Yeasts are

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

, single-celled

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s classified as members of the

fungus A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

are currently recognized. They are estimated to constitute 1% of all described fungal species. Some yeast species have the ability to develop multicellular characteristics by forming strings of connected budding cells known as

pseudohyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

or false

hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

, or quickly evolve into a

multicellular A multicellular organism is an organism that consists of more than one cell (biology), cell, unlike unicellular organisms. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few organism ...

cluster with specialised

cell organelles In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...

function. Yeast sizes vary greatly, depending on species and environment, typically measuring 3–4

μm The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System ...

diameter In geometry, a diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest Chord (geometry), chord of the circle. Both definitions a ...

, although some yeasts can grow to 40 μm in size. Most yeasts reproduce asexually by

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

, and many do so by the asymmetric division process known as

budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...

. With their single-celled growth habit, yeasts can be contrasted with

mold A mold () or mould () is one of the structures that certain fungus, fungi can form. The dust-like, colored appearance of molds is due to the formation of Spore#Fungi, spores containing Secondary metabolite#Fungal secondary metabolites, fungal ...

s, which grow

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

e. Fungal species that can take both forms (depending on temperature or other conditions) are called

dimorphic fungi A dimorphic fungus is a fungus that can exist in the form of both mold and yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of mill ...

. The yeast species ''

Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungal microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have be ...

'' converts

carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...

s to

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

and

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

s through the process of

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

. The products of this reaction have been used in

baking Baking is a method of preparing food that uses dry heat, typically in an oven, but it can also be done in hot ashes, or on hot Baking stone, stones. Bread is the most commonly baked item, but many other types of food can also be baked. Heat is ...

and the production of alcoholic beverages for thousands of years. ''S. cerevisiae'' is also an important

model organism A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...

in modern

cell biology Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living an ...

research, and is one of the most thoroughly studied eukaryotic microorganisms. Researchers have cultured it in order to understand the biology of the eukaryotic cell and ultimately human biology in great detail. Other species of yeasts, such as ''

Candida albicans ''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is usu ...

'', are

opportunistic pathogen An opportunistic infection is an infection that occurs most commonly in individuals with an immunodeficiency disorder and acts more severe on those with a weakened immune system. These types of infections are considered serious and can be caused b ...

s and can cause

infections An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...

in humans. Yeasts have recently been used to generate electricity in

microbial fuel cell Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell system also known as micro fuel cell that generates electric current by diverting electrons produced from the microbial oxidation of reduced compounds (also known as fuel or ele ...

s and to produce

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

for the

biofuel Biofuel is a fuel that is produced over a short time span from Biomass (energy), biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricu ...

industry. Yeasts do not form a single

taxonomic 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme of classes (a taxonomy) and the allocation ...

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

grouping. The term "yeast" is often taken as a

synonym A synonym is a word, morpheme, or phrase that means precisely or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are a ...

for ''Saccharomyces cerevisiae'', but the phylogenetic diversity of yeasts is shown by their placement in two separate

phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...

: the

Ascomycota Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The def ...

and the

Basidiomycota 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. Members are known as basidiomycetes. More specifically, Basi ...

. The budding yeasts or "true yeasts" are classified in the

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Saccharomycetales Saccharomycetales belongs to the kingdom of Fungi and the division Ascomycota. It is the only order in the class Saccharomycetes. There are currently 13 families recognized as belonging to Saccharomycetales. GBIF also includes; Alloascoideaceae ...

, within the phylum Ascomycota.

History

The word "yeast" comes from

Old English Old English ( or , or ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the Early Middle Ages. It developed from the languages brought to Great Britain by Anglo-S ...

''gist'', ''gyst'', and from the

Indo-European The Indo-European languages are a language family native to the northern Indian subcontinent, most of Europe, and the Iranian plateau with additional native branches found in regions such as Sri Lanka, the Maldives, parts of Central Asia (e. ...

root ''yes-'', meaning "boil", "foam", or "bubble". Yeast microbes are probably one of the earliest

domesticated Domestication is a multi-generational mutualistic relationship in which an animal species, such as humans or leafcutter ants, takes over control and care of another species, such as sheep or fungi, to obtain from them a steady supply of reso ...

organisms. Archaeologists digging in

Egyptian ''Egyptian'' describes something of, from, or related to Egypt. Egyptian or Egyptians may refer to: Nations and ethnic groups * Egyptians, a national group in North Africa ** Egyptian culture, a complex and stable culture with thousands of year ...

ruins found early grinding stones and baking chambers for yeast-raised bread, as well as drawings of 4,000-year-old bakeries and

breweries A brewery or brewing company is a business that makes and sells beer. The place at which beer is commercially made is either called a brewery or a beerhouse, where distinct sets of brewing equipment are called plant. The commercial brewing of be ...

