Yeasts are eukaryotic, single-celled microorganisms classified as members of the funguskingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitute 1% of all described fungal species.
Some yeast species have the ability to develop multicellular
characteristics by forming strings of connected budding cells known as pseudohyphae or false hyphae, or quickly evolve into a multicellular cluster with specialised cell organelles function. Yeast sizes vary greatly, depending on species and environment, typically measuring 3–4 µm in diameter, although some yeasts can grow to 40 µm in size. Most yeasts reproduce asexually by mitosis, and many do so by the asymmetric division process known as budding. With their single-celled growth habit, yeasts can be contrasted with molds, which grow hyphae. Fungal species that can take both forms (depending on temperature or other conditions) are called dimorphic fungi.
The yeast species Saccharomyces cerevisiae converts carbohydrates to carbon dioxide and alcohols through the process of fermentation. The products of this reaction have been used in baking and the production of alcoholic beverages for thousands of years. S. cerevisiae is also an important model organism in modern cell biology
research, and is one of the most thoroughly studied eukaryotic
microorganisms. Researchers have cultured it in order to understand the
biology of the eukaryotic cell and ultimately human biology in great
detail. Other species of yeasts, 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 to 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 or "true yeasts" are classified in the orderSaccharomycetales, within the phylum Ascomycota.
The word "yeast" comes from Old Englishgist, gyst, and from the Indo-European root yes-, meaning "boil", "foam", or "bubble".
Yeast microbes are probably one of the earliest domesticated organisms.
Archaeologists digging in Egyptian ruins found early grinding stones
and baking chambers for yeast-raised bread, as well as drawings of
4,000-year-old bakeries and breweries.
Vessels studied from several archaeological sites in Israel (dating to
around 5,000, 3,000 and 2,500 years ago), which were believed to have
contained alcoholic beverages (beer and mead), were found to contain
yeast colonies that had survived over the millennia, providing the first
direct biological evidence of yeast use in early cultures. In 1680, 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 as researchers were doubtful whether yeasts were algae or fungi. Theodor Schwann recognized them as fungi in 1837.
In 1857, French microbiologist Louis 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". In the paper "Mémoire sur la fermentation alcoolique," Pasteur proved that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst.
By the late 18th century two yeast strains used in brewing had been identified: Saccharomyces cerevisiae (top-fermenting yeast) and S. pastorianus (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.
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.
The mechanical refrigerator (first patented in the 1850s in Europe) liberated brewers and winemakers from seasonal constraints for the first time and allowed them to exit cellars and other earthen environments. For John Molson, who made his livelihood in Montreal
prior to the development of the fridge, the brewing season lasted from
September through to May. The same seasonal restrictions formerly
governed the distiller's art.
Yeasts vary in regard to the temperature range in which they grow best. For example, Leucosporidium frigidum grows at −2 to 20 °C (28 to 68 °F), Saccharomyces telluris at 5 to 35 °C (41 to 95 °F), and Candida slooffi at 28 to 45 °C (82 to 113 °F). The cells can survive freezing under certain conditions, with viability decreasing over time.
In general, yeasts are grown in the laboratory on solid growth media or in liquid broths. Common media used for the cultivation of yeasts include potato dextrose agar or potato dextrose broth, Wallerstein Laboratories nutrient agar, yeast peptonedextrose agar, and yeast mould agar or broth. Home brewers who cultivate yeast frequently use dried malt extract and agar as a solid growth medium. The fungicidecycloheximide 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.
The appearance of a white, thready yeast, commonly known as kahm
yeast, is often a byproduct of the lactofermentation (or pickling) of
certain vegetables. It is usually the result of exposure to air.
Although harmless, it can give pickled vegetables a bad flavor and must
be removed regularly during fermentation.
Ecology
Yeasts are very common in the environment, and are often isolated
from sugar-rich materials. 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. Yeasts from the soil and from the skins of fruits and berries have been shown to dominate fungal succession during fruit decay. The ecological function and biodiversity of yeasts are relatively unknown compared to those of other microorganisms. Yeasts, including Candida albicans, Rhodotorula rubra, Torulopsis and Trichosporon cutaneum, have been found living in between people's toes as part of their skin flora. Yeasts are also present in the gut flora of mammals and some insects and even deep-sea environments host an array of yeasts.
An Indian study of seven bee species and nine plant species found 45 species from 16 genera colonize the nectaries of flowers and honey stomachs of bees. Most were members of the genus Candida; 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 has a
negative effect on the bacteria that normally produce antibiotics to
kill the parasite, so may affect the ants' health by allowing the
parasite to spread.
