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.[1][2][3] They are estimated to constitute 1% of all described fungal species.[4] Yeasts are unicellular organisms which evolved from multicellular ancestors,[5] with some species having the ability to develop multicellular characteristics by forming strings of connected budding cells known as pseudohyphae or false hyphae.[6] 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.[7] 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.[8] 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.[9] 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,[10] 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,[11] 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,[12] within the phylum Ascomycota.

S cerevisiae under DIC microscopy
Yeast of the species Saccharomyces cerevisiae
Cross-sectional 2D diagram of a yeast cell
Cross-sectional labelled diagram of a typical yeast cell
Scientific classification
Phyla and Subphyla


The word "yeast" comes from Old English gist, gyst, and from the Indo-European root yes-, meaning "boil", "foam", or "bubble".[13] 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.[14] 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[15] as researchers were doubtful whether yeasts were algae or fungi.[16] Theodor Schwann recognized them as fungi in 1837.[17][18]

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][19]

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.[20] 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.[21] 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.[22]

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.[23]

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,[24] 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).[25] 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.[26]


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.[27][28] The ecological function and biodiversity of yeasts are relatively unknown compared to those of other microorganisms.[29] 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.[30] Yeasts are also present in the gut flora of mammals and some insects[31] and even deep-sea environments host an array of yeasts.[32][33]

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.[34] 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.[29][35] 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.[36]

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).[37]

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.[38] 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.[39] Following this discovery, various other marine yeasts have been isolated from around the world from different sources, including seawater, seaweeds, marine fish and mammals.[40] 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.[39] However, no sufficient evidence has been found to explain the indispensability of seawater for obligate marine yeasts.[38] 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.[38] Marine yeast was successfully used to produce bioethanol using seawater-based media which will potentially reduce the water footprint of bioethanol.[41]


Yeast lifecycle
The yeast cell's life cycle:
  1. Budding
  2. Conjugation
  3. Spore

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,[42] 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.[43] 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,[42] 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.[44]

The haploid fission yeast Schizosaccharomyces pombe is a facultative sexual microorganism that can undergo mating when nutrients are limiting.[3][45] 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.[46] 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.[47] Haploid cells may then reproduce asexually by mitosis. Katz Ezov et al.[48] 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.[49] 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.[49] 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.[50] Rather, a short-term benefit, such as recombinational repair during meiosis,[51][52] 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.[53]


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.[54] 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.[55]

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.


NM.0019545 Jästkrans
Yeast ring used by Swedish farmhouse brewers in the 19th century to preserve yeast between brewing sessions.
2009-03-21 Beer brewing bubbles
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, 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.[58]

The most common top-cropping brewer's yeast, S. cerevisiae, is the same species as the common baking yeast.[59] Brewer's yeast is also very rich in essential minerals and the B vitamins (except B12).[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/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.[61] 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.[62] Shortly after, the formation of ascospores was observed and the genus Dekkera, which reproduces sexually (teleomorph form), was introduced as part of the taxonomy.[63] 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.[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]


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",[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]


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.[74]

Compressed fresh yeast - 1
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.

Dry yeast
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 flavour, but the yeast can fail to raise the bread in the final stages if it is left for too long initially.


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.[75] It can also tolerate high concentrations of salt and heavy metals,[76] and is being investigated for its potential as a heavy metal biosorbent.[77] Saccharomyces cerevisiae has potential to bioremediate toxic pollutants like arsenic from industrial effluent.[78] Bronze statues are known to be degraded by certain species of yeast.[79] Different yeasts from Brazilian gold mines bioaccumulate free and complexed silver ions.[80]

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.[81]

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.[82][83] 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.[84]

Nonalcoholic beverages

Kombucha jar
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.

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[88] (contrary to some claims, it contains little or no vitamin B12[60] ), 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.[89]

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.[90]

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.[91]


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,[92] reduce the chance of infection by Clostridium difficile (often identified simply as C. difficile or C. diff),[93] reduce bowel movements in diarrhea-predominant IBS patients,[94] and reduce the incidence of antibiotic-, traveler's-, and HIV/AIDS-associated diarrheas.[95]

Aquarium hobby

Yeast is often used by aquarium hobbyists to generate carbon dioxide (CO2) to nourish plants in planted aquaria.[96] 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.[96]

Yeast extract

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

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.[97] 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

Yeast cell english
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,[98] synthetic genetic array analysis,[99] 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.[100]