. Vessels studied from several archaeological sites in

Israel Israel, officially the State of Israel, is a country in West Asia. It Borders of Israel, shares borders with Lebanon to the north, Syria to the north-east, Jordan to the east, Egypt to the south-west, and the Mediterranean Sea to the west. Isr ...

(dating to around 5,000, 3,000 and 2,500 years ago), which were believed to have contained alcoholic beverages (

beer Beer is an alcoholic beverage produced by the brewing and fermentation of starches from cereal grain—most commonly malted barley, although wheat, maize (corn), rice, and oats are also used. The grain is mashed to convert starch in the ...

and

mead Mead (), also called honey wine, and hydromel (particularly when low in alcohol content), is an alcoholic beverage made by fermenting honey mixed with water, and sometimes with added ingredients such as fruits, spices, grains, or hops. The alco ...

), were found to contain yeast colonies that had survived over the millennia, providing the first direct biological evidence of yeast use in early cultures. In 1680,

Dutch Dutch or Nederlands commonly refers to: * Something of, from, or related to the Netherlands ** Dutch people as an ethnic group () ** Dutch nationality law, history and regulations of Dutch citizenship () ** Dutch language () * In specific terms, i ...

naturalist

Anton van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch art, science and technology. A largely self-taught man in science, he is commonly known as " ...

first

microscopically Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical, e ...

observed yeast, but at the time did not consider them to be

living organisms An organism is any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been pro ...

, but rather globular structures as researchers were doubtful whether yeasts were

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

or fungi.

Theodor Schwann Theodor Schwann (; 7 December 181011 January 1882) was a German physician and physiology, physiologist. His most significant contribution to biology is considered to be the extension of cell theory to animals. Other contributions include the d ...

recognized them as fungi in 1837. In 1857, French microbiologist

Louis Pasteur Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist, pharmacist, and microbiologist renowned for his discoveries of the principles of vaccination, Fermentation, microbial fermentation, and pasteurization, the la ...

showed that by bubbling oxygen into the yeast broth,

cell growth Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

could be increased, but fermentation was inhibited – an observation later called the "

Pasteur effect The Pasteur effect describes how available oxygen inhibits ethanol fermentation, driving yeast to switch toward aerobic respiration for increased generation of the energy carrier adenosine triphosphate (ATP). More generally, in the medical liter ...

". In the paper "''Mémoire sur la fermentation alcoolique,''" Pasteur proved that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst. By the late 18th century two yeast strains used in brewing had been identified: ''

'' (top-fermenting yeast) and '' S. pastorianus'' (bottom-fermenting yeast). ''S. cerevisiae'' has been sold commercially by the Dutch for bread-making since 1780; while, around 1800, the Germans started producing ''S. cerevisiae'' in the form of cream. In 1825, a method was developed to remove the liquid so the yeast could be prepared as solid blocks. The industrial production of yeast blocks was enhanced by the introduction of the filter press in 1867. In 1872, Baron Max de Springer developed a manufacturing process to create granulated yeast from

beetroot The beetroot (British English) or beet (North American English) is the taproot portion of a ''Beta vulgaris'' subsp. ''vulgaris'' plant in the Conditiva Group. The plant is a root vegetable also known as the table beet, garden beet, dinner bee ...

molasses Molasses () is a viscous byproduct, principally obtained from the refining of sugarcane or sugar beet juice into sugar. Molasses varies in the amount of sugar, the method of extraction, and the age of the plant. Sugarcane molasses is usuall ...

,Miniac (de) M., 1988. Conduite des ateliers de fermentation alcoolique de produits sucriers (mélasses et égouts). Industries alimentaires et Agricoles 105, 675–688. 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 The Centennial International Exhibition, officially the International Exhibition of Arts, Manufactures, and Products of the Soil and Mine, was held in Philadelphia, Pennsylvania, from May 10 to November 10, 1876. It was the first official wo ...

in 1876 in Philadelphia, where Charles L. Fleischmann exhibited the product and a process to use it, as well as serving the resultant baked bread. The mechanical refrigerator (first patented in the 1850s in Europe) liberated brewers and

winemakers A winemaker or vintner is a person engaged in winemaking. They are generally employed by wineries or wine companies, where their work includes: *Cooperating with viticulturists *Monitoring the maturity of grapes to ensure their quality and to de ...

from seasonal constraints for the first time and allowed them to exit cellars and other earthen environments. For

John Molson John Molson (28 December 1763 – 11 January 1836) was an English people, English-born brewer and entrepreneur in colonial Province of Quebec (1763–91), Quebec, which during his lifetime became Lower Canada. In addition to founding Molson Brewe ...

, who made his livelihood in

Montreal Montreal is the List of towns in Quebec, largest city in the Provinces and territories of Canada, province of Quebec, the List of the largest municipalities in Canada by population, second-largest in Canada, and the List of North American cit ...

prior to the development of the fridge, the brewing season lasted from September through to May. The same seasonal restrictions formerly governed the

distiller Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...

