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Saccharomyces cerevisiae is a
species of budding yeast. It is perhaps the most relevant
yeast for mankind, both for its use since ancient times in
baking and brewing, and for being one of the most
intensively studied eukaryotic model organisms in molecular
and cell biology, much like Escherichia coli as the model
prokaryote. It is the microorganism behind the most common
type of fermentation. Saccharomyces cerevisiae cells are
round to ovoid, 5-10 Micrometres in diameter. It reproduces
by a division process known as budding.
It is useful in studying the cell cycle because it is easy
to culture but as a eukaryote, it shares the complex
internal cell structure of plants and animals. S. cerevisiae
was the first eukaryotic genome that was completely
sequenced. The yeast genome database [1] is highly annotated
and remains a very important tool for developing basic
knowledge about the function and organization of eukaryotic
cell genetics and physiology. Another important S.
cerevisiae database is maintained by the Munich Information
Center for Protein Sequences [2]. The genome is composed of
about 13,000,000 base pairs and 6,275 genes. It is estimated
that yeast shares about 23% of its genome with humans.
Brewer's
yeast
Brewer's yeast can mean any live yeast used in brewing. It
can also mean yeast obtained as a by-product of brewing,
dried and killed, and used as nutritional yeast. S.
cerevisiae is the usual species for this purpose.[1]
"Brewer's yeast" can also refer loosely to any nutritional
yeast.
Beer brewers classify yeasts as top-fermenting and
bottom-fermenting. This distinction was introduced by the
Dane Emil Christian Hansen.
Top-fermenting yeasts (so-called because they float to the
top of the beer) can produce higher alcohol concentrations
and prefer higher temperatures. An example is Saccharomyces
cerevisiae, known to brewers as ale yeast. They produce
fruitier, sweeter, real ale-type beers. Bottom-fermenting
yeasts ferment more sugars, leaving a crisper taste, and
work well at low temperatures. An example is Saccharomyces
uvarum, formerly known as Saccharomyces carlsbergensis. They
are used in producing lager-type beers. Brewers of wheat
beers often use varieties of Torulaspora delbrueckii.
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