'Yeasts' are a growth form of
eukaryotic microorganisms classified in the
kingdom Fungi, with approximately 1,500
species described.
[1] Most reproduce
asexually by
budding, although a few do by
binary fission. Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation of a string of connected budding cells known as ''
pseudohyphae'', or ''
true hyphae'' as seen in most
molds.
[Kurtzman, C.P., Fell, J.W. 2006. Yeast systematics and phylogeny - implications of molecular identification methods for studies in ecology. In: Rosa, C.A. and Peter, G., editors. The Yeast Handbook. Germany:Springer-Verlag Berlin Herdelberg. p. 11-30.] Yeast size can vary greatly depending on the species, typically measuring 3–4
µm in
diameter, although some yeasts can reach over 40 µm.
[Walker K, Skelton H, Smith K., Cutaneous lesions showing giant yeast forms of Blastomyces dermatitidis., J Cutan Pathol. 2002 Nov;29(10):616-8.]
The yeast species ''
Saccharomyces cerevisiae'' has been used in
baking and
fermenting alcoholic beverages for thousands of years. It is also extremely important as a
model organism in modern
cell biology research, and is the most thoroughly researched eukaryotic microorganism. Researchers can use it to gather information into the biology of the eukaryotic cell and ultimately human biology.
[ Ostergaard, S., Olsson, L., Nielsen, J., Metabolic Engineering of Saccharomyces cerevisiae, Microbiol. Mol. Biol. Rev. 2000 64: 34-50] Other species of yeast, such as ''
Candida albicans'', are
opportunistic pathogens and can cause
infection in humans. Yeasts have recently been used to generate electricity in
microbial fuel cells,
[2] and produce ethanol for the
biofuel industry.
Yeasts do not form a specific
taxonomic or
phylogenetic grouping. At present it is estimated that only 1% of all yeast species have been described.
[Kurtzman, C.P., Piskur, J. 2006. Taxonomy and phylogenetic diversity among the yeasts. In: Sunnerhagen, P. and Piskur, J., editors. Comparative Genomics: Using Fungi as Models. Vol. 15. Berlin: Springer-Verlag, Berlin. p. 29-46.] The term "''yeast''" is often taken as a
synonym for ''S. cerevisiae'',
[Kurtzman C.P., Molecular taxonomy of the yeasts., Yeast. 1994 Dec;10(13):1727-40] however the phylogenetic diversity of yeasts is shown by their placement in both
divisions
Ascomycota and
Basidiomycota. The budding yeasts ("true yeasts") are classified in the
order Saccharomycetales.
[3]
History
The word "''yeast''" comes from the
Old English language "''gist''", "''gyst''", ultimately from the
Indo-European root "''yes-''", meaning ''boil'', ''foam'', or ''bubble''.
[4] Yeast microbes are probably one of the earliest domesticated organisms. People have used yeast for fermentation and baking throughout history. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted bread, as well as drawings of 4,000-year-old bakeries and breweries.
[Planets in a Bottle, More about Yeast, Science@NASA, Retrieved 6 January 2007.] In 1680 the
Dutch naturalist
Anton van Leeuwenhoek first
microscopically observed yeast, but at the time did not consider them to be living organisms but rather globular structures.
[Yeast, The Contemporary Review (1871), Collected Essays VIII.. Retrieved 6 January 2007.] In 1857
French microbiologist
Louis Pasteur proved in the paper "''Mémoire sur la fermentation alcoolique''" that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst.
[Barnett, James A., Beginnings of microbiology and biochemistry: the contribution of yeast research, Microbiology '149' (2003), 557-567] Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, but the fermentation inhibited - an observation later called the ''Pasteur effect''.
Growth and nutrition
Yeasts are chemoorganotrophs as they use organic compounds as a source of energy and do not require light to grow. The main source of carbon is obtained by hexose sugars such as glucose, or disaccharides such as sucrose and maltose. Some species can metabolize pentose sugars such as fructose, alcohols, and organic acids. Yeast species either require oxygen for aerobic cellular respiration (obligate aerobes), or are anaerobic but also have aerobic methods of energy production (facultative anaerobes). Unlike bacteria, there are no known yeast species that grow only anaerobically (obligate anaerobes).
