Three types of yeast are commonly available to the home baker: fresh, or cake yeast; active dry yeast; and instant yeast. Both instant and active dry yeasts are dry yeasts, and these are by far the most common yeasts available. However, you may think of either cake yeast or active dry yeast as "regular" yeast.
Manufacturing
Red Star Original, a yeast manufacturer, says yeast is made using a carefully selected, laboratory grown and purified seed yeast. The seed yeast is grown in large tanks with added air and the sugars in sterilized molasses. To harvest, machinery passes the resulting fermented liquid into centrifugal pumps, where the yeast separates from the liquid; the result of this is "cream yeast." Further drying and processing turns the cream yeast into cake yeast, then active dry yeast, then instant yeast. Therefore, instant yeast is merely a drier, more processed version of active dry or cake yeast.
Preparation
Active dry and cake yeasts are prepared by "proofing," in which the yeast is combined with warm water and possibly sugar, and left to ferment for several minutes. Instant yeast is ready to use immediately; you may mix it, unhydrated, directly into your other ingredients, according to the recipe.
Time Frame
Active dry and cake yeasts typically require two rises in breadmaking; each rise may take a couple of hours, depending on the recipe's requirements. Instant yeast, which includes bread machine yeast and fast rising yeast, may include ascorbic acid as a dough conditioner, allowing the dough to rise much faster, requiring only one rise, and increasing faster bread loaf volume.
Substitutions
Allrecipes.com advises that you can substitute 25 percent less instant yeast for the amount of active dry yeast a recipe calls for. According to the yeast manufacturer Fleischmann's, you can also substitute equal amounts of active dry yeast for instant yeast as long as you proof it first. You may use cake yeast instead of active dry or instant yeast by dissolving it in warm water; no proofing is necessary. Home bakers do not need to adjust the liquid in the recipes for yeast substitutions because of the small amounts involved.
Shelf Life
Cake yeast comes in small blocks, whereas instant and active dry yeast come in jars or ¼-ounce packets. Red Star Original says cake yeast has the shortest shelf life, at about eight weeks from the packaging date when unopened; unopened active dry and instant yeast lasts two years from the date of packaging. Cooking for Engineers reports that opened active dry and instant yeast will last three months in the refrigerator and six months in the freezer, though Fleischmann's does not recommend freezing yeast. If unsure if your yeast is still good, proof it with warm water and sugar; if it foams enough to double in volume, it is fine for baking.
