Ascomycota (sac fungi)Saccharomycotina (true yeasts) Taphrinomycotina Schizosaccharomycetes (fission yeasts)
Basidiomycota (club fungi)Urediniomycetes Sporidiales
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with about 1,500 species currently described; they dominate fungal diversity in the oceans. Most reproduce asexually by budding, although a few do so by binary fission. Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation of a string of connected budding cells known as pseudohyphae, or false hyphae as seen in most molds. Yeast size can vary greatly depending on the species, typically measuring 3â€“4 Âµm in diameter, although some yeasts can reach over 40 Âµm.
The yeast species Saccharomyces cerevisiae has been used in baking and fermenting alcoholic beverages for thousands of years. It is also extremely important as a model organism in modern cell biology research, and is one of the most thoroughly researched eukaryotic microorganisms. Researchers have used it to gather information about the biology of the eukaryotic cell and ultimately human biology. Other species of yeast, such as Candida albicans, are opportunistic pathogens and can cause infections in humans. Yeasts have recently been used to generate electricity in microbial fuel cells, and produce ethanol for the biofuel industry.
Yeasts do not form an exact taxonomic or phylogenetic grouping. At present it is estimated that only 1% of all yeast species have been described. The term ``yeast`` is often taken as a synonym for S. cerevisiae, but the phylogenetic diversity of yeasts is shown by their placement in both divisions Ascomycota and Basidiomycota. The budding yeasts (``true yeasts``) are classified in the order Saccharomycetales.//
The word ``yeast`` comes to us from Old English gist, gyst, and from the Indo-European root yes-, meaning boil, foam, or bubble. Yeast microbes are probably one of the earliest domesticated organisms. People have used yeast for fermentation and baking throughout history. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted bread, as well as drawings of 4,000-year-old bakeries and breweries. In 1680 the Dutch naturalist Antonie van Leeuwenhoek first microscopically observed yeast, but at the time did not consider them to be living organisms, but rather globular structures. In 1857 French microbiologist Louis Pasteur proved in the paper ``MÃ©moire sur la fermentation alcoolique`` that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst. Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, but the fermentation inhibited â€“ an observation later called the Pasteur effect.
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.
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 like ribose, alcohols, and organic acids. Yeast species either require oxygen for aerobic cellular respiration (obligate aerobes), or are anaerobic but also have aerobic methods of energy production (facultative anaerobes). Unlike bacteria, there are no known yeast species that grow only anaerobically (obligate anaerobes). Yeasts grow best in a neutral or slightly acidic pH environment.
Yeasts will grow over a temperature range of 10 to 37 Â°C (50 to 99 Â°F), with an optimal temperature range of 30 to 37 Â°C (86 to 99 Â°F), depending on the type of species (S. cerevisiae works best at about 30 Â°C (86 Â°F). Above 37 Â°C (99 Â°F) yeast cells become stressed and will not divide properly. Most yeast cells die above 50 Â°C (122 Â°F). If the solution reaches 105 Â°C (221 Â°F) the yeast will disintegrate. There is little activity in the range of 0 to 10 Â°C (32 to 50 Â°F). The cells can survive freezing under certain conditions, with viability decreasing over time.
Yeasts are very common in the environment, but are usually isolated from sugar-rich material. Some good 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. Yeast are generally grown in the laboratory on solid growth media or liquid broths. Common media used for the cultivation of yeasts include; potato dextrose agar (PDA) or potato dextrose broth, Wallerstein Laboratories nutrient (WLN) agar, yeast peptone dextrose agar (YPD), and yeast mould agar or broth (YM). 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.
A white thready yeast commonly known as kahm yeast is often a harmless byproduct of the lactofermentation (or pickling) of certain vegetables, usually the result of exposure to air.
Yeasts have asexual and sexual reproductive cycles, however the most common mode of vegetative growth in yeast is asexual reproduction by budding or fission. Here a small bud, or daughter cell, is formed on the parent cell. The nucleus of the parent cell splits into a daughter nucleus and migrates into the daughter cell. The bud continues to grow until it separates from the parent cell, forming a new cell.
Under high stress conditions haploid cells will generally die, however under the same conditions 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.
Yeast of the species Schizosaccharomyces pombe reproduce by binary fission instead of budding.
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, yeast in wine fermentation and for xylitol production. Yeasts are also one of the most widely used model organisms for genetics and cell biology.
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 yeast under anaerobic or low-oxygen conditions. Beverages such as wine, beer, or distilled spirits all use yeast at some stage of their production.
Brewer's yeast (also known as brewing yeast) can mean any live yeast used in brewing. It can also mean yeast obtained as a by-product of brewing, dried and killed, and used as a dietary supplement for its B vitamin content.
Brewers classify yeasts as top-fermenting and bottom-fermenting. This distinction was introduced by the Dane Emil Christian Hansen. ``Top-fermenting yeasts`` are so called because they form a foam at the top of the wort during fermentation. They can produce higher alcohol concentrations and prefer higher temperatures, typically 16 to 24 Â°C (61 to 75 Â°F), producing fruitier, sweeter, ale-type beers. An example of a top-fermenting yeast is Saccharomyces cerevisiae, known to brewers as ale yeast. ``Bottom-fermenting yeasts`` are typically used to produce lager-type beers, though can also produce ale-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low temperatures. An example of bottom fermenting yeast is Saccharomyces pastorianus, formerly known as Saccharomyces carlsbergensis.
For both types, yeast is fully distributed through the beer while it is fermenting, and both equally flocculate (clump together and precipitate to the bottom of the vessel) when it is finished. By no means do all top-fermenting yeasts demonstrate this behaviour, but it features strongly in many English ale yeasts which may also exhibit chain forming (the failure of budded cells to break from the mother cell) which is technically different from true flocculation.Fermenting tanks with yeast being used to brew beer.
In industrial brewing, to ensure purity of strain, a 'clean' sample of the yeast is stored refrigerated in a laboratory. After a certain number of fermentation cycles, a full scale propagation is produced from this laboratory sample. Typically, it is grown up in about three or four stages using sterile brewing wort and oxygen.
The most common top-fermenting brewer's yeast, Saccharomyces cerevisiae, belongs to the same species as the common baking yeast. However, baking and brewing yeasts typically belong to different strains, cultivated to favor different characteristics: baking yeast strains are more aggressive, in order to carbonate dough in the shortest amount of time possible; brewing yeast strains act slower, but tend to produce fewer off-flavors and tolerate higher alcohol concentrations (in some cases, up to 20%).Brettanomyces
Brettanomyces is a genus of wild yeast important in brewing lambic, a beer produced not by the deliberate addition of brewer's yeasts, but by spontaneous fermentation by wild yeasts and bacteria. Brettanomyces lambicus, B. bruxellensis and B. claussenii are native to the Senne Valley region of Belgium, where lambic beer is produced.
A distilled beverage is a beverage that contains 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 account for the flavor of the beverage.Fresh grapes Main article: Fermentation (wine)
Yeast is used in winemaking where it converts the sugars present in grape juice or must into alcohol. Yeast is normally already invisibly present on the grapes. The fermentation can be done with this endogenous (or wild) yeast; however, this may give unpredictable results depending on the exact types of yeast species that are present. For this reason a pure yeast culture is generally added to the must, which rapidly predominates the fermentation as it proceeds. This represses the wild yeasts and ensures a reliable and predictable fermentation. Most added wine yeasts are strains of Saccharomyces 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.