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 uvarum.
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 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 beerIn 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.
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Wine
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 contributer to wine faults within the wine industry.
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Baking
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.
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 tastier than the normal flat, hard cake.
Today there are several retailers of baker's yeast, one of the best-known being 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.
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.
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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 amylase enzymes, to break down the starches in to fermentable sugars. Yeasts are then added to convert the fermentable 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.
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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.
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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.
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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.
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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).
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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).
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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.
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