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.[101] 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.[102] The second yeast species to have its genome sequenced was Schizosaccharomyces pombe, which was completed in 2002.[103][104] 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.[105]

Genomic and functional gene annotation of the two major yeast models can be accessed via their respective model organism databases: SGD[106][107] and PomBase.[108][109]

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.[110] About 20% of biopharmaceuticals are produced in S. cerevisiae, including insulin, vaccines for hepatitis, and human serum albumin.[111]

Pathogenic yeasts

C albicans en
A photomicrograph of Candida albicans showing hyphal outgrowth and other morphological characteristics

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.[112] 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.[113]

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.[114] 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.[115] Candida glabrata is the second most common Candida pathogen after C. albicans, causing infections of the urogenital tract, and of the bloodstream (candidemia).[116]

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.[117] 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.[118] 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.[117] 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,[71] representing some of the commonly used food preservation methods. Methylene blue is used to test for the presence of live yeast cells.[119] In oenology, the major spoilage yeast is Brettanomyces bruxellensis.

Candida blankii has been detected in Iberian ham and meat.[120]


An Indian study of seven bee species and 9 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 C. blankii are present.[34]

See also

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.


  1. ^ Molecular Mechanisms in Yeast Carbon Metabolism The second completely sequenced yeast genome came 6 years later from the fission yeast Schizosaccharomyces pombe, which diverged from S. cerevisiae probably more than 300 million years ago
  2. ^ Kurtzman CP, Fell JW (2006). "Yeast Systematics and Phylogeny—Implications of Molecular Identification Methods for Studies in Ecology". Biodiversity and Ecophysiology of Yeasts, The Yeast Handbook. Springer.
  3. ^ a b Hoffman CS, Wood V, Fantes PA (October 2015). "An Ancient Yeast for Young Geneticists: A Primer on the Schizosaccharomyces pombe Model System". Genetics. 201 (2): 403–23. doi:10.1534/genetics.115.181503. PMC 4596657. PMID 26447128.
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External links

Baker's yeast

Baker's yeast is the common name for the strains of yeast commonly used in baking bread and bakery products, serving as a leavening agent which causes the bread to rise (expand and become lighter and softer) by converting the fermentable sugars present in the dough into carbon dioxide and ethanol. Baker's yeast is of the species Saccharomyces cerevisiae, and is the same species (but a different strain) as the kind commonly used in alcoholic fermentation, which is called brewer's yeast. Baker's yeast is also a single-cell microorganism found on and around the human body.

The use of steamed or boiled potatoes, water from potato boiling, or sugar in a bread dough provides food for the growth of yeasts; however, too much sugar will dehydrate them. Yeast growth is inhibited by both salt and sugar, but more so by salt than sugar. Some sources say fats, such as butter and eggs, slow down yeast growth; others say the effect of fat on dough remains unclear, presenting evidence that small amounts of fat are beneficial for baked bread volume.Saccharomyces Exiguus (also known as S. minor) is a wild yeast found on plants, fruits, and grains that is occasionally used for baking; however, in general, it is not used in a pure form but comes from being propagated in a sourdough starter.


Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi.

More specifically, Basidiomycota includes these groups: mushrooms, puffballs, stinkhorns, bracket fungi, other polypores, jelly fungi, boletes, chanterelles, earth stars, smuts, bunts, rusts, mirror yeasts, and the human pathogenic yeast Cryptococcus.

Basidiomycota are filamentous fungi composed of hyphae (except for basidiomycota-yeast; refer yeast for more information) and reproduce sexually via the formation of specialized club-shaped end cells called basidia that normally bear external meiospores (usually four). These specialized spores are called basidiospores. However, some Basidiomycota reproduce asexually in addition or exclusively. Basidiomycota that reproduce asexually (discussed below) can be recognized as members of this division by gross similarity to others, by the formation of a distinctive anatomical feature (the clamp connection – see below), cell wall components, and definitively by phylogenetic molecular analysis of DNA sequence data.


