Yeasts are eukaryotic micro-organisms classified in the kingdom fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding. 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. Yeast size can vary greatly depending on the species, typically measuring 3–4 µm in diameter, although some yeasts can reach over 40 µm.

By fermentation, the yeast species ''Saccharomyces cerevisiae'' converts carbohydrates to carbon dioxide and alcohols – for thousands of years the carbon dioxide has been used in baking and the alcohol in alcoholic beverages. 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. 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, and produce ethanol for the biofuel industry.

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

History

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

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

An Indian study of seven bee species and 9 plant species found 45 species from 16 genera colonise the nectaries of flowers and honey stomachs of bees. Most were members of the ''Candida'' genus; the most common species in honey stomachs was ''Dekkera intermedia'' and in flower nectaries, ''Candida blankii''. Yeast colonising nectaries of the stinking hellebore have been found to raise the temperature of the flower, which may aid in attracting pollinators by increasing the evaporation of volatile organic compounds. A black yeast has been recorded as a partner in a complex relationship between ants, their mutualistic fungus, a fungal parasite of the fungus and a bacterium that kills the parasite. The yeast have a negative effect on the bacteria that normally produce antibiotics to kill the parasite and so may affect the ants' health by allowing the parasite to spread.

Reproduction

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

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

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

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. So-called red rice yeast is actually a mold, ''Monascus purpureus''. Yeasts include some 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 mead, wine, beer, or distilled spirits all use yeast at some stage of their production. A distilled beverage is a beverage containing ethanol that has been purified by distillation. Carbohydrate-containing plant material is fermented by yeast, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. Components other than ethanol are collected in the condensate, including water, esters, and other alcohols, which (in addition to that provided by the oak it is aged in) account for the flavour of the beverage.

==== Beer ====

Brewing yeasts may be classed as "top cropping" (or "top-fermenting") and "bottom-cropping" (or "bottom-fermenting"). Top cropping yeasts are so called because they form a foam at the top of the wort during fermentation. An example of a top-cropping yeast is ''Saccharomyces cerevisiae'', sometimes called an "ale yeast". Bottom-cropping yeasts are typically used to produce lager-type beers, though they can also produce ale-type beers. These yeasts ferment well at low temperatures. An example of bottom-cropping yeast is ''Saccharomyces pastorianus'', formerly known as ''S. carlsbergensis''.

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

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

''Dekkera/Brettanomyces'' is a genus of yeast known for their important role in the production of ''Lambic'' and specialty ''sour ales'', along with the secondary conditioning of a particular Belgian Trappist beer. The taxonomy of the genus ''Brettanomyces'' has been debated since its early discovery and has seen many re-classifications over the years. Early classification was based on a few species that reproduced asexually (anamorph form) through multipolar budding. Shortly after, the formation of ascospores was observed and the genus Dekkera, which reproduces sexually (teleomorph form), was introduced as part of the taxonomy. The current taxonomy includes five species within the genera of Dekkera/Brettanomyces. Those are the anamorphs ''Brettanomyces bruxellensis'', ''Brettanomyces anomalus'', ''Brettanomyces custersianus'', ''Brettanomyces naardenensis'', and ''Brettanomyces nanus'', with teleomorphs existing for the first two species, ''Dekkera bruxellensis'' and ''Dekkera anomala''. The distinction between ''Dekkera'' and ''Brettanomyces'' is arguable with Oelofse et al. (2008) citing Loureiro and Malfeito-Ferreira from 2006 when they affirmed that current molecular DNA detection techniques have uncovered no variance between the anamorph and teleomorph states. Over the past decade, ''Brettanomyces'' spp. have seen an increasing use in the craft-brewing sector of the industry with a handful of breweries having produced beers that were primary fermented with pure cultures of ''Brettanomyces'' spp. This has occurred out of experimentation as very little information exists regarding pure culture fermentative capabilities and the aromatic compounds produced by various strains. ''Dekkera''/''Brettanomyces'' spp. have been the subjects of numerous studies conducted over the past century, although a majority of the recent research has focused on enhancing the knowledge of the wine industry. Recent research on 8 ''Brettanomyces'' strains available in the brewing industry focused on strain specific fermentations and identified the major compounds produced during pure culture anaerobic fermentation in wort.

Wine

Yeast is used in winemaking, where it converts the sugars present in grape juice (must) into ethanol. Yeast is normally already present on grape skins. Fermentation can be done with this endogenous "wild yeast," but this procedure gives unpredictable results, which depend upon the exact types of yeast species present. For this reason, a pure yeast culture is usually added to the must; this yeast quickly dominates the fermentation. The wild yeasts are repressed, which ensures a reliable and predictable fermentation.

Most added wine yeasts are strains of ''S. cerevisiae'', though not all strains of the species are suitable. Different ''S. cerevisiae'' yeast strains have differing physiological and fermentative properties, therefore the actual strain of yeast selected can have a direct impact on the finished wine. Significant research has been undertaken into the development of novel wine yeast strains that produce atypical flavour profiles or increased complexity in wines.

