Yeasts constitute a group of unicellular fungi, which convert sugar to alcohol like in the fermentation of alcoholic beverages process.
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with about 1,500 species currently described; they dominate fungal diversity in the oceans. Most reproduce asexually by budding, although a few do so 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 false hyphae as seen in most molds. Yeast size can vary greatly depending on the species, typically measuring 3–4 µm 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 one of the most thoroughly researched eukaryotic microorganisms. Researchers have used it to gather information about the biology of the eukaryotic cell and ultimately human biology. Other species of yeast, such as Candida albicans, are opportunistic pathogens and can cause infections 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, but 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 Old English gist, gyst, and 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 Antonie 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 sunlight to grow. Carbon is obtained mostly from hexose sugars such as glucose and fructose, or disaccharides such as sucrose and maltose. Some species can metabolize pentose sugars like ribose, 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 grow best in a neutral or slightly acidic pH environment.
Yeasts will grow over a temperature range of 10 °C (50 °F) to 37 °C (99 °F), with an optimal temperature range of 30 °C (86 °F) to 37 °C (99 °F), depending on the type of species (S. cerevisiae works best at about 30 °C (86 °F). Above 37 °C (99 °F) yeast cells become stressed and will not divide properly. Most yeast cells die above 50 °C (122 °F). If the solution reaches 105 °C (221 °F) the yeast will disintegrate. There is little activity in the range of 0 °C (32 °F) - 10 °C (50 °F). The cells can survive freezing under certain conditions, with viability decreasing over time.
Yeasts are very common in the environment, but are usually isolated from sugar-rich material. Some good examples include naturally occurring yeasts on the skins of 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, Wallerstein Laboratories Nutrient (WLN) agar, 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. This will change the yeast process.
Reproduction: 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.
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.
Yeast extract is the common name for various forms of processed yeast products made by extracting the cell contents (removing the cell walls); they are used as food additives or flavourings, or as nutrients for bacterial culture media. They are often used to create savory flavors and umami taste sensations. Monosodium glutamate (MSG) is used for umami, but has no flavor. Yeast extract, like MSG, often contains free glutamic acid. Yeast extracts in liquid form can be dried to a light paste or a dry powder. Glutamic acid in yeast extracts are produced from an acid-base fermentation cycle, only found in some yeasts, typically ones bred for use in baking.
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 spoiled. 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 used food preservation methods. Methylene Blue is used to test for the presence of live yeast cells.
Topics of Interest
Ethanol fermentation, also referred to as alcoholic fermentation, is a biological process in which sugars such as glucose, fructose, and sucrose are converted into cellular energy and thereby produce ethanol and carbon dioxide as metabolic waste products. Because yeasts perform this process in the absence of oxygen, ethanol fermentation is classified as anaerobic. Ethanol fermentation occurs in the production of alcoholic beverages and ethanol fuel, and in the rising of bread dough.
Baker's yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and bakery products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol. 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 Saccharomyces cerevisiae, which is the same species commonly used in alcoholic fermentation, and so is also called brewer's yeast. 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, however, generally used in a pure form, but comes from being propagated in a sourdough starter.
Nutritional yeast, similar to brewer's yeast, is a deactivated yeast, usually Saccharomyces cerevisiae. It is popular with vegans and vegetarians and may be used as an ingredient in recipes or as a condiment. Nutritional yeast is produced by culturing the yeast with a mixture of sugarcane and beet molasses, then harvesting, washing, drying and packaging the yeast. It is commercially available 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.
Red yeast rice, red fermented rice, red kojic rice, red koji rice, anka, or ang-kak, is a bright reddish purple fermented rice, which acquires its colour from being cultivated with the mold Monascus purpureus.
Candidiasis (Commonly referred to as a yeast infection) or thrush is a fungal infection (mycosis) of any of the Candida species, of which Candida albicans is the most common. Candidiasis encompasses infections that range from superficial, such as oral thrush and vaginitis, to systemic and potentially life-threatening diseases. Candida infections of the latter category are also referred to as candidemia and are usually confined to severely immunocompromised persons, such as cancer, transplant, and AIDS patients.
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