yeast

1. 1. Any of various unicellular fungi of the genus Saccharomyces, especially S. cerevisiae, reproducing by budding and from ascospores and capable of fermenting carbohydrates.
   2. Any of various similar fungi.
2. Froth consisting of yeast cells together with the carbon dioxide they produce in the process of fermentation, present in or added to fruit juices and other substances in the production of alcoholic beverages.
3. A powdered or compressed commercial preparation, having yeast cells and inert material such as meal and used chiefly as a leavening agent or as a dietary supplement.
4. Foam; froth.
5. An agent of ferment or activity: political agitators who are the yeast of revolution.

1. To ferment.
2. To froth or foam.

[Middle English yeest, from Old English gist.]

A collective name for those fungi which possess, under normal conditions of growth, a vegetative body (thallus) consisting, at least in part, of simple, single cells. The cells making up the thallus occur in pairs, in groups of three, or in straight or branched chains consisting of as many as 12 or more cells. Vegetative reproduction is characterized by budding or fission. Sexual reproduction also occurs in yeast, and is differentiated from that of other fungi by sexual states that are not enclosed in a fruiting body. Yeasts are a phylogenetically diverse group of organisms that occur in two divisions of fungi (Ascomycotina and Basidiomycotina) and 100 genera. The 700 or more species that have been described possibly represent only 1% of the species in nature, so the majority of the yeasts have yet to be discovered. Yeast plays a large part in industrial fermentation processes such as the production of industrial enzymes and chemicals, food products, industrial ethanol, and malt beverage and wine; in diseases of humans, animals and plants; in food spoilage; and as a model of molecular genetics. See also Distilled spirits; Food microbiology; Genetic engineering; Malt beverage; Medical mycology; Wine.

The shape and size of the individual cells of some species vary slightly, but in other species the cell morphology is extremely heterogeneous. The shape of yeast cells may be spherical, globose, ellipsoidal, elongate to cylindrical with rounded ends, more or less rectangular, pear-shaped, apiculate or lemon-shaped, ogival or pointed at one end, or tetrahedral. The diameter of a spherical cell may vary from 2 to 10 micrometers. The length of cylindrical cells is often 20–30 μm and, in some cases, even greater.

The asexual multiplication of yeast cells occurs by a budding process, by the formation of cross walls or fission, and sometimes by a combination of these two processes. Yeast buds are sometimes called blastospores or blastoconidia. When yeast reproduces by a fission mechanism, the resulting cells are termed arthrospores or arthroconidia.

Yeasts are categorized into two groups, based on their methods of sexual reproduction: the ascomycetous (Division Ascomycotina) and basidiomycetous (Division Basidiomycotina) yeasts.

The sexual spores of the ascomycetous yeasts are termed ascospores, which are formed in simple structures, often a vegetative cell. Such asci are called naked asci because of the absence of an ascocarp, which is a more complex fruiting body found in the higher Ascomycetes. If the vegetative cells are diploid, a cell may transform directly into an ascus after the 2n nucleus undergoes a reduction or meiotic division. See also Ascomycota.

Certain yeasts have been shown to be heterothallic; that is, sporulation occurs when strains of opposite mating type (usually indicated by “a” and α) are mixed on sporulation media. However, some strains may be homothallic (self-fertile), and reduction division and karyogamy (fusion of two haploid nuclei) take place during formation of the sexual spore. Yeasts that produce sporogenous cells represent the teleomorphic form of the life cycle. In cases, in which sexual cycles are unknown, the yeast represents the asexual or anamorphic form. A species of yeast may be originally discovered in the anamorphic form and named accordingly; subsequently, the sexual state may be found and a name applied to represent the teleomorph. Consequently, the anamorphic and teleomorphic names will differ.

Basidiospores and teliospores are the sexual spores that are produced in the three classes of basidiomycetous yeasts: Urediniomycetes, Hymenomycetes, and Ustilaginomycetes. Sexual reproduction and life cycle in these yeasts is typical of other basidiomycetes in that it can include both unifactorial (bipolar) and bifactorial (tetrapolar) mating systems. See also Basidiomycota.

