Yeast
2008/9 Schools Wikipedia Selection. Related subjects: Agriculture; Organisms
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 Yeast of the species Saccharomyces cerevisiae.
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Ascomycota (sac fungi)
Basidiomycota (club fungi)
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Yeasts are a growth form of eukaryotic microorganisms classified in the kingdom Fungi. Approximately 1,500 species of yeasts have been described, most of which reproduce asexually by budding, although in a few cases 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 true hyphae as seen in most molds. Yeasts size can vary greatly depending on the species, typically measuring 3 to 7 micrometres in diameter, although some yeasts can reach over 40 µm.
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 the most thoroughly researched eukaryotic microorganism. Researchers can use it to gather information into the biology of the eukaryotic cell and ultimately human biology. Other species of yeast, such as Candida albicans, are opportunistic pathogens and can cause infection in humans. Yeasts have recently been used to generate electricity in microbial fuel cells, and produce ethanol for the biofuel industry.
Yeasts do not form a specific 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, however 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.
History
The word "yeast" comes from the Old English language "gist", "gyst", ultimately 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 Anton 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 "Mémoire sur la fermentation alcoolique" that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst. Pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, but the fermentation inhibited - an observation later called the Pasteur effect.
Growth and nutrition
Yeasts are chemoorganotrophs as they use organic compounds as a source of energy and do not require light to grow. The main source of carbon is obtained by hexose sugars such as glucose and fructose, or disaccharides such as sucrose and maltose. Some species can metabolize pentose sugars, 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 are ubiquitous in the environment, but are most frequently isolated from sugar-rich samples. Some good examples include 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, Wallerstien Laboratories Nutrient agar (WLN), 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.
Reproduction
1. Budding
2. Conjugation
3. Spore
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. The bud can develop on different parts of the parent cell depending on the genus of the yeast.
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.
Uses
The useful physiological properties of yeast have led to their use in the field of biotechnology. Fermentation of sugars by yeast is the oldest and largest application of this technology. Many types of yeasts are used for making many foods: Baker's yeast in bread production, brewer's yeast in beer fermentation, 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
Alcoholic beverages are loosely defined as a beverage that contains ethanol (CH3CH2OH). This ethanol is almost always produced by fermentation - the metabolism of carbohydrates by certain species of yeast. Beverages such as wine, beer, or distilled spirits all use yeast at some stage of their production.
Beer
Beer 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, 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 used to produce lager-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low temperatures. An example of a bottom-fermenting yeast is Saccharomyces pastorianus.
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.
Brewers of Bavarian-style wheat beers often use varieties of Torulaspora delbrueckii, which contribute to the distinctive flavour profile. Lambic, a style of Belgian beer, is fermented spontaneously by wild yeasts primarily of the genus Brettanomyces.
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.
Distilled beverages
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.
Wine
Yeast is used in winemaking where it converts the sugars present in grape juice or must into alcohol. Yeast is normally already present on the grapes, often visible as a powdery film (also known as the bloom or blush) on their exterior. The fermentation can be done with this indigenous (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, however 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 develoment 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.
Baking
Yeast, specifically Saccharomyces cerevisiae, is used in baking as a leavening agent, where it converts the fermentable sugars present in the dough into carbon dioxide. This causes the dough to expand or rise as the carbon dioxide forms pockets or bubbles. When the dough is baked it "sets" and the pockets remain, 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. Salt and fats such as butter slow down yeast growth. The majority of the yeast used in baking is of the same species common in alcoholic fermentation. Additionally, Saccharomyces exiguus (also known as S. minor) is a wild yeast found on plants, fruits, and grains that is occasionally used for baking.
It is not known when yeast was first used to bake bread. The first records that show this use come from Ancient Egypt. Researchers speculate that 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 more tasty than the normal flat, hard cake.
Today there are several retailers of baker's yeast; one of the best-known is Fleischmann’s Yeast, which was developed in 1868. During World War II Fleischmann's developed a granulated active dry yeast, which did not require refrigeration and had a longer shelf life than fresh yeast. The company created yeast that would rise twice as fast, cutting down on baking time. Baker's yeast is also sold as a fresh yeast compressed into a square "cake". This form perishes quickly, and must be used soon after production in order to maintain viability. A weak solution of water and sugar can be used to determine if yeast is expired. When dissolved in the solution, active yeast will foam and bubble as it ferments the sugar into ethanol and carbon dioxide.
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. Some bread doughs are knocked back after one rising and left to rise again. 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. The dough is then shaped into loaves, left to rise until it is the correct size, and then baked. Dried yeast is always used for bread made in a bread machine.
Industrial ethanol production
The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry, which has various uses including ethanol fuel. The process starts by milling a feedstock, such as sugar cane, sweetcorn, or cheap cereal grains, and then adding dilute sulfuric acid, or fungal alpha amylase enzymes, to break down the starches in to complex sugars. A gluco amylase 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 concentration.
