Yeast are simple, unicellular fungi. The most common forms of yeast — baker’s and brewer’s yeast — are strains of the species Saccaromyces cerevisiae. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins, niacin, and folic acid.
Yeast microbes are probably one of the earliest domesticated organisms. People have used yeast for fermentation and baking throughout history. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted bread, as well as drawings of 4,000-year-old bakeries and breweries. Only in the last 150 years, since the experiments of Louis Pasteur, have scientist begun to explore how yeast works. Pasteur first proposed the production of carbon dioxide from yeast as responsible for raising a loaf of bread in 1859.
As little as two pounds of yeast starter can raise 500 pounds of bread dough.
Wild yeast spores are constantly floating in the air and landing on uncovered foods and liquids. These wild varieties contributed some of the earliest kinds of sourdough bread mixes which did not depend on adding starter cultures.
Yeast is also a popular organism for studying genetics. Baker’s yeast is one of only a half-dozen microbes on Earth whose unique gene script has recently been comprehensively deciphered. Notable in the yeast gene is a host of signals that trigger the microbe to protect itself against extremes in cold and heat, called thermal shock proteins. Hopes now run high in the biological community that over the next several years, more than 50 to 100 additional microbes will also provide comprehensive genetic scripts for their lifecycles, including how these organisms might survive under relentless swings in near boiling water, deep ice, or even in the core of an active volcano vent and nuclear reactors.
The yeasts, like most fungi, respire oxygen (aerobic respiration), but in the absence of air they derive energy by fermenting sugars and carbohydrates to produce ethanol and carbon dioxide. When yeast are supplied with both sugar and oxygen, the colonies grow up to 20 times faster through cell division than without oxygen.
In 1815, Guy-Lussac understood how yeasts convert the simplest sugar, glucose (C6H12O6) to ethanol:
C6H12O6 (glucose)--->2CO2 (carbon dioxide) + 2C2H5OH (ethanol)
The great medical microbiologist, Louis Pasteur, played a central role in proving this conversion to ethanol required living organisms, rather than a chemical catalyst. Pasteur showed that by bubbling oxygen into the yeast broth, the cells could be made to stop growing, but ferment vigorously–an observation later called the Pasteur Effect.
Many higher animals share this property of oxygen balance with yeasts. When given nutrient (sugar) and oxygen, they will burn fuel quickly like a stoked fire, but when deprived of oxygen, they will reproduce by cell multiplication and division (rather than metabolize). This kind of behavior–burn fuel or divide–is common to many biochemistries and these kinds of organisms are classified as facultative anaerobes; they essentially scrounge a meager living out of whatever particular circumstances are handed to them.
Unlike many kinds of fermenting bacteria (such as yogurt making or lactic acid microbes), yeasts don’t require anything but sugar and water to maintain fermentation and growth. For example, their nutrient broth can be free from other complex molecules such as amino acids, minerals or vitamins, since the yeasts’ history of austere conditions in nature has brought them to a unique state of self-sufficiency, even by microbial standards. The ingeniousness of adaptation makes yeasts one of the most studied and robust microbes.
The Early Days of Yeast Genetics. (1993) edited by Michael N. Hall and Patrick Linder. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.
The Molecular and Cellular Biology of the Yeast Saccharomyces cerevisiae: Gene Expression. (1992) edited by Elizabeth W. Jones, John R. Pringle, and James R. Broach. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.
Mortimer, R.K. and Schold, D. 1985. Genetic map of Saccharomyces cerevisiae, Edition 9. Microbiological Reviews 49: 181-212.