipedia.org/wiki/Yeast Yeast] are single-celled eukaryotic microorganisms that are classified, along with molds and mushrooms as members of the kingdom [http://en.wikipedia.org/wiki/Fungus Fungi].  Yeasts are phylogenetically diverse, and as such are classified in two separate phyla, the [http://en.wikipedia.org/wiki/Ascomycota ''Ascomycota''] and the [http://en.wikipedia.org/wiki/Basidiomycota Basidiomycota].  Budding yeast (also called “true yeasts”), such as the well-known species, [http://en.wikipedia.org/wiki/Saccharomyces_cerevisiae ''Saccharomyces cerevisiae''] are classified in the order [http://en.wikipedia.org/wiki/Saccharomycetales ''Saccharomycetales''], which is under Ascomycota. Such classifications are based on characteristics of the cell, ascospore and colony as well as physiology. One of the most well known characteristics of yeast is the ability to ferment sugars for the production of ethanol and carbon dioxide.   
 
[http://en.wikipedia.org/wiki/Yeast Yeast] are single-celled eukaryotic microorganisms that are classified, along with molds and mushrooms as members of the kingdom [http://en.wikipedia.org/wiki/Fungus Fungi].  Yeasts are phylogenetically diverse, and as such are classified in two separate phyla, the [http://en.wikipedia.org/wiki/Ascomycota ''Ascomycota''] and the [http://en.wikipedia.org/wiki/Basidiomycota Basidiomycota].  Budding yeast (also called “true yeasts”), such as the well-known species, [http://en.wikipedia.org/wiki/Saccharomyces_cerevisiae ''Saccharomyces cerevisiae''] are classified in the order [http://en.wikipedia.org/wiki/Saccharomycetales ''Saccharomycetales''], which is under Ascomycota. Such classifications are based on characteristics of the cell, ascospore and colony as well as physiology. One of the most well known characteristics of yeast is the ability to ferment sugars for the production of ethanol and carbon dioxide.   
 
[[File:EM yeast.jpeg|thumb|150px|right|'''EM image of ''S. cerevisiae''''' Credit: ''UC Berkeley'']]
 
[[File:EM yeast.jpeg|thumb|150px|right|'''EM image of ''S. cerevisiae''''' Credit: ''UC Berkeley'']]
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fuse with adjoining nuclei and multiply through vegetative division
 
fuse with adjoining nuclei and multiply through vegetative division
 
or, as with certain yeast, fuse with other ascospores.
 
or, as with certain yeast, fuse with other ascospores.
[[File:Pombe cerevisiae.jpg|thumb|left|200px|'''Fission (''S. pombe'') and budding (''S. cerevisiae'') yeasts''' Credit: ''Susan L. Forsberg, Nat Rev Gen '''2''':659'']]
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[[File:Pombe cerevisiae.jpg|thumb|right|200px|'''Fission (''S. pombe'') and budding (''S. cerevisiae'') yeasts''' Credit: ''Susan L. Forsberg, Nat Rev Gen '''2''':659'']]
  
Commercial Application
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Yeast are characterized by a wide dispersion of natural habitats. They are common on plant leaves, flowers, and fruits, as well as in soil.  Yeast are also found on the skin surfaces and in the intestinal tracts of
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warm-blooded animals, where they may live symbiotically or as parasites.  The common "yeast infection" [http://en.wikipedia.org/wiki/Candidiasis ''Candidiasis'']
 +
is typically caused by [http://en.wikipedia.org/wiki/Candida_albicans ''Candida albicans''].  In addition to being the causative agent in vaginal yeast infections ''Candida'' is also a cause of diaper rash and
 +
thrush of the mouth and throat.
 +
 
 +
====Commercial Applications====
 
''Saccharomyces cerevisiae'' are one of the most well-known and commercially significant species of yeast. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages, and to expand and raise dough to make bread.  
 
''Saccharomyces cerevisiae'' are one of the most well-known and commercially significant species of yeast. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages, and to expand and raise dough to make bread.  
 
