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yeast:defined
Dictionary:yeast (yēst)
n.
Any of various unicellular fungi of the genus Saccharomyces, especially S. cerevisiae, reproducing by budding and from ascospores and capable of fermenting carbohydrates.
Any of various similar fungi.
Froth consisting of yeast cells together with the carbon dioxide they produce in the process of fermentation, present in or added to fruit juices and other substances in the production of alcoholic beverages.
A powdered or compressed commercial preparation, having yeast cells and inert material such as meal and used chiefly as a leavening agent or as a dietary supplement.
Foam; froth.
An agent of ferment or activity: political agitators who are the yeast of revolution.
intr.v., yeast·ed, yeast·ing, yeasts.
To ferment.
To froth or foam.
[Middle English yeest, from Old English gist.]
Yeast
A collective name for those fungi which possess, under normal conditions of growth, a vegetative body (thallus) consisting, at least in part, of simple, single cells. The cells making up the thallus occur in pairs, in groups of three, or in straight or branched chains consisting of as many as 12 or more cells. Vegetative reproduction is characterized by budding or fission. Sexual reproduction also occurs in yeast, and is differentiated from that of other fungi by sexual states that are not enclosed in a fruiting body. Yeasts are a phylogenetically diverse group of organisms that occur in two divisions of fungi (Ascomycotina and Basidiomycotina) and 100 genera. The 700 or more species that have been described possibly represent only 1% of the species in nature, so the majority of the yeasts have yet to be discovered. Yeast plays a large part in industrial fermentation processes such as the production of industrial enzymes and chemicals, food products, industrial ethanol, and malt beverage and wine; in diseases of humans, animals and plants; in food spoilage; and as a model of molecular genetics. See also Distilled spirits; Food microbiology; Genetic engineering; Malt beverage; Medical mycology; Wine.
The shape and size of the individual cells of some species vary slightly, but in other species the cell morphology is extremely heterogeneous. The shape of yeast cells may be spherical, globose, ellipsoidal, elongate to cylindrical with rounded ends, more or less rectangular, pear-shaped, apiculate or lemon-shaped, ogival or pointed at one end, or tetrahedral. The diameter of a spherical cell may vary from 2 to 10 micrometers. The length of cylindrical cells is often 20–30 ?m and, in some cases, even greater.
The asexual multiplication of yeast cells occurs by a budding process, by the formation of cross walls or fission, and sometimes by a combination of these two processes. Yeast buds are sometimes called blastospores or blastoconidia. When yeast reproduces by a fission mechanism, the resulting cells are termed arthrospores or arthroconidia.
Yeasts are categorized into two groups, based on their methods of sexual reproduction: the ascomycetous (Division Ascomycotina) and basidiomycetous (Division Basidiomycotina) yeasts.
The sexual spores of the ascomycetous yeasts are termed ascospores, which are formed in simple structures, often a vegetative cell. Such asci are called naked asci because of the absence of an ascocarp, which is a more complex fruiting body found in the higher Ascomycetes. If the vegetative cells are diploid, a cell may transform directly into an ascus after the 2n nucleus undergoes a reduction or meiotic division. See also Ascomycota.
Certain yeasts have been shown to be heterothallic; that is, sporulation occurs when strains of opposite mating type (usually indicated by “a” and ?) are mixed on sporulation media. However, some strains may be homothallic (self-fertile), and reduction division and karyogamy (fusion of two haploid nuclei) take place during formation of the sexual spore. Yeasts that produce sporogenous cells represent the teleomorphic form of the life cycle. In cases, in which sexual cycles are unknown, the yeast represents the asexual or anamorphic form. A species of yeast may be originally discovered in the anamorphic form and named accordingly; subsequently, the sexual state may be found and a name applied to represent the teleomorph. Consequently, the anamorphic and teleomorphic names will differ.
Basidiospores and teliospores are the sexual spores that are produced in the three classes of basidiomycetous yeasts: Urediniomycetes, Hymenomycetes, and Ustilaginomycetes. Sexual reproduction and life cycle in these yeasts is typical of other basidiomycetes in that it can include both unifactorial (bipolar) and bifactorial (tetrapolar) mating systems. See also Basidiomycota.