Beer is one of the oldest and most widely consumed alcoholic drinks in the world, and the third most popular drink overall after water and tea. Beer is brewed from cereal grains—most commonly from malted barley, though wheat, maize (corn), and rice are also used. During the brewing process, fermentation of the starch sugars in the wort produces ethanol and carbonation in the resulting beer. Most modern beer is brewed with hops, which add bitterness and other flavours and act as a natural preservative and stabilizing agent. Other flavouring agents such as gruit, herbs, or fruits may be included or used instead of hops. In commercial brewing, the natural carbonation effect is often removed during processing and replaced with forced carbonation.Some of humanity's earliest known writings refer to the production and distribution of beer: the Code of Hammurabi included laws regulating beer and beer parlours, and "The Hymn to Ninkasi", a prayer to the Mesopotamian goddess of beer, served as both a prayer and as a method of remembering the recipe for beer in a culture with few literate people.Beer is distributed in bottles and cans and is also commonly available on draught, particularly in pubs and bars. The brewing industry is a global business, consisting of several dominant multinational companies and many thousands of smaller producers ranging from brewpubs to regional breweries. The strength of modern beer is usually around 4% to 6% alcohol by volume (ABV), although it may vary between 0.5% and 20%, with some breweries creating examples of 40% ABV and above.Beer forms part of the culture of many nations and is associated with social traditions such as beer festivals, as well as a rich pub culture involving activities like pub crawling and pub games.


Bread is a staple food prepared from a dough of flour and water, usually by baking. Throughout recorded history it has been a prominent food in large parts of the world and is one of the oldest man-made foods, having been of significant importance since the dawn of agriculture.

Bread may be leavened by processes such as reliance on naturally occurring sourdough microbes, chemicals, industrially produced yeast, or high-pressure aeration. Commercial bread commonly contains additives to improve flavor, texture, color, shelf life, nutrition, and ease of manufacturing.

Bread plays essential roles in religious rituals and secular culture.


Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and fermenting the resulting sweet liquid with yeast. It may be done in a brewery by a commercial brewer, at home by a homebrewer, or by a variety of traditional methods such as communally by the indigenous peoples in Brazil when making cauim. Brewing has taken place since around the 6th millennium BC, and archaeological evidence suggests that emerging civilizations including ancient Egypt and Mesopotamia brewed beer. Since the nineteenth century the brewing industry has been part of most western economies.

The basic ingredients of beer are water and a fermentable starch source such as malted barley. Most beer is fermented with a brewer's yeast and flavoured with hops. Less widely used starch sources include millet, sorghum and cassava. Secondary sources (adjuncts), such as maize (corn), rice, or sugar, may also be used, sometimes to reduce cost, or to add a feature, such as adding wheat to aid in retaining the foamy head of the beer. The proportion of each starch source in a beer recipe is collectively called the grain bill.

Steps in the brewing process include malting, milling, mashing, lautering, boiling, fermenting, conditioning, filtering, and packaging. There are three main fermentation methods, warm, cool and spontaneous. Fermentation may take place in an open or closed fermenting vessel; a secondary fermentation may also occur in the cask or bottle. There are several additional brewing methods, such as barrel aging, double dropping, and Yorkshire Square.


Candidiasis is a fungal infection due to any type of Candida (a type of yeast). When it affects the mouth, it is commonly called thrush. Signs and symptoms include white patches on the tongue or other areas of the mouth and throat. Other symptoms may include soreness and problems swallowing. When it affects the vagina, it is commonly called a yeast infection. Signs and symptoms include genital itching, burning, and sometimes a white "cottage cheese-like" discharge from the vagina. Yeast infections of the penis are less common and typically present with an itchy rash. Very rarely, yeast infections may become invasive, spreading to other parts of the body. This may result in fevers along with other symptoms depending on the parts involved.More than 20 types of Candida can cause infection with Candida albicans being the most common. Infections of the mouth are most common among children less than one month old, the elderly, and those with weak immune systems. Conditions that result in a weak immune system include HIV/AIDS, the medications used after organ transplantation, diabetes, and the use of corticosteroids. Other risks include dentures and following antibiotic therapy. Vaginal infections occur more commonly during pregnancy, in those with weak immune systems, and following antibiotic use. Individuals at risk for invasive candidiasis include low birth weight babies, people recovering from surgery, people admitted to an intensive care units, and those with an otherwise compromised immune systems.Efforts to prevent infections of the mouth include the use of chlorhexidine mouth wash in those with poor immune function and washing out the mouth following the use of inhaled steroids. Little evidence supports probiotics for either prevention or treatment even among those with frequent vaginal infections. For infections of the mouth, treatment with topical clotrimazole or nystatin is usually effective. By mouth or intravenous fluconazole, itraconazole, or amphotericin B may be used if these do not work. A number of topical antifungal medications may be used for vaginal infections including clotrimazole. In those with widespread disease, an echinocandin such as caspofungin or micafungin is used. A number of weeks of intravenous amphotericin B may be used as an alternative. In certain groups at very high risk, antifungal medications may be used preventatively.Infections of the mouth occur in about 6% of babies less than a month old. About 20% of those receiving chemotherapy for cancer and 20% of those with AIDS also develop the disease. About three-quarters of women have at least one yeast infection at some time during their lives. Widespread disease is rare except in those who have risk factors.


