What Is Yeast?

Yeast are a group of unicellular fungi a few species of which are commonly used to leaven bread and ferment alcoholic beverages. Most yeasts belong to the division Ascomycota. A few yeasts, such as Candida albicans can cause infection in humans. More than one-thousand species of yeasts have been described. The most commonly used yeast is Saccharomyces cerevisiae, which was domesticated for wine, bread and beer production thousands of years ago. See Yeast (baking).

Yeast physiology can be either obligately aerobic or facultatively fermentative. There is no known obligately anaerobic yeast. In the absence of oxygen, fermentative yeasts produce their energy by converting sugars into carbon dioxide and ethanol (alcohol). In brewing, the ethanol is used, while in baking the carbon dioxide raises the bread and the ethanol evaporates.

An example with glucose as the substrate is

C6H12O6 (glucose) →2C2H5OH + 2CO2 Yeasts can reproduce asexually through budding or sexually through the formation of ascospores. During asexual reproduction a new bud grows out of the parent yeast when the condition is right, then after the bud reaches an adult size, it separates from the parent yeast. Under low nutrient conditions, yeasts that are capapable of sexual reproduction will form ascospores. Yeasts that are not capable of going through the full sexual cycle are classified in the genus Candida.

Yeasts for leavening bread may be produced commercially or caught from the environment. Many yeasts can be isolated from sugar-rich environmental samples. Some good examples include fruits and berries (such as grapes, apples or peaches), exudates from plants (such as plant saps or cacti). Some yeasts are found in association with insects.

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 yeast growth down. A common medium used for the cultivation of yeasts is called potato dextrose agar (PDA) or potato dextrose broth. Potato extract is made by autoclaving cut-up potatoes with water for 5 to 10 minutes and then decanting off the broth. Dextrose (glucose) is then added (10 g/L), and the medium is sterilized by autoclaving.

Yeast fermentations comprise the oldest and largest application of microbial technology. They are used for beer and wine fermentations and bread production. Beer brewers classify yeasts as top-fermenting and bottom-fermenting. This distinction was introduced by the Dane Emil Christian Hansen.

Top-fermenting yeasts (so-called because they float to the top of the beer) can produce higher alcohol concentrations and prefer higher temperatures. An example is Saccharomyces cerevisiae, known to brewers as ale yeast. They produce fruitier, sweeter, real ale type beers. Bottom-fermenting yeasts ferment more sugars leaving a crisper taste and work well at low temperatures. An example is Saccharomyces uvarum, formerly known as Saccharomyces carlsbergensis. They are used in producing lager-type beers. Brewers of wheat beers often use varieties of Torulaspora delbrueckii.

Winemakers use a variety of different yeasts depending on the type of wine and the condition of the grapes. Too high a sugar or alcohol concentration slows the growth of yeast, so for very ripe grapes with lots of sugar he or she would use a yeast tolerant of those conditions. If the yeast dies before all the fermentable sugar has been converted to alcohol, the result is a "stuck" fermentation. Some yeast is chosen because it tends to develop certain aromas, such as the distinctive "banana" smells of Beaujolais from Georges Duboeuf. Wild yeast are naturally present on the skins of grapes, so grape juice will spontaneously ferment unless the wild yeast are arrested by cold temperature or sulfates. Depending on the strain of indigenous yeast, the result may be unpalatable or possibly more complex than if a single cultured strain were used. In general, natural yeasts are riskier than cultured, and tend to be used by tradition-oriented, Old World-style winemakers.

Saccharomyces cerevisiae is also known as budding or baker's yeast. It is used as a model organism by biologists studying genetics and molecular biology (in particular the cell cycle) because it is easy to culture but as a eukaryote, it shares the complex internal cell structure of plants and animals.

Saccharomyces cerevisiae was the first eukaryotic genome that was completely sequenced. The yeast genome database [1] (http://www.yeastgenome.org/) is highly annotated and remains a very important tool for developing basic knowledge about the function and organization of eukaryotic cell genetics and physiology. Another important S. cerevisiae database is maintained by the Munich Information Center for Protein Sequences [2] (http://mips.gsf.de/genre/proj/yeast/index.jsp).

Another important experimental model is Schizosaccharomyces pombe or fission yeast.