 
 
 
 
 
 
 
Model Organism
 
The awsome power of yeast genetics is partially due to the ability to
 
quickly map a phenotype producing gene to a region of the ''S.
 
cerevisiae'' genome.  For the past two decades ''S. cerevisiae''
 
has been the model system for much of molecular genetic research
 
because the basic cellular mechanics of replication, recombination,
 
cell division and metabolism are generally conserved between yeast and
 
larger eukaryotes, including mammals.
 
  
 
[[File:Hansen-emil-christian.jpeg|thumb|x125px|left|'''Dr. Emil Christian Hansen''', ''1842-1909'']]
 
[[File:Hansen-emil-christian.jpeg|thumb|x125px|left|'''Dr. Emil Christian Hansen''', ''1842-1909'']]
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more information about sourdough see [http://www.faqs.org/faqs/food/sourdough/faq/preamble.html rec.food.sourdough FAQ].
 
more information about sourdough see [http://www.faqs.org/faqs/food/sourdough/faq/preamble.html rec.food.sourdough FAQ].
 
[[File:Sourdough-bread.jpeg|thumb|right|x100px|'''Wild yeast and acid-generating bacteria are used in making sourdough bread''']]
 
[[File:Sourdough-bread.jpeg|thumb|right|x100px|'''Wild yeast and acid-generating bacteria are used in making sourdough bread''']]
 
  
 
The fermentation of wine is initiated by naturally occurring yeast
 
The fermentation of wine is initiated by naturally occurring yeast
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[[File:Candida albicans.jpeg|thumb|left|150px|'''''C. albicans''''' Credit: ''E. Gueho/Science Photo Library'']]
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Yeast are characterized by a wide dispersion of natural habitats.
+
====Model Organism====
Common on plant leaves and flowers, soil and salt water.  Yeast are
+
The awesome power of yeast genetics is partially due to the ability to
also found on the skin surfaces and in the intestinal tracts of
+
quickly map a phenotype producing gene to a region of the ''S.
warm-blooded animals, where they may live symbiotically or as
+
cerevisiae'' genome. For the past two decades ''S. cerevisiae''
parasites.  The common "yeast infection" [http://en.wikipedia.org/wiki/Candidiasis ''Candidiasis'']
+
has been the model system for much of molecular genetic research
is typically caused by [http://en.wikipedia.org/wiki/Candida_albicans ''Candida albicans''].  In addition to being the causative agent in vaginal yeast infections ''Candida'' is also a cause of diaper rash and
+
because the basic cellular mechanics of replication, recombination,
thrush of the mouth and throat.
+
cell division and metabolism are generally conserved between yeast and
 +
larger eukaryotes, including mammals.
 +
 
 +
 
 +
 
  
 
==Suggested Reading:==
 
==Suggested Reading:==

Revision as of 14:13, 11 April 2012

Introduction to Fungi Credit: Kandis Elliot, Mo Fayyaz, University of Wisconsin, Madison

General Information

Yeast are single-celled eukaryotic microorganisms that are classified, along with molds and mushrooms as members of the kingdom Fungi. Yeasts are phylogenetically diverse, and as such are classified in two separate phyla, the Ascomycota and the Basidiomycota. Budding yeast (also called “true yeasts”), such as the well-known species, Saccharomyces cerevisiae are classified in the order Saccharomycetales, which is under Ascomycota. Such classifications are based on characteristics of the cell, ascospore and colony as well as physiology. One of the most well known characteristics of yeast is the ability to ferment sugars for the production of ethanol and carbon dioxide.

EM image of S. cerevisiae Credit: UC Berkeley

Yeast multiply as single cells that divide by asymmetric budding (eg. Saccharomyces) or by direct division (fission, eg. Schizosaccharomyces); they may also grow as simple irregular filaments (mycelium). In the budding, a small bud emerges from the surface of the parent cell and enlarges until it is almost the size of the parent. In sexual reproduction most yeast form asci, which contain up to eight haploid ascospores. These ascospores may fuse with adjoining nuclei and multiply through vegetative division or, as with certain yeast, fuse with other ascospores.

