Most bread bakers are accustomed to using yeast in the form of dry, tan-colored granules. Occasionally, though, a vintage or artisanal recipe calls for the use of fresh cake yeast. Cake yeast is sold in the refrigerated section of some supermarkets, but it's not always easy to find or convenient to keep on hand. Adapting your recipe for dry yeast is a simple conversion.
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The Basic Conversion
Cake yeast is typically sold in a single 2-ounce cake, though some brands subdivide that cake into thirds. Each third is roughly equivalent to one standardized pouch of dry active yeast, or 2 1/4 teaspoons if you buy your yeast in bulk. If the recipe you wish to use has its measurements in weight, rather than volume, use a simple numerical conversion. Multiply the specified weight of fresh yeast by .4 to arrive at the correct quantity of dry active yeast for your recipe. Professionals prefer these weight-based recipes, because it's easier to scale them up or down as needed.
Vive la Difference
The two forms of yeast are not exactly identical. Cake yeast is the original form of commercial yeast, made up of live, active microorganisms that are ready to get to work as soon as you provide them with moisture and food. Unfortunately, like other fresh foods, fresh yeast is perishable and only lasts for 2 to 3 weeks. Dry active yeast, on the other hand, is dormant. It's surrounded by a protective coating of dead yeast cells, giving it a shelf life of months at room temperature if it's stored in an airtight container. This makes it a more convenient option for home bakers.
Adapting Your Recipe
Most recipes suggest crumbling the fresh yeast into warm water to "proof" and become foamy. The water you use becomes part of the liquids in the recipe. Follow the same technique with dry active yeast, but increase the water's temperature. Fresh yeast is best in lukewarm water, approximately 85 to 95 degrees Fahrenheit. Dry yeast, because its protective coating must be dissolved, requires temperatures 20 to 25 degrees higher. Aside from that detail, you'll mix and knead the bread as usual. Fresh yeast tends to provide a faster and more vigorous rise than dry active yeast, which is why it's favored by professional bakers, so you might need to let the dough rise for longer than your recipe indicates.
An Alternative Calculation
Vintage recipes often don't specify how large a "cake" of yeast to use, but you can judge this pretty accurately by the quantity of flour in the recipe. A full 2-ounce cake will raise 8 to 12 cups of flour, enough for a large old-school recipe. One-third of the cake will raise up to 4 cups of flour, roughly the size of most modern bread recipes. When in doubt, then, simply use one pouch of yeast for every 3 to 4 cups of flour in the recipe.