Saccharomyces cerevisiae (), also called brewer's yeast or baker's yeast, is a species of yeast (single-celled fungal microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been originally isolated from the skin of grapes. It is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the model bacterium. It is the microorganism which causes many common types of fermentation. S. cerevisiae cells are round to ovoid, 5–10 μm in diameter. It reproduces by budding. 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. S. cerevisiae is currently the only yeast cell known to have Berkeley bodies present, which are involved in particular secretory pathways. Antibodies against S. cerevisiae are found in 60–70% of patients with Crohn's disease and 10–15% of patients with ulcerative colitis, and may be useful as part of a panel of serological markers in differentiating between inflammatory bowel diseases (e.g. between ulcerative colitis and Crohn's disease), their localization, and severity.
Etymology "Saccharomyces" derives from Latinized Greek and means "sugar-mold" or "sugar-fungus", with saccharon (σάκχαρον) being the combining form of 'sugar' and myces (μύκης) being 'fungus'. Cerevisiae comes from Latin and means 'of beer'. Other names for the organism are:
Brewer's yeast, though other species are also used in brewing Baker's yeast, though other species are also used in baking Ale yeast Top-fermenting yeast Ragi yeast, in connection to making tapai Budding yeast This species is also the main source of nutritional yeast and yeast extract.
History
In antiquity, pure yeasts were unavailable due to a lack of understanding of microbiology necessary to produce them. Instead, mixtures of wild bacteria (especially Lactobacillus) and yeasts were used for brewing and leavening, resulting in acidic goods. However, empirical testing by beer makers around the 15th century led to the discovery that first boiling a wort containing hops gave a non acidic beverage. The reason for this was unknown at the time, but boiling killed unwanted bacteria, and the hops contain natural chemicals that suppressed the regrowth of acid-making bacteria while the yeast flourished. Thus in the 19th century, bread bakers obtained their yeast from beer brewers, and this led to sweet-fermented breads such as the Imperial "Kaisersemmel" roll, which in general lacked the sourness created by the acidification typical of Lactobacillus. However, many beer brewers slowly switched from top-fermenting (S. cerevisiae) to bottom-fermenting (S. pastorianus) yeast. The Vienna Process was developed in 1846. While the innovation is often popularly credited for using steam in baking ovens, leading to a different crust characteristic, it is notable for including procedures for high milling of grains (see Vienna grits), cracking them incrementally instead of mashing them with one pass; as well as better processes for growing and harvesting top-fermenting yeasts, known as press-yeast. Refinements in microbiology following the work of Louis Pasteur led to more advanced methods of culturing pure strains. In 1879, Great Britain introduced specialized growing vats for the production of S. cerevisiae, and in the United States around the turn of the 20th century centrifuges were used for concentrating the yeast, turning yeast production into a major industrial process which simplified its distribution, reduced unit costs, and contributed to the commercialization and commoditization of bread and beer. Fresh "cake yeast" became the standard leaven for bread bakers in much of the Western world during the early 20th century. During World War II, Fleischmann's developed a granulated active dry yeast for the United States armed forces, which did not require refrigeration and had a longer shelf-life and better temperature tolerance than fresh yeast; it is still the standard yeast for US military recipes. The company created yeast that would rise twice as fast, cutting down on baking time. Lesaffre would later create instant yeast in the 1970s, which has gained considerable use and market share at the expense of both fresh and dry yeast in their various applications.
Biology
Ecology In nature, yeast cells are found primarily on ripe fruits such as grapes (before maturation, grapes are almost free of yeasts). S. cerevisiae can also be found year-round in the bark of oak trees. Since S. cerevisiae is not airborne, it requires a vector to move. Queens of social wasps overwintering as adults (Vespa crabro and Polistes spp.) can harbor yeast cells from autumn to spring and transmit them to their progeny. The intestine of Polistes dominula, a social wasp, hosts S. cerevisiae strains as well as S. cerevisiae × S. paradoxus hybrids. Stefanini et al. (2016) showed that the intestine of Polistes dominula favors the mating of S. cerevisiae strains, both among themselves and with S. paradoxus cells by providing environmental conditions prompting cell sporulation and spores germination. The optimum temperature for growth of S. cerevisiae is 30–35 °C (86–95 °F).[Citation needed]
Life cycle Two forms of yeast cells can survive and grow: haploid and diploid. The haploid cells undergo a simple lifecycle of mitosis and growth and, under conditions of high stress, will, in general, die. This is the asexual form of the fungus. The diploid cells (the preferential 'form' of yeast) similarly undergo a simple lifecycle of mitosis and growth. The rate at which the mitotic cell cycle progresses often differs substantially between haploid and diploid cells. Under conditions of stress, diploid cells can undergo sporulation, entering meiosis and producing four haploid spores, which can subsequently mate. This is the sexual form of the fungus. Under optimal conditions, yeast cells can double their population every 100 minutes. However, growth rates vary enormously between strains and between environments. Mean replicative lifespan is about 26 cell divisions. In the wild, recessive deleterious mutations accumulate during long periods of asexual reproduction of diploids, and are purged during selfing: this purging has been termed "genome renewal".
Nutritional requirements
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