Certain strains of some species of yeasts produce proteins called
yeast killer toxins that allow them to eliminate competing strains.
(See main article on killer yeast.)
This can cause problems for winemaking but could potentially also be
used to advantage by using killer toxin-producing strains to make the
wine. Yeast killer toxins may also have medical applications in treating
yeast infections (see "Pathogenic yeasts" section below).
Marine yeasts, defined as the yeasts that are isolated from
marine environments, are able to grow better on a medium prepared using
seawater rather than freshwater. The first marine yeasts were isolated by Bernhard Fischer in 1894 from the Atlantic Ocean, and those were identified as Torula sp. and Mycoderma sp.
Following this discovery, various other marine yeasts have been
isolated from around the world from different sources, including
seawater, seaweeds, marine fish and mammals.
Among these isolates, some marine yeasts originated from terrestrial
habitats (grouped as facultative marine yeast), which were brought to
and survived in marine environments. The other marine yeasts were
grouped as obligate or indigenous marine yeasts, which are confined to
marine habitats. However, no sufficient evidence has been found to explain the indispensability of seawater for obligate marine yeasts.
It has been reported that marine yeasts are able to produce many
bioactive substances, such as amino acids, glucans, glutathione, toxins,
enzymes, phytase, and vitamins with potential applications in the food,
pharmaceutical, cosmetic, and chemical industries as well as for marine
culture and environmental protection. Marine yeast was successfully used to produce bioethanol using seawater-based media which will potentially reduce the water footprint of bioethanol.
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, where 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 then 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, and thereby creating two identically sized daughter cells.
In general, under high-stress conditions such as 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.
The haploid fission yeast Schizosaccharomyces pombe is a facultative sexual microorganism that can undergo mating when nutrients are limited. Exposure of S. pombe
to hydrogen peroxide, an agent that causes oxidative stress leading to
oxidative DNA damage, strongly induces mating and the formation of
meiotic spores. The budding yeast Saccharomyces cerevisiae
reproduces by mitosis as diploid cells when nutrients are abundant, but
when starved, this yeast undergoes meiosis to form haploid spores. Haploid cells may then reproduce asexually by mitosis. Katz Ezov et al. presented evidence that in natural S. cerevisiae
populations clonal reproduction and selfing (in the form of intratetrad
mating) predominate. In nature, the mating of haploid cells to form
diploid cells is most often between members of the same clonal
population and out-crossing is uncommon. Analysis of the ancestry of natural S. cerevisiae strains led to the conclusion that out-crossing occurs only about once every 50,000 cell divisions.
These observations suggest that the possible long-term benefits of
outcrossing (e.g. generation of diversity) are likely to be insufficient
for generally maintaining sex from one generation to the next. Rather, a short-term benefit, such as recombinational repair during meiosis, may be the key to the maintenance of sex in S. cerevisiae.
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 yeasts 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 in which it may be aged) account for the flavour of the beverage.
Yeast ring used by Swedish farmhouse brewers in the 19th century to preserve yeast between brewing sessions.Bubbles of carbon dioxide forming during beer-brewing
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. cerevisiae, noting that the only distinct difference between the two is metabolic. Lager strains of S. cerevisiae secrete an enzyme called melibiase, allowing them to hydrolyse melibiose, a disaccharide, into more fermentable monosaccharides.
Top- and bottom-cropping and cold- and warm-fermenting distinctions are
largely generalizations used by 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), a feature exploited in food products made from leftover (by-product) yeast from brewing.
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 more
slowly 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 its 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
reclassifications 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 primarily
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 eight 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.
Yeast is used in winemaking, where it converts the sugars present (glucose and fructose) 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 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.
Researchers from the 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% of people are sensitive to biogenic amines, such as histamines.
Yeast, most commonly S. cerevisiae, is used in baking as a leavening agent, converting the fermentable sugars present in dough into 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 yeast. 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 breadmaking, the yeast initially respires aerobically,
producing carbon dioxide and water. When the oxygen is depleted, fermentation begins, producing ethanol as a waste product; however, this evaporates during baking.
A block of compressed 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.
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.
Active dried yeast, a granulated form in which yeast is commercially sold
Today, there are several retailers of baker's yeast; one of the earlier developments in North America is Fleischmann's Yeast, 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 rose twice as fast. Baker's yeast is also sold as
a fresh yeast compressed into a square "cake". This form perishes
quickly, so must 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 a sourdough starter is used, 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. The dough is then shaped into loaves. Some bread doughs
are knocked back after one rising and left to rise again (this is
called dough proofing)
and then baked. A longer rising time gives a better flavor, but the
yeast can fail to raise the bread in the final stages if it is left for
too long initially.
