Baking a fresh loaf of bread can be tricky if you don't have a good understanding about yeast. Yeast comes in 3 different forms: Compressed (Fresh) yeast, Active Dry yeast, and Instant (or Rapid Rise) yeast.
Compressed yeast is a mixture of yeast and starch with a moisture content of about 70%. It is also referred to as fresh yeast and must be kept refrigerated. It should be creamy white and crumbly with a fresh, yeasty smell. Compressed yeast should softened in warm water at 100°F before being added to bread dough. Under proper storage conditions, compressed yeast has a shelf life of 2 to 3 weeks.
Active Dry Yeast
Active dry yeast differs from compressed yeast in that virtually all the moisture has been removed by hot air. The absence of moisture renders the organism dormant and allows the yeast to be stored without refrigeration for several months. When used in preparing doughs, dry yeast is generally rehydrated in a lukewarm (110°F) liquid before being added to the other ingredients. It should be stored in a cool, dry place and refrigerated after opening.
Instant Dry Yeast
Instant dry yeast has gained popularity because of its ease of use; it is added directly to the dry ingredients in a bread formula without rehydrating. The water in the formula activates it. Like all yeasts, instant dry yeast is a living organism and is destroyed at temperatures above 138°F.
You can use whichever type of yeast you prefer just remember that dry yeasts are at least twice as strong as compressed yeast. You'll need to use the formulas below if you have to substitute yeasts. For example, if a recipe calls for compressed yeast, but all you have is Active Dry yeast at home.
Compressed (fresh) Yeast x 0.5 = Active Dry Yeast
Compressed (fresh) Yeast x 0.33 = Instant Dry Yeast
Active Dry Yeast x 2 = Compressed (fresh) Yeast
Active Dry Yeast x 0.75 = Instant Yeast
Instant Yeast x 3 = Compressed (fresh) Yeast
Instant Yeast x 1.33 = Active Dry Yeast
Use these formulas to convert from one type of yeast to another. Happy baking!