's art.

Nutrition and growth

Yeasts are

chemoorganotroph Primary nutritional groups are groups of organisms, divided in relation to the nutrition mode according to the sources of energy and carbon, needed for living, growth and reproduction. The sources of energy can be light or chemical compounds; the ...

s, as they use

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s as a source of energy and do not require sunlight to grow. Carbon is obtained mostly from

hexose In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is , and their molecular weight is 180.156 g/mol. Hexoses exist in two forms, open-chain or cyclic, that easily convert into ...

sugars, such as

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

and

fructose Fructose (), or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and gal ...

, or disaccharides such as

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...

and

maltose } Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the tw ...

. Some species can metabolize

pentose In chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atoms. The chemical formula of many pentoses is , and their molecular weight is 150.13 g/mol. alcohols, and

organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are re ...

s. Yeast species either require oxygen for aerobic

cellular respiration Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cell ...

(

obligate aerobe An obligate aerobe is an organism that requires oxygen to grow. Through cellular respiration, these organisms use oxygen to metabolise substances, like sugars or fats, to obtain energy. In this type of respiration, oxygen serves as the terminal ...

s) or are anaerobic, but also have aerobic methods of energy production (

facultative anaerobe A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent. Some examples of facultatively anaerobic bacteria are ''Staphylococcus' ...

s). Unlike

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

, no known yeast species grow only anaerobically (

obligate anaerobe Obligate anaerobes are microorganisms killed by normal Atmosphere of Earth, atmospheric concentrations of oxygen (20.95% O2). Oxygen tolerance varies between species, with some species capable of surviving in up to 8% oxygen, while others lose vi ...

s). 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 , '' Saccharomyces telluris'' at , and '' Candida slooffi'' at . 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 A growth medium or culture medium is a solid, liquid, or semi-solid designed to support the growth of a population of microorganisms or cells via the process of cell proliferation or small plants like the moss ''Physcomitrella patens''. Differe ...

or in liquid

broth Broth, also known as bouillon (), is a savory liquid made of water in which meat, fish, or vegetables have been simmered for a short period of time. It can be eaten alone, but it is most commonly used to prepare other dishes, such as soups ...

s. Common media used for the cultivation of yeasts include

potato dextrose agar Potato dextrose agar (BAM Media M127
from the ...

potato dextrose broth Potato dextrose agar (BAM Media M127
from the ...

, Wallerstein Laboratories nutrient

agar Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from " ogonori" and " tengusa". As found in nature, agar is a mixture of two components, t ...

, yeast

peptone Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty ami ...

dextrose Glucose is a sugar with the molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae during photosynthesis from water an ...

agar, and yeast mould agar or broth. Home brewers who cultivate yeast frequently use dried

malt extract Malt is any cereal grain that has been made to germinate by soaking in water and then stopped from germinating further by drying with hot air, a process known as " malting". Malted grain is used to make beer, whisky, malted milk, malt vinegar ...

and agar as a solid growth medium. The

fungicide Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...

cycloheximide Cycloheximide is a naturally occurring fungicide produced by the bacterium '' Streptomyces griseus''. Cycloheximide exerts its effects by interfering with the translocation step in protein synthesis (movement of two tRNA molecules and mRNA in re ...

is sometimes added to yeast growth media to inhibit the growth of ''

Saccharomyces ''Saccharomyces'' is a genus of fungi that includes many species of yeasts. ''Saccharomyces'' is from Greek σάκχαρον (sugar) and μύκης (fungus) and means ''sugar fungus''. Many members of this genus are considered very important in f ...

'' yeasts and select for wild/indigenous yeast species. This will change the yeast process. The appearance of a white, thready yeast, commonly known as kahm yeast, is often a byproduct of the lactofermentation (or pickling) of certain vegetables. It is usually the result of exposure to air. Although harmless, it can give pickled vegetables a bad flavor and must be removed regularly during fermentation.

Ecology

Yeasts are very common in the environment, and are often isolated from sugar-rich materials. Examples include naturally occurring yeasts on the skins of fruits and berries (such as grapes, apples, or

peach The peach (''Prunus persica'') is a deciduous tree first domesticated and Agriculture, cultivated in China. It bears edible juicy fruits with various characteristics, most called peaches and the glossy-skinned, non-fuzzy varieties called necta ...

es), and

exudate An exudate is a fluid released by an organism through pores or a wound, a process known as exuding or exudation. ''Exudate'' is derived from ''exude'' 'to ooze' from Latin language, Latin 'to (ooze out) sweat' (' 'out' and ' 'to sweat'). Medi ...