Yeasts are ubiquitous in the environment, but are most frequently isolated from sugar-rich samples. Some good examples include fruits and berries (such as grapes, apples or peaches), and exudates from plants (such as plant saps or cacti). Some yeasts are found in association with soil and insects.[5][6] Yeast are generally grown in the laboratory on solid growth media or liquid broths. Common media used for the cultivation of yeasts include; potato dextrose agar (PDA) or potato dextrose broth, Wallerstien Laboratories Nutrient agar (WLN), Yeast Peptone Dextrose agar (YPD), and Yeast Mould agar or broth (YM). The antibiotic cycloheximide is sometimes added to yeast growth media to inhibit the growth of ''Saccharomyces'' yeasts and select for wild/indigenous yeast species.
Reproduction
The yeast life cycle.
1. Budding
2. Conjugation
3. Spore
Yeasts have asexual and sexual reproductive cycles; however the most common mode of vegetative growth in yeast is asexual reproduction by budding or fission.[ Comparative analysis of cytokinesis in budding yeast, fission yeast and animal cells, Balasubramanian M, Bi E, Glotzer M, , , Curr Biol, 2004 ] Here a small bud, or daughter cell, is formed on the parent cell. The nucleus of the parent cell splits into a daughter nucleus and migrates into the daughter cell. The bud continues to grow until it separates from the parent cell, forming a new cell.[7] The bud can develop on different parts of the parent cell depending on the genus of the yeast.
Under high stress conditions haploid cells will generally die, however under the same conditions diploid cells can undergo sporulation, entering sexual reproduction (meiosis) and producing a variety of haploid spores, which can go on to mate (conjugate), reforming the diploid.[8]
Yeast of the species ''Schizosaccharomyces pombe'' reproduce by binary fission instead of budding.
Uses
The useful physiological properties of yeast have led to their use in the field of biotechnology. Fermentation of sugars by yeast is the oldest and largest application of this technology. Many types of yeasts are used for making many foods: Baker's yeast in bread production, brewer's yeast in beer fermentation, yeast in wine fermentation and for xylitol[9] production. Yeasts are also one of the most widely used model organisms for genetics and cell biology.
Alcoholic beverages
Alcoholic beverages are loosely defined as a beverage that contains ethanol (CH3CH2OH). This ethanol is almost always produced by fermentation - the metabolism of carbohydrates by certain species of yeast. Beverages such as wine, beer, or distilled spirits all use yeast at some stage of their production.
Beer
Beer brewers classify yeasts as top-fermenting and bottom-fermenting. This distinction was introduced by the Dane Emil Christian Hansen. ''Top-fermenting yeasts'' are so-called because they form a foam at the top of the wort during fermentation. They can produce higher alcohol concentrations and prefer higher temperatures, producing fruitier, sweeter, ale-type beers. An example of a top-fermenting yeast is ''Saccharomyces cerevisiae'', known to brewers as ale yeast. ''Bottom-fermenting yeasts'' are used to produce lager-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low temperatures. An example of a bottom-fermenting yeast is ''Saccharomyces pastorianus''.
For both types, yeast is fully distributed through the beer while it is fermenting, and both equally flocculate (clump together and precipitate to the bottom of the vessel) when it is finished. By no means do all top-fermenting yeasts demonstrate this behaviour, but it features strongly in many English ale yeasts which may also exhibit chain forming (the failure of budded cells to break from the mother cell) which is technically different from true flocculation.
Brewers of Bavarian-style wheat beers often use varieties of ''Torulaspora delbrueckii'', which contribute to the distinctive flavour profile. Lambic, a style of Belgian beer, is fermented spontaneously by wild yeasts primarily of the genus ''Brettanomyces''.
Fermenting tanks with yeast being used to
brew beer.