Some yeasts have the ability to carry out an alcoholic fermentation. Other yeasts lack this property. In addition to the fermentative type of metabolism, fermentative yeasts as a rule have a respiratory type of metabolism, whereas nonfermentative yeasts have only a respiratory, or oxidative, metabolism. Both reactions produce energy, with respiration producing by far the most, which is used in part for synthetic reactions, such as assimilation and growth. Part is lost as heat. In addition, small or sometimes large amounts of by-products are formed, including organic acids, esters, aldehydes, glycerol, and higher alcohols. When a fermenting yeast culture is aerated, fermentation is suppressed and respiration increases. This phenomenon is called the Pasteur effect. See also Fermentation.

Yeasts are ubiquitous in nature. They exist on plants and animals; in waters, sediments, and soils; and in terrestrial, aquatic, and marine habitats. Yeasts require oxygen for growth and reproduction; therefore they do not inhabit anaerobic environments such as anoxic sediments. Many species have highly specific habitats, whereas others are found on a variety of substrates in nature.

Unicellular organisms, grouped with the fungi; they have more complex subcellular organization than bacteria. Some types are of major importance in the food industry. Saccharomyces cerevisiae and S. carlsbergensis are used in brewing, wine making, and baking. Varieties such as Candida utilis (formerly Torula utilis) are grown on carbohydrate or hydrocarbon media as animal feed and potential human food, since they contain about 50% protein (dry weight) and are very rich in B vitamins.

Some yeasts are pathogenic (especially Candida spp., which cause thrush); many are used in biotechnology for production of hormones and other proteins.

[YEEST] Yeast is a living, microscopic, single-cell organism that, as it grows, converts its food (through a process known as fermentation) into alcohol and carbon dioxide. This trait is what endears yeast to winemakers, brewmasters and breadbakers. In the making of wine and beer, the yeast's manufacture of alcohol is desired and necessary for the final product; and carbon dioxide is what makes beer and champagne effervescent. The art of breadmaking needs the carbon dioxide produced by yeast in order for certain doughs to rise. To multiply and grow, all yeast needs is the right environment, which includes moisture, food (in the form of sugar or starch) and a warm, nurturing temperature (70° to 85°F is best). Wild yeast spores are constantly floating in the air and landing on uncovered foods and liquids. No one's sure when these wild spores first interacted with foods but it's known that the Egyptians used yeast as a leavening agent more than 5,000 years ago. Wine and other fermented beverages were made for millennia before that. Today, scientists have been able to isolate and identify the various yeasts that are best for winemaking, beermaking and baking. The two types commercially available are baker's yeast and brewer's yeast. Baker's yeast, as the name implies, is used as a leavener. It's catagorized into three basic types-active dry yeast, compressed fresh yeast and yeast starters. Active dry yeast is in the form of tiny, dehydrated granules. The yeast cells are alive but dormant because of the lack of moisture. When mixed with a warm liquid (105° to 115°F), the cells once again become active. Active dry yeast is available in two forms, regular and quick-rising, of which the latter takes about half as long to leaven bread. They may be used interchangeably (with adjustments in rising time) and both are available in 1⁄4-ounce envelopes. Regular active dry yeast may also be purchased in 4-ounce jars or in bulk in some natural food stores. It should be stored in a cool, dry place, but can also be refrigerated or frozen. It should always be at room temperature before being dissolved in liquid. Properly stored, it's reliable when used by the expiration date, which should be stamped on the envelope or jar label. One package of dry yeast is equal to 1 scant tablespoon dry yeast or 1 cake of compressed fresh yeast. Compressed fresh yeast, which comes in tiny (0.06-ounce), square cakes, is moist and extremely perishable. It must be refrigerated and used within a week or two, or by the date indicated on the package. It can be frozen, but should be defrosted at room temperature and used immediately. One cake of fresh yeast can be substituted for one envelope of dry yeast. The use of compressed fresh yeast has been primarily replaced by the more convenient active dry yeast. All baker's yeast should be given a test called proofing to make sure it's still alive. To proof yeast, dissolve it in warm water and add a pinch of sugar. Set the mixture aside in a warm place for 5 to 10 minutes. If it begins to swell and foam, the yeast is alive, active and capable of leavening bread. Brewer's yeasts are special non-leavening yeasts used in beermaking. Because it's a rich source of B vitamins, brewer's yeast is also used as a food supplement. It's available in natural food stores. Brewer's yeasts are also marketed in specialty beermaking equipment shops, with different strains used for different beers.

