   #copyright

Yeast

2007 Schools Wikipedia Selection. Related subjects: Food and agriculture;
Organisms

            How to read a taxoboxYeasts
   Yeast of the species Saccharomyces cerevisiae.
   Yeast of the species Saccharomyces cerevisiae.
             Scientific classification

   Domain:  Eukaryota
   Kingdom: Fungi

                              Typical divisions

   Ascomycota (sac fungi)
     * Saccharomycotina (true yeasts)
     * Taphrinomycotina
          + Schizosaccharomycetes (fission yeasts)

   Basidiomycota (club fungi)
     * Urediniomycetes
          + Sporidiales

   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

   The yeast life cycle. 1. Budding 2. Conjugation 3. Spore
   The yeast life cycle.
   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 (CH[3]CH[2]OH). 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

   A mixture of diatomaceous earth and yeast after filtering beer.
   A mixture of diatomaceous earth and yeast after filtering 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.
   Fermenting tanks with yeast being used to brew beer.
   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.

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

   Grapes covered in yeast growth observable as a white film, also known
   as the "blush".
   Grapes covered in yeast growth observable as a white film, also known
   as the "blush".

   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

   Bread showing pockets left by carbon dioxide.
   Bread showing pockets left by carbon dioxide.

   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.
   A block of fresh yeast.
   A block of fresh yeast.

   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.
   Active dried yeast, a granulated form in which yeast is commercially
   sold.
   Active dried yeast, a granulated form in which yeast is commercially
   sold.

   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

   A Kombucha culture fermenting in a jar
   A Kombucha culture fermenting in a jar

   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

   Diagram showing a yeast cell
   Diagram showing a yeast cell

   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

                       Marmite and Vegemite have a distinctive dark colour
                       Marmite and Vegemite have a distinctive dark colour

                    Vegemite and Marmite, products made from yeast extract
                    Vegemite and Marmite, products made from yeast extract

   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

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

   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.

   Retrieved from " http://en.wikipedia.org/wiki/Yeast"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