In industrial brewing, to ensure purity of strain, a 'clean' sample of the yeast is stored refrigerated in a laboratory. After a certain number of fermentation cycles, a full scale propagation is produced from this laboratory sample. Typically, it is grown up in about three or four stages using sterile brewing wort and oxygen.
Root Beer and Sodas
Root Beer and Sodas can be produced using the same methods as Beer only the carbonation process created by the active yeast is stopped sooner producing only trace amounts of alcohol (consumable by all ages) and a significant amount of sugar is left in the drink. Low calorie or nearly sugar free root beer and soda can be produced using small amounts of sugar and replacing the majority with artificial sweeteners.
Distilled beverages
A distilled beverage is a beverage that contains ethanol that has been purified by distillation. Carbohydrate-containing plant material is fermented by yeast, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. Components other than ethanol are collected in the condensate, including water, esters, and other alcohols which account for the flavor of the beverage.
Wine
Grapes covered in yeast growth observable as a white film, also known as the "''blush''".
Yeast is used in winemaking where it converts the sugars present in grape juice or must into alcohol. Yeast is normally already present on the grapes, often visible as a powdery film (also known as the ''bloom'' or ''blush'') on their exterior. The fermentation can be done with this indigenous (or ''wild'') yeast;[Jordan P. Ross, Going wild: wild yeast in winemaking Wines & Vines, Sept, 1997. Retrieved 10 January 2007.] however, this may give unpredictable results depending on the exact types of yeast species that are present. For this reason a pure yeast culture is generally added to the must, which rapidly predominates the fermentation as it proceeds. This represses the wild yeasts and ensures a reliable and predictable fermentation.[A. González Techera, S. Jubany, F.M. Carrau, C. Gaggero, 'Differentiation of industrial wine yeast strains using microsatellite markers', Letters in Applied Microbiology 2001 33:1 71.] Most added wine yeasts are strains of ''Saccharomyces cerevisiae'', however not all strains of the species are suitable.[ 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.][Dunn, B., Levine, R.P., Sherlock, G., Microarray karyotyping of commercial wine yeast strains reveals shared, as well as unique, genomic signatures, BMC Genomics. 2005 Apr 16;6(1):53.] Significant research has been undertaken into the develoment of ''novel'' wine yeast strains that produce atypical flavour profiles or increased complexity in wines.[Research enables yeast suppliers to expand options. Retrieved 10 January 2007.][McBryde, Colin, Gardner, Jennifer M., de Barros Lopes, Miguel, Jiranek, Vladimir, [Generation of Novel Wine Yeast Strains by Adaptive Evolution], Am. J. Enol. Vitic. 2006 57: 423-430]
The growth of some yeasts such as ''Zygosaccharomyces'' and ''Brettanomyces'' in wine can result in wine faults and subsequent spoilage.[10] ''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.[BRETTANOMYCES. Retrieved 10 January 2007.]
Baking
Yeast, specifically ''Saccharomyces cerevisiae'', is used in baking as a leavening agent, where it converts the fermentable sugars present in the dough into carbon dioxide. This causes the dough to expand or rise as the carbon dioxide forms pockets or bubbles. When the dough is baked it "sets" and the pockets remain, giving the baked product a soft and spongy texture. The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Salt and fats such as butter slow down yeast growth. The majority of the yeast used in baking is of the same species common in alcoholic fermentation. Additionally, ''Saccharomyces exiguus'' (also known as ''S. minor'') is a wild yeast found on plants, fruits, and grains that is occasionally used for baking.[ Crust and Crumb: Master formulas for serious bread bakers Peter Reinhart ]
A block of fresh yeast.
It is not known when yeast was first used to bake bread. The first records that show this use came from Ancient Egypt.[11] Researchers speculate that a mixture of flour meal and water was left longer than usual on a warm day and the yeasts that occur in natural contaminants of the flour caused it to ferment before baking. The resulting bread would have been lighter and more tasty than the normal flat, hard cake.
Active dried yeast, a granulated form in which yeast is commercially sold.