s from plants (such as plant saps or cacti). Some yeasts are found in association with soil and insects. Yeasts from the soil and from the skins of fruits and berries have been shown to dominate fungal

succession Succession is the act or process of following in order or sequence. Governance and politics *Order of succession, in politics, the ascension to power by one ruler, official, or monarch after the death, resignation, or removal from office of ...

during fruit decay. The ecological function and

biodiversity Biodiversity is the variability of life, life on Earth. It can be measured on various levels. There is for example genetic variability, species diversity, ecosystem diversity and Phylogenetics, phylogenetic diversity. Diversity is not distribut ...

of yeasts are relatively unknown compared to those of other

s. Yeasts, including ''

'', ''

Rhodotorula ''Rhodotorula'' is a genus of fungi in the class (biology), class Microbotryomycetes. Most species are known in their yeast states which produce orange to red colony (biology), colonies when grown on Sabouraud agar, Sabouraud's dextrose agar (SD ...

rubra'', '' Torulopsis'' and ''

Trichosporon cutaneum ''Trichosporon'' is a genus of anamorphic fungi in the family Trichosporonaceae. All species of ''Trichosporon'' are yeasts with no known teleomorphs (sexual states). Most are typically isolated from soil, but several species occur as a natur ...

'', have been found living in between people's toes as part of their

skin flora Skin flora, also called skin microbiota, refers to microbiota (community (ecology), communities of microorganisms) that reside on the skin, typically human skin. Many of them are bacterium, bacteria of which there are around 1,000 species upon hu ...

. Yeasts are also present in the

gut flora Gut microbiota, gut microbiome, or gut flora are the microorganisms, including bacteria, archaea, fungi, and viruses, that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the g ...

of mammals and some insects and even deep-sea environments host an array of yeasts. An Indian study of seven

bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfamil ...

species and nine plant species found 45 species from 16 genera colonize the

nectaries Nectar is a viscous, sugar-rich liquid produced by plants in glands called nectaries, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualists, ...

of flowers and honey stomachs of bees. Most were members of the genus ''

Candida Candida, or Cándida (Spanish), may refer to: Biology and medicine * ''Candida'' (fungus), a genus of yeasts ** Candidiasis, an infection by ''Candida'' organisms * Malvasia Candida, a variety of grape Places * Candida, Campania, a ''comu ...

''; the most common species in honey stomachs was ''

Dekkera intermedia ''Brettanomyces'' is a non-spore forming genus of yeast in the family Saccharomycetaceae, and is often colloquially referred to as "Brett". The genus name ''Dekkera'' is used interchangeably with ''Brettanomyces'', as it describes the teleomorph ...

'' and in flower nectaries, ''

Candida blankii ''Candida blankii'' is a species of budding yeast (Saccharomycotina) in the family Saccharomycetaceae. The yeast may be a dangerous pathogen and resistant to treatment in human hosts. Research on this fungus has therapeutic, medical, and industr ...

''. Yeast colonising nectaries of the

stinking hellebore ''Helleborus foetidus'', known variously as stinking hellebore , dungwort, setterwort and bear's foot, is a species of flowering plant in the buttercup family Ranunculaceae, native to the mountainous regions of Central and Southern Europe and As ...

have been found to raise the temperature of the flower, which may aid in attracting pollinators by increasing the evaporation of

volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapor pressure at room temperature. They are common and exist in a variety of settings and products, not limited to Indoor mold, house mold, Upholstery, upholstered furnitur ...

s. A

black yeast Black yeasts, sometimes also black fungi, dematiaceous fungi, microcolonial fungi or meristematic fungi is a diverse group of slow-growing microfungi which reproduce mostly asexually ( fungi imperfecti). Only few genera reproduce by budding cells ...

has been recorded as a partner in a complex relationship between

ant Ants are Eusociality, eusocial insects of the Family (biology), family Formicidae and, along with the related wasps and bees, belong to the Taxonomy (biology), order Hymenoptera. Ants evolved from Vespoidea, vespoid wasp ancestors in the Cre ...

s, their mutualistic fungus, a fungal

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

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. A different yeast species is grown by

fungus-growing ants Fungus-growing ants (tribe Attini) comprise all the known fungus-growing ant species participating in ant–fungus mutualism. They are known for cutting grasses and leaves, carrying them to their colonies' nests, and using them to grow fungus on ...

of the genus '' Cyphomyrmex''. 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 A killer yeast is a yeast, such as ''Saccharomyces cerevisiae'', which is able to secrete one of a number of toxic proteins which are lethal to susceptible cells. These "killer toxins" are polypeptides that kill sensitive cells of the same or relat ...