Candida albicans, a diploid sexual fungus (a form of yeast) is the causal agent of opportunistic infections in humans, the most common being oral and vaginal infections. Systemic fungal infections have emerged as important causes of morbidity and mortality in immunocompromised patients (e.g., AIDS, cancer chemotherapy, organ or bone marrow transplantation). In addition, hospital-related infections in patients not previously considered at risk (e.g. patients on an intensive care unit) have become a cause of major health concern.

Among the many organisms that live in the human mouth and digestive tract is the yeast Candida albicans, which under normal circumstances lives in 80 percent of the human population with no harmful effects. However, overgrowth results in candidiasis. Candidiasis is often observed in immunocompromised individuals such as HIV-positive patients. Candidiasis also may occur in the blood and in the genital tract. Candidiasis is commonly known as "thrush", and is a common condition that is usually easily cured in people who are not immunocompromised. To infect host tissue, the usual unicellular yeast-like form of Candida albicans reacts to environmental cues and switches into an invasive, multicellular filamentous form. This switching between two cell-types is known as dimorphism.

In a process that superficially resembles dimorphism, Candida albicans undergoes a process called "phenotypic switching", in which different cellular morphologies are generated spontaneously. One of the classically studied strains that undergoes phenotypic switching is WO-1, which consists of two phases - one that grows as smooth white colonies and one that is rod-like and grows as flat gray colonies. The other strain known to undergo switching is 3153A; this strain produces at least seven different colony morphologies. In both the WO-1 and 3153A strains, the different phases convert spontaneously to the other(s) at a low frequency. The switching is reversible, and colony type can be inherited from one generation to another. While several genes that are expressed differently in different colony morphologies have been identified, some recent efforts have focussed on what might be controlling these changes. Further, whether there is a potential molecular link between dimorphism and phenotypic switching is a tantalizing question.

In the 3153A strain, a gene called SIR2 (for silent information regulator) has been found that seems to be important for phenotypic switching. SIR2 was originally found in Saccharomyces cerevisiae (brewer's yeast), where it is involved in chromosomal silencing - a form of transcriptional regulation in which regions of the genome are reversibly inactivated by changes in chromatin structure (chromatin is the complex of DNA and proteins that make chromosomes). In yeast, genes involved in the control of mating type are found in these silent regions, and SIR2 represses their expression by maintaining a silent-competent chromatin structure in this region. The discovery of a Candida albicans SIR2 that is implicated in phenotypic switching suggests that it too has silent regions controlled by SIR2, in which the phenotype-specific genes may perhaps reside.

Another potential regulatory molecule is Efg1p, a transcription factor found in the WO-1 strain that regulates dimorphism, and more recently has been suggested to help regulate phenotypic switching. Efg1p is expressed only in the white and not in the gray cell-type, and overexpression of Efg1p in the gray form causes a rapid conversion to the white form.

So far there are few data that says that dimorphism and phenotypic switching use common molecular components. However, it is not inconceivable that phenotypic switching may occur in response to some change in the environment as well as being a spontaneous event. How SIR2 itself is regulated in S. cerevisiae may yet provide clues as to the switching mechanisms of Candida albicans.

One of the most interesting features of the Candida albicans genome is the occurrence of numeric and structural chromosomal rearrangements as means of generating genetic diversity, named chromosome length polymorphisms (contraction/expansion of repeats), reciprocal translocations, chromosome deletions and trisomy of individual chromosomes. These karyotypic alterations lead to changes in the phenotype, which is an adaptation strategy of this fungus. These mechanisms will be better understood with the complete analysis of the Candida albicans genome.

The Candida albicans genome is being sequenced at the Stanford DNA Sequencing and Technology Center. Funding for this project is provided by National Institute of Dental and Craniofacial Research and Burroughs-Wellcome Fund. A pilot sequencing program is also being carried on by The Sanger Center. Funding for this project is provided by Beowulf Genomics.

Saccharomyces cerevisiae is a species of budding yeast. It is perhaps the most relevant yeast for mankind, both for its use since ancient times in baking and brewing, and for being one of the most intensively studied eukaryotic model organisms in molecular and cell biology.

