Yeast is a microscopic plant of fungous growth, and is the lowest form of vegetable life. It consists of spores, or germs, found floating in air, and belongs to a family of which there are many species. These spores grow by budding and division, and multiply very rapidly under favorable conditions, and produce fermentation.

Fermentation is the process by which, under influence of air, warmth, moisture, and some ferment, sugar (or dextrose, starch converted into sugar) is changed into alcohol (CsH5HO) and carbon-dioxide (CO.). The product of all fermentation is the same. Three kinds are considered, — alcoholic, acetic, and lactic. Where bread dough is allowed to ferment by addition of yeast, the fermentation is alcoholic; where alcoholic fermentation continues too long, acetic fermentation sets in, which is a continuation of alcoholic. Luetic fermentation is fermentation which takes place when milk sours.

Liquid, dry or compressed yeast, may be used for raising bread. The compressed yeast cakes done up in tin foil have long proved satisfactory, and are now almost universally used, having replaced the home-made liquid yeast. Never use a yeast cake unless perfectly fresh, which may be determined by its light color and absence of dark streaks.

The yeast plant is killed at 212° F.; life is suspended, but not entirely destroyed, at 32° F. The temperature best suited for its growth is from 65° to 68° F. The most favorable conditions for the growth of yeast are a warm, moist, sweet, nitrogenous soil. These must be especially considered in bread making.

The most well-known and commercially significant yeast are the related species and strains of Saccharomyces cerevisiae. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to expand, or raise, dough. Saccharomyces cerevisiae is commonly used as baker's yeast and for some types of fermentation. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins such as niacin, folic acid, riboflavin, and biotin.

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 is inhibited by both salt and sugar, but more so with salt than sugar. Fats such as butter or eggs slow down yeast growth, however others say the effect of fat on dough remains unclear, presenting evidence that small amounts of fat are beneficial for baked bread volume.

History

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 either have been cultivated separately or transferred from batch to batch by means of previously mixed ("old") dough. Alternatively, 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.

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 generally lacked the sourness created by the acidification typical of Lactobacillus. However, beer brewers slowly switched from top-fermenting to bottom-fermenting yeast (both S. cerevisiae) and this created a shortage of yeast for making bread, so the Vienna Process was developed in 1846. While the innovation is often popularly credited for using steam in baking ovens leading to a different crust characteristic, it notably included 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, this was known as press-yeast.

Refinements in microbiology following the work of Louis Pasteur led to more advanced methods of culturing pure strains. In 1879, Great Britain introduced specialized growing vats for the production of S. 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.

Varieties

Active Dry Yeast = Dry Yeast   

Equivalents: One package = 2 1/4 teaspoons = 1/4 ounce

Notes: This is the yeast called for by most bread recipes.  It's largely displaced the fresh yeast our grandparents used since it has a longer shelf life and is more tolerant of mishandling.   To activate it, sprinkle it on water that's 105° - 115° F and wait for it to begin foaming (about five minutes).  Look for it in the dairy case--it's usually sold in strips of three packages or in 4-ounce jars.  Always check the expiration date to make sure it's fresh.  Dry yeast can be stored at room temperature until the expiration date--or within 4 months of opening--but it lasts even longer in the refrigerator or freezer.  Always bring yeast to room temperature before you use it.   It's important to keep stored yeast away from air and moisture, so use the smallest container you can find and seal it well.

Substitutes: Fresh yeast (Substitute one cake for each package or 2 1/4 teaspoons of active dry yeast.) OR instant yeast (Substitute measure for measure, but don't dissolve it in liquid first.  Your bread will only need to rise once.) OR bread machine yeast (Substitute measure for measure, but don't dissolve it in liquid first.  Your bread will only need to rise once.)

Baker's yeast = baking yeast = bread yeast   

Equivalents: 1 tablespoon = 1 package = 1 cake

Notes: This is used as a leaven in breads, coffeecakes, and pastries like croissants and brioche.  It works by converting sugar into carbon dioxide, which causes the dough to rise so the bread will be light and airy.  Yeast comes either as dry granules or moist cakes.   It becomes less potent after the expiration date stamped on the package, so dough made with it may take longer to rise, or not rise at all.  If the potency of the yeast is in doubt, test or "proof" it by putting some of it in warm water (105° - 115° F) mixed with a bit of sugar.  If it doesn't get foamy within ten minutes, you'll need to get fresher yeast.  

Beer yeast = brewer's yeast   

Notes: This is used to produce alcohol and bubbles in beer.  There are several varieties, each matched to specific varieties of beer.   It's available either as a liquid or powder at beer-making supply stores.  Don't confuse this with the brewer's yeast that's used as a nutritional supplement.  That type of yeast is deactivated, so it won't produce any alcohol or bubbles.  

Bread Machine Yeast 

Equivalents: One package active dry yeast = 2 1/4 teaspoons bread machine yeast

Notes: This type of dry yeast is highly active and very finely granulated so that it hydrates quickly.  Breads made with this yeast require only a single rise, so this yeast is handy to use in a bread machine.  Most machines will have you add this yeast last, on top of the dry ingredients.  If you're not using a bread machine, add this yeast to the flour and other dry ingredients.   It's often sold in 4-ounce jars.  You can store unopened jars at room temperature until the expiration date stamped on the jar, but the yeast lasts even longer in the refrigerator or freezer.  If you freeze yeast, let it come to room temperature before using.