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. Saccharomyces cerevisiae has potential to bioremediate toxic pollutants like arsenic from industrial effluent. Bronze statues are known to be degraded by certain species of yeast. Different yeasts from Brazilian gold mines bioaccumulate free and complexed silver ions.
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 purity.
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.[83][84] 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.
A number of sweet carbonated beverages
can be produced by the same methods as beer, except the fermentation is
stopped sooner, producing carbon dioxide, but only trace amounts of
alcohol, leaving a significant amount of residual sugar in the drink.
Yeast is used as an ingredient in foods for its umami flavor, in much of the same way that monosodium glutamate (MSG) is used and, like MSG, often contain free glutamic acid. Examples include:
Yeast extract, made from the intracellular contents of yeast and used as food additives or flavours. The general method for making yeast extract for food products such as Vegemite and Marmite on a commercial scale is heat autolysis,
i.e. 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
removed. Yeast autolysates are used in Vegemite and Promite (Australia); Marmite (the United Kingdom); the unrelated Marmite (New Zealand); Vitam-R (Germany); and Cenovis (Switzerland).
Nutritional yeast flakes are yellow in colourNutritional yeast, which is whole dried, deactivated yeast cells, usually S. cerevisiae. Usually in the form of yellow flake or powder, its nutty and umami flavor makes it a vegan substitute for cheese powder. 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. In Australia, it is sometimes sold as "savoury yeast flakes".
Both types of yeast foods above are rich in B-complex vitamins (besides vitamin B12 unless fortified), making them an attractive nutritional supplement to vegans. The same vitamins are also found in some yeast-fermented products mentioned above, such as kvass. Nutritional yeast in particular is naturally low in fat and sodium and a source of protein and vitamins as well as other minerals and cofactors required for growth. Many brands of nutritional yeast and yeast extract spreads, though not all, are fortified with vitamin B12, which is produced separately by bacteria.
In 1920, the Fleischmann Yeast Company
began to promote yeast cakes in a "Yeast for Health" campaign. They
initially emphasized yeast as a source of vitamins, good for skin and
digestion. Their later advertising claimed a much broader range of
health benefits, and was censured as misleading by the Federal Trade Commission. The fad for yeast cakes lasted until the late 1930s.
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, reduce the chance of infection by Clostridium difficile (often identified simply as C. difficile or C. diff), 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 aquaria. CO2 levels from yeast are more difficult to regulate than those from pressurized CO2 systems. However, the low cost of yeast makes it a widely used alternative.
Scientific research
Diagram showing a yeast cell
Several yeasts, in particular S. cerevisiae and S. pombe, have been widely used in genetics and cell biology, largely because they are simple eukaryotic cells, 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 the work of 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 2014, over 50 yeast species have had
their genomes sequenced and published.
Genomic and functional gene annotation of the two major yeast models can be accessed via their respective model organism databases: SGDand PomBase.
Genetically engineered biofactories
Various yeast species have been genetically engineered to efficiently produce various drugs, a technique called metabolic engineering. S. cerevisiae
is easy to genetically engineer; its physiology, metabolism and
genetics are well known, and it is amenable for use in harsh industrial
conditions. A wide variety of chemical in different classes can be
produced by engineered yeast, including phenolics, isoprenoids, alkaloids, and polyketides. About 20% of biopharmaceuticals are produced in S. cerevisiae, including insulin, vaccines for hepatitis, and human serum albumin.
Gram stain of Candida albicans from a vaginal swab. The small oval chlamydospores are 2–4 µm in diameter.A photomicrograph of Candida albicans showing hyphal outgrowth and other morphological characteristics
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
surfaces, as in cheeses or meats, or by the fermentation of sugars in
beverages, such as juices, and semiliquid products, such as syrups and jams. The yeast of the genus Zygosaccharomyces have had a long history as spoilage yeasts within the food industry. This is mainly because these species can grow in the presence of high sucrose, ethanol, acetic acid, sorbic acid, benzoic acid, and sulfur dioxide concentrations, representing some of the commonly used food preservation methods. Methylene blue is used to test for the presence of live yeast cells. In oenology, the major spoilage yeast is Brettanomyces bruxellensis.
An Indian study of seven bee species and nine plant species found 45 yeast species from 16 genera colonise the nectaries of flowers and honey stomachs of bees. Most were members of the genus Candida; the most common species in honey bee stomachs was Dekkera intermedia, while the most common species colonising flower nectaries was Candida blankii. Although the mechanics are not fully understood, it was found that A. indica flowers more if Candida blankii is present.
