Yeast

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For other uses, see Yeast (disambiguation).

Yeast
Saccharomyces cerevisiae, a species of yeast
Saccharomyces cerevisiae, a species of yeast
Cross-sectional 2D diagram of a yeast cell
Cross-sectional labelled diagram of a typical yeast cell
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Fungi
Phyla and subphyla with yeast species

Yeasts are

eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized.[1][2][3] They are estimated to constitute 1% of all described fungal species.[4]

Some yeast species have the ability to develop multicellular characteristics by forming strings of connected budding cells known as

dimorphic fungi
.

The yeast species

infections in humans. Yeasts have recently been used to generate electricity in microbial fuel cells[10] 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,[11] 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 order Saccharomycetales,[12] within the phylum Ascomycota.

History

See also: History of bread, History of wine, and History of beer

The word "yeast" comes from

Anton van Leeuwenhoek first microscopically observed yeast, but at the time did not consider them to be living organisms, but rather globular structures[16] as researchers were doubtful whether yeasts were algae or fungi.[17] Theodor Schwann recognized them as fungi in 1837.[18][19]

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.[14][20]

By the late 18th century two yeast strains used in brewing had been identified: Saccharomyces cerevisiae (top-fermenting yeast) and

Charles L. Fleischmann exhibited the product and a process to use it, as well as serving the resultant baked bread.[23]

The

distiller's art.[24]

Nutrition and growth

Yeasts are

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).[26]
The cells can survive freezing under certain conditions, with viability decreasing over time.

In general, yeasts are grown in the laboratory on solid

malt extract and agar as a solid growth medium. The fungicide 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. 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.[27]

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

gut flora of mammals and some insects[33] and even deep-sea environments host an array of yeasts.[34][35]

An Indian study of seven

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.[38]

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).[39]

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.[40] 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.[41] Following this discovery, various other marine yeasts have been isolated from around the world from different sources, including seawater, seaweeds, marine fish and mammals.[42] 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.[41] However, no sufficient evidence has been found to explain the indispensability of seawater for obligate marine yeasts.[40] 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.[40] Marine yeast was successfully used to produce bioethanol using seawater-based media which will potentially reduce the water footprint of bioethanol.[43]

Reproduction

The yeast cell's life cycle:
  1. Budding
  2. Conjugation
  3. Spore
See also: Mating of yeast

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,[44] 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.[45] 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,[44] and thereby creating two identically sized daughter cells.

In general, under high-stress conditions such as

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.[46]

The haploid fission yeast Schizosaccharomyces pombe is a facultative sexual microorganism that can undergo mating when nutrients are limited.[3][47] 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.[48] 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.[49] Haploid cells may then reproduce asexually by mitosis. Katz Ezov et al.[50] 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.[51] 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.[51] 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.[citation needed] Rather, a short-term benefit, such as recombinational repair during meiosis,[52] may be the key to the maintenance of sex in S. cerevisiae.

Some

Sporidiobolus and Sporobolomyces, produce aerially dispersed, asexual ballistoconidia.[53]

Uses

The useful physiological properties of yeast have led to their use in the field of

mold, Monascus purpureus. Yeasts include some of the most widely used model organisms for genetics and cell biology.[55]

Alcoholic beverages

Alcoholic beverages are defined as

flavour
of the beverage.

Beer

Main article: Brewing
See also: Barm
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[9]

Brewing yeasts may be classed as "top-cropping" (or "top-fermenting") and "bottom-cropping" (or "bottom-fermenting").[56] 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".[57] 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,[vague] taxonomists reclassified S. carlsbergensis (uvarum) as a member of S. cerevisiae, noting that the only distinct difference between the two is metabolic. [dubious – discuss] 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.[58]

The most common top-cropping brewer's yeast, S. cerevisiae, is the same species as the common baking yeast.