.) 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 The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

, and those were identified as ''Torula'' sp. and ''Mycoderma'' sp. Following this discovery, various other marine yeasts have been isolated from around the world from different sources, including seawater, seaweeds, marine fish and mammals. Among these isolates, some marine yeasts originated from terrestrial habitats (grouped as facultative marine yeast), which were brought to and survived in marine environments. The other marine yeasts were grouped as obligate or indigenous marine yeasts, which are confined to marine habitats. However, no sufficient evidence has been found to explain the indispensability of seawater for obligate marine yeasts. This article contains quotations from this source, which is available under a Creative Commons Attribution license. It has been reported that marine yeasts are able to produce many bioactive substances, such as amino acids, glucans, glutathione, toxins, enzymes, phytase, and vitamins with potential applications in the food, pharmaceutical, cosmetic, and chemical industries as well as for marine culture and environmental protection. Marine yeast was successfully used to produce bioethanol using seawater-based media which will potentially reduce the

water footprint A water footprint shows the extent of water use in relation to Consumption (economics), consumption by people. The water footprint of an individual, community, or business is defined as the total volume of fresh water used to produce the goods an ...

of bioethanol.

Reproduction

Yeasts, like all fungi, may have

asexual Asexual or Asexuals may refer to: *Asexual reproduction **Asexual reproduction in starfish *Asexuality, the lack of sexual attraction to anyone or lack of interest in or desire for sexual activity. **Gray asexuality, the spectrum between asexualit ...

and sexual reproductive cycles. The most common mode of vegetative growth in yeast is asexual reproduction by

, where a small bud (also known as a bleb or daughter cell) is formed on the parent cell. The

nucleus Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucleu ...

of the parent cell splits into a daughter nucleus and migrates into the daughter cell. The bud then continues to grow until it separates from the parent cell, forming a new cell. The daughter cell produced during the budding process is generally smaller than the mother cell. Some yeasts, including ''

Schizosaccharomyces pombe ''Schizosaccharomyces pombe'', also called "fission yeast", is a species of yeast used in traditional brewing and as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. Cells typically meas ...

'', reproduce by fission instead of budding, and thereby creating two identically sized daughter cells. In general, under high-stress conditions such as

nutrient A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

starvation,

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

cells will die; under the same conditions, however,

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

cells can undergo sporulation, entering sexual reproduction (

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

) and producing a variety of haploid

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

s, which can go on to

mate Mate may refer to: Science * Mate, one of a pair of animals involved in: ** Mate choice, intersexual selection *** Mate choice in humans ** Mating * Multi-antimicrobial extrusion protein, or MATE, an efflux transporter family of proteins Pers ...

(conjugate), reforming the diploid. The haploid fission yeast ''Schizosaccharomyces pombe'' is a facultative sexual microorganism that can undergo mating when nutrients are limited. Exposure of ''S. pombe'' to hydrogen peroxide, an agent that causes oxidative stress leading to oxidative DNA damage, strongly induces mating and the formation of meiotic spores. The budding yeast ''Saccharomyces cerevisiae'' reproduces by mitosis as diploid cells when nutrients are abundant, but when starved, this yeast undergoes meiosis to form haploid spores. Haploid cells may then reproduce asexually by mitosis. Katz Ezov et al. presented evidence that in natural ''S. cerevisiae'' populations clonal reproduction and selfing (in the form of intratetrad mating) predominate. In nature, the mating of haploid cells to form diploid cells is most often between members of the same clonal population and

out-crossing Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity. Outcrossing in animals Out ...

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 ''Sporobolomyces'' is a genus of fungi in the subdivision Pucciniomycotina. Species produce both yeast states and hyphal states. The latter form teliospores from which auricularioid (tubular and laterally septate) basidia emerge, bearing bas ...

'', produce aerially dispersed, asexual ballistoconidia.

Uses

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

biotechnology Biotechnology is a multidisciplinary field that involves the integration of natural sciences and Engineering Science, engineering sciences in order to achieve the application of organisms and parts thereof for products and services. Specialists ...

Fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

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 Baker's yeast is the common name for the strains of yeast commonly used in baking bread and other bakery products, serving as a leavening agent which causes the bread to rise (expand and become lighter and softer) by converting the fermentable ...

in bread production, brewer's yeast in

beer fermentation 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, ...

, and yeast in wine fermentation and for

xylitol Xylitol is a chemical compound with the formula , or HO(CH2)(CHOH)3(CH2)OH; specifically, one particular Stereoisomerism, stereoisomer with that structural formula. It is a colorless or white crystalline solid. It is classified as a polyalcoho ...

production. So-called red rice yeast is actually a

, ''

Monascus purpureus ''Monascus purpureus'' (syn. ''M. albidus'', ''M. anka'', ''M. araneosus'', ''M. major'', ''M. rubiginosus'', and ''M. vini''; zh, s= 红曲霉, t=紅麴黴, p=hóng qū méi, lit. "red yeast") is a species of mold that is purplish-red in colo ...