In another example, Spathaspora passalidarum, found in the digestive tract of bess beetles, aids the digestion of plant cells by fermenting xylose.
The pharyngeal arches, also known as visceral arches, are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill arches.
In the human embryo, the arches are first seen during the fourth week of development. They appear as a series of outpouchings of mesoderm on both sides of the developing pharynx. The vasculature of the pharyngeal arches is known as the aortic arches.
In vertebrates, the pharyngeal arches are derived from all three germ layers (the primary layers of cells that form during embryogenesis). Neural crest cells enter these arches where they contribute to features of the skull and facial skeleton such as bone and cartilage.
However, the existence of pharyngeal structures before neural crest
cells evolved is indicated by the existence of neural crest-independent
mechanisms of pharyngeal arch development. The first, most anterior pharyngeal arch gives rise to the oral jaw. The second arch becomes the hyoid and jaw support.
In fish, the other posterior arches contribute to the branchial
skeleton, which support the gills; in tetrapods the anterior arches
develop into components of the ear, tonsils, and thymus. The genetic and developmental basis of pharyngeal arch development is well characterized. It has been shown that Hox genes and other developmental genes such as DLX are important for patterning the anterior/posterior and dorsal/ventral axes of the branchial arches. Some fish species have a second set of jaws in their throat, known as pharyngeal jaws, which develop using the same genetic pathways involved in oral jaw formation.
During human, and all vertebratedevelopment, a series of pharyngeal arch pairs form in the developing embryo.
These project forward from the back of the embryo toward the front of
the face and neck. Each arch develops its own artery, nerve that
controls a distinct muscle group, and skeletal tissue. The arches are
numbered from 1 to 6, with 1 being the arch closest to the head of the
embryo, and arch 5 existing only transiently.
These grow and join in the ventral midline. The first arch, as the first to form, separates the mouth pit or stomodeum from the pericardium. By differential growth the neck elongates and new arches form, so the pharynx has six arches ultimately.
Each pharyngeal arch has a cartilaginous stick, a muscle component that differentiates from the cartilaginous tissue, an artery, and a cranial nerve. Each of these is surrounded by mesenchyme. Arches do not develop simultaneously but instead possess a "staggered" development.
Pharyngeal pouches form on the endodermal side between the arches, and pharyngeal grooves (or clefts) form from the lateral ectodermal surface of the neck region to separate the arches. In fish the pouches line up with the clefts, and these thin segments become gills. In mammals the endoderm and ectoderm not only remain intact but also continue to be separated by a mesoderm layer.
The development of the pharyngeal arches provides a useful
landmark with which to establish the precise stage of embryonic
development. Their formation and development corresponds to Carnegie stages 10 to 16 in mammals, and Hamburger–Hamilton stages 14 to 28 in the chicken. Although there are six pharyngeal arches, in humans the fifth arch exists only transiently during embryogenesis.
First arch
The first pharyngeal arch also mandibular arch (corresponding to the first branchial arch or gill arch of fish), is the first of six pharyngeal arches that develops during the fourth week of development. It is located between the stomodeum and the first pharyngeal groove.
Processes
This arch divides into a maxillary process and a mandibular process, giving rise to structures including the bones of the lower two-thirds of the face and the jaw. The maxillary process becomes the maxilla (or upper jaw, although there are large differences among animals), and palate while the mandibular process becomes the mandible or lower jaw. This arch also gives rise to the muscles of mastication.
Meckel's cartilage
Meckel's cartilage forms in the mesoderm of the mandibular process and eventually regresses to form the incus and malleus of the middle ear, the anterior ligament of the malleus and the sphenomandibular ligament. The mandible or lower jaw forms by perichondral ossification using Meckel's cartilage as a 'template', but the maxillary does not arise from direct ossification of Meckel's cartilage.
Derivatives
The skeletal elements and muscles are derived from mesoderm of the pharyngeal arches.
The mandibular and maxillary branches of the trigeminal nerve (CN V)
innervate the structures derived from the corresponding processes of
the first arch. In some lower animals, each arch is supplied by two
cranial nerves. The nerve of the arch itself runs along the cranial side
of the arch and is called post-trematic nerve of the arch. Each arch
also receives a branch from the nerve of the succeeding arch called the
pre-trematic nerve which runs along the caudal border of the arch. In
human embryo, a double innervation is seen only in the first pharyngeal
arch. The mandibular nerve is the post-trematic nerve of the first arch
and chorda tympani
(branch of facial nerve) is the pre-trematic nerve. This double
innervation is reflected in the nerve supply of anterior two-thirds of tongue which is derived from the first arch.