Today there are several retailers of baker's yeast; one of the best-known is Fleischmann’s Yeast, which was developed in 1868. During World War II Fleischmann's developed a active dry yeast, which did not require refrigeration and had a longer shelf life than fresh yeast. The company created yeast that would rise twice as fast, cutting down on baking time. Baker's yeast is also sold as a fresh yeast compressed into a square "cake". This form perishes quickly, and must be used soon after production in order to maintain viability. A weak solution of water and sugar can be used to determine if yeast is expired. When dissolved in the solution, active yeast will foam and bubble as it ferments the sugar into ethanol and carbon dioxide.
When yeast is used for making bread, it is mixed with flour, salt, and warm water (or milk). The dough is kneaded until it is smooth, and then left to rise, sometimes until it has doubled in size. Some bread doughs are knocked back after one rising and left to rise again. A longer rising time gives a better flavour, but the yeast can fail to raise the bread in the final stages if it is left for too long initially. The dough is then shaped into loaves, left to rise until it is the correct size, and then baked. Dried yeast is always used for bread made in a bread machine.
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,[ Palm oil mill effluent treatment by a tropical marine yeast., , N, Oswal, Bioresour Technol., 2002 ] TNT (an explosive material),[ 2,4,6-trinitrotoluene transformation by a tropical marine yeast, Yarrowia lipolytica NCIM 3589., , MR, Jain, Mar Pollut Bull., 2004 ] and other hydrocarbons such as alkanes, fatty acids, fats and oils.[ Hydrophobic substrate utilisation by the yeast Yarrowia lipolytica, and its potential applications., , P, Fickers, FEMS Yeast Res., 2005 ]
Industrial ethanol production
The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry, which has various uses including ethanol fuel. The process starts by milling a feedstock, such as sugar cane, sweetcorn, or cheap cereal grains, and then adding dilute sulfuric acid, or fungal alpha amylase enzymes, to break down the starches in to complex sugars. A gluco amylase is then added to break the complex sugars down into simple sugars. After this, yeasts are added to convert the simple sugars to ethanol, which is then distilled off to obtain ethanol up to 96% in concentration.[12]
''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.[13] Such a development means that ethanol can be efficiently produced from more inexpensive feedstocks, making cellulosic ethanol fuel a more competitively priced alternative to gasoline fuels.[14]
Kombucha
Yeast in symbiosis with acetic acid bacteria is used in the preparation of Kombucha, a fermented sweetened tea. Species of yeast found in the tea can vary, and may include: ''Brettanomyces bruxellensis'', ''Candida stellata'', ''Schizosaccharomyces pombe'', ''Torulaspora delbrueckii'' and ''Zygosaccharomyces bailii''.[15]
Nutritional supplements
Yeast is used in nutritional supplements popular with vegans and the health conscious, where it is often referred to as "nutritional yeast". It is a deactivated yeast, usually ''Saccharomyces cerevisiae''. It is an excellent source of protein and vitamins, especially the B-complex vitamins, whose functions are related to metabolism as well as other minerals and cofactors required for growth. It is also naturally low in fat and sodium. Some brands of nutritional yeast, though not all, are fortified with vitamin B12, which is produced separately from bacteria. Nutritional yeast, though it has a similar appearance to brewer's yeast, is very different and has a very different taste.
Nutritional yeast has a nutty, cheesy, creamy flavor which makes it popular as an ingredient in cheese substitutes. It is often used by vegans in place of parmesan cheese. Another popular use is as a topping for popcorn. Some movie theaters are beginning to offer it along with salt or cayenne pepper as a popcorn condiment. It comes in the form of flakes, or as a yellow powder similar in texture to cornmeal, and can be found in the bulk aisle of most natural food stores. In Australia it is sometimes sold as "savory yeast flakes". Though "nutritional yeast" usually refers to commercial products, inadequately fed prisoners have used "home-grown" yeast to prevent vitamin deficiency.[16]
Probiotics
Some probiotic supplements use the yeast ''Saccharomyces boulardii'' to maintain and restore the natural flora in the large and small gastrointestinal tract. ''S. boulardii'' has been shown to reduce the symptoms of acute diarrhea in children,[17][17] prevent reinfection of ''Clostridium difficile'',[17] reduce bowel movements in diarrhea predominant IBS patients,[17] and reduce the incidence of antibiotic,[17] traveler's,[17] and HIV/AIDS[17] associated diarrheas.