Saccharomyces cerevisiae was the first eukaryote to have its genome sequenced (published in 1996). The genome is composed of about 13,000,000 base pairs and 6,275 genes. It is estimated that yeast shares about 23% of its genome with humans.

Schizosaccharomyces pombe, also called "fission yeast," is a species of yeast. It is used as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. These cells maintain their shape by growing exclusively through the cell tips and divide by medial fission to produce two daughter cells of equal sizes.

Fission yeast was isolated in 1893 by Lindner from East African beer. The species name is derived from the Swahili word for beer (Pombe).

The sequence of the S. pombe genome was published in 2002.

The fission yeast researcher Paul Nurse won the 2001 Nobel Prize in Physiology or Medicine, together with Lee Hartwell and Tim Hunt, for work on the cell cycle.

Yeast, any of a number of microscopic, one-celled fungi important for their ability to ferment carbohydrates in various substances. Yeasts in general are widespread in nature, occurring in the soil and on plants. Most cultivated yeasts belong to the genus Saccharomyces; those known as brewer's yeasts are strains of Saccharomyces cerevisiae. For a discussion of the physiology and reproductive behaviour of yeasts, see Fungi: Classification: Ascomycota.

Yeasts have been used since prehistoric times in the making of bread and wine, but their cultivation and use in large quantities was put on a scientific basis by the work of the French microbiologist Louis Pasteur in the 19th century. Today they are used industrially in a wide range of fermentation processes; medicinally, as a source of B-complex vitamins and thiamine and as a stage in the production of various antibiotics and steroid hormones; and as animal feed and foodstuff for humans.

Pure yeast cultures are grown in a medium of sugars, nitrogen sources, minerals, and water. The final product may take the form of dried yeast cells, or the yeast may be pressed into cakes with some starchy material. When a batch of yeast for baking, medicinal, or food purposes is completed, the medium in which the yeast was grown is discarded. In the making of wines, beers, spirits, and industrial alcohol, however, the fermented medium is the desired product, and the yeast itself is discarded or used to make animal feed. See also Brewing.

A baker's yeast is a yeast culture which has been bred specifically for the purpose of leavening breads.

Members of the Division Ascomycota are known as the Sac Fungi and are fungi that produce spores in a distinctive type of microscopic sporangium called an ascus (Greek for a "bag" or "wineskin"). This monophyletic grouping was formerly known as the Ascomycetae and is an extremely significant and successful group of organisms (12,000 species in 1950), accounting for some 75% of all described fungi. Included are most of the fungi that combine with algae to form lichens. The majority of fungi that lack morphological evidence of sexual reproduction are placed here. Better known examples of sac fungi are yeasts, morels, truffles, and Penicillium. The majority of plant-pathogenic fungi belong to this group, or the related Deuteromycota. Species of ascomycetes are also popular in the laboratory. Sordaria fimicola, Neurospora crassa and several species of yeasts are used in many genetics and cell biology experiments.

An ascomycete produces great numbers of asci at any one time, and these may be contained in a structure called an ascocarp. Each ascus contains eight (or a multiple of 8) ascospores, the result of one round of mitosis following meiosis. The resulting haploid nuclei are surrounded by membranes (from the plasma membrane in Euascomycetes; from the nuclear membrane in Hemiascomycetes) and eventually a spore wall.

An exception to the structure described above are the yeasts, which are secondarily unicellular.

An ascospore is a spore contained in an ascus or that was produced inside an ascus. This kind of spore is specific to ascomycetes.

Yeast are small single cell organisms. It is everywhere you can imagine. When making beer the yeast we are referring to is Saccharomyces cerevisiae for ales and Saccharomyces uvarum for lagers. There are many strains of this yeast to obtain different flavor profiles. There are other types of yeast and bacteria that one can use in brewing. Lambic styles call for some different bacteria that wouldn't normally be used in making beer, thus lending the sour taste that lambics are known for. Yeast can be purchased in dry or liquid suspension forms. The liquid yeast gives the brewer more control over the flavor profile. You can also culture your own yeast from starters or obtained from other beers.

Ale yeast, which is top fermenting, tends to make a soft or sometimes fruity beer, while lager yeast, which is bottom fermenting, can make it dry and crisp. Ales ferment at higher temperatures (66 - 74° F) than do lagers (45 - 65° F). The lower temperature prevents the fruity flavors from being absorbed.