The second pharyngeal arch or hyoid arch, is the second of fifth pharyngeal arches that develops in fetal life during the fourth week of development and assists in forming the side and front of the neck.
Reichert's cartilage
Cartilage
in the second pharyngeal arch is referred to as Reichert's cartilage
and contributes to many structures in the fully developed adult. In contrast to the Meckel's cartilage of the first pharyngeal arch
it does not constitute a continuous element, and instead is composed of
two distinct cartilaginous segments joined by a faint layer of mesenchyme. Dorsal ends of Reichert's cartilage ossify during development to form the stapes of the middle ear
before being incorporated into the middle ear cavity, while the ventral
portion ossifies to form the lesser cornu and upper part of the body of
the hyoid bone. Caudal to what will eventually become the stapes, Reichert's cartilage also forms the styloid process of the temporal bone. The cartilage between the hyoid bone and styloid process will not remain as development continues, but its perichondrium will eventually form the stylohyoid ligament.
The artery of the second arch is the second aortic arch, which gives origin to the stapedial artery in some mammals but atrophies in most humans.
Muscles derived from the pharyngeal arches
Pharyngeal muscles or Branchial muscles are striated muscles of the head and neck. Unlike skeletal muscles that developmentally come from somites, pharyngeal muscles are developmentally formed from the pharyngeal arches.
Most of the skeletal musculature supplied by the cranial nerves (special visceral efferent) is pharyngeal. Exceptions include, but are not limited to, the extraocular muscles and some of the muscles of the tongue. These exceptions receive general somatic efferent innervation.
All of the pharyngeal muscles of the second pharyngeal arch are innervated by the facial nerve. These muscles include the muscles of facial expression, the posterior belly of the digastric, the stylohyoid muscle, the auricular muscle and the stapedius muscle of the middle ear.
Third arch
There is only one muscle of the third pharyngeal arch, the stylopharyngeus. The stylopharyngeus and other structures from the third pharyngeal arch are all innervated by the glossopharyngeal nerve.
Fourth and sixth arches
All
the pharyngeal muscles of the fourth and sixth arches are innervated by
the superior laryngeal and the recurrent laryngeal branches of the vagus nerve. These muscles include all the muscles of the palate (exception of the tensor veli palatini which is innervated by the trigeminal nerve), all the muscles of the pharynx (except stylopharyngeus which is innervated by the glossopharyngeal nerve), and all the muscles of the larynx.
In humans
Amniotes have five arches, numbered 1 to 5.
Older literature reports the fifth arch as the sixth arch, the fifth
being absent. More is known about the fate of the first arch than the
remaining four. The first three contribute to structures above the
larynx, whereas the last two contribute to the larynx and trachea.
The recurrent laryngeal nerves
are produced from the nerve of arch 5, and the laryngeal cartilages
from arches 4 and 5. The superior laryngeal branch of the vagus nerve
arises from arch 4. Its arteries, which project between the nerves of
the fourth and fifth arches, become the left-side arch of the aorta and
the right subclavian artery. On the right side, the artery of arch 5 is obliterated while, on the left side, the artery persists as the ductus arteriosus; circulatory changes immediately following birth cause the vessel to close down, leaving a remnant, the ligamentum arteriosum. During growth, these arteries descend into their ultimate positions in the chest, creating the elongated recurrent paths.
Important classes of inorganic compounds are the oxides, the carbonates, the sulfates, and the halides. Many inorganic compounds are characterized by high melting points. Many inorganic compounds have high melting point and ease of crystallization. Some salts (e.g., NaCl) are very soluble in water, others (e.g., FeS) are not.
The simplest inorganic reaction is double displacement when in mixing of two salts the ions are swapped without a change in oxidation state. In redox reactions one reactant, the oxidant, lowers its oxidation state and another reactant, the reductant, has its oxidation state increased. The net result is an exchange of electrons. Electron exchange can occur indirectly as well, e.g., in batteries, a key concept in electrochemistry.
When one reactant contains hydrogen atoms, a reaction can take place by exchanging protons in acid-base chemistry. In a more general definition, any chemical species capable of binding to electron pairs is called a Lewis acid; conversely any molecule that tends to donate an electron pair is referred to as a Lewis base. As a refinement of acid-base interactions, the HSAB theory takes into account polarizability and size of ions.
Inorganic compounds are found in nature as minerals. Soil may contain iron sulfide as pyrite or calcium sulfate as gypsum. Inorganic compounds are also found multitasking as biomolecules: as electrolytes (sodium chloride), in energy storage (ATP) or in construction (the polyphosphate backbone in DNA).