Substitutes:  Instant yeast (This is very similar.  One envelope active dry yeast = 2 1/4 teaspoons bread machine yeast) OR active dry yeast (One envelope active dry yeast = 2 1/4 teaspoons bread machine yeast.   Ordinary active dry yeast needs to be dissolved in water first, and the bread will need to rise more than once.) OR compressed yeast (Substitute one cake for each package or 2 1/4 teaspoons of dry yeast.  This needs to be dissolved in water first, and the bread will need to rise more than once.) 

Brewer's Yeast 

Notes: This inactive yeast is rich in protein and B vitamins, and it's used a nutritional supplement.  It's a by-product of beer-making, which gives it a slightly bitter flavor.  If you object to the bitterness, try nutritional yeast, which is made from the same yeast strain but grown on molasses.  It's more expensive but has a more pleasant flavor.  You can also buy debittered brewer's yeast.  Brewer's yeast comes powdered (the most potent form), in flakes (best for health shakes), and in tablets.  Don't confuse this with active forms of yeast, like the kinds bakers, brewers, and winemakers use.  If you eat them, active yeasts will continue to grow in your intestine, robbing your body of valuable nutrients.

Substitutes:  nutritional yeast (better, nuttier flavor, lighter color) OR yeast extract

Fresh yeast = compressed yeast = active fresh yeast = cake yeast = baker's compressed yeast = wet yeast

Equivalents:   2-ounce cake = 3 X 0.6-ounce cakes

Notes: This form of yeast usually comes in 0.6-ounce or 2-ounce foil-wrapped cakes.   It works faster and longer than active dry yeast, but it's very perishable and loses potency a few weeks after it's packed.  It's popular among commercial bakers, who can keep ahead of the expiration dates, but home bakers usually prefer dry yeast.  To use, soften the cake in a liquid that's 70° - 80° F.  Store fresh yeast in the refrigerator, well wrapped, or in the freezer, where it will keep for up to four months.  If you freeze it, defrost it for a day in the refrigerator before using.

Substitutes: Active dry yeast (Substitute one package or 2 1/4 teaspoons for each .6-ounce cake of compressed yeast) OR instant yeast (Substitute one package or 2 1/4 teaspoons for each cake of compressed yeast) OR bread machine yeast (Substitute 2 1/4 teaspoons for each cake of compressed yeast)

Instant yeast = quick yeast = rapid rise active dry yeast = quick rise active dry yeast = fast-rising active dry yeast = fast rising yeast   

Equivalents:  One package = 2 1/4 teaspoons = 1/4 ounce

Notes: This very active strain of yeast allows you to make bread with only one rise.  The trade-off is that some flavor is sacrificed, though this doesn't matter much if the bread is sweetened or heavily flavored with other ingredients.  Unlike ordinary active dry yeast, instant yeast doesn't need to be dissolved in liquid first--you just add it to the dry ingredients.  Look for it in the dairy case--it's usually sold in strips of three packages or in 4-ounce jars.   Before buying it, check the expiration date to make sure it's fresh.  Dry yeast can be stored at room temperature until the expiration date stamped on the jar, but it lasts even longer in the refrigerator.

Substitutes: Bread machine yeast (very similar; substitute measure for measure.) OR active dry yeast (Substitute measure for measure. Active dry yeast needs to be dissolved in water first, and the bread will need to rise more than once.) OR fresh yeast (Substitute one cake for each package or 2 1/4 teaspoons of dry yeast.  This needs to be dissolved in water first, and the bread will need to rise more than once.)

Nutritional Yeast

Equivalents: 1 tablespoon powdered = 2 tablespoons flakes

Notes: This nutritional supplement has a pleasant nutty-cheesy flavor and is packed with protein and B vitamins.  It comes in flakes or powder and is popular with vegans and health buffs who use it to make cheese substitutes, gravies, and many other dishes.  It's also a great topping for popcorn.  Nutritional yeast is very similar to brewer's yeast, which is also used as a nutritional supplement and is made from the same strain of yeast.  The difference is that brewer's yeast is a by-product of beer production and retains some of the bitter flavor of hops.   Don't confuse nutritional yeast, which is deactivated, with active forms of yeast, like the kinds bakers, brewers, and winemakers use.  If you eat them, active yeasts will continue to grow in your intestine, robbing your body of valuable nutrients.  Look for nutritional yeast at health food stores.  Get fortified nutritional yeast if you're taking it as a source of vitamin B12.   Substitutes:  brewer's yeast (inferior flavor, darker color) OR Parmesan cheese (as a condiment; higher in fat, less nutritious) OR wheat germ (works well in baked goods or sprinkled on cereals) OR yeast extract.

Smoked yeast = bacon yeast = hickory-smoked yeast

Notes: This is yeast that's been smoked, giving it a bacon-like flavor.  It's used to flavor other dishes.  Don't confuse it with active forms of yeast, like the kinds bakers, brewers, and winemakers use.  If you eat them, active yeasts will continue to grow in your intestine, robbing your body of valuable nutrients. 

Tips on Yeast

Always ensure your yeast is fresh by checking its expiration date. Ideally, yeast should be used several months before the expiration date. Once a package or jar of yeast is opened, it is important that the remaining contents be immediately resealed and refrigerated or frozen for future use. Often dough that fails to rise is due to stale yeast.

Conversion Chart for Quick-Acting Yeast

  ¾ tsp. active dry yeast = ½ tsp. quick-acting yeast
1 tsp. active dry yeast = ¾ tsp. quick-acting yeast
1½ tsp. active dry yeast = 1 tsp. quick-acting yeast
2¼ tsp. active dry yeast = 1½ tsp. quick-acting yeast

From Wikipedia, the free encyclopedia (Baker's Yeast) 
The Cooks Thesaurus (Yeast)
Cooking For Engineers (Baker's Yeast)