Yeast is an important organism employed by humans for its ability to digest sugars. The digesting itself is not as interesting as some of its waste products, alcohol and carbon dioxide. Beer, wine and Australians are three things that wouldn't exist were it not for yeast. See also, marmite.

Any of many small, single-celled fungi in the phylum Ascomycota that reproduce by budding; the yeasts used to produce alcohol through fermentation or to leaven bread are all in the genus Saccharomyces.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

In the pastry station and in professional bread and pastry formulas when a recipe calls for yeast, it almost always mean fresh yeast. For those who plan to use said formulas with active dry yeast or instant yeast the following conversion factors may be used.

To convert amount from fresh yeast to:

  • Active dry yeast - multiply by .5
  • Instant dry yeast - multiply by .35

I have successfully used this conversion when I made baguettes at the hotel I took an internship in, slightly altering the cordon bleu formula for baguettes for our use.

I'll correct some misinformation provided above. 

Fungi are not plants and do not derive energy from sunlight.  Yeast is a cellular morphology that can be found in multiple fungal phyla including Basidiomycota (also includes mushrooms), Ascomycota and Zygomycota. It's generally believed that all fungi of these phyla can produce a yeast morphology under the right environmental conditions.  

The most commonly encountered yeasts reproduce by budding but there are also fission yeasts that reproduce by dividing themselves in half with a central cross wall and then separating.

Saccharomyces spp. are most commonly used for alcohol and bread fermentations but the fission yeast Schizosaccharomyces pombe is used to make some african beers (pombe is swahili for beer).  

Not all alcoholic beverages are fermented with yeasts or even fungi.  Pulque (mescal. tequila) is a product of bacterial fermentation.

Yeast (?), n. [OE. [yogh]eest, [yogh]est, AS. gist; akin to D. gest, gist, G. gischt, gascht, OHG. jesan, jerian, to ferment, G. gischen, gaschen, gahren, Gr. boiled, zei^n to boil, Skr. yas. &root;111.]

1.

The foam, or troth (top yeast), or the sediment (bottom yeast), of beer or other in fermentation, which contains the yeast plant or its spores, and under certain conditions produces fermentation in saccharine or farinaceous substances; a preparation used for raising dough for bread or cakes, and making it light and puffy; barm; ferment.

2.

Spume, or foam, of water.

They melt thy yeast of waves, which mar Alike the Armada's pride, or spoils of Trafalgar. Byron.

<-- 3.

A form of fungus which grows as indvidual rounded cells, rather than in a mycelium, and reproduces by budding; esp. members of the orders Endomycetales and Moniliales. Some fungi may grow both as a yeast or as a mycelium, depending on the conditions of growth.

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Yeast cake, a mealy cake impregnated with the live germs of the yeast plant, and used as a conveniently transportable substitute for yeast. -- Yeast plant Bot., the vegetable organism, or fungus, of which beer yeast consists. The yeast plant is composed of simple cells, or granules, about one three-thousandth of an inch in diameter, often united into filaments which reproduce by budding, and under certain circumstances by the formation of spores. The name is extended to other ferments of the same genus. See Saccharomyces. -- Yeast powder, a baling powder, -- used instead of yeast in leavening bread.

 

© Webster 1913.

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