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, and yeast in wine fermentation and for xylitol production. so-called red rice yeast is actually a mold, monascus purpureus. yeasts include some of the most widely used model organisms for genetics and cell biology. alcoholic beverages 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 yeasts under anaerobic or low-oxygen conditions. beverages such as mead, wine, beer, or distilled spirits all use yeast at some stage of their production. a distilled beverage is a beverage containing 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 (in addition to that provided by the oak in which it may be aged) account for the flavour of the beverage. beer yeast ring used by swedish farmhouse brewers in the 19th century to preserve yeast between brewing sessions. bubbles of carbon dioxide forming during beer-brewing 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 during fermentation. an example of a top-cropping yeast is saccharomyces cerevisiae, sometimes called an "ale yeast". bottom-cropping yeasts are typically used to produce lager-type beers, though they can also produce ale-type beers. these yeasts ferment well at low temperatures. an example of bottom-cropping yeast is saccharomyces pastorianus, 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, a disaccharide, into more fermentable monosaccharides. 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 vitamins (except b12), a feature exploited in food products made from leftover (by-product) 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 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' and specialty sour ales, along with the secondary conditioning of a particular belgian trappist beer. 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 anomalus, brettanomyces custersianus, brettanomyces naardenensis, and brettanomyces nanus, with teleomorphs existing for the first two species, dekkera bruxellensis 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 main article: yeast in winemaking yeast in a bottle during sparkling wine production at schramsberg vineyards, napa yeast is used in winemaking, where it converts the sugars present (glucose and fructose) in grape juice (must) into ethanol. yeast is normally already present on grape skins. 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 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 fermentable sugars present in dough into carbon dioxide. 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, 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 begins, producing ethanol as a waste product; however, this evaporates during baking. a block of compressed fresh yeast 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. active dried yeast, a granulated form in which yeast is commercially sold 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 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 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. 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. the dough is then shaped into loaves. some bread doughs are knocked back after one rising and left to rise again (this is called 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, 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 metals, and is being investigated for its potential as a heavy metal 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 bioaccumulate free and 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 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 kombucha culture fermenting in a jaryeast and bacteria in kombucha at 400× a number of sweet 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 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: brettanomyces bruxellensis, candida stellata, schizosaccharomyces pombe, torulaspora delbrueckii and zygosaccharomyces bailii. also popular in eastern europe and some former soviet republics under the name chajnyj grib (russian: чайный гриб), which means "tea mushroom". kefir and kumis are made by fermenting milk with yeast and bacteria. mauby (spanish: mabí), made by fermenting sugar with the wild yeasts naturally present on the bark of the colubrina elliptica tree, popular in the caribbean foods and nutritional supplements marmite and vegemite, products made from yeast extractmarmite and vegemite are dark in colour 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, often contain free glutamic acid. examples include: yeast extract, made from the intracellular contents of yeast and used as food additives