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Yeasts are eukaryotic microorganisms classified as members of the fungus kingdom with 1,500 species currently identified〔 and are estimated to constitute 1% of all described fungal species.〔 Yeasts are unicellular, although some species may also develop multicellular characteristics by forming strings of connected budding cells known as pseudohyphae or false hyphae.〔 Yeast sizes vary greatly, depending on species and environment, typically measuring 3–4 µm in diameter, although some yeasts can grow to 40 µm in size.〔 Most yeasts reproduce asexually by mitosis, and many do so by the asymmetric division process known as budding. By fermentation, the yeast species ''Saccharomyces cerevisiae'' converts carbohydrates to carbon dioxide and alcohols – for thousands of years the carbon dioxide has been used in baking and the alcohol in alcoholic beverages.〔 It is also a centrally important 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 yeasts, 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 single taxonomic or phylogenetic grouping. The term "yeast" is often taken as a synonym for ''Saccharomyces cerevisiae'',〔 but the phylogenetic diversity of yeasts is shown by their placement in two separate phyla: the Ascomycota and the 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. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeast-raised bread, as well as drawings of 4,000-year-old bakeries and breweries.〔 In 1680, 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 fermentation was inhibited – an observation later called the "Pasteur effect". By the late 18th century, two yeast strains used in brewing had been identified: ''Saccharomyces cerevisiae'' (top-fermenting yeast) and ''S. carlsbergensis'' (bottom-fermenting yeast). ''S. cerevisiae'' has been sold commercially by the Dutch for bread-making since 1780; while, around 1800, the Germans started producing ''S. cerevisiae'' in the form of cream. In 1825, a method was developed to remove the liquid so the yeast could be prepared as solid blocks.〔 The industrial production of yeast blocks was enhanced by the introduction of the filter press in 1867. In 1872, Baron Max de Springer developed a manufacturing process to create granulated yeast, a technique that was used until the first World War.〔 In the United States, naturally occurring airborne yeasts were used almost exclusively until commercial yeast was marketed at the Centennial Exposition in 1876 in Philadelphia, where Charles L. Fleischmann exhibited the product and a process to use it, as well as serving the resultant baked bread.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「yeast」の詳細全文を読む スポンサード リンク
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