is the common name for the strains of yeast
commonly used as a leavening agent
in baking bread
and related products, where it converts the fermentable sugars
present in the dough
into carbon dioxide
. The use of potatoes
, water from potato boiling, eggs
, or sugar
in a bread dough accelerates the growth of yeasts. Salt
such as butter
slow down yeast growth. The majority of the yeast used in baking is Saccharomyces cerevisiae
, the same species commonly used in alcoholic fermentation
. 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
It is not known when yeast was first used to bake bread. The first records that show this use came 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 more tasty than the normal flat, hard cake. It is generally assumed that the earliest forms of leavening were likely very similar to modern sourdough; that is, 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 breadmaking.
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.
Today there are several retailers of baker's yeast; the biggest producers are Fleischmann's Yeast and Lesaffre (who also owns the popular American label Red Star Yeast). During World War II Fleischmann's developed a granulate 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.
Types of baker's yeast
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.
- Cream yeast is the closest form to the yeast slurries of the 19th century, being essentially a suspension of yeast cells in liquid, siphoned off from the growth medium. Its primary use is in industrial bakeries with special high-volume dispensing and mixing equipment, and it is not readily available to small bakeries or home cooks.
- Compressed yeast is essentially cream yeast with most of the liquid removed. It is best known in the form of cake yeast, which is essentially a soft solid, beige in color, but is also available in crumbled form for bulk usage. It is highly perishable; though formerly widely available for the consumer market, it has become less common in supermarkets in some countries due to its poor keeping properties, having been obsoleted in some such markets by active dry and instant yeast. It is still widely available for commercial use, and is somewhat more tolerant of low temperatures than other forms of commercial yeast; however, even there, instant yeast has made significant market inroads.
- Active dry yeast is the form of yeast most commonly available to noncommercial bakers, as well as the yeast of choice for situations where long travel or uncontrolled storage conditions are likely. It consists of coarse oblong granules of yeast, with live yeast cells encapsulated in a thick jacket of dry, dead cells with some growth medium. Under most conditions, active dry yeast must be proofed or rehydrated first and, despite its better keeping qualities than other forms, is generally considered more sensitive than other forms to thermal shock when actually used in recipes.
- Instant yeast appears similar to active dry yeast, but has smaller granules with substantially higher percentages of live cells. It is more perishable than active dry yeast, but also does not require rehydration, and can usually be added directly to all but the driest doughs. Instant yeast generally has a small amount of ascorbic acid added as a preservative. Some producers provide two or more forms of instant yeast in their product portfolio; for example, LeSaffre's "SAF Instant Gold" is designed specifically for doughs with high sugar contents.
- Rapid-rise yeast is a variety of yeast (usually a form of instant yeast) designed to provide greater carbon dioxide output to allow faster rising at the expense of shortened fermentation times. There is considerable debate as to the value of such a product; while most baking experts believe it reduces the flavor potential of the finished product, Cook's Illustrated magazine, among others, feels that at least for direct-rise recipes, it makes little difference. Rapid-rise yeast is often marketed specifically for use in bread machines.
- Flake yeast is dead yeast, sold primarily as a nutritional supplement. It has little to no leavening power.
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. A single dose (reckoned for the average bread recipe of between 500g and 1000g of dough) is generally about 2.5 tsp or about 7g, though comparatively lesser amounts are used when the yeast is used in a pre-ferment.
- Corriher, Shirley, Cookwise. New York: William Morrow and Co., 1997, ISBN 0688102298.
- Editors of Cook's Illustrated Magazine, Baking Illustrated. Brookline, MA:Boston Common Press, 2004, ISBN 0936184752.
- The King Arthur Flour Baker's Companion. Woodstock, VT: Countryman Press, 2003, ISBN 0881505811.
- Simmons, Amelia. American Cookery, Hartford, 1798. Text at Feeding America and Project Gutenberg
- Sloat, Caroline (ed.), Old Sturbridge Village Cookbook 2ed.. Old Saybrook: Globe Pequot Press, 1995, ISBN 1564407284.