A crumpet (listen) is a small griddle cake made from an unsweetened batter of water or milk, flour and yeast, eaten in the United Kingdom, the Republic of Ireland, and some areas of the Commonwealth.

Crumpets are regionally known as pikelets, a name also applied to a thinner, more pancake-like griddle cake: a type of the latter is referred to as a crumpet in Scotland.


Kombucha (also tea mushroom, tea fungus, or Manchurian mushroom when referring to the culture; botanical name Medusomyces gisevii Lindau) is a fermented, slightly alcoholic, lightly effervescent, sweetened black or green tea drink commonly intended as a functional beverage for its supposed health benefits. Sometimes the beverage is called kombucha tea to distinguish it from the culture of bacteria and yeast. Juice, spices, or other flavorings are often added to enhance the taste of the beverage. The exact origins of kombucha are not known. It is thought to have originated in Northeastern China where the drink is traditionally consumed. Kombucha is also likely to have originated in Russia and Eastern Europe. Kombucha is now homebrewed globally, and also bottled and sold commercially by various companies.Kombucha is produced by fermenting sugared tea using a symbiotic culture of bacteria and yeast (SCOBY) commonly called a "mother" or "mushroom". The microbial populations in a SCOBY vary; the yeast component generally includes Saccharomyces cerevisiae, along with other species; the bacterial component almost always includes Gluconacetobacter xylinus to oxidize yeast-produced alcohols to acetic acid (and other acids). Although the SCOBY is commonly called "tea fungus" or "mushroom", it is actually "a symbiotic growth of acetic acid bacteria and osmophilic yeast species in a zoogleal mat [biofilm]". The living bacteria are said to be probiotic, one of the reasons for the drink's popularity.Numerous implausible health benefits have been attributed to drinking kombucha. These include claims for treating AIDS, aging, anorexia, arthritis, atherosclerosis, cancer, constipation, and diabetes, but there is no evidence to support any of these claims. Moreover, the beverage has caused rare cases of serious adverse effects, including fatalities, possibly arising from contamination during home preparation. Therefore, the potential harms from drinking kombucha outweigh any unclear benefits, so doctors do not recommend its use for therapeutic purposes.


Lager is a type of beer conditioned at low temperatures. Lagers can be pale, amber, or dark. Pale lager is the most widely consumed and commercially available style of beer. Well-known brands include Pilsner Urquell, Molson Canadian, Miller, Stella Artois, Beck's, Brahma, Budweiser Budvar, Corona, Snow, Tsingtao, Singha, Kirin, Heineken, Carling, Foster's, Carlsberg, Birra Moretti and Tennents.

As well as maturation in cold storage, most lagers are also distinguished by the use of Saccharomyces pastorianus yeast, a "bottom-fermenting" yeast that also ferments at relatively cold temperatures. It is possible to use lager yeast in a warm fermentation process, such as with American steam beer; while German Altbier and Kölsch are brewed with Saccharomyces cerevisiae top-fermenting yeast at a warm temperature, but with a cold-storage finishing stage, and classified as obergäriges lagerbier (top-fermented lager beer).Until the 19th century, the German word lagerbier (de) referred to all types of bottom-fermented, cool-conditioned beer in normal strengths. In Germany today, it mainly refers to beers in southern Germany, "Helles" (pale) and "Dunkel" (dark). Pilsner, a more heavily hopped pale lager, is most often known as "Pilsner", "Pilsener", or "Pils". Other lagers are Bock, Märzen, and Schwarzbier. In the United Kingdom, the term commonly refers to pale lagers derived from the Pilsner style.

List of breads

This is a list of baked or steamed bread varieties. It does not include cakes, pastries, or fried dough foods, which are listed in separate Wikipedia articles. It also does not list foods in which bread is an ingredient which is processed further before serving.


Marmite ( MAR-myte) is a British food spread produced by Unilever. Marmite is made from yeast extract, a by-product of beer brewing. Other similar products include the Australian Vegemite (the name of which is derived partly from that of Marmite), the Swiss Cenovis, and the German Vitam-R.