noun

1. A mass of bubbles in or on the surface of a liquid: foam, froth, head, lather, spume, suds. See solid/liquid/consistency.
2. An agent that stimulates or precipitates a reaction, development, or change: catalyst, ferment, leaven, leavening. See change/persist.

verb

To form or cause to form foam: bubble, cream, effervesce, fizz, foam, froth, lather, spume, suds. See solid/liquid/consistency.

A general term denoting true fungi of the family Saccharomycetaceae. Because of their ability to ferment carbohydrates, some yeasts are important to the brewing and baking industries.

Yeast are single-celled eukaryotic organisms related to fungi. The baker's yeast Saccahromyces cerevisiae and the distantly related Schizosaccharomyces pombe are favored model organisms for genetic research. The interest in yeast research stems from the fact that, as eukaryotic organisms, the sub-cellular organization of yeast is similar to that of cells of more complex organisms. Thus, understanding how a particular gene functions in yeast frequently correlates to how similar genes function in mammals, including humans.

Yeast Genetics

Yeast have many advantages as a genetic research tool. First, yeast are nonpathogenic (they do not cause diseases) and are therefore easy and safe to grow. Yeast can divide by simple fission (mitosis) or by budding and, like bacteria, they can be rapidly grown on solid agar plates or in liquid media. After just a few days in culture, a single yeast cell can produce millions of identical copies of itself, giving scientist a large supply of a genetically pure research tool.

Second, yeast grow as either haploids (having only one set of chromosomes) or diploids (with two chromosome sets). Thus, genetically recessive mutations can be readily identified by phenotypic (visually observable) changes in the haploid strain. In addition, complementation can be performed by simply mating two haploid strains, where one does not contain the mutation. The resulting diploid strain contains both the functional and nonfunctional version of a gene responsible for a phenotype. The addition of the functional gene complements for the defect caused by the nonfunctional gene in the haploid strain. Diploid strains can be induced to undergo meiosis, a process in which the cell divides and passes one-half of its chromosomes to each of the resulting cells. After two such divisions, reproductive structures called asci are produced that contain four haploid offspring, called ascospores. The asci can be dissected and each of the ascospores isolated. In this way, scientists can easily mate different yeast strains and obtain new haploid genotypes through sexual reproduction and meiosis.

Third, the genome of yeast is small, about 3.5 times larger than that of bacteria and 200 times smaller than that of mammals. The yeast genome is arranged in 16 linear chromosomes that range from 200 to 2,200 kilobases in length. Unlike mammals, the yeast genome is very compact, with only 12 million base pairs, very few introns, and very little spacer DNA between functional genes. As a result, in 1996 baker's yeast was the first eukaryotic organism to have its entire genome sequenced.

Genetic Transformation

Finally, one of the most useful properties of yeast for genetic studies is the ease with which DNA can be introduced into them, in a process called transformation. The introduced DNA can be maintained on self-replicating, circular strands of DNA called plasmids, or it can integrate into the yeast genome. Most importantly, integration usually occurs by a process called homologous recombination, whereby the introduced DNA replaces chromosomal DNA that contains the same sequence. This process permits scientists to readily mutate any yeast gene and replace the native gene in the cells with the mutated version. Since yeast can be grown as haploids, the phenotypic changes caused by the introduced gene can be readily identified. In addition, the function of a cloned piece of DNA (e.g., a gene) can be identified by transforming yeast in which the DNA is carried on a circular plasmid. The introduced gene may either functionally replace a defective gene or cause a phenotypic defect in the cells indicating a function for that gene.