B vitamins (except B12), a feature exploited in food products made from leftover (by-product) yeast from brewing.[60] 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 bruxellensis and Dekkera anomala.[64] 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.[65]

Wine

Main article: Yeast in winemaking
Yeast in a bottle during sparkling wine production at Schramsberg Vineyards, Napa

Yeast is used in

Fermentation can be done with this endogenous "wild yeast",[66] 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.[67]

Most added wine yeasts are strains of S. cerevisiae, though not all strains of the species are suitable.[67] 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.[68] Significant research has been undertaken into the development of novel wine yeast strains that produce atypical flavour profiles or increased complexity in wines.[69][70]

The growth of some yeasts, such as Zygosaccharomyces and Brettanomyces, in wine can result in wine faults and subsequent spoilage.[71] 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.[72]

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.[73]

Baking

Main article: Baker's yeast

Yeast, most commonly S. cerevisiae, is used in baking as a

fermentation begins, producing ethanol as a waste product; however, this evaporates during baking.[74]

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.[8] 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.[75] 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.[citation needed]

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

heavy metals,[77] and is being investigated for its potential as a heavy metal biosorbent.[78] Saccharomyces cerevisiae has potential to bioremediate toxic pollutants like arsenic from industrial effluent.[79] Bronze statues are known to be degraded by certain species of yeast.[80] Different yeasts from Brazilian gold mines bioaccumulate free and complexed silver ions.[81]

Industrial ethanol production

See also: Bioethanol

The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry to produce

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.[82]

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.[85]

Nonalcoholic beverages

A kombucha culture fermenting in a jar
Yeast and bacteria in kombucha at 400×

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.

See also: Tibicos

Foods and nutritional supplements

Marmite and Vegemite, products made from yeast extract
Marmite and Vegemite are dark in colour

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:[89]

Both types of yeast foods above are rich in

B-complex vitamins (besides vitamin B12 unless fortified),[60] making them an attractive nutritional supplement to vegans.[90] The same vitamins are also found in some yeast-fermented products mentioned above, such as kvass.[92] 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.[93]

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.[94]

Probiotics

Some

Clostridium difficile (often identified simply as C. difficile or C. diff),[96] reduce bowel movements in diarrhea-predominant IBS patients,[97] and reduce the incidence of antibiotic-, traveler's-, and HIV/AIDS-associated diarrheas.[98]

Aquarium hobby

Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO2) to nourish plants in planted aquaria.[99] 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.[99]

Scientific research

Diagram showing a yeast cell

Several yeasts, in particular

cell cycle proteins, signaling proteins, and protein-processing enzymes.[102]

On 24 April 1996, S. cerevisiae was announced to be the first eukaryote to have its

Genome Project.[103] 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.[104] The second yeast species to have its genome sequenced was Schizosaccharomyces pombe, which was completed in 2002.[105][106] 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.[107]

Genomic and functional gene annotation of the two major yeast models can be accessed via their respective

model organism databases: SGD[108][109] and PomBase.[110][111]

Genetically engineered biofactories

Various yeast species have been genetically engineered to efficiently produce various drugs, a technique called

isoprenoids, alkaloids, and polyketides.[113] About 20% of biopharmaceuticals are produced in S. cerevisiae, including insulin, vaccines for hepatitis, and human serum albumin.[114]

Pathogenic yeasts

Main article: Fungal infection
Gram stain of Candida albicans from a vaginal swab. The small oval chlamydospores are 2–4 µm in diameter.
photomicrograph of Candida albicans
showing hyphal outgrowth and other morphological characteristics

Some species of yeast are

immunocompromised people. They are the species primarily responsible for cryptococcosis, a fungal infection that occurs in about one million HIV/AIDS patients, causing over 600,000 deaths annually.[115] 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.[116]

Yeasts of the genus

candidemia).[119] C. auris
has been more recently identified.

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.

jams.[120] 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,[71] representing some of the commonly used food preservation methods. Methylene blue is used to test for the presence of live yeast cells.[122] In oenology, the major spoilage yeast is Brettanomyces bruxellensis
.

Iberian ham and meat.[123]

Symbiosis

Main article: Symbiosis

An Indian study of seven

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 mechanism is not fully understood, it was found that A. indica flowers more if Candida blankii is present.[36]

In another example, Spathaspora passalidarum, found in the digestive tract of bess beetles, aids the digestion of plant cells by fermenting xylose.[124]

Many fruits produce different types of sugars that attract yeasts, which ferment the sugar and turns it into alcohol. Fruit eating mammals find the scent of alcohol attractive as it indicates a ripe, sugary fruit which provides more nutrition. In turn, the mammals helps disperse both the fruit's seeds and the yeast's spores.[125][126]

See also

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Further reading

External links

Look up yeast in Wiktionary, the free dictionary.
Wikimedia Commons has media related to Yeasts.
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