Types
Fresh 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: 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: Also 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.)