You may find some discrepancies in the yeast names. In some places the name Saccharomyces carlsbergensis is used. This is just an older name for Saccharomyces uvarum. In other places, you may not see either the calsbergenesis or uvarum names. Recently the powers that be did some regrouping of the names and put them together under cerevisiae.

As yeast feeds on the sugars in the wort they reproduce. After fermentation starts a layer of foam will be present in the fermenting vessel. This will soon turn into a rocky, dirty looking head that can vary in size. A lot of carbon dioxide is being produced and if the fermenter is not able to expel this it can explode. A blow off tube may be a good idea if you have high gravity wort.

Yeasts are single-celled fungi. As fungi, they are related to the other fungi that people are more familiar with. These include edible mushrooms available at the supermarket, common baker’s yeast used to leaven bread, molds that ripen blue cheese and the molds that produce antibiotics for medical and veterinary use. Many consider edible yeast and fungi to be as natural as fruits and vegetables.

Over 600 different species of yeast are known and they are widely distributed in nature. They are found in association with other microorganisms as part of the normal inhabitants of soil, vegetation, marine and other aqueous environments. Some yeast species are also natural inhabitants of man and animals. While some species are highly specialized and found only in certain habitats at certain times of the year, other species are generalists and can be isolated from many different sources.

Baker’s yeast is used to leaven bread throughout the world and it is the type of yeast that people are most familiar with. Baker’s yeast is produced from the genus and species of yeast called Saccharomyces cerevisiae. The scientific name of the genus of baker’s yeast, Saccharomyces, refers to “saccharo” meaning sugar and “myces” meaning fungus. The species name, cerevisiae, is derived from the name Ceres, the Roman goddess of agriculture. Baker’s yeast products are made from strains of this yeast selected for their special qualities relating to the needs of the baking industry.

The typical yeast cell is approximately equal in size to a human red blood cell and is spherical to ellipsoidal in shape. Because of its small size, it takes about seven billion yeast cells to make up to one gram of compressed baker’s yeast. Yeast reproduce vegetatively by budding, a process during which a new bud grows from the side of the existing cell wall. This bud eventually breaks away from the mother cell to form a separate daughter cell. Each yeast cell, on average, undergoes this budding process 12 to 15 times before it is no longer capable of reproducing. During commercial production, yeast is grown under carefully controlled conditions on a sugar containing media typically composed of beet and cane molasses. Under ideal growth conditions a yeast cell reproduces every two to three hours.

Yeast is the essential ingredient in many bakery products. It is responsible for leavening the dough and imparting a delicious yeast fermentation flavor to the product. It is used in rather small amounts in most bakery products, but having good yeast and using the yeast properly often makes the difference between success and something less than success in a bakery operation.

Candida albicans is a dimorphic fungus that grows at 37oC. Its normal habitat is the mucosal membranes of humans and other warm-blooded animals, where it grows as a yeast (Fig. C) and causes little or no damage. In fact, it can be isolated from the mucosa of up to 50% of humans - from the mouth, the gut, the vagina or, less often, from the surface of the skin.

In some circumstances, however, the same strains of C. albicans that grow as harmless commensals can become pathogenic, invading the mucosa and causing significant damage. This usually happens when a variety of predisposing factors cause the yeast population to multiply, escaping the normal competition from resident bacteria which keep the yeast population in check. Then the yeast cells sprout a hyphal outgrowth (Fig. D) which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues.

One of the best examples of this is the disease termed thrush - a white speckling of the tongue and the back of the throat, resembling the speckling on the bird's chest. This is common in newborn babies, perhaps resulting from passage through an infected birth canal. It is also common in AIDS patients and people who have had a prolonged course of antibacterial therapy, reducing the normal resident bacterial population.

C. albicans also causes vaginitis - inflammation and invasion of the vaginal mucosa, especially during the third trimester of pregnancy and in women who take the pill. The predisposing factors seem to be hormonal, associated with changes in the balance of cell types in the lining epithelium of the vagina. A similar condition termed stomatitis is common in people who wear dentures. Candida can adhere to denture resin, and high sugar levels in the diet can also increase the adhesion by enhancing the production of a mannoprotein adhesive on the yeast cell surface. Systemic candidosis is a more serious condition, when yeast cells proliferate in the circulatory system. This can occur after invasive surgical techniques, including the insertion of intravenous catheters to which the yeast cells adhere, providing a base from which the cells can bud and be disseminated.