The ability to complement yeast defects with cloned pieces of DNA has been extended to mammalian genes. Recognizing that some genes have similar sequences and functions in both mammals and yeast, scientists sometimes use yeast as a tool to identify the functions of mammalian genes. Not many mammalian genes can directly substitute for a yeast gene, however. More frequently, scientists study the yeast gene itself to understand how its protein functions in the cell. The knowledge gained can often lead to an understanding of how similar genes might function in mammals. Now that the yeast genome has been completely sequenced and the results have been deposited in a public databank for all to use, rapid progress is being made in identifying all yeast genes and their functions.

An important method for studying mammalian genes in yeast is called the two-hybrid system. This system is used to determine if two proteins functionally interact with each other. Both genes are cloned into yeast plasmids and transformed into the cells. A special detection system is used that is active only when both cloned proteins physically contact each other in the cell. When that happens, scientist can identify which proteins need to interact with each other in order to function.

Yeast are also being used in the laboratory and commercial production of important nonyeast proteins. Foreign genes are transformed into yeast and, after transcription and translation, the foreign proteins can be isolated. Because of the ease of growing large quantities of cells, yeast can produce a large amount of the protein. While similar protein production can be performed by bacteria, eukaryotic proteins often do not function when made in bacteria. This is because most eukaryotic proteins are normally altered after translation by the addition of short sugar chains, and these modifications are often required for proper function, but bacteria do not carry out these necessary post-translational modifications. Yeast, however, does permit these modifications, and is thus more likely to produce a functional protein.

Bibliography

Sherman, Fred. "Getting Started with Yeast." In Methods in Enzymology, vol. 194, Christine Guthrie and Gerald R. Fink, eds. New York: Academic Press, 1991.

Watson, James D., Michael Gilman, Jan Witowski, and Mark Zoller. Recombinant DNA. New York: Scientific American Books, 1992.

—Suzanne Bradshaw

For more information on yeast, visit Britannica.com.

[YEEST] A living, microscopic, single-cell organism. Wild yeast spores are always floating in the air. Just when these wild spores first interacted with foods and liquids is uncertain, but we do know that Egyptians used yeast as a leavening agent over 5,000 years ago and that wine and other fermented beverages were made for millennia before that. It was in 1857 that France's famous microbiologist Louis Pasteur discovered that fermentation was caused by yeasts. During fermentation, yeast converts food (in the form of sugar or starch) into alcohol and carbon dioxide. In the production of wine, the conversion of yeast to alcohol is necessary for the final product, and carbon dioxide is what makes sparkling wines effervescent. To multiply and grow, all yeast needs is the right environment-moisture, food, and a warm, nurturing temperature. Today, scientists have been able to isolate and identify the specific yeasts that are best for winemaking. Modern winemakers carefully choose the yeasts they use in combination with different varieties of grapes. Various yeasts have specific properties and are better suited for particular winemaking styles. For example, some yeasts produce less foam and are therefore well suited for barrel fermentation. Those styles of yeast that are resistant to cold temperatures are best for making white wines. Other yeasts ferment more rapidly, tolerate alcohol better, or impart flavors to the wine (some desirable, others not). Popular commercially available yeasts used today include Champagne, Epernay, Montrachet, Pasteur Champagne, and Steinberg. Rather than resorting to using cultivated yeasts, some winemakers prefer native yeast fermentation, which relies simply on natural wild yeast spores. See also brettanomyces.

A general term including unicellular, nucleated, usually rounded fungi that reproduce by budding; some are fermenters of carbohydrates, and a few are pathogenic for animals. See also mastitis.

• y. artificial chromosome — cloning vectors developed for the cloning of large (200–500 kbp) DNA fragments; YAC libraries permit the cloning of large genes with their flanking regulatory sequences as well as families of contiguous genes. They are difficult to work with and have the further disadvantage that the cloned sequences are unstably retained.
• brewer's y. — Saccharomyces cerevisiae used in brewing beer, making alcoholic liquors, and baking bread. See also dried yeast (below).
• dried y. — dried cells of any suitable strain of Saccharomyces cerevisiae, usually a by-product of the brewing industry; used as a natural source of protein and B-complex vitamins.
• y. two-hybrid system — an experimental technique for identifying genes whose protein product interacts with another particular protein of interest.

