Because your homemade focaccia is about to be the hottest thing since sliced bread.

Baking sourdough, cinnamon rolls, banana bread, and other forms of carb-filled quarantine comfort foods from scratch has become the official pandemic pastime. As a result, we’ve been delightfully inundated with questions about better baking, flour-free desserts, creative ways to cook with shelf-stable foods, sweet recipes to relieve stress, storage strategies, and how to extend the life of fresh fruits and vegetables.

But one of the most common issues we’ve all been faced with is identifying adequate ingredient substitutions. The more supermarkets sell out of traditional baked good ingredients—flour, eggs, sugar, sour cream, evaporated milk, and so on—the more we’re being forced to consider alternative types of each.

Take yeast, for instance. Most of us are highly familiar with active dry yeast, or could at least spot the packets on a grocery store shelf if pressed (but good luck finding it). Beyond that, many are unaware that there are several types of yeast you can use when baking, and the one you choose will make a difference in your final product.

First, a key question: what is yeast, exactly? 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 try 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.

Now that we’ve outlined the basics of what yeast is, on to the differences between the three most common types: active dry yeast, instant yeast, and fresh yeast.

Active Dry Yeast

This is the most popular type 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 105ºF and 115ºF). It’s finishing 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.

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.”

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 breads that require 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.

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 one of these delicious yeast-free bread recipes instead.