Understanding Yeast

What does yeast do?

Yeast makes bread rise. Just as baking soda and baking powder make your muffins and cakes rise, yeast makes breads of all kinds rise—sandwich loaves, rolls, pizza crust, artisan hearth breads, and more.

How does yeast work?

Since yeast doesn’t reproduce without a good supply of oxygen, it stops reproducing once it’s in bread dough. Instead, it starts to eat: sugar (sucrose and fructose) is its favorite food. If there is sugar in the dough, that’s what the yeast eats first; once that’s gone, yeast converts the starch in flour into sugar; thus flour is capable of providing yeast with a continuous food source.

The byproducts of feeding yeast are CO2, alcohol, and organic acids. CO2 released by yeast is trapped in bread dough’s elastic web of gluten; think of blowing up a balloon. Alcohol and organic acids disperse throughout the dough, enhancing baked bread’s flavor.

As long as moisture and food are available, yeast will continue to eat and produce CO2, alcohol, and organic acids. If your bread stops rising, it’s usually not because the yeast isn’t working (or has died); it’s because the gluten has somehow become “leaky,” failing to retain CO2.

What is yeast, and how is it made?

Yeast is a single-cell organism, part of the fungi kingdom. The yeast we use most often today, Saccharomyces cerevisiae, is one of the oldest domesticated organisms known to mankind: it’s been helping humans bake bread and brew alcohol for thousands of years. Fittingly, the Latin translation of Saccharomyces cerevisiae is “sweet fungi of beer.”

Saccharomyces cerevisiae is just one strain of the more than 1,500 identified species of yeast. 1,500 strains of yeast? But wait, there’s more—literally. Those 1,500 identified yeasts are just an estimated 1% of the yeast population in the world; most species remain as yet unnamed.

In order to have a reliable supply of yeast on hand for all of our baking needs, it’s necessary for manufacturers to “domesticate” wild yeast—stabilizing it, and in the process making it 200 times stronger than its wild counterpart.

Plant scientists working with a yeast manufacturer identify certain characteristics of wild yeast that they decide are desirable; isolate them, and then replicate them. The resulting yeast is given a “training” diet to make it reproduce. Formerly based on molasses, most manufacturers now feed their growing yeast with corn syrup. Once the cells have replicated to a critical mass—a process that generally takes about a week—they’re filtered, dried, packaged, and sent off to the market.

What’s the best way to store yeast?

We recommend transferring yeast to an airtight container (glass or acrylic), and storing it in the freezer for up to a year. If you buy yeast in bulk (e.g., a 1-pound vacuum-packed brick), open it up; divide it into 3 or 4 smaller portions, and store each in a tightly closed container. A zip-top freezer bag works well.

When you’re ready to use yeast, remove the bag or jar from the freezer, spoon out what you need, and quickly return it to the freezer. Yeast manufacturers say you should let frozen yeast rest at room temperature for 30 to 45 minutes before using; frankly, we’re usually too impatient to do that, and have never experienced any problem using yeast straight from the freezer.

Can I use active dry and instant yeasts interchangeably?

Yes, they can be substituted for one another 1:1. We’ve found that active dry yeast is a little bit slower off the mark than instant, as far as dough rising goes; but in a long (2- to 3-hour) rise, the active dry yeast catches up. If a recipe using instant yeast calls for the dough to “double in size, about 1 hour,” you may want to mentally add 15 to 20 minutes to this time if you’re using active dry yeast.

When dough is rising, you need to judge it by how much it’s risen, not how long it takes; cold weather, low barometric pressure, how often you bake, and a host of other factors affect dough rising times, so use them as a guide, not an unbreakable rule.

Remember, bread-baking involves living things (yeast), your own personal touch in kneading technique, and the atmosphere of your kitchen; there are so many variables that it’s impossible to say that “Dough X will double in size in 60 minutes.” Baking with yeast is a combination of art, science and a bit of magic; stay flexible, and your bread (and you!) will be just fine.

One time when you might not want to use instant and active dry yeasts interchangeably is when you’re baking bread in a bread machine. Since bread machines use a higher temperature to raise dough, substituting instant for active dry yeast 1:1 may cause bread to over-rise, then collapse. When baking in the bread machine, and substituting instant yeast for active dry, reduce the amount of instant yeast by 25%.

How much is a “packet” of yeast?

