Baker's yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and bakery products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol. The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Salt and fats such as butter slow down yeast growth[citation needed]. The majority of the yeast used in baking is Saccharomyces cerevisiae, which is the same species commonly used in alcoholic fermentation, and so is also called brewer's yeast. Additionally, Saccharomyces exiguus (also known as S. minor) is a wild yeast found on plants, fruits, and grains that is occasionally used for baking; it is not, however, generally used in a pure form, but comes from being propagated in a sourdough starter.
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It is not known when yeast was first used to bake bread. The first records that show this use came from Ancient Egypt.[1] Researchers speculate that a mixture of flour meal and water was left longer than usual on a warm day and the yeasts that occur in natural contaminants of the flour caused it to ferment before baking. The resulting bread would have been lighter and more tasty than the previous flat, hard flatbreads. It is generally assumed that the earliest forms of leavening were likely very similar to modern sourdough; the leavening action of yeast would have been discovered from its action on flatbread doughs, and would either have been cultivated separately or transferred from batch to batch by means of previously mixed ("old") dough. Alternately, the development of leavened bread seems to have developed in close proximity to the development of beer brewing, and barm from the beer fermentation process can also be used in bread making.
Without a full understanding of microbiology, early bakers would have had little ability to directly control yeast cultures, but still kept locally interesting cultures by reusing doughs and starters to leaven later batches. However, it became possible to isolate and propagate favored yeast strains in the same manner as was done in the beer industry, and it eventually became practical to propagate yeast in a slurry with a composition similar to beer wort, usually including barley malt and wheat flour. Such cultures (sometimes referred to in old American cookery as "emptins", from their origins as the dregs of beer or cider fermentation) would become the ancestors of modern baker's yeast, as they generally were carefully maintained to avoid what would later be discovered to be bacterial contamination, including using preservatives such as hops as well as boiling the growth medium.
Later refinements in microbiology following the work of Louis Pasteur led to more advanced methods of culturing pure strains, making modern commercial yeast possible, and turning yeast production into a major industrial endeavor. The slurry yeast made by small bakers and grocery shops became cream yeast, a suspension of live yeast cells in growth medium, and then compressed yeast, the fresh cake yeast that became the standard leaven for bread bakers in much of the Westernized 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.
Baker's yeast is available in a number of different forms. Though each version has certain advantages over the others, the choice of which form to use is largely a question of the requirements of the recipe at hand and the training of the cook preparing it. With occasional allowances for liquid content and temperature, the different forms of commercial yeast are generally considered interchangeable.
For most commercial uses, yeast of any form is packaged in bulk (blocks or freezer bags for fresh yeast; vacuum-packed brick bags for dry or instant); however, yeast for home use is often packaged in pre-measured doses, either small squares for compressed yeast or sealed packets for dry or instant. For active dry and instant yeast, a single dose (reckoned for the average bread recipe of between 500 g and 1000 g of dough) is generally about 2.5 tsp (~12 mL) or about 7 g (1/4 ounce), though comparatively lesser amounts are used when the yeast is used in a pre-ferment.
Because it is readily available and easy to culture, baker's yeast has long been used in chemical, biological, and genetic research. In 1996, after 6 years of work, S. cerevisiae became the first eucaryote to have its entire genome sequenced. It has over 12 million base pairs and around 6000 genes. Since then it has remained on the forefront of genetic research. For example, most of our knowledge of the cell division cycle was worked out from experiments with yeast.
Baker's yeast contains enzymes which can reduce a carbonyl group into a hydroxyl group in fairly high yield, thus making it a useful bio-reagent in chemical syntheses.[2] It is known to reduce organometallic carbonyl compounds in very high yield.
Baker's yeast can also be used to produce ethanol via fermentation for use in chemical synthesis, although doing so in some places requires permits.
   | "The History of Bread Yeast" - BBC - h2g2 - The History of Bread Yeast |
| | Project Gutenberg - American Cookery by Amelia Simmons - Project Gutenberg |
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