Inorganic
chemistry is a highly practical area of science. Traditionally, the
scale of a nation's economy could be evaluated by their productivity of sulfuric acid.
The manufacturing of fertilizers, which often begins with the Haber-Bosch process, is another practical application of industrial inorganic chemistry.
Descriptive inorganic chemistry
Descriptive
inorganic chemistry focuses on the classification of compounds based on
their properties. Partly the classification focuses on the position in
the periodic table of the heaviest element (the element with the
highest atomic weight) in the compound, partly by grouping compounds by
their structural similarities.
Coordination compounds
EDTAchelates an octahedrally coordinated Co3+ ion in [Co(EDTA)]−
Classical coordination compounds feature metals bound to "lone pairs" of electrons residing on the main group atoms of ligands such as H2O, NH3, Cl−, and CN−.
In modern coordination compounds almost all organic and inorganic
compounds can be used as ligands. The "metal" usually is a metal from
the groups 3–13, as well as the trans-lanthanides and trans-actinides, but from a certain perspective, all chemical compounds can be described as coordination complexes.
The stereochemistry of coordination complexes can be quite rich, as hinted at by Werner's separation of two enantiomers of [Co((OH)2Co(NH3)4)3]6+,
an early demonstration that chirality is not inherent to organic
compounds. A topical theme within this specialization is supramolecular
coordination chemistry.
These species feature elements from groups
I, II, III, IV, V,VI, VII, 0 (excluding hydrogen) of the periodic
table. Due to their often similar reactivity, the elements in group 3 (Sc, Y, and La) and group 12 (Zn, Cd, and Hg) are also generally included, and the lanthanides and actinides are sometimes included as well.
Main group compounds have been known since the beginnings of chemistry, e.g., elemental sulfur and the distillable white phosphorus. Experiments on oxygen, O2, by Lavoisier and Priestley not only identified an important diatomic gas, but opened the way for describing compounds and reactions according to stoichiometric ratios. The discovery of a practical synthesis of ammonia using iron catalysts by Carl Bosch and Fritz Haber in the early 1900s deeply impacted mankind, demonstrating the significance of inorganic chemical synthesis.
Typical main group compounds are SiO2, SnCl4, and N2O. Many main group compounds can also be classed as "organometallic", as they contain organic groups, e.g., B(CH3)3). Main group compounds also occur in nature, e.g., phosphate in DNA,
and therefore may be classed as bioinorganic. Conversely, organic
compounds lacking (many) hydrogen ligands can be classed as "inorganic",
such as the fullerenes, buckytubes and binary carbon oxides.
Noble gases are elements which have filled valence electron
shells in their neutral state, and are thus stable as lone atoms.
Historically known as being inert, methods were discovered to react with
them. The trend within the group is for the larger elements to be more
reactive. Xenon and krypton are more easily ionized, and can combine with extremely electronegative elements to make fluorides and oxides and form solid ionic compounds. Argon, neon, and helium are much less reactive, though in cosmochemistry ArH+
has been observed spectroscopically in interstellar gas. Noble gases
can also be trapped in solids while not being directly coordinated in clathrates or in endohedral fullerenes.
Compounds
containing metals from group 4 to 11 are considered transition metal
compounds. Compounds with a metal from group 3 or 12 are sometimes also
incorporated into this group, but also often classified as main group
compounds.
Transition metal compounds show a rich coordination chemistry, varying from tetrahedral for titanium (e.g., TiCl4)
to square planar for some nickel complexes to octahedral for
coordination complexes of cobalt. A range of transition metals can be
found in biologically important compounds, such as iron in hemoglobin.
Usually, organometallic compounds are considered to contain the M-C-H group.
The metal (M) in these species can either be a main group element or a
transition metal. Operationally, the definition of an organometallic
compound is more relaxed to include also highly lipophilic complexes such as metal carbonyls and even metal alkoxides.
Organometallic compounds are mainly considered a special category
because organic ligands are often sensitive to hydrolysis or oxidation,
necessitating that organometallic chemistry employs more specialized
preparative methods than was traditional in Werner-type complexes.
Synthetic methodology, especially the ability to manipulate complexes in
solvents of low coordinating power, enabled the exploration of very
weakly coordinating ligands such as hydrocarbons, H2, and N2.
Because the ligands are petrochemicals in some sense, the area of
organometallic chemistry has greatly benefited from its relevance to
industry.
Clusters can be found in all classes of chemical compounds.