Marmite is a sticky, dark brown food paste with a distinctive, powerful flavour, which is extremely salty. This distinctive taste is represented in the marketing slogan: "Love it or hate it." Such is its prominence in British popular culture that the product's name is often used as a metaphor for something that is an acquired taste or tends to polarise opinions.The image on the front of the jar shows a "marmite" (French: [maʁmit]), a French term for a large, covered earthenware or metal cooking pot. Marmite was originally supplied in earthenware pots, but since the 1920s has been sold in glass jars.

A similar spread, also named Marmite, has been manufactured in New Zealand since 1919. This is the only product sold as Marmite in Australasia and the Pacific, whereas elsewhere in the world the British version predominates.

Nutritional yeast

Nutritional yeast is a deactivated yeast, often a strain of Saccharomyces cerevisiae, which is sold commercially as a food product. It is sold in the form of flakes or as a yellow powder and can be found in the bulk aisle of most natural food stores. It is popular with vegans and vegetarians and may be used as an ingredient in recipes or as a condiment.It is a significant source of some B-complex vitamins and contains trace amounts of several other vitamins and minerals. Sometimes nutritional yeast is fortified with vitamin B12.

Nutritional yeast has a strong flavor that is described as nutty, cheesy, or creamy, which makes it popular as an ingredient in cheese substitutes. It is often used by vegans in place of cheese, for example in mashed and fried potatoes, in scrambled tofu, or as a topping for popcorn.In Australia, it is sometimes sold as "savoury yeast flakes." In New Zealand, it has long been known as Brufax. In the United States, it is sometimes referred to as "hippie dust" or "nooch". Though "nutritional yeast" usually refers to commercial products, inadequately fed prisoners of war have used "home-grown" yeast to prevent vitamin deficiency.

Nutritional yeast is different from yeast extract, which has a very strong flavour and comes in the form of a dark brown paste.

Proofing (baking technique)

Proofing (also called proving or more rarely blooming), as the term is used by bakers, is the final rise of shaped bread dough before baking. It refers to a specific rest period within the more generalized process known as fermentation. Fermentation is a step in creating yeast breads and baked goods where the yeast is allowed to leaven the dough.

Fermentation rest periods are not always explicitly named, and can appear in recipes as "Allow dough to rise." When they are named, terms include "bulk fermentation," "first rise," "second rise," "final proof" and "shaped proof."

Proofing yeast (as opposed to proofing the shaped bread dough) refers to the process of first dissolving yeast in warm water, a needed hydration step when using active dry yeast. Proofing can also refer to testing the viability of yeast by dissolving it in water and feeding it sugar or carbohydrate. If the yeast is viable, it will feed on the sugar and produce a visible layer of bubbles on the surface of the water mixture.

Red yeast rice

Red yeast rice (simplified Chinese: 红曲米; traditional Chinese: 紅麴米; pinyin: hóng qū mǐ; literally: 'red yeast rice'), red rice koji (べにこうじ, lit. 'red koji'), red fermented rice, red kojic rice, red koji rice, anka, or angkak, is a bright reddish purple fermented rice, which acquires its colour from being cultivated with the mold Monascus purpureus.

Red yeast rice is what is referred to as a "koji" in Japanese, meaning "grain or bean overgrown with a mold culture", a food preparation tradition going back to ca. 300 BC. In both the scientific and popular literature in English that draws principally on Japanese, red yeast rice is most often referred to as "red rice koji". English language articles favoring Chinese literature sources prefer the translation "red yeast rice."

Saccharomyces cerevisiae

Saccharomyces cerevisiae () is a species of yeast. It has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been originally isolated from the skin of grapes (one can see the yeast as a component of the thin white film on the skins of some dark-colored fruits such as plums; it exists among the waxes of the cuticle). It is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the model bacterium. It is the microorganism behind the most common type of fermentation. S. cerevisiae cells are round to ovoid, 5–10 μm in diameter. It reproduces by a division process known as budding.Many proteins important in human biology were first discovered by studying their homologs in yeast; these proteins include cell cycle proteins, signaling proteins, and protein-processing enzymes. S. cerevisiae is currently the only yeast cell known to have Berkeley bodies present, which are involved in particular secretory pathways. Antibodies against S. cerevisiae are found in 60–70% of patients with Crohn's disease and 10–15% of patients with ulcerative colitis (and 8% of healthy controls). S. cerevisiae, a yeast, have been found to contribute to the smell of bread by Schieberle (1990); proline, and ornithine present in yeast are precursors of 2‐acetyl‐l‐pyrroline, a roast‐smelling odorant, in the bread crust.


