Despite indications to the contrary—created by the commercial largesse of the yeast companies—there are only three types of yeast: fresh, active dry, and instant. All are derived from the powerful brewer's yeast known as Saccharomyces cerevisiae, but each is processed from a slightly different strain of this protypical yeast.
Fresh, Compressed, or Cake Yeast
The original commercial yeast, known as fresh, compressed, or cake yeast is about 70 percent water by weight and is composed of 100 percent living cells. It is soft and crumbly and requires no proofing—fresh yeast will dissolve if it is simply rubbed into sugar or dropped into warm liquid. Owing to qualities associated with its strain, fresh yeast will produce the most carbon dioxide of all three types of yeasts during fermentation. Fresh yeast is considered fast, potent, and reliable, but it has a drawback: it is highly perishable and must be refrigerated and used before its expiry date.
Active dry yeasts arrive at their granular state by undergoing processes that reduce them to 95 percent dry matter. Traditional active dry yeast is exposed to heat so high that many of its cells are destroyed in the process. Because the spent outer cells encapsulate living centers, active dry yeast must first be dissolved in a relatively hot liquid (proofed) to slough off dead cells and reach the living centers.
Instant Yeasts (called "Instant," "Rapid Rise," or "Bread")
Instant yeasts are also processed to 95 percent dry matter, but are subjected to a gentler drying process than active dry. As a result, every dried particle is living, or active. This means the yeast can be mixed directly with recipe ingredients without first being dissolved in water or proofed. It is in this context that the yeast is characterized as "instant." We prefer instant yeast in the test kitchen. It combines the potency of fresh yeast with the convenience of active dry, and it is considered by some to have a cleaner flavor than active dry because it contains no dead cells. (In our months of testing, we found this to be true when we made a lean baguette dough but could detect no difference in flavor when using the two yeasts in doughs made with milk, sugar, and butter.)
To substitute active dry for instant (or rapid rise) yeast: Use 25 percent more active dry. For example, if the recipe calls for 1 teaspoon of instant yeast, use 1 1/4 teaspoons of active dry. And don't forget to "prove" the yeast, i.e. dissolving it in a portion of the water from the recipe, heated to 105 degrees.
To substitute instant (or rapid rise) yeast for active dry: Use about 25 percent less. For example if the recipe calls for 1 packet or 2 1/4 teaspoons of active dry yeast, use 1 3/4 teaspoons of instant yeast. And you do not need to prove the yeast, just add it to the dry ingredients.
To substitute fresh yeast for active dry yeast, use a ratio of roughly 2:1, i.e. use one small cake (0.6 ounce) of compressed fresh yeast in lieu of 1 packet (.25 ounces) of active dry yeast.
Note a packet of active dry or instant yeast contains about 2 1/4 teaspoons (.25 ounces) of yeast.