File:Pombe cerevisiae.jpg
Fission (S. pombe) and budding (S. cerevisiae) yeasts Credit: Susan L. Forsberg, Nat Rev Gen 2:659

Yeast are characterized by a wide dispersion of natural habitats. They are common on plant leaves, flowers, and fruits, as well as in soil. Yeast are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. The common "yeast infection" Candidiasis is typically caused by Candida albicans. In addition to being the causative agent in vaginal yeast infections Candida is also a cause of diaper rash and thrush of the mouth and throat.

Commercial Applications

Saccharomyces cerevisiae are one of the most well-known and commercially significant species of yeast. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages, and to expand and raise dough to make bread.

File:Hansen-emil-christian.jpeg
Dr. Emil Christian Hansen, 1842-1909

In brewing, Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen, where it was first isolated in pure culture by Dr. Emil Christian Hansen (1842-1909) in 1883, is used in the production of several types of beers including lagers. S. carlsbergensis is used for bottom fermentation. S. cerevisiae used for the production of ales and conducts top fermentation, in which the yeast rise to the surface of the brewing vessel. In modern brewing many of the original top fermentation strains have been modified to be bottom fermenters. Currently the S. carlsbergensis designation is not used, the S. cerevisiae classification is used instead.


The yeast's function in baking is to ferment sugars present in the flour or added to the dough. This fermentation gives off carbon dioxide and ethanol. The carbon dioxide is trapped within tiny bubbles and results in the dough expanding, or rising. Sourdough bread, is not produced with baker's yeast, rather a combination of wild yeast (often Candida milleri) and an acid-generating bacteria (Lactobacillus sanfranciscensis sp. nov). It has been reported that the ratio of wild yeast to bacteria in San Francisco sourdough cultures is about 1:100. The C. milleri strengthens the gluten and the L. sanfrancisco ferments the maltose. For more information about sourdough see rec.food.sourdough FAQ.

Wild yeast and acid-generating bacteria are used in making sourdough bread

The fermentation of wine is initiated by naturally occurring yeast present in the vineyards. Many wineries still use natural strains, however many use modern methods of strain maintenance and isolation. The bubbles in sparkling wines is trapped carbon dioxide, the result of yeast fermenting sugars in the grape juice. One yeast cell can ferment approximately its own weight of glucose per hour. Under optimal conditions S. cerevisiae can produce up to 18 percent, by volume, ethanol with 15 to 16 percent being the norm. The sulfur dioxide present in commercially produced wine is actually added just after the grapes are crushed to kill the naturally present bacteria, mold, and yeast.

Bunches of grapes on vines at Trinity Hill vineyard in Hawkes Bay NZ


Model Organism

The awesome power of yeast genetics is partially due to the ability to quickly map a phenotype producing gene to a region of the S. cerevisiae genome. For the past two decades S. cerevisiae has been the model system for much of molecular genetic research because the basic cellular mechanics of replication, recombination, cell division and metabolism are generally conserved between yeast and larger eukaryotes, including mammals.



Suggested Reading:

Campbell, I., and Duffus, J.H., eds., Yeast (1988)

Pfaff, Herman Jan, et al., The Life of Yeasts (1978)

The Early Days of Yeast Genetics. (1993) edited by Michael N. Hall and Patrick Linder. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.

Mortimer, R.K., Contopoulou, C.R. and J.S. King (1992) Genetic and physical maps of Saccharomyces cerevisiae, Edition 11. Yeast 8:817-902.

The Molecular and Cellular Biology of the Yeast Saccharomyces cerevisiae: Gene Expression. (1992) edited by Elizabeth W. Jones, John R. Pringle, and James R. Broach. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.

The Molecular and Cellular Biology of the Yeast Saccharomyces cerevisiae: Genome Dynamics, Protein Synthesis, and Energetics. (1991) edited by James R. Broach, John R. Pringle, and Elizabeth W. Jones. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.