Sourdough bread is made by the fermentation of dough using naturally occurring lactobacilli and yeast. Sourdough bread has a more sour taste and better inherent keeping qualities than breads made with baker's yeast, due to the lactic acid produced by the lactobacilli.

Two-hybrid screening

Two-hybrid screening (originally known as yeast two-hybrid system or Y2H) is a molecular biology technique used to discover protein–protein interactions (PPIs) and protein–DNA interactions by testing for physical interactions (such as binding) between two proteins or a single protein and a DNA molecule, respectively.

The premise behind the test is the activation of downstream reporter gene(s) by the binding of a transcription factor onto an upstream activating sequence (UAS). For two-hybrid screening, the transcription factor is split into two separate fragments, called the DNA-binding domain (DBD or often also abbreviated as BD) and activating domain (AD). The BD is the domain responsible for binding to the UAS and the AD is the domain responsible for the activation of transcription. The Y2H is thus a protein-fragment complementation assay.

Vaginal yeast infection

Vaginal yeast infection, also known as candidal vulvovaginitis and vaginal thrush, is excessive growth of yeast in the vagina that results in irritation. The most common symptom is vaginal itching, which may be severe. Other symptoms include burning with urination, a thick, white vaginal discharge that typically does not smell bad, pain during sex, and redness around the vagina. Symptoms often worsen just before a woman's period.Vaginal yeast infections are due to excessive growth of Candida. These yeast are normally present in the vagina in small numbers. It is not classified as a sexually transmitted infection; however, it may occur more often in those who are frequently sexually active. Risk factors include taking antibiotics, pregnancy, diabetes, and HIV/AIDS. Eating a diet high in simple sugar may also play a role. Tight clothing, type of underwear, and personal hygiene do not appear to be factors. Diagnosis is by testing a sample of vaginal discharge. As symptoms are similar to that of the sexually transmitted infections, chlamydia and gonorrhea, testing may be recommended.Despite the lack of evidence, wearing cotton underwear and loose fitting clothing is often recommended as a preventative measure. Avoiding douching and scented hygiene products is also recommended. Treatment is with an antifungal medication. This may be either as a cream such as clotrimazole or with oral medications such as fluconazole. Probiotics have not been found to be useful for active infections.Around 75% of women have at least one vaginal yeast infection at some point in their lives, while nearly half have at least two. Around 5% have more than three infections in a single year. It is the second most common cause of vaginal inflammation after bacterial vaginosis.

Yeast in winemaking

The role of yeast in winemaking is the most important element that distinguishes wine from grape juice. In the absence of oxygen, yeast converts the sugars of wine grapes into alcohol and carbon dioxide through the process of fermentation. The more sugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation to dryness. Sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with dessert wines. This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterile filtering the wine to remove the yeast or fortification with brandy or neutral spirits to kill off the yeast cells. If fermentation is unintentionally stopped, such as when the yeasts become exhausted of available nutrients and the wine has not yet reached dryness, this is considered a stuck fermentation.The most common yeast associated with winemaking is Saccharomyces cerevisiae which has been favored due to its predictable and vigorous fermentation capabilities, tolerance of relatively high levels of alcohol and sulfur dioxide as well as its ability to thrive in normal wine pH between 2.8 and 4. Despite its widespread use which often includes deliberate inoculation from cultured stock, S. cerevisiae is rarely the only yeast species involved in a fermentation. Grapes brought in from harvest are usually teeming with a variety of "wild yeast" from the Kloeckera and Candida genera. These yeasts often begin the fermentation process almost as soon as the grapes are picked when the weight of the clusters in the harvest bins begin to crush the grapes, releasing the sugar-rich must. While additions of sulfur dioxide (often added at the crusher) may limit some of the wild yeast activities, these yeasts will usually die out once the alcohol level reaches about 15% due to the toxicity of alcohol on the yeast cells physiology while the more alcohol tolerant Saccharomyces species take over. In addition to S. cerevisiae, Saccharomyces bayanus is a species of yeast that can tolerate alcohol levels of 17–20% and is often used in fortified wine production such as ports and varieties such as Zinfandel and Syrah harvested at high Brix sugar levels. Another common yeast involved in wine production is Brettanomyces whose presence in a wine may be viewed by different winemakers as either a wine fault or in limited quantities as an added note of complexity.

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