''. Yeasts include some of the most widely used

s for

genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinians, Augustinian ...

and

Alcoholic beverages

Alcoholic beverages are defined as

beverages A drink or beverage is a liquid intended for human consumption. In addition to their basic function of satisfying thirst, drinks play important roles in human culture. Common types of drinks include plain drinking water, milk, juice, smoothie ...

that contain

(C₂H₅OH). This ethanol is almost always produced by

– the

metabolism Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...

s by certain species of yeasts under anaerobic or low-oxygen conditions. Beverages such as mead, wine, beer, or

distilled spirits Liquor ( , sometimes hard liquor), spirits, distilled spirits, or spiritous liquor are alcoholic drinks produced by the distillation of grains, fruits, vegetables, or sugar that have already gone through alcoholic fermentation. While the w ...

all use yeast at some stage of their production. A distilled beverage is a beverage containing ethanol that has been purified by

distillation Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...

. Carbohydrate-containing plant material is fermented by yeast, producing a dilute solution of ethanol in the process. Spirits such as

whiskey Whisky or whiskey is a type of liquor made from Fermentation in food processing, fermented grain mashing, mash. Various grains (which may be Malting, malted) are used for different varieties, including barley, Maize, corn, rye, and wheat. Whisky ...

and

rum Rum is a liquor made by fermenting and then distilling sugarcane molasses or sugarcane juice. The distillate, a clear liquid, is often aged in barrels of oak. Rum originated in the Caribbean in the 17th century, but today it is produced i ...

are prepared by distilling these dilute solutions of ethanol. Components other than ethanol are collected in the condensate, including water,

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

s, 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 Wort () is the liquid extracted from the mashing process during the brewing of beer or whisky. Wort contains the sugars, the most important being maltose and maltotriose, that will be Ethanol fermentation, fermented by the brewing yeast to prod ...

during fermentation. An example of a top-cropping yeast is ''

'', sometimes called an "ale yeast". Bottom-cropping yeasts are typically used to produce

lager Lager (; ) is a Type of beer, style of beer brewed and Brewing#Conditioning, conditioned at low temperature. Lagers can be Pale lager, pale, Amber lager, amber, or Dark lager, dark. Pale lager is the most widely consumed and commercially availab ...

-type beers, though they can also produce

ale Ale is a style of beer, brewed using a warm fermentation method. In medieval England, the term referred to a drink brewed without hops. As with most beers, ale typically has a bittering agent to balance the malt and act as a preservative. Ale ...

-type beers. These yeasts ferment well at low temperatures. An example of bottom-cropping yeast is ''

Saccharomyces pastorianus ''Saccharomyces pastorianus'' is a yeast used industrially for the production of lager beer, and was named in honour of Louis Pasteur by the German Max Reess in 1870. This yeast's complicated genome appears to be the result of Hybridisation (bio ...

'', 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 Melibiose is a reducing disaccharide formed by an α-1,6 linkage between galactose and glucose (D-Gal-(α1→6)-D-Glc). It differs from lactose in the chirality of the carbon where the galactose ring is closed and that the galactose is linked to a ...

, a

disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, ...

, into more fermentable

monosaccharide Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. Chemically, monosaccharides are polyhy ...

s. 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 vitamin B vitamins are a class of water-soluble vitamins that play important roles in cell metabolism and synthesis of red blood cells. They are a chemically diverse class of compounds. Dietary supplements containing all eight are referred to as a vit ...

s (except B₁₂), a feature exploited in food products made from leftover (

by-product A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be cons ...

) yeast from brewing. However, baking and brewing yeasts typically belong to different strains, cultivated to favour different characteristics: baking yeast strains are more aggressive, to carbonate

dough Dough is a malleable, sometimes elastic paste made from flour (which itself is made from grains or from leguminous or chestnut crops). Dough is typically made by mixing flour with a small amount of water or other liquid and sometimes includes ...

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 Lambic ( , ; ) is a type of beer brewed in the Pajottenland region of Belgium southwest of Brussels since the 13th century. Types of lambic beer include gueuze, kriek lambic, and framboise. Lambic differs from most other beers in that it is Bre ...

' and specialty

sour ale Sour beer is beer which has an intentionally acidic, tart, or sour taste. Sour beer styles include Belgian lambics and Flanders red ale and German Gose and Berliner Weisse. Brewing Unlike modern brewing, which is done in a sanitary environment to ...

s, along with the secondary conditioning of a particular Belgian

Trappist beer Trappist beer is brewed by Trappist monks. Thirteen Trappist monasteries—six in Belgium, two in the Netherlands, and one each in Austria, Italy, England, France, and Spain— produce beer, but the ''Authentic Trappist Product'' label is assigned ...

. 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 bruxellensis'' (the anamorph of ''Dekkera bruxellensis'') is a yeast associated with the Senne valley near Brussels, Belgium. Despite its Latin species name, ''B. bruxellensis'' is found all over the globe. In the wild, it is oft ...