Science
Diagram showing a yeast cell
Several yeasts, particularly ''Saccharomyces cerevisiae'', have been widely used in genetics and cell biology. This is largely because the cell cycle in a yeast cell is very similar to the cell cycle in humans, and therefore the basic cellular mechanics of DNA replication, recombination, cell division and metabolism are comparable.[ Also many proteins important in human biology were first discovered by studying their homologs in yeast; these proteins include cell cycle proteins, signaling proteins, and protein-processing enzymes. ]
On 24 April 1996 ''S. cerevisiae'' was announced to be the first eukaryote to have its genome, consisting of 12 million base pairs, fully sequenced as part of the Genome project.[24] At the time it was the most complex organism to have its full genome sequenced and took 7 years and the involvement of more than 100 laboratories to accomplish.[25] The second yeast species to have its genome sequenced was ''Schizosaccharomyces pombe'', which was completed in 2002.[26] It was the 6th eukaryotic genome sequenced and consists of 13.8 million base pairs.
Yeast extract
Main articles: Yeast extract
 Marmite and Vegemite have a distinctive dark colour |
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 acids. 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'', where 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 which would give poor texture) are separated. Yeast autolysates are used in Vegemite and Promite (Australia), Marmite and Bovril (the United Kingdom and Republic of Ireland), Oxo (South Africa, United Kingdom, and Republic of Ireland), and Cenovis (Switzerland).
Pathogenic yeasts
Some species of yeast are opportunistic pathogens, where they can cause infection in people with compromised immune systems.
''Cryptococcus neoformans'', is a significant pathogen of immunocompromised people, causing the disease termed Cryptococcosis. This disease occurs in about 7–8% of AIDS patients in the USA, and a slightly smaller percentage (3–6%) in western Europe.[27] The cells of the yeast are surrounded by a rigid polysaccharide capsule, which helps to prevent them from being recognised and engulfed by white blood cells in the human body.
Yeasts of the ''Candida'' genus are another group of opportunistic pathogens, which causes oral and vaginal infections in humans, known as Candidiasis. ''Candida'' is commonly found as a commensal yeast in the mucus membranes of humans and other warm-blooded animals. However, sometimes these same strains can become pathogenic. Here the yeast cells sprout a hyphal outgrowth, which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues.[ 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''.[28] ''Candida glabrata'' is the second most common ''Candida'' pathogen after ''C. albicans'', causing infections of the urogenital tract, and of the bloodstream (Candidemia).[29]]
Non-pathogenic yeast such as ''S. cerevisiae'' are also implicated in disease; anti saccharomyces cerevisiae antibodies (ASCA) have been found at relatively high frequencies in familial crohn's disease and at higher frequencies in other forms of colitis.[30]
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.[Kurtzman, C.P. 2006. Detection, identification and enumeration methods for spoilage yeasts. In: Blackburn, C. de. W, editor. Food spoilage microorganisms. Cambridge, England: Woodhead Publishing. p. 28-54.] During their growth, yeasts metabolize some food components and produce metabolic end products. This causes the physical, chemical, and sensory properties of a food to change, and the food is spoilt.[Fleet, G.H., and Praphailong, W., 'Yeasts', In: Spoilage of Processed Foods: Causes and Diagnosis, AIFST (2001), Southwood Press. p 383-397] The growth of yeast within food products is often seen on their surface, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semi-liquid products, such as syrups and jams.[ The yeast of the ''Zygosaccharomyces'' genus have had a long history as a spoilage yeast within the food industry. This is mainly due to the fact that these species can grow in the presence of high sucrose, ethanol, acetic acid, sorbic acid, benzoic acid, and sulfur dioxide concentrations,][Fugelsang, K.C., Zygosaccharomyces, A Spoilage Yeast Isolated from Wine, California Agriculture Technology Institute. Retrieved 10 January 2007.] representing some of the commonly utilised food preservation methods. Methylene Blue is used to test for the presence of live yeast cells.
