Baker's yeast
Baker's yeast is the common name for the strains of yeast commonly used in baking bread and other bakery products, serving as a leavening agent which causes the bread to rise (expand and become lighter and softer) by converting the fermentable sugars present in the dough into carbon dioxide and ethanol. Baker's yeast is of the species Saccharomyces cerevisiae, and is the same species (but a different strain) as the kind commonly used in alcoholic fermentation, which is called brewer's yeast or the deactivated form nutritional 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 as S. 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 a sourdough 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 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 have been either cultivated separately or transferred from batch to batch by means of previously mixed ("old") dough.