Saccharomyces yeasts have been genetically engineered to ferment xylose, one of the major fermentable sugars present in cellulosic biomasses, such as agriculture residues, paper wastes, and wood chips. Such a development means that ethanol can be efficiently produced from more inexpensive feedstocks, making cellulosic ethanol fuel a more competitively priced alternative to gasoline fuels.
Kombucha
Yeast in symbiosis with acetic acid bacteria is used in the preparation of Kombucha, a fermented sweetened tea. Species of yeast found in the tea can vary, and may include: Brettanomyces bruxellensis, Candida stellata, Schizosaccharomyces pombe, Torulaspora delbrueckii and Zygosaccharomyces bailii.
Nutritional supplements
Yeast is used in nutritional supplements popular with vegans and the health conscious, where it is often referred to as "nutritional yeast". It is a deactivated yeast, usually Saccharomyces cerevisiae. It is an excellent source of protein and vitamins, especially the B-complex vitamins, whose functions are related to metabolism as well as other minerals and cofactors required for growth. It is also naturally low in fat and sodium. Some brands of nutritional yeast, though not all, are fortified with vitamin B12, which is produced separately from bacteria. Nutritional yeast, though it has a similar appearance to brewer's yeast, is very different and has a very different taste.
Nutritional yeast has a nutty, cheesy, creamy flavor which makes it popular as an ingredient in cheese substitutes. It is often used by vegans in place of parmesan cheese. Another popular use is as a topping for popcorn. Some movie theaters are beginning to offer it along with salt or cayenne pepper as a popcorn condiment. It comes in the form of flakes, or as a yellow powder similar in texture to cornmeal, and can be found in the bulk aisle of most natural food stores. In Australia it is sometimes sold as "savory yeast flakes". Though "nutritional yeast" usually refers to commercial products, inadequately fed prisoners have used "home-grown" yeast to prevent vitamin deficiency.
Probiotics
Some probiotic supplements use the yeast Saccharomyces boulardii to maintain and restore the natural flora in the large and small gastrointestinal tract. S. boulardii has been shown to reduce the symptoms of acute diarrhea in children, prevent reinfection of Clostridium difficile, reduce bowel movements in diarrhea predominant IBS patients, and reduce the incidence of antibiotic, traveler's, and HIV/AIDS associated diarrheas.
Science
Several yeasts, particularly Saccharomyces cerevisiae, have been widely used in genetics and cell biology. This is largely because the cell cycle in a yeast cell is very similar to the cell cycle in humans, and therefore the basic cellular mechanics of DNA replication, recombination, cell division and metabolism are comparable. Also 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.
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 took 7 years and the involvement of over 100 laboratories to accomplish. The second yeast species to have its genome sequenced was Schizosaccharomyces pombe, which was completed in 2002. It was the 6th eukaryotic genome sequenced and consists of 13.8 million base pairs.
Yeast extract
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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 acids. 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, where 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 which would give poor texture) are separated. Yeast autolysates are used in Vegemite and Promite (Australia), Marmite and Bovril (the United Kingdom and Republic of Ireland), Oxo (South Africa, United Kingdom, and Republic of Ireland), and Cenovis (Switzerland).
Pathogenic yeasts
Some species of yeast are opportunistic pathogens, where they can cause infection in people with compromised immune systems.
Cryptococcus neoformans, is a significant pathogen of immunocompromised people, causing the disease termed Cryptococcosis. This disease occurs in about 7-8% of AIDS patients in the USA, and a slightly smaller percentage (3-6%) in western Europe. The cells of the 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 Candida genus are another group of opportunistic pathogens, which causes oral and vaginal infections in humans, known as Candidiasis. Candida is commonly found as a commensal yeast in the mucus membranes of humans and other warm-blooded animals. However, sometimes these same strains can become pathogenic. Here the yeast cells sprout a hyphal outgrowth, which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues. The pathogenic yeasts of candidiasis in probable descending order of virulence for humans are: C. albicans, C. tropicalis, C. stellatoidea, C. glabrata, C. krusei, C. parapsilosis, C. guilliermondii, C. viswanathii, C. lusitaniae and Rhodotorula mucilaginosa. Candida glabrata is the second most common Candida pathogen after C. albicans, causing infections of the urogenital tract, and of the bloodstream ( Candidemia).
Food spoilage
Yeasts are able to grow in foods with a low pH, (5.0 or lower) and in the presence of sugars, organic acids and other easily metabolized carbon sources. During their growth, yeasts metabolize some food components and produce metabolic end products. This causes the physical, chemical, and sensory properties of a food to change, and the food is spoilt. The growth of yeast within food products is often seen on their surface, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semi-liquid products, such as syrups and jams. The yeast of the Zygosaccharomyces genus have had a long history as a spoilage yeast within the food industry. This is mainly due to the fact that 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 utilised food preservation methods. Methylene Blue is used to test for the presence of live yeast cells.