Yeast
Yeast of the species Saccharomyces cerevisiae.
Scientific classification
Domain:	Eukaryota
Kingdom:	Fungi
Typical divisions
Ascomycota (sac fungi) Saccharomycotina (true yeasts) Taphrinomycotina Schizosaccharomycetes (fission yeasts) Basidiomycota (club fungi) Urediniomycetes Sporidiales

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with about 1,500 species currently described;^[1] they dominate fungal diversity in the oceans.^[2] Most reproduce asexually by budding, although a few do so 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 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]

The yeast open 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 one of the most thoroughly researched eukaryotic microorganisms. Researchers have used it to gather information about the biology of the eukaryotic cell and ultimately human biology.^[5] Other species of yeast, such as Candida albicans, are opportunistic pathogens and can cause infections in humans. Yeasts have recently been used to generate electricity in microbial fuel cells,^[6] 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.^[7] The term "yeast" is often taken as a synonym for S. cerevisiae,^[8] but 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.^[9]

Contents 1 History 2 Growth and nutrition 3 Reproduction 4 Uses 4.1 Alcoholic beverages 4.1.1 Beer 4.1.1.1 Brettanomyces 4.1.2 Distilled beverages 4.2 Baking 4.3 Bioremediation 4.4 Industrial ethanol production 4.5 Kombucha 4.6 Nutritional supplements 4.7 Probiotics 4.8 Root beer and sodas 4.9 Aquarium hobby 4.10 Science 4.11 Yeast extract 5 Pathogenic yeasts 6 Food spoilage 7 See also 8 References 9 External links

History

See also: History of wine and History of beer

The word "yeast" comes from Old English gist, gyst, and from the Indo-European root yes-, meaning boil, foam, or bubble.^[10] 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.^[11] In 1680 the Dutch naturalist Antonie van Leeuwenhoek first microscopically observed yeast, but at the time did not consider them to be living organisms but rather globular structures.^[12] 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.^[11][13] Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, but the fermentation inhibited - an observation later called the Pasteur effect.

In the United States, naturally occurring airborne yeasts were used almost exclusively until commercial yeast was marketed at the Centennial Exposition in 1876 in Philadelphia, where Charles L. Fleischmann exhibited the product and a process to use it, as well as serving the resultant baked bread.

Growth and nutrition

Yeasts are chemoorganotrophs as they use organic compounds 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 like ribose,^[14] 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 grow best in a neutral or slightly acidic pH environment.

Yeasts will grow over a temperature range of 10 °C (50 °F) to 37 °C (99 °F), with an optimal temperature range of 30 °C (86 °F) to 37 °C (99 °F), depending on the type of species (S. cerevisiae works best at about 30 °C (86 °F). Above 37 °C (99 °F) yeast cells become stressed and will not divide properly. Most yeast cells die above 50 °C (122 °F). If the solution reaches 105 °C (221 °F) the yeast will disintegrate. There is little activity in the range of 0 °C (32 °F) - 10 °C (50 °F). The cells can survive freezing under certain conditions, with viability decreasing over time.

Yeasts are very common in the environment, but are usually isolated from sugar-rich material. Some good examples include naturally occurring yeasts on the skins of 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.^[15][16] 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, Wallerstein Laboratories Nutrient (WLN) agar, 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. This will change the yeast process.

Reproduction

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

See also: Mating of yeast

Yeasts have asexual and sexual reproductive cycles, however the most common mode of vegetative growth in yeast is asexual reproduction by budding or fission.^[17] 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.^[18]

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.^[19]

Yeast of the species Schizosaccharomyces pombe reproduce by binary fission instead of budding.^[17]

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 production.^[20] Yeasts are also one of the most widely used model organisms for genetics and cell biology.

Alcoholic beverages

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 yeast under anaerobic or low-oxygen conditions. Beverages such as wine, beer, or distilled spirits all use yeast at some stage of their production.

Beer

A mixture of diatomaceous earth and yeast after filtering beer.

Brewer's yeast (also known as brewing yeast) can mean any live yeast used in brewing. It can also mean yeast obtained as a by-product of brewing, dried and killed, and used as a dietary supplement for its B vitamin content.

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, typically 16 °C (61 °F) - 24 °C (75 °F), 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 typically used to produce lager-type beers, though can also produce ale-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low temperatures. An example of bottom fermenting yeast is Saccharomyces pastorianus, formerly known as Saccharomyces carlsbergensis.

