Baker's yeast

Baker yeastis the common name for the strains ofyeastcommonly used in baking bread and other bakery products, serving as aleavening agentwhich causes the bread to rise (expand and become lighter and softer) by converting thefermentablesugars present in the dough intocarbon dioxideandethanol. Baker's yeast is of the speciesSaccharomyces cerevisiae,and is the same species (but a differentstrain) as the kind commonly used in alcoholic fermentation, which is calledbrewer's yeastor the deactivated formnutritional yeast.Baker's yeast is also a single-cell microorganism found on and around the human body.
The use of steamed or boiled potatoes,water from potato boiling,or sugar in a bread dough provides food for the growth of yeasts; however, too much sugar will dehydrate them.Yeast growth is inhibited by both salt and sugar, but more so by salt than sugar.Some sources say fats, such as butter and eggs, slow down yeast growth;others say the effect of fat on dough remains unclear, presenting evidence that small amounts of fat are beneficial for baked bread volume.
Saccharomyces exiguus(also known asS. minor) is a wild yeast found on plants, grains, and fruits that is occasionally used for baking; however, in general, it is not used in a pure form but comes from being propagated in asourdough starter.
It is not known when yeast was first used to bake bread; the earliest definite records come from Ancient Egypt.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 tastier than the previous hardflatbreads. 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 have been either cultivated separately or transferred from batch to batch by means of previously mixed ("old") dough. Also, the development of leavened bread seems to have developed in close proximity to the development of beer brewing, andbarmfrom the beer fermentation process can also be used in bread making.
Without an understanding ofmicrobiology, 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 beerwort, usually includingmalted barleyand wheat flour. Such cultures (sometimes referred to in old American cookery as "emptins", from their origins as the dregs of beer or cider fermentation) became the ancestors of modern baker's yeast, as, in general, they were carefully maintained to avoid what was later discovered to be bacterial contamination, including using preservatives such ashopsas well as boiling the growth medium.
In the 19th century, bread bakers obtained their yeast from beer brewers, and this led to sweet-fermented breads such as the Imperial "Kaiser-Semmel" roll,which in general lacked the sourness created by the acidification typical ofLactobacillus. However, beer brewers slowly switched from top-fermenting to bottom-fermenting yeast (Saccharomyces pastorianus) and this created a shortage of yeast for making bread, so theVienna Processwas developed in 1846.While the innovation is often popularly credited for using steam in baking ovens, leading to a different crust characteristic, it is notable for including procedures for high milling of grains (see Vienna grits), cracking them incrementally instead of mashing them with one pass; as well as better processes for growing and harvesting top-fermenting yeasts, known as press-yeast.
Refinements in microbiology following the work ofLouis Pasteurled to more advanced methods of culturing pure strains. In 1879, Great Britain introduced specialized growing vats for the production ofS. cerevisiae, and in the United States around the turn of the century centrifuges were used for concentrating the yeast,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.
DuringWorld War II,Fleischmann'sdeveloped agranulatedactive 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.
In 1973,Lesaffrecreated instant yeast (also called "quick rise" or "fast acting" yeast), which has gained considerable use and market share at the expense of both fresh and active dry yeast in their various applications.  Instant yeast differs from active dry yeast in several ways:  Instant yeast rises faster than active dry yeast; instant yeast can be directly added to the dry ingredients, whereas active dry yeast should be mixed with liquid (water, milk or beer) and proofed before mixing; instant yeast has a lower moisture content;  and instant yeast is formed of smaller granules.
Modern baker's yeast is the speciesSaccharomyces cerevisiae. One of its properties is that it is not inhibited bypropionates, which are commonly added to baked goods like bread dough to inhibit mold development and bacterial growth.Conversely,sorbatesdo inhibit yeast fermentation activity, so are not added directly to yeast-leavened dough but may be sprayed onto finished products or even incorporated into packing materials.
Baker's yeast is available in a number of different forms, the main differences being the moisture contents.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. Dry yeast forms are good choices for longer-term storage, often lasting more than a year at room temperatures without significant loss of viability.In general, with occasional allowances for liquid content and temperature, the different forms of commercial yeast are considered interchangeable.
Instant and active dry yeast are essentially the same ingredient, just in slightly different forms and applications. Consumers should keep sealed packets of both yeast types at room temperature and store partially used packets in an airtight container in the refrigerator. The main differences between the two are:
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, in general a single dose (reckoned for the average bread recipe of between 500 g and 1000 g of dough) is about 2.5tsp(~12 mL) or about7 g (1⁄4oz), though comparatively lesser amounts are used when the yeast is used in apre-ferment. In general, a yeast flavor in the baked bread is not noticeable when thebakers' percentof added yeast is less than 2.5%.
Notable commercial brands of baker's yeast includeLesaffre's SAF red and SAF gold,Fleischmann's, andRed Star Yeast.
Because it is readily available and easy to culture, baker's yeast has long been used in chemical, biological, and genetic research as amodel organism.Saccharomyces cerevisiaeis a facultative anaerobe and undergoes aerobic fermentation in the presences of oxygen and sugars.  In 1996, after six years of work,S. cerevisiaebecame the firsteukaryoteto have its entiregenomesequenced. It has over 12 millionbase pairsand around 6000genes. Since then, it has remained in the forefront of genetic research. For example, most of our knowledge of thecell division cyclewas worked out from experiments with yeast.
Baker's yeast containsenzymesthat canreduceacarbonylgroup into ahydroxylgroup in fairly high yield, thus making it useful forbiotransformationsinorganic syntheses.It is known to reduceorganometalliccarbonyl compounds in very high yield.
The baking industry relies on industrial production of its ingredients, including baking yeasts. Much effort has been put into developing and marketing yeasts that will perform reliably in mass production. Since the end of the nineteenth century, baker's yeast has been produced by companies that specialize in its production.
The main ingredients for industrial production are yeast cultures, sugar fromcaneandsugar beet; but a number of minerals, nitrogen and vitamins are also needed.
Fermentation happens in several phases, which vary depending on the manufacturer:
The yeast grows from hundreds of kilograms in the intermediate and stock fermentor to tens of thousands of kilograms in the trade fermentor, where most yeast is produced.The earlier stages produce more ethanol and other alcohols, while in the final stages ethanol production is suppressed up to 95% by controlling the amount of oxygen and sugar, in order to increase the yeast production instead.
The industry is highly concentrated, with five companies holding up to 80% of the worldwide market for dry yeast as of 2006. While dry yeast is exported over long distances and mostly sold in the developing countries, industrial customers often prefer to supply fresh yeast from local facilities, with a single wholesaler having up to 90% of the liquid yeast market in the UK in 2006.In the US companies likeLesaffre Group, AB Vista, GB Plange andAB Mauriproduced hundreds of thousands of metric tons of yeast in 2012.