TYPES OF YEAST
There are three types of bakers yeast; Compressed (fresh) yeast, active dry and instant. There is a yeast called brewer’s yeast, this is a nutritional yeast and has no leavening ability.
This is a mixture of yeast and starch with a moisture content of approximately 70 percent. This is commonly referred to as fresh yeast and must be kept refrigerated. It should appear creamy white and crumbly with a fresh yeasty aroma. If it has developed a sour odor, has a brown color or slimy film, do not use. While some bakers add this directly into the dry bread mix, it is usually softened in twice its weight in warm water at 100° F (38° C) before adding to a bread dough. Under proper storage conditions, it has a shelf-life of 2-3 weeks. It can be frozen for up to one month if it is fresh (when thawed, frozen compressed yeast loses about 5 percent of its activity).
ACTIVE DRY YEAST
Virtually, all of the moisture has been removed from this particular type of yeast using hot air. The absence of moisture renders the organism dormant and can be stored, unopened for several months without needing to be refrigerated. It is usually re-hydrated in lukewarm water 110° F (43° C) liquid before adding to other ingredients. It should be stored in a cool, dry place and refrigerated after opening.
INSTANT DRY YEAST
This yeast is added directly into the dry ingredients without rehydrating. This has made it a very popular choice. It is activated by the water in the formula. Like the other yeasts, it is destroyed in temperatures greater than 138° F (59° C). When you are making a dough that requires little mixing or are very firm (such as a bagel or croissant), this particular yeast may not fully-dissolve during mixing. It would best to moisten the yeast in four to five times its weight of water. Then you must deduct this amount of water called for in the recipe.
You need to remember, dry yeasts are generally twice as strong as fresh yeast. Too much yeast can ruin a bread, always remember to reduce the amount of dry yeast or active yeast in a recipe that calls for fresh (See cheat sheet - YEAST SUBSTITUTIONS). There is virtually no difference in taste but it can drastically ruin your bread.