Some yeasts have the ability to carry out an alcoholic fermentation. Other yeasts lack this property. In addition to the fermentative type of metabolism, fermentative yeasts as a rule have a respiratory type of metabolism, whereas nonfermentative yeasts have only a respiratory, or oxidative, metabolism. Both reactions produce energy, with respiration producing by far the most, which is used in part for synthetic reactions, such as assimilation and growth. Part is lost as heat. In addition, small or sometimes large amounts of by-products are formed, including organic acids, esters, aldehydes, glycerol, and higher alcohols. When a fermenting yeast culture is aerated, fermentation is suppressed and respiration increases. This phenomenon is called the Pasteur effect. See also Fermentation.
Yeasts are ubiquitous in nature. They exist on plants and animals; in waters, sediments, and soils; and in terrestrial, aquatic, and marine habitats. Yeasts require oxygen for growth and reproduction; therefore they do not inhabit anaerobic environments such as anoxic sediments. Many species have highly specific habitats, whereas others are found on a variety of substrates in nature.
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brewers companion:yeast
[YEEST] Yeast is a living, microscopic, single-cell organism that, as it grows, converts its food (through a process known as fermentation) into alcohol and carbon dioxide. This trait is what endears yeast to winemakers, brewmasters and breadbakers. In the making of wine and beer, the yeast's manufacture of alcohol is desired and necessary for the final product; and carbon dioxide is what makes beer and champagne effervescent. The art of breadmaking needs the carbon dioxide produced by yeast in order for certain doughs to rise. To multiply and grow, all yeast needs is the right environment, which includes moisture, food (in the form of sugar or starch) and a warm, nurturing temperature (70° to 85°F is best). Wild yeast spores are constantly floating in the air and landing on uncovered foods and liquids. No one's sure when these wild spores first interacted with foods but it's known that the Egyptians used yeast as a leavening agent more than 5,000 years ago. Wine and other fermented beverages were made for millennia before that. Today, scientists have been able to isolate and identify the various yeasts that are best for winemaking, beermaking and baking. The two types commercially available are baker's yeast and brewer's yeast. Baker's yeast, as the name implies, is used as a leavener. It's catagorized into three basic types-active dry yeast, compressed fresh yeast and yeast starters. Active dry yeast is in the form of tiny, dehydrated granules. The yeast cells are alive but dormant because of the lack of moisture. When mixed with a warm
liquid (105° to 115°F), the cells once again become active. Active dry yeast is available in two forms, regular and quick-rising, of which the latter takes about half as long to leaven bread. They may be used interchangeably (with adjustments in rising time) and both are available in 1⁄4-ounce envelopes. Regular active dry yeast may also be purchased in 4-ounce jars or in bulk in some natural food stores. It should be stored in a cool, dry place, but can also be refrigerated or frozen. It should always be at room temperature before being dissolved in liquid. Properly stored, it's reliable when used by the expiration date, which should be stamped on the envelope or jar label. One package of dry yeast is equal to 1 scant tablespoon dry yeast or 1 cake of compressed fresh yeast. Compressed fresh yeast, which comes in tiny (0.06-ounce), square cakes, is moist and extremely perishable. It must be refrigerated and used within a week or two, or by the date indicated on the package. It can be frozen, but should be defrosted at room temperature and used immediately. One cake of fresh yeast can be substituted for one envelope of dry yeast. The use of compressed fresh yeast has been primarily replaced by the more convenient active dry yeast. All baker's yeast should be given a test called proofing to make sure it's still alive. To proof yeast, dissolve it in warm water and add a pinch of sugar. Set the mixture aside in a warm place for 5 to 10 minutes. If it begins to swell and foam, the yeast is alive, active and capable of leavening bread. Brewer's yeasts are special non-leavening yeasts used in beermaking. Because it's a rich source of B vitamins, brewer's yeast is also used as a food supplement. It's available in natural food stores. Brewer's yeasts are also marketed in specialty beermaking equipment shops, with different strains used for different beers.
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