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.

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

When I pour the pints of flocculated yeast left in the bottom of my carboy down the sink, I feel a twinge of regret. As the scummy sludge burbles its way down the drain, I say a brief prayer, thanking the little guys for their aid, hoping each bud survives its passage through the sewage system.

While the E2 database provides the scientific knowledge essential to allow other noders to "understand" yeast, impressing folks at parties discussing the differences between Saccharomyces cerevisae and S. uvarum (or for the truly aroused, S. carlsbergensis), and while much of what I am going to discuss will not help you on your GRE's or (God forbid) your MCAT's, my soul cannot rest until this node on yeast stirs up a modicum of the passion these little guys deserve.

I am a homebrewer, a yeast farmer. I have felt the cool release of carbon dioxide (CO2) on my palm, as I held it above my airlock as it rhythmically clacks away, marking the work of millions upon millions of yeasties, busy converting sunlight into alcohol. I love the end result as much as anyone who breathes, but I cannot romanticize away the burden I put these critters through. So pour yourself an ale, let the foam caress your upper lip, and relax--it is time you know just how much that beer cost a fellow organism.

Yeast is a facultative anaerobe (there, help on your MCATs); that does not mean, however, that it does not appreciate a breath of oxygen now and then. As any decent homebrewer will tell you (and I confess few of us rise to the category of "decent"), the wort (the lovely malt, hopped extract that feeds the yeast--foetal beer, if you will) should be aerated prior to pitching the yeast.

Despite the high falutin' language, "aerating" (at least in these parts) involves nothing more than sloshing the wort around, rolling the carboy around the kitchen floor for a few minutes (a poor substitute for youthful indiscretions, but then again, I am no longer a youth). With good nutrients, and a dose of good air, the yeast go into an asexual frenzy, budding like there's no tomorrow, producing gazillions of fellow yeasties, so that each cc of wort contains 50 million yeasties (you can look this up)!

But (and this is THE key point)--happy yeast with lots of nutrients and oxygen do not make ethanol. They (like you and me) breathe, respire, and convert carbohydrates into water and CO2. They screw like mad, play and live and (perhaps) whistle delightedly to themselves but (again I will repeat the take home message, as a good teacher will), no ethanol. No hooch. No brewski. No demon alcohol.

Making alcohol requires stress, contributing to the karma that allows alcohol to relieve stress. To make alcohol, the yeasties must be put in an environment that has little oxygen. So I torture them. I put an airlock on my carboy, and the little critters consume the oxygen they have. In order to survive, they switch over to anaerobic metabolism--you and I, we'd take the easy way out and suffocate, but the yeasties are more evolved than we are.

So they say, "Ha, ha, ha, Mr. Doyle, despite your diabolical airlock, we can still screw with impunity in your carboy, happy, happy, happy critters we are, with our sophisticated facultative anerobic metabolism, that will allow us to supercede humans when the atmosphere is nothing but charred CO2 and sulfur dust...."

This makes me sad. If I were power hungry, I suppose I might glean some glee from this, but sadly, I know this will end with me sharing bottles of ale with my obligatory aerobic friends. You see, as the yeasties play joyfully among themselves, they forget a key fact. When they switch to anaerobic metabolism, they make alcohol. And while the yeasties are more advanced than most humans, like most humans, they are susceptible to ethanol. Once the wort reaches a certain level of alcohol, the yeasties pass out and sink to the bottom of the carboy, dazed but not quite dead. And like the male of our species, a drunk yeastie can no more reproduce than a tonerless xerox machine (and I know dannye is going to nail me for that poor analogy...sigh).

So a toast to my finely evolved buddies, and I hope a smidgeon of appreciation for the complex life cycles these little guys have gone through so that you may enjoy your ale. And I hope that an ale will never be just an ale to you, my friend, for knowing the struggle that went into that bottle can only make you more versed in the complexity and relatedness of this universe, too beautiful to comprehend.

(Dedicated to iceowl, who loves the universe perhaps even more than me...)

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.