'', '' Brettanomyces anomalus'', '' Brettanomyces custersianus'', '' Brettanomyces naardenensis'', and '' Brettanomyces nanus'', with teleomorphs existing for the first two species, ''

Dekkera bruxellensis ''Brettanomyces bruxellensis'' (the anamorph of ''Dekkera bruxellensis'') is a yeast associated with the Senne valley near Brussels, Belgium. Despite its Latin species name, ''B. bruxellensis'' is found all over the globe. In the wild, it is oft ...

'' 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 Winemaking, wine-making, or vinification is the production of wine, starting with the selection of the fruit, its Ethanol fermentation, fermentation into alcohol, and the bottling of the finished liquid. The history of wine-making stretches over ...

, where it converts the sugars present (

and

) in grape juice (must) into ethanol. Yeast is normally already present on grape skins. Fermentation (wine), 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 Wine color, 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, most commonly ''S. cerevisiae'', is used in baking as a leavening agent, converting the fermentation (food), fermentable sugars present in dough into

. 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, egg (food), eggs, or sugar in a bread dough accelerates the growth of yeast. Most yeasts used in baking are of the same species common in alcoholic fermentation. In addition, ''Saccharomyces exiguus'' (also known as ''S. minor''), a wild yeast found on plants, fruits, and grains, is occasionally used for baking. In breadmaking, the yeast initially respires aerobically, producing carbon dioxide and water. When the oxygen is depleted, fermentation (biochemistry), fermentation begins, producing ethanol as a waste product; however, this evaporates during baking. Compressed fresh yeast - 1

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 wikt:granulate, 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 (baking technique), 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 and dough, sponge. When yeast is used for making bread, it is mixed with flour, salt, and warm water or milk. The dough is Kneading, 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 proofing (baking technique), dough proofing) and then baked. A longer rising time gives a better flavor, but the yeast can fail to raise the bread in the final stages if it is left for too long initially.

Bioremediation

Some yeasts can find potential application in the field of bioremediation. One such yeast, ''Yarrowia lipolytica'', is known to degrade palm oil mill effluent, Trinitrotoluene, 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 metal (chemistry), heavy metals, and is being investigated for its potential as a heavy metal Biosorption, 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 bioaccumulation, bioaccumulate free and Coordination complex, 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 Lignocellulosic biomass, 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 soft drink, 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 in the United States, 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: ''

'', ''Candida stellata'', ''

'', ''Torulaspora delbrueckii'' and ''Zygosaccharomyces bailii''. Also popular in Eastern Europe and some Post-Soviet states, former Soviet republics under the name ''chajnyj grib'' (), which means "tea mushroom". * Kefir and kumis are made by fermenting milk with yeast and bacteria. * Mauby (), made by fermenting sugar with the wild yeasts naturally present on the bark of the ''Colubrina elliptica'' tree, popular in the Caribbean

Foods and

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, yeast often contains free glutamic acid. Examples include: * Yeast extract, made from the intracellular contents of yeast and used as food additives or Flavoring, flavours. The general method for making yeast extract for food products such as Vegemite and Marmite on a commercial scale is ''heat autolysis'', i.e. to add salt to a suspension of yeast, making the solution hypertonic, which leads to the cells' shrivelling up. This triggers autolysis (biology), autolysis, wherein the yeast's digestion, digestive enzymes break their own proteins down into simpler compounds, a process of self-destruction. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeast with thick cell walls that would give poor texture) are removed. Yeast autolysates are used in Vegemite and Promite (Australia); Marmite (the United Kingdom); the unrelated Marmite (New Zealand), Marmite (New Zealand); Vitam-R (Germany); and Cenovis (Switzerland). * Hefeflocken Naturata

Nutritional yeast, which is whole dried, deactivated yeast cells, usually ''S. cerevisiae''. Usually in the form of yellow flake or powder, its nutty and umami flavor makes it a vegan substitute for Powdered cheese, cheese powder. Another popular use is as a topping for popcorn. It can also be used in mashed and fried potatoes, as well as in scrambled eggs. It comes in the form of flakes, or as a yellow powder similar in texture to cornmeal. In Australia, it is sometimes sold as "savoury yeast flakes". Both types of yeast foods above are rich in Vitamin B, B-complex vitamins (besides vitamin B₁₂ unless fortified), making them an attractive nutritional supplement to vegans. The same vitamins are also found in some yeast-fermented products mentioned above, such as kvass. Nutritional yeast in particular is naturally low in fat and sodium and a source of protein and vitamins as well as other minerals and cofactor (biochemistry), cofactors required for growth. Many brands of nutritional yeast and yeast extract spreads, though not all, are fortified with Cyanocobalamin, vitamin B₁₂, which is produced separately by

. In 1920, the Fleischmann's Yeast, Fleischmann Yeast Company began to promote yeast cakes in a "Yeast for Health" campaign. They initially emphasized yeast as a source of vitamins, good for skin and digestion. Their later advertising claimed a much broader range of health benefits, and was censured as misleading by the Federal Trade Commission. The fad for yeast cakes lasted until the late 1930s.

