Saccharomyces cerevisiae

Naming

"''Saccharomyces''" derives from Latinized Greek and means "sugar-mould" or "sugar-fungus", ''saccharon'' being the combining form "sugar" and ''myces'' 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
⤷  Ale yeast
⤷  Top-fermenting yeast
⤷  Baker's yeast
⤷  Ragi yeast, in connection to making tapai
⤷  Budding yeast

This species is also the main source of nutritional yeast and yeast extract.

Behavior

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 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".Approaches that can be applied in many different fields of biological and medicinal science have been developed by yeast scientists. These include yeast two-hybrid for studying protein interactions and tetrad analysis. Other resources, include a gene deletion library including ~4,700 viable haploid single gene deletion strains. A GFP fusion strain library used to study protein localisation and a TAP tag library used to purify protein from yeast cell extracts.

Stanford University's yeast deletion project created knockout mutations of every gene in the ''S. cerevisiae'' genome to determine their function.Among other microorganisms, a sample of living ''S. cerevisiae'' was included in the Living Interplanetary Flight Experiment, which would have completed a three-year interplanetary round-trip in a small capsule aboard the Russian Fobos-Grunt spacecraft, launched in late 2011. The goal was to test whether selected organisms could survive a few years in deep space by flying them through interplanetary space. The experiment would have tested one aspect of transpermia, the hypothesis that life could survive space travel, if protected inside rocks blasted by impact off one planet to land on another. Fobos-Grunt's mission ended unsuccessfully, however, when it failed to escape low Earth orbit. The spacecraft along with its instruments fell into the Pacific Ocean in an uncontrolled re-entry on January 15, 2012. The next planned exposure mission in deep space using ''S. cerevisiae'' is BioSentinel.

Habitat

In nature, yeast cells are found primarily on ripe fruits such as grapes . ''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 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. 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 .

Evolution

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, beer brewers slowly switched from top-fermenting to bottom-fermenting 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 ,
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 commercialisation and commoditisation 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.

Uses

Owing to the high cost of commercial CO2 cylinder systems, CO2 injection by yeast is one of the most popular DIY approaches followed by aquaculturists for providing CO2 to underwater aquatic plants. The yeast culture is, in general, maintained in plastic bottles, and typical systems provide one bubble every 3–7 seconds. Various approaches have been devised to allow proper absorption of the gas into the water.