What Is Yeast?
Yeast is a simple single-celled organism called Saccharomyces cerevisiae. Yeast cells are egg-shaped and are only visible with a microscope—it takes 20,000,000,000 yeast cells to weigh one gram. They’re technically a member of the fungi kingdom, and over 500 species of yeast actually exist. (Don’t worry, we’re only going to explain three here.) Yeast is the driving force not only behind bread-baking, but also in fermentation, which is the chemical process behind making everything from beer and wine to pickles, chocolate, and kombucha. Yeast cells require three things to thrive: food, warmth, and moisture. In the presence of warmth and moisture, yeast converts its food—sugar and starch—into carbon dioxide and alcohol through fermentation. It’s the carbon dioxide that makes baked goods rise.
Active Dry Yeast
This is the most popular type of yeast for home bakers and is typically sold in quarter-ounce packets or jars. Its texture is granular, not unlike cornmeal or very finely-ground coffee. Because active dry yeast is alive but dormant in its packaged state, you must rehydrate it via proofing or dissolving the granules in warm water (ideally between 105 F and 115 F). It finishes blooming when the yeast is dissolved and small bubbles rise to the top of the water glass. If the mixture doesn’t bloom, this is a telltale sign that your yeast is dead.
Instant Yeast
Instant yeast is another form of dormant dry yeast with smaller granules than active dry and a faster absorption rate. Instant yeast does not need to be proofed or rehydrated before you bake with it, so you can mix it straight into your dry ingredients. Rapid-rise and quick-rise yeasts are two forms of instant yeast that may contain extra enzymes and additives to make your dough rise faster.
Fresh Yeast
Fresh yeast is active. You’ll find it in the refrigerated sections of many supermarkets, often in the form of small-sized cakes. Fresh yeast is light brown, soft, and crumbly. It requires proofing in lukewarm water and is best for bread that requires a long, cool rise. Fresh yeast only lasts for a couple of weeks in the fridge, so if you notice any mold on it (or it turns dark brown and feels dry and rough), discard it immediately.
Substitutions
According to Nathan Myhrvold, author of Modernist Bread and bread-baking expert, you can use a few simple equations to convert one style of yeast into another. “If you have instant yeast, but need active dry, simply multiply by 1.33; if you have active dry yeast, but need instant, multiply by 0.75, he says. “The conversions are easy, and the difference is so nominal in small batches that it will have little or no effect on the bread.” Bottom line: if you can’t track down one of the above, consider your other options (just make sure you’re following a recipe that calls for the specific style of yeast you have). Another simple solution? Try making delicious yeast-free bread recipes instead.