All these examples illustrate that C. albicans is a classic opportunistic pathogen, normally kept in check but capable of flaring up in specific, predisposing conditions. It can be identified quite readily from clinical specimens by its ability to sprout hyphae when yeast cells at 37oC are transferred to tubes of horse serum and incubated for 3-5 hours (Fig. D). Only C. albicans and a few related pathogenic Candida species do this. But the fungus has a strong tendency to revert to the yeast phase after only a short period of hyphal growth. Figures E and F show this for horse serum incubated for 24 hours - the hyphae themselves have a beaded appearance, and they give rise to budding yeast cells at the sites where the hyphae of other fungi would form branches.

Cryptococcus albidus is another budding yeast, shown here by phase-contrast microscopy but also with negative staining (the cells are suspended in India ink). Various stages of bud development are seen. The cells are surrounded by a rigid polysaccharide capsule, typical of the genus Cryptococcus, and seen as distinct haloes where the India ink particles have been excluded.

Cryptococcus species are common on leaf surfaces. But the most important species from the human standpoint is C. neoformans, 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. The capsule is a significant virulence determinant of C. neoformans because it helps to prevent the cells from being recognised and engulfed by white blood cells. Additionally, C. neoformans is unique among Cryptococcus species in producing a phenoloxidase. This enzyme acts on phenolic compounds to produce melanin, which might help to protect the cells against the antimicrobial effects of oxidants in host tissues.

C. neoformans grows commonly on old "weathered" bird droppings in cities, but does not compete well with bacteria in wet droppings. It infects through the lungs, where it causes a mild or chronic, persistent pneumonia, depending on the person's degree of immunity. Random testing of people for skin reactions to C. neoformans antigens in Britain, Australia and the USA indicates that many people have unknowingly been exposed to the fungus with no serious effect. However, in a small proportion of the population the fungus can disseminate "silently" in the central nervous system, causing fatality.

For many years it was assumed that yeast cells inhaled in dried, powdered bird droppings were the source of lung infection. But a sexual stage of the fungus has now been discovered in laboratory culture; it is typical of the fungal group Basidiomycota (which includes the mushroom fungi) but is microscopic, and it leads to the release of small (about 3 micrometre) airborne basidiospores. These are the ideal size for deposition in the lungs (see Airborne Microbes). They are thought to be the main means of infection, but their environmental source is unknown - perhaps a yeast stage growing on vegetation.

Saccharomyces cerevisiae is the budding yeast used for bread-making, where the carbon dioxide produced by growth in the dough causes the bread to rise. Essentially similar yeasts, but now given different species names, are used for production of beers, wines and other alcoholic drinks. This phase-contrast micrograph shows cells in various stages of budding. The buds are small at first, but enlarge progressively and eventually separate from the mother cell by formation of a septum (cross wall).

Few of the cellular organelles can be seen by light microscopy, unless they are stained specifically. The only conspicuous organelle seen in Fig. A is the large central vacuole which contributes to cell expansion. S. cerevisiae is a member of the fungal group Ascomycota (the ascus-forming fungi).

Yeasts are fungi that grow as single cells, producing daughter cells either by budding (the budding yeasts) or by binary fission (the fission yeasts). They differ from most fungi, which grow as thread-like hyphae. But this distinction is not a fundamental one, because some fungi can alternate between a yeast phase and a hyphal phase, depending on environmental conditions. Such fungi are termed dimorphic (with two shapes) and they include several that cause disease of humans.

Yeasts grow typically in moist environments where there is a plentiful supply of simple, soluble nutrients such as sugars and amino acids. For this reason they are common on leaf and fruit surfaces, on roots and in various types of food. With few exceptions, they are unable to degrade polymers, such as starch and cellulose which are used by many hyphal fungi.

Yeast are unicellular fungi. The precise classification is a field that uses the characteristics of the cell, ascospore and colony. Physiological characteristics are also used to identify species. One of the more well known characteristics is the ability to ferment sugars for the production of ethanol. Budding yeasts are true fungi of the phylum Ascomycetes, class Hemiascomycetes. The true yeasts are separated into one main order Saccharomycetales.