The growth of some yeasts, such as ''Zygosaccharomyces'' and ''Brettanomyces'', in wine can result in wine faults and subsequent spoilage. ''Brettanomyces'' produces an array of metabolites when growing in wine, some of which being 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.

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

Baking

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

It is not known when yeast was first used to bake bread. The first records that show this use came from Ancient Egypt. Researchers speculate a mixture of flour meal and water was left longer than usual on a warm day and the yeasts that occur in natural contaminants of the flour caused it to ferment before baking. The resulting bread would have been lighter and tastier than the normal flat, hard cake.

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

When yeast is used for making bread, it is mixed with flour, salt, and warm water or milk. The dough is kneaded until it is smooth, and then left to rise, sometimes until it has doubled in size. Some bread doughs are knocked back after one rising and left to rise again. A longer rising time gives a better flavour, but the yeast can fail to raise the bread in the final stages if it is left for too long initially. The dough is then shaped into loaves, left to rise until it is the correct size, and then baked. Bread machine recipies usually call for dried yeast; 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, TNT (an explosive material), and other hydrocarbons, such as alkanes, fatty acids, fats and oils. It can also tolerate high concentrations of salt and heavy metals, and is being investigated for its potential as a heavy metal biosorbent.

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

''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. Such a development means ethanol can be efficiently produced from more inexpensive feedstocks, making cellulosic ethanol fuel a more competitively priced alternative to gasoline fuels.

Nonalcoholic beverages

Root beer and other sweet carbonated beverages can be produced using the same methods as beer, except the fermentation is stopped sooner, producing carbon dioxide, but only trace amounts of alcohol, and a significant amount of sugar is left in the drink. ''Kvass'', a fermented drink made from rye, is popular in Eastern Europe; it has a recognizable, but low alcoholic content. 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''. ''Kombucha'' is a popular beverage in Eastern Europe and some former Soviet republics under the name ''chajnyj grib'' (Чайный гриб), which means "tea mushroom". ''Kefir'' and ''kumis'' are made by fermenting milk with yeast and bacteria.

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 ''S. 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 by bacteria. Nutritional yeast, though it has a similar appearance to brewer's yeast, is very different and has a very different taste. Brewer's yeast is a good source of B-complex vitamins but, contrary to some claims, it contains little or no vitamin B₁₂.

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

Probiotics

Some probiotic supplements use the yeast ''S. boulardii'' to maintain and restore the natural flora in the gastrointestinal tract. ''S. boulardii'' has been shown to reduce the symptoms of acute diarrhea in children, prevent reinfection of ''Clostridium difficile'', reduce bowel movements in diarrhea-predominant IBS patients, and reduce the incidence of antibiotic, traveler's, and HIV/AIDS associated diarrheas.

Aquarium hobby

Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO₂) to nourish 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: Baker's yeast, with sugar, baking soda, and water, are added to a plastic bottle. A few drops of vegetable oil at the start reduces surface tension and speeds the release of CO₂. This will produce CO₂ for about 2 or 3 weeks; the use of a bubble counter determines production. The CO₂ is injected in the aquarium through a narrow hose and released through a diffuser that helps dissolve the gas in the water. The CO₂ is used by plants in the photosynthesis process.

Science

Several yeasts, in particular ''S. 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 laboratory, which has allowed for the development of powerful standard techniques, such as yeast two-hybrid, synthetic genetic array analysis, and tetrad analysis. Many proteins important in human biology were first discovered by studying their homologues in yeast; these proteins include cell cycle proteins, signaling proteins, and protein-processing enzymes.

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. At the time, it was the most complex organism to have its full genome sequenced, and took seven years and the involvement of more than 100 laboratories to accomplish. The second yeast species to have its genome sequenced was ''Schizosaccharomyces pombe'', which was completed in 2002. It was the sixth eukaryotic genome sequenced and consists of 13.8 million base pairs. As of 2012, over 30 yeast species have had their genomes sequenced and published.

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'', wherein the yeast's digestive enzymes break their own proteins down into simpler compounds, a process of self-destruction. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeast with thick cell walls that would give poor texture) are separated. Yeast autolysates are used in Vegemite and Promite (Australia); Marmite, Bovril and Oxo (the United Kingdom, Republic of Ireland and South Africa); and Cenovis (Switzerland).

Pathogenic yeasts

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

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. During their growth, yeasts metabolize some food components and produce metabolic end products. This causes the physical, chemical, and sensible properties of a food to change, and the food is spoiled. The growth of yeast within food products is often seen on their surface, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semi-liquid products, such as syrups and jams. The yeast of the ''Zygosaccharomyces'' genus have had a long history as a spoilage yeast within the food industry. This is due mainly to the fact that these species can grow in the presence of high sucrose, ethanol, acetic acid, sorbic acid, benzoic acid, and sulphur dioxide concentrations, representing some of the commonly used food preservation methods. Methylene blue is used to test for the presence of live yeast cells.

See also

Mycosis (Fungal infection in animals)

Bioaerosol

Ethanol fermentation

Start point (yeast)

References

Cited texts

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

Category:Brewing ingredients Category:Food additives Category:Leavening agents Category:Medicinal fungi Category:Vegan cuisine * * Category:Fermentation (food)

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