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.

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.

Brettanomyces

Brettanomyces is a genus of wild yeast important in brewing lambic, a beer produced not by the deliberate addition of brewer's yeasts but by spontaneous fermentation by wild yeasts and bacteria. Brettanomyces lambicus, B. bruxellensis and B. claussenii are native to the Senne Valley region of Belgium, where lambic beer is produced.

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.

Fresh grapes

Main article: Fermentation (wine)

Yeast is used in winemaking where it converts the sugars present in grape juice or must into alcohol. Yeast is normally already invisibly present on the grapes. The fermentation can be done with this endogenous (or wild) yeast;^[21] 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.^[22] Most added wine yeasts are strains of Saccharomyces cerevisiae, though not all strains of the species are suitable.^[22] 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.^[23] Significant research has been undertaken into the development of novel wine yeast strains that produce atypical flavour profiles or increased complexity in wines.^[24][25]

The growth of some yeasts such as Zygosaccharomyces and Brettanomyces in wine can result in wine faults and subsequent spoilage.^[26] 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.^[27]

Baking

Main article: Baker's yeast

Yeast, most commonly 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 the yeast dies off and the air pockets "sets", 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. Sugar and vinegar are the best conditions for yeast to ferment. In bread making the yeast respires aerobically at first producing carbon dioxide and water. When the oxygen is used up anaerobic respiration is used producing ethanol as a waste product; however, this is evaporated during the baking process.

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.^[28] 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 tastier 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 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 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. 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 and this is referred to as proofing the sponge.

When yeast is used for making bread, it is mixed with flour, salt, and warm water (or milk). The dough is kneaded until it is smooth, and then left to rise, sometimes until it has doubled in size. Some bread doughs are knocked back after one rising and left to rise again. A longer rising time gives a better flavour, but the yeast can fail to raise the bread in the final stages if it is left for too long initially. The dough is then shaped into loaves, left to rise until it is the correct size, and then baked. Dried yeast is usually specified for use in a bread machine, however a (wet) sourdough starter can also work.

Bioremediation

Some yeasts can find potential application in the field of bioremediation. One such yeast Yarrowia lipolytica is known to degrade palm oil mill effluent,^[29] TNT (an explosive material),^[30] and other hydrocarbons such as alkanes, fatty acids, fats and oils.^[31]

Industrial ethanol production

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 cheap cereal grains, and then adding dilute sulfuric acid, or fungal alpha amylase enzymes, to break down the starches into complex sugars. A gluco amylase is then added to break the complex sugars down into simple sugars. After this, yeasts are added to convert the simple sugars to ethanol, which is then distilled off to obtain ethanol up to 96% in concentration.^[32]

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. ^[33][34] 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.^[35]

Kombucha

A Kombucha culture fermenting in a jar

Yeast in symbiosis with acetic acid bacteria is used in the preparation of Kombucha, a fermented sweetened tea. Species of yeast found in the tea can vary, and may include: Brettanomyces bruxellensis, Candida stellata, Schizosaccharomyces pombe, Torulaspora delbrueckii and Zygosaccharomyces bailii.^[36]

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. It can also be used in mashed and fried potatoes, as well as putting it into 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.^[37]

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,^[38][39] prevent reinfection of Clostridium difficile,^[40] reduce bowel movements in diarrhea predominant IBS patients,^[41] and reduce the incidence of antibiotic,^[42] traveler's,^[43] and HIV/AIDS^[44] associated diarrheas.

Root beer and sodas

Root beer and other sweet carbonated beverages can be produced using the same methods as beer, except that fermentation is stopped sooner, producing carbon dioxide, but only trace amounts of alcohol, and a significant amount of sugar is left in the drink.

Aquarium hobby

Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO₂) to fertilize plants in planted aquariums. 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.

There are several recipes for homemade CO₂, but they are variations of the basic recipe: Baking yeast is inserted in a plastic bottle together with sugar, baking soda and water. This produces CO₂ for about 2 or 3 weeks. The CO₂ is injected in the aquarium via a narrow hose and released through a CO₂ diffuser that helps dissolve the gas in the water. The CO₂ is used by plants in the photosynthesis process.