See also
★ Bioaerosol
★ Brewing
★ Ethanol fermentation
★ Fermentation (food)
★ Fungi
★ ''Saccharomyces cerevisiae''
★ Winemaking
References
1. Kurtzman, C.P., Fell, J.W. 2006. "Yeast Systematics and Phylogeny — Implications of Molecular Identification Methods for Studies in Ecology.", Biodiversity and Ecophysiology of Yeasts, The Yeast Handbook, Springer. Retrieved January 7 2007.
2. "Biofuelcell". ''Helsinki University of Technology''. Retrieved December 24 2006.
3. "What are yeasts?". ''Saccharomyces Genome Database''. Retrieved December 24 2006.
4. American Heritage Dictionary."yes-". Retrieved January 22, 2007.
5. The beetle gut: a hyperdiverse source of novel yeasts, Suh S, McHugh J, Pollock D, Blackwell M, , , Mycol Res, 2005
6. The diversity of yeasts in the agricultural soil, Sláviková E, Vadkertiová R, , , J Basic Microbiol, 2003
7. Severing all ties between mother and daughter: cell separation in budding yeast, Yeong F, , , Mol Microbiol, 2005
8. Ascospore formation in the yeast Saccharomyces cerevisiae, Neiman A, , , Microbiol Mol Biol Rev, 2005
9. R. Sreenivas Rao, R.S. Prakasham, K. Krishna Prasad, S. Rajesham,P.N. Sarma, L. Venkateswar Rao (2004) Xylitol production by Candida sp.: parameter optimization using Taguchi approach, Process Biochemistry 39:951-956
10. Spoilage yeasts in the wine industry, Loureiro V, Malfeito-Ferreira M, , , Int J Food Microbiol, 2003
11. "The History of Bread Yeast". British Broadcasting Company. Retrieved December 24 2006.
12. "Fuel Ethanol Production". ''Genomics:GTL''. Retrieved December 24 2006.
13. "Genetically Engineered Saccharomyces Yeast Capable of Effective Cofermentation of Glucose and Xylose". ''American Society for Microbiology''. Retrieved December 24 2006.
14. "Yeast rises to a new occasion". ''American Society for Microbiology''. Retrieved December 24 2006.
15. Yeast ecology of Kombucha fermentation, , Ai Leng, Teoh, International Journal of Food Microbiology,
16. "Harukoe (Haruku)". ''Children of Far East Prisoners of War''. Retrieved December 24 2006.
17.
18.
19.
20.
21.
22.
23.
24. Genome Projects: Yeast Genome Sequence Ferments New Research, , N, Williams, Science,
25. COMPLETE DNA SEQUENCE OF YEAST. Retrieved on 31 January 2007.
26. Schizosaccharomyces pombe: Second yeast genome sequenced. Retrieved on 31 January 2007.
27. "The Microbial World: Yeasts and yeast-like fungi". ''Institute of Cell and Molecular Biology''. Retrieved December 24 2006.
28. Hurley, R., J. de Louvois, and A. Mulhall. 1987. Yeast as human and animal pathogens, p. 207-281. In A. H. Rose and J. S. Harrison (ed.), The yeasts, vol. 1. Academic Press, Inc., New York, N.Y.
29.
30. The value of serologic markers in indeterminate colitis: a prospective follow-up study, Joossens S, Reinisch W, Vermeire S, ''et al'', , , Gastroenterology, 2002
External links
★ Cell cycle and metabolic cycle regulated transcription in yeast
★ Yeast Resource Center
★ Yeast growth and the cell cycle
★ About Nutritional Yeast, Recipes Using Nutritional Yeast
★ Information about Yeast Allergies
★ Yeast virtual library
★ Ancient Egyptian Bread Making
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