Probiotics

Some probiotic supplements use the yeast ''Saccharomyces boulardii, 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 (bacteria), Clostridium difficile'' (often identified simply as C. difficile or C. diff), reduce bowel movements in diarrhea-predominant irritable bowel syndrome, IBS patients, and reduce the incidence of antibiotic-associated diarrhea, antibiotic-, travelers' diarrhea, 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 Aquascaping, planted aquaria. CO₂ levels from yeast are more difficult to regulate than those from pressurized CO₂ systems. However, the low cost of yeast makes it a widely used alternative.

Scientific research

Several yeasts, in particular ''S. cerevisiae'' and ''S. pombe'', have been widely used in genetics and cell biology, largely because they are simple eukaryote, 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, genetic recombination, 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 (genetics), tetrad analysis. Many proteins important in human biology were first discovered by studying their homology (biology), homologues in yeast; these proteins include cell cycle proteins, Cell signaling, signaling proteins, and protein-processing enzymes. On 24 April 1996, ''S. cerevisiae'' was announced to be the first eukaryote to have its genome, consisting of 12 million base pairs, fully sequenced as part of the Genome Project. At the time, it was the most complex organism to have its full genome sequenced, and the work of seven years and the involvement of more than 100 laboratories to accomplish. The second yeast species to have its genome sequenced was ''Schizosaccharomyces pombe'', which was completed in 2002. It was the sixth eukaryotic genome sequenced and consists of 13.8 million base pairs. As of 2014, over 50 yeast species have had their genomes sequenced and published. Genomic and functional gene annotation of the two major yeast models can be accessed via their respective model organism databases: 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 phenols, 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

s 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 infection 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 ''

'', another group of opportunistic pathogens, cause Oral candidiasis, 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

l outgrowth, which locally penetrates the mucous membrane, mucosal membrane, causing irritation and shedding of the tissues. A book from the 1980s listed the pathogenic yeasts of candidiasis in probable descending order of virulence for humans as: ''Candida albicans, C. albicans'', ''Candida tropicalis, C. tropicalis'', ''Candida stellatoidea, C. stellatoidea'', ''Candida glabrata, C. glabrata'', ''Candida krusei, C. krusei'', ''Candida parapsilosis, C. parapsilosis'', ''Candida guilliermondii, C. guilliermondii'', ''Candida viswanathii, C. viswanathii'', ''Candida lusitaniae, C. lusitaniae'', and ''Rhodotorula, 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). ''Candida auris, 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. During their growth, yeasts metabolize some food components and produce metabolic end products. This causes the physical, chemical, and sensible properties of a food to change, and the food is spoiled. The growth of yeast within food products is often seen on their surfaces, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semiliquid products, such as syrups and jams. The yeast of the genus ''Zygosaccharomyces'' have had a long history as spoilage yeasts within the food industry. This is mainly because these species can grow in the presence of high sucrose, ethanol, acetic acid, sorbic acid, benzoic acid, and sulfur dioxide concentrations, representing some of the commonly used food preservation methods. Methylene blue is used to test for the presence of live yeast cells. In oenology, the major spoilage yeast is ''

''. ''

'' has been detected in Iberian ham and meat.

Symbiosis

An Indian study of seven

species and nine plant species found 45 yeast species from 16 genera colonise the

of flowers and honey stomachs of bees. Most were members of the genus ''

''; the most common species in honey bee stomachs was ''

'', while the most common species colonising flower nectaries was ''

''. Although the mechanism is not fully understood, it was found that ''A. indica'' flowers more if ''

'' is present. In another example, ''Spathaspora passalidarum'', found in the digestive tract of Passalidae, bess beetles, aids the digestion of plant cells by fermenting xylose. 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. Yeast and small hive beetle have mutualistic relationship. While small hive beetle is attracted by the pheromone released by the host honeybee, yeast can produce a similar pheromone which have the same attractive effect to the small hive beetle. Therefore, yeast facilitates SHB's infestation if the beehive contains yeast inside.

References

External links

''Saccharomyces'' genome database

Yeast growth and the cell cycle
(archived 21 July 2007)
Yeast virtual library
{{Authority control Yeasts, Abortifacients Brewing ingredients Fermentation in food processing Food additives Fungus common names Leavening agents Medicinal fungi

History

Nutrition and growth

Ecology

Reproduction

Uses

Alcoholic beverages

Beer

Wine

Baking

Bioremediation

Industrial ethanol production

Nonalcoholic beverages

Foods and

Probiotics

Aquarium hobby

Scientific research

Genetically engineered biofactories

Pathogenic yeasts

Food spoilage

Symbiosis

See also

References

Further reading

External links