CO₂ injection is very important to plant growth in planted aquariums^[45].

Science

Diagram showing a yeast cell

Several yeasts, particularly Saccharomyces cerevisiae, have been widely used in genetics and cell biology. This is 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 lab 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 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.^[46] 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.^[47] The second yeast species to have its genome sequenced was Schizosaccharomyces pombe, which was completed in 2002.^[48] It was the 6th eukaryotic genome sequenced and consists of 13.8 million base pairs.

Yeast extract

Main article: Yeast extract

Marmite and Vegemite have a distinctive dark colour

Vegemite and Marmite, products made from yeast extract

Yeast extract is the common name for various forms of processed yeast products that are used as food 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. 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, Bovril and Oxo (the United Kingdom, Republic of Ireland and South Africa); and Cenovis (Switzerland).

Pathogenic yeasts

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

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 7–9% of AIDS patients in the USA, and a slightly smaller percentage (3–6%) in western Europe.^[49] 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.^[49] 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.^[50] Candida glabrata is the second most common Candida pathogen after C. albicans, causing infections of the urogenital tract, and of the bloodstream (Candidemia).^[51]

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.^[52] During their growth, yeasts metabolize some food components and produce metabolic end products. This causes the physical, chemical, and sensory properties of a food to change, and the food is spoiled.^[53] 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.^[52] 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,^[54] representing some of the commonly used food preservation methods. Methylene Blue is used to test for the presence of live yeast cells.

See also

Fungi portal

Look up yeast in Wiktionary, the free dictionary.

Wikimedia Commons has media related to: Yeast

• Bioaerosol
• Candidiasis (yeast infection)
• Ethanol fermentation
• Tetrad (genetics)
• Winemaking

References

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External links

• Yeast Development, Different Forms of Yeast...
• Cell cycle and metabolic cycle regulated transcription in yeast
• Yeast Resource Center
• Yeast growth and the cell cycle
• Yeast virtual library
• Ancient Egyptian Bread Making
• Current research on Yeasts at the Norwich Research Park
• How To Build An Inexpensive CO2 Unit For Your Aquarium

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Dansk (Danish)
n. - gær
v. intr. - gære

idioms:

• yeast extract    gærudtræk

Nederlands (Dutch)
gist

Français (French)
n. - levure
v. intr. - écumer ou mousser

idioms:

• yeast extract    extrait de levure

Deutsch (German)
n. - Hefe
v. - gären

idioms:

• yeast extract    Hefeextrakt

Ελληνική (Greek)
n. - μαγιά, προζύμι

idioms:

• yeast extract    μαγιά μπίρας

Italiano (Italian)
lievito

idioms:

• yeast extract    estratto di lievito

Português (Portuguese)
n. - fermento (m)

idioms:

• yeast extract    fermento (m) biológico (Culin.)

Русский (Russian)
дрожжи, палочка сухих дрожжей, дрожжевая клетка, пена, брожение, бродить, покрываться пеной

idioms:

• yeast extract    дрожжевой экстракт

Español (Spanish)
n. - levadura
v. intr. - levar

idioms:

• yeast extract    extracto de levadura

Svenska (Swedish)
n. - jäst, fradga, kraftkälla (bildl.)

中文（简体）(Chinese (Simplified))
酵母, 发酵粉, 发酵, 起泡沫

idioms:

• yeast extract    酵母提取物

中文（繁體）(Chinese (Traditional))
n. - 酵母, 發酵粉
v. intr. - 發酵, 起泡沫

idioms:

• yeast extract    酵母提取物

한국어 (Korean)
n. - 효모, 고체 이스트, 거품, 큰 소동
v. intr. - ~에 이스트를 넣다

日本語 (Japanese)
n. - イースト, 酵母, パン種, 刺激

idioms:

• yeast extract    酵母エキス, 酵母抽出物

العربيه (Arabic)
‏(الاسم) خميرة‏

עברית (Hebrew)
n. - ‮שמרים, מיני פטריות‬
v. intr. - ‮תסס, העלה קצף‬

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