According to the commonly accepted definition, a cluster consists
minimally of a triangular set of atoms that are directly bonded to each
other. But metal-metal bonded dimetallic complexes are highly relevant
to the area. Clusters occur in "pure" inorganic systems, organometallic
chemistry, main group chemistry, and bioinorganic chemistry. The
distinction between very large clusters and bulk solids is increasingly
blurred. This interface is the chemical basis of nanoscience or nanotechnology and specifically arise from the study of quantum size effects in cadmium selenide
clusters. Thus, large clusters can be described as an array of bound
atoms intermediate in character between a molecule and a solid.
By definition, these compounds occur in nature, but the subfield includes anthropogenic species, such as pollutants (e.g., methylmercury) and drugs (e.g., Cisplatin).
The field, which incorporates many aspects of biochemistry, includes
many kinds of compounds, e.g., the phosphates in DNA, and also metal
complexes containing ligands that range from biological macromolecules,
commonly peptides, to ill-defined species such as humic acid, and to water (e.g., coordinated to gadolinium complexes employed for MRI).
Traditionally bioinorganic chemistry focuses on electron- and
energy-transfer in proteins relevant to respiration. Medicinal
inorganic chemistry includes the study of both non-essential and essential elements with applications to diagnosis and therapies.
This important area focuses on structure, bonding, and the physical properties of materials. In practice, solid state inorganic chemistry uses techniques such as crystallography
to gain an understanding of the properties that result from collective
interactions between the subunits of the solid. Included in solid state
chemistry are metals and their alloys or intermetallic derivatives. Related fields are condensed matter physics, mineralogy, and materials science.
An alternative perspective on the area of inorganic chemistry begins with the Bohr model of the atom and, using the tools and models of theoretical chemistry and computational chemistry,
expands into bonding in simple and then more complicated molecules.
Precise quantum mechanical descriptions for multielectron species, the
province of inorganic chemistry, is difficult. This challenge has
spawned many semi-quantitative or semi-empirical approaches including molecular orbital theory and ligand field theory, In parallel with these theoretical descriptions, approximate methodologies are employed, including density functional theory.
Exceptions to theories, qualitative and quantitative, are extremely important in the development of the field. For example, CuII2(OAc)4(H2O)2 is almost diamagnetic below room temperature whereas crystal field theory
predicts that the molecule would have two unpaired electrons. The
disagreement between qualitative theory (paramagnetic) and observation
(diamagnetic) led to the development of models for magnetic coupling,
such as the exchange interaction. These improved models led to the
development of new magnetic materials and new technologies.
Inorganic chemistry has greatly benefited from qualitative theories.
Such theories are easier to learn as they require little background in
quantum theory. Within main group compounds, VSEPR theory powerfully predicts, or at least rationalizes, the structures of main group compounds, such as an explanation for why NH3 is pyramidal whereas ClF3 is T-shaped. For the transition metals, crystal field theory allows one to understand the magnetism of many simple complexes, such as why [FeIII(CN)6]3− has only one unpaired electron, whereas [FeIII(H2O)6]3+
has five. A particularly powerful qualitative approach to assessing
the structure and reactivity begins with classifying molecules according
to electron counting, focusing on the numbers of valence electrons, usually at the central atom in a molecule.
A central construct in inorganic chemistry is the theory of molecular symmetry. Mathematical group theory provides the language to describe the shapes of molecules according to their point group symmetry. Group theory also enables factoring and simplification of theoretical calculations.
Spectroscopic features are analyzed and described with respect to the symmetry properties of the, inter alia,
vibrational or electronic states. Knowledge of the symmetry properties
of the ground and excited states allows one to predict the numbers and
intensities of absorptions in vibrational and electronic spectra. A
classic application of group theory is the prediction of the number of
C-O vibrations in substituted metal carbonyl complexes. The most common
applications of symmetry to spectroscopy involve vibrational and
electronic spectra.
Group theory highlights commonalities and differences in the
bonding of otherwise disparate species. For example, the metal-based
orbitals transform identically for WF6 and W(CO)6, but the energies and populations of these orbitals differ significantly. A similar relationship exists CO2 and molecular beryllium difluoride.
Thermodynamics and inorganic chemistry
An
alternative quantitative approach to inorganic chemistry focuses on
energies of reactions. This approach is highly traditional and empirical, but it is also useful. Broad concepts that are couched in thermodynamic terms include redox potential, acidity, phase changes. A classic concept in inorganic thermodynamics is the Born–Haber cycle, which is used for assessing the energies of elementary processes such as electron affinity, some of which cannot be observed directly.
Mechanistic inorganic chemistry
An important aspect of inorganic chemistry focuses on reaction pathways, i.e. reaction mechanisms.
