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Yeast bread | Wikipedia audio article

August 27, 2019


Yeasts are eukaryotic, single-celled microorganisms
classified as members of the fungus kingdom. The first yeast originated hundreds of millions
of years ago, and 1,500 species are currently identified. They are estimated to constitute 1% of all
described fungal species. Yeasts are unicellular organisms which evolved
from multicellular ancestors, with some species having the ability to develop multicellular
characteristics by forming strings of connected budding cells known as pseudohyphae or false
hyphae. 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. Yeasts, with their single-celled growth habit,
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 (“dimorphic” means “having
two forms”). 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 a centrally important 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 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 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, within the phylum Ascomycota.==History==The word “yeast” comes 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. 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. 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. Researchers were doubtful whether yeasts were
algae or fungi, but in 1837 Theodor Schwann recognized them as fungi.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 (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. 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.==Nutrition and growth==
Yeasts are chemoorganotrophs, as they use organic compounds as a source of energy and
do not require sunlight to grow. Carbon is obtained mostly from hexose sugars,
such as glucose and fructose, or disaccharides such as sucrose and maltose. Some species can metabolize pentose sugars
such as ribose, alcohols, and organic acids. Yeast species either require oxygen for aerobic
cellular respiration (obligate aerobes) or are anaerobic, but also have aerobic methods
of energy production (facultative anaerobes). Unlike bacteria, no known yeast species grow
only anaerobically (obligate anaerobes). Most yeasts grow best in a neutral or slightly
acidic pH environment. 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 peptone dextrose 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 antibiotic cycloheximide is sometimes
added to yeast growth media to inhibit the growth of Saccharomyces yeasts and select
for wild/indigenous yeast species. This will change the yeast process. The appearance of a white, thready yeast,
commonly known as kahm yeast, is often a byproduct of the lactofermentation (or pickling) of
certain vegetables, 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. 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 9 plant species found 45 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 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 confine 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
application 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 WF of bioethanol .==
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 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 limiting. 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, 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. 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, and 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 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.====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. 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). 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 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.====Wine====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.===Baking===Yeast, the most common one being S. cerevisiae,
is used in baking as a leavening agent, where it converts the food/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 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. 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 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 flavour,
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.===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 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. 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===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. Root beer, originally made by Native Americans,
commercialized in the United States by Charles Elmer Hires and especially popular during
Prohibition Kvass, a fermented drink made from rye, popular
in Eastern Europe. It has a recognizable, but low alcoholic content. Kombucha, a fermented sweetened tea. Yeast in symbiosis with acetic acid bacteria
is used in its preparation. Species of yeasts found in the tea can vary,
and may include: Brettanomyces bruxellensis, Candida stellata, Schizosaccharomyces pombe,
Torulaspora delbrueckii and Zygosaccharomyces bailii. Also popular in Eastern Europe and some former
Soviet republics under the name chajnyj grib (Russian: Чайный гриб), which means
“tea mushroom”. Kefir and kumis are made by fermenting milk
with yeast and bacteria. Mauby (Spanish: mabí), made by fermenting
sugar with the wild yeasts naturally present on the bark of the Colubrina elliptica tree,
popular in the Caribbean===
Nutritional supplements===Yeast is used in nutritional supplements,
especially those marketed to vegans. It is often referred to as “nutritional yeast”
when sold as a dietary supplement. Nutritional yeast is a deactivated yeast,
usually S. cerevisiae. It is naturally low in fat and sodium as well
as an excellent source of protein and vitamins, especially most B-complex vitamins (contrary
to some claims, it contains little or no vitamin B12 ), as well as other minerals and cofactors
required for growth. Some brands of nutritional yeast, 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.Nutritional yeast has a nutty, cheesy flavor and is often used as an ingredient
in cheese substitutes. 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”. 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, 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.===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 (the United Kingdom); the unrelated Marmite (New Zealand); Vitam-R
(Germany); and Cenovis (Switzerland).===Scientific research===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 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: SGD and
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.==Pathogenic yeasts==Some species of yeast are opportunistic pathogens
that can cause infection in people with compromised immune systems. Cryptococcus neoformans and Cryptococcus gattii
are significant pathogens of immunocompromised people. They are the species primarily responsible
for cryptococcosis, a fungal disease that occurs in about one million HIV/AIDS patients,
causing over 600,000 deaths annually. The cells of these 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 genus Candida, another
group of opportunistic pathogens, cause oral and vaginal infections in humans, known as
candidiasis. Candida is commonly found as a commensal yeast
in the mucous membranes of humans and other warm-blooded animals. However, sometimes these same strains can
become pathogenic. 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 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
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. In oenology, the major spoilage yeast is Brettanomyces
bruxellensis.==See also==Bioaerosol
Ethanol fermentation Mycosis (fungal infection in animals)
Start point (yeast) WHI3
Zymology Evolution of aerobic fermentation
Plasmid#Yeast plasmids==
Further reading==Alexopoulos CJ, Mims CW, Blackwell M (1996). Introductory Mycology. New York, New York: Wiley. ISBN 978-0-471-52229-4. Kirk PM, Cannon PF, Minter DW, Stalpers JA
(2008). Dictionary of the Fungi (10th ed.). Wallingford, UK: CAB International. ISBN 978-0-85199-826-8. Kurtzman CP; Fell JW; Boekhout T, eds. (2011). The Yeasts: A Taxonomic Study. 1 (5th ed.). Amsterdam, etc.: Elsevier. ISBN 978-0-12-384708-9. Moore-Landecker E (1996). Fundamentals of the Fungi. Englewood Cliffs, New Jersey: Prentice Hall. ISBN 978-0-13-376864-0. Priest FG, Stewart GG (2006). Handbook of Brewing (2nd ed.). CRC Press. p. 691. ISBN 978-1-4200-1517-1

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