Yeasts are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, soil and salt water. Yeasts 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" is typically Candidiasis is caused by the yeast-like fungus 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.

Yeasts multiply as single cells that divide by budding (eg Saccharomyces) or direct division (fission, eg. Schizosaccharomyces), or they may grow as simple irregular filaments (mycelium). In sexual reproduction most yeasts 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 yeasts, fuse with other ascospores.

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.

The most well-known and commercially significant yeasts are the related species and strains of Saccharomyces cerevisiae. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to expand, or raise, dough. Saccharomyces cerevisiae is commonly used as baker's yeast and for some types of fermentation. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins, niacin, and folic acid.

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 sanfrancisco 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.

The fermentation of wine is initiated by naturally occurring yeasts present in the vineyards. Many wineries still use nature 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, molds, and yeasts.

The yeastlike fungus, Candida albicans, is commonly found in the mouth, vagina, and intestinal tract. Candida is a normal inhabitant of humans and normally causes no ill effects. However, among infants and individuals with other illness a variety of conditions can occur. Candidiasis of the mucous membranes of the mouth is known as thrush. Candidiasis of the vagina is called vaginitis. Candida also causes severe disease in persons with AIDS and chemotherapy patients.

Budding yeasts are true fungi of the phylum Ascomycetes, class Hemiascomycetes. The true yeasts are separated one main order Saccharomycetales, which includes at least ten families. Yeasts are heterotrophic, lack chlorophyll, and are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, yeasts are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. k, g, d, b, l. In women, who are pregnant or taking antibiotics, an infection of the vagina and vulva caused by a yeastlike fungus Candida albicans, is common. Yeasts are also found in soil and saltwater, where they contribute to the decomposition of plant and algal matter.

Are yeasts Fungi?

Yes, the 700 known yeasts really are fungi, although their colonies often look more like those of bacteria.

Most fungi explore their surroundings by producing miles of fine, branching filaments called hyphae, but most yeasts have become more or less unicellular, with rounded cells. This is often an adaptation to living in a liquid medium of high osmotic pressure. This usually means media with a high sugar content, such as is found in the nectaries of flowers or on the surface of fruits, where if they present the least possible surface area (as close to spherical as possible), it makes it easier for them to control the movement of dissolved substances in and out of their cells.

The cells of most yeasts can be regarded as asexual propagules, and they produce more of the same by a variety of methods similar to those found in moulds.

Some yeasts are related to ascomycetes, others to basidiomycetes, and even zygomycetes sometimes take on a yeast-like appearance.

Some yeasts make hyphae as well as unicells, and some are even exclusively hyphal, being recognizable as yeasts only by biochemical characters.

Yeasts are, of course, among the most important fungi, because they raise bread, ferment sugars to make beer, wine, and spirits, and represent a concentrated food and a source of B vitamins.

A few yeasts cause serious diseases of people.

Yeast name applied specifically to a certain group of microscopic fungi and to commercial products consisting of masses of dried yeast cells or of yeast mixed with a starchy material and pressed into yeast cakes. Although a number of fungi are sometimes called yeasts, the true yeasts are unicellular, consist of oval or round cells, and reproduce chiefly by budding. Under certain conditions some yeast cells secrete a thickened wall, and the cytoplasm of the single cell within divides to form four or eight cells, or spores, known as ascospores, which emerge when the wall ruptures. In a few species two cells fuse before undergoing spore formation. There are about 500 species in all. 1 Yeasts, especially those of the genus Saccharomyces, have long been of commercial importance because they are the chief agents in alcoholic fermentation. Because of this they are essential to the making of beer, wine, and other alcoholic beverages and industrial alcohol. Wild yeasts, those found in nature and probably carried by insects from the soil to fruits, are frequently active in the fermentation process. In breadmaking the yeasts act upon the carbohydrates in the dough, forming carbon dioxide and ethyl alcohol, which are driven off in the baking process; the escaping carbon dioxide causes the bread to rise. Since early times yeast has been used in treating various ailments; brewer’s yeast has a high content of thiamine and other vitamins of the B-complex group. Yeasts are classified in the kingdom Fungi, phyla (divisions) Ascomycota and Basidiomycota.