Main group elements and lanthanides
The
mechanisms of main group compounds of groups 13-18 are usually
discussed in the context of organic chemistry (organic compounds are
main group compounds, after all). Elements heavier than C, N, O, and F
often form compounds with more electrons than predicted by the octet rule, as explained in the article on hypervalent molecules. The mechanisms of their reactions differ from organic compounds for this reason. Elements lighter than carbon (B, Be, Li) as well as Al and Mg often form electron-deficient structures that are electronically akin to carbocations.
Such electron-deficient species tend to react via associative
pathways. The chemistry of the lanthanides mirrors many aspects of
chemistry seen for aluminium.
Transition metal complexes
Transition metal and main group compounds often react differently.
The important role of d-orbitals in bonding strongly influences the
pathways and rates of ligand substitution and dissociation. These
themes are covered in articles on coordination chemistry and ligand. Both associative and dissociative pathways are observed.
An overarching aspect of mechanistic transition metal chemistry
is the kinetic lability of the complex illustrated by the exchange of
free and bound water in the prototypical complexes [M(H2O)6]n+:
[M(H2O)6]n+ + 6 H2O* → [M(H2O*)6]n+ + 6 H2O
where H2O* denotes isotopically enriched water, e.g., H217O
The rates of water exchange varies by 20 orders of magnitude across
the periodic table, with lanthanide complexes at one extreme and Ir(III)
species being the slowest.
Redox reactions
Redox
reactions are prevalent for the transition elements. Two classes of
redox reaction are considered: atom-transfer reactions, such as
oxidative addition/reductive elimination, and electron-transfer. A fundamental redox reaction is "self-exchange", which involves the degenerate reaction between an oxidant and a reductant. For example, permanganate and its one-electron reduced relative manganate exchange one electron:
[MnO4]− + [Mn*O4]2− → [MnO4]2− + [Mn*O4]−
Reactions at ligands
Coordinated ligands display reactivity distinct from the free ligands. For example, the acidity of the ammonia ligands in [Co(NH3)6]3+ is elevated relative to NH3
itself. Alkenes bound to metal cations are reactive toward
nucleophiles whereas alkenes normally are not. The large and
industrially important area of catalysis hinges on the ability of metals to modify the reactivity of organic ligands. Homogeneous catalysis occurs in solution and heterogeneous catalysis occurs when gaseous or dissolved substrates interact with surfaces of solids. Traditionally homogeneous catalysis is considered part of organometallic chemistry and heterogeneous catalysis is discussed in the context of surface science,
a subfield of solid state chemistry. But the basic inorganic chemical
principles are the same. Transition metals, almost uniquely, react with
small molecules such as CO, H2, O2, and C2H4.
The industrial significance of these feedstocks drives the active area
of catalysis. Ligands can also undergo ligand transfer reactions such
as transmetalation.
Characterization of inorganic compounds
Because
of the diverse range of elements and the correspondingly diverse
properties of the resulting derivatives, inorganic chemistry is closely
associated with many methods of analysis. Older methods tended to
examine bulk properties such as the electrical conductivity of
solutions, melting points, solubility, and acidity. With the advent of quantum theory
and the corresponding expansion of electronic apparatus, new tools have
been introduced to probe the electronic properties of inorganic
molecules and solids. Often these measurements provide insights
relevant to theoretical models. Commonly encountered techniques are:
Ultraviolet-visible spectroscopy: Historically, this has been an important tool, since many inorganic compounds are strongly colored
NMR spectroscopy: Besides 1H and 13C many other NMR-active nuclei (e.g., 11B, 19F, 31P, and 195Pt)
can give important information on compound properties and structure.
The NMR of paramagnetic species can provide important structural
information. Proton (1H) NMR is also important because the light hydrogen nucleus is not easily detected by X-ray crystallography.
Although
some inorganic species can be obtained in pure form from nature, most
are synthesized in chemical plants and in the laboratory.
Inorganic synthetic methods can be classified roughly according to the volatility or solubility of the component reactants. Soluble inorganic compounds are prepared using methods of organic synthesis. For metal-containing compounds that are reactive toward air, Schlenk line and glove box
techniques are followed. Volatile compounds and gases are manipulated
in "vacuum manifolds" consisting of glass piping interconnected through
valves, the entirety of which can be evacuated to 0.001 mm Hg or less.
Compounds are condensed using liquid nitrogen (b.p. 78K) or other cryogens.
Solids are typically prepared using tube furnaces, the reactants and
products being sealed in containers, often made of fused silica
(amorphous SiO2) but sometimes more specialized materials
such as welded Ta tubes or Pt "boats". Products and reactants are
transported between temperature zones to drive reactions.
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