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 warmliquid (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 health-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 health-food stores. Brewer's yeasts are also marketed in specialty beermaking equipment shops, with different strains used for different beers.

Prior to the evolution of commercially available baking powders and yeasts during the 19th century, yeast starters were the LEAVENERS used in breadmaking. Such starters are a simple mixture of flour, water, sugar and YEAST. (At one time, airborne yeast was the only source used, but today convenient commercially packaged baker's yeast is more common.) This batter is set aside in a warm place until the yeast ferments and the mixture is foamy. A portion of the starter-usually about 2 cups-is removed and used as the base and leavener for some bread recipes. Once fermented, yeast starters-the most famous of which is sourdough starter-can be kept going in the right environment for years simply by adding equal parts flour and water. Herman starter is a colloquialism (of unknown origin) for a honey- or sugar-sweetened starter used primarily for sweet breads. Starter should be refrigerated and can be stored this way indefinitely as long as it's replenished every 2 weeks. Before using or replenishing, it should be brought to room temperature. If a starter turns orange or pink and develops an unpleasantly acrid odor, undesirable bacteria have invaded it and the mixture must be discarded. Two cups of the foamy starter mixture can be substituted for each package of yeast called for in a recipe.

yeast, name applied specifically to a certain group of microscopic fungi and to commercial products consisting of masses of dried yeast cells or of yeast mixed with a starchy material and pressed into yeast cakes. Although a number of fungi are sometimes called yeasts, the true yeasts are unicellular, consist of oval or round cells, and reproduce chiefly by budding. Under certain conditions some yeast cells secrete a thickened wall, and the cytoplasm of the single cell within divides to form four or eight cells, or spores, known as ascospores, which emerge when the wall ruptures. In a few species two cells fuse before undergoing spore formation. There are about 500 species in all.

Yeasts multiply as single cells that divide by budding or direct division (fission), or they may grow as simple irregular filaments (mycelium). In sexual reproduction most yeasts 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 yeasts, fuse with other ascospores. 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. The most well-known and commercially significant yeasts are the related species and strains of Saccharomyces cerevisiae. k, b, b, c, k. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to expand, or raise, dough. Saccharomyces cerevisiae is commonly used as baker's yeast and for some types of fermentation. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins, niacin, and folic acid.

Yeasts, especially those of the genus Saccharomyces, have long been of commercial importance because they are the chief agents in alcoholic fermentation. Because of this they are essential to the making of beer, wine, and other alcoholic beverages and industrial alcohol. Wild yeasts, those found in nature and probably carried by insects from the soil to fruits, are frequently active in the fermentation process. In breadmaking the yeasts act upon the carbohydrates in the dough, forming carbon dioxide and ethyl alcohol, which are driven off in the baking process; the escaping carbon dioxide causes the bread to rise. Since early times yeast has been used in treating various ailments; brewer's yeast has a high content of thiamine and other vitamins of the B-complex group. Yeasts are classified in the kingdom Fungi, phyla (divisions) Ascomycota and Basidiomycota.

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 degrees to 85 degrees 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 degrees to 115 degrees), 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 health-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 (0.6-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 health-food stores. Brewer's yeasts are also marketed in specialty beermaking equipment shops, with different strains used for different beers.

Budding yeasts are true fungi of the phylum Ascomycetes, class Hemiascomycetes. The true yeasts comprise the family Saccharomycetes, which has but one genus Saccharomyces, but includes at least ten species. The classification of yeasts is a specialized field using cell, ascospore, and colony characteristics for distinguishing genera, and physiological characteristics - particularly the ability to ferment individual sugars - to identify species. Yeasts are heterotrophic, lack chlorophyll, and are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, yeasts are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. In women, who are pregnant or taking antibiotics, an infection of the vagina and vulva caused by a yeast like fungus Candida albicans, is common. Yeasts are also found in soil and saltwater, where they contribute to the decomposition of plant and algal matter. Yeasts multiply as single cells that divide by budding or direct division, or they may grow as simple irregular filaments. In sexual reproduction most yeasts 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 yeasts, fuse with other ascospores. The most well-known and commercially significant yeasts are the related species and strains of Saccharomyces cerevisiae. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to exp