You may find older recipes calling for “1 packet active dry yeast.” A packet used to include 1 tablespoon of yeast; currently, it’s closer to 2 generous teaspoons, a tribute to improved manufacturing methods that produce stronger, more active yeast.

Can I vary the amount of yeast in a recipe?

The amount of yeast you use in your bread dough has a significant bearing on how quickly it’ll rise, and thus on your own schedule. By reducing the yeast, you ensure a long, slow rise, one more likely to produce a strong dough able to withstand the rigors of baking.

The more yeast in a recipe initially, the quicker it produces CO2, alcohol, and organic acids. Alcohol, being acidic, weakens the gluten in the dough, and eventually the dough becomes “porous,” and won’t rise; or won’t rise very well.

By starting with a smaller amount of yeast, you slow down the amount of CO2, alcohol, and organic acids being released into the dough, thus ensuring the gluten remains strong and the bread rises well—from its initial rise in the bowl, to its final rise in the oven.

Remember that this slow rise extends to the shaped loaf, as well as dough in the bowl. Once you’ve shaped your loaf, covered it, and set it aside to rise again, it may take 2 hours or more, rather than the usual 1 to 1 1/2, to rise fully and be ready for the oven.

Keep in mind, also, the characteristics of your own kitchen. If you bake bread all the time, your kitchen is full of wild yeast, and any dough you make there will rise vigorously. If you seldom bake bread, or are just beginning, your kitchen will be quite “sterile;” your dough won’t be aided by wild yeast, and will rise more slowly than it would in a more “active” kitchen.

Here are some guidelines to get you started. If you’re an occasional bread baker, cut back the usual 2 to 2 ½ teaspoons of instant yeast to 1/2 to 1 teaspoon, depending on how long you want to let the dough ferment before the final shape-rise-bake process. 1/2 teaspoon would give you lots of flexibility, such as letting the dough “rest” for 16 to 20 hours; 1 teaspoon would be a good amount for an all-day or overnight rise (10 hours or so, at cool room temperature).

If you’re using active dry yeast, which isn’t as vigorous as instant yeast, we’d up the range to 3/4 to 1 1/2 teaspoons.

We’ve found that here in our King Arthur kitchen, where we bake bread every day, we can cut the yeast all the way back to 1/16 to 1/8 teaspoon in a 3-cup-of-flour recipe, and get a good overnight or all-day rise.

Use your judgment in rating your own kitchen as to “yeast friendliness.”

What about long, slow rises for dough made with perishable ingredients?

Basic flour-water-yeast-salt doughs (which may also contain a bit of oil and/or sugar), such as those for baguettes, focaccia, and pizza, are the best candidates for an all-day countertop rise. Doughs that contain dairy products (and shouldn’t, for food safety reasons, be left at room temperature all day) should be refrigerated if you want to slow them down.

Sweet doughs are notoriously slow risers, anyway; by cutting back on the yeast, you’re just slowing them down even more. Sweet doughs are best slowed down by refrigeration, rather than by reducing the amount of yeast.

And what about whole-grain dough? That rises slowly anyway, doesn’t it?

Whole-grain doughs are naturally slow rising, due to the bran in the grain, which interferes with gluten development. If you’d like to slow down a familiar whole-grain recipe, cut back on the yeast; but if you’re making a particular whole-grain recipe for the first time, we recommend using the amount of yeast indicated, and seeing just how long it takes the dough to rise fully. Often it takes longer than the directions say, and there’s probably no need to slow things down even more.

My yeast didn’t work!

There are all kinds of reasons why bread fails to rise; weak or dead yeast is one of them. In spite of the fact you may have just purchased your yeast, it may not have been stored or rotated correctly prior to your purchasing it so that it isn’t, in fact, as new as you think it is.

A vacuum-sealed bag of yeast stored at room temperature will remain fresh indefinitely. Once the seal is broken, it should go into the freezer for optimum shelf life.

A vacuum-sealed bag of yeast stored at high temperatures, however—e.g., in a hot kitchen over the summer, or in a hot warehouse before delivery—will fairly quickly lose its effectiveness. After awhile, if stored improperly, yeast cells will die. And if you use dead (or dying) yeast in your bread, it won’t rise.

Another reason yeast might not work—you may have killed it by using overly hot water in your recipe; water hotter than 139°F will kill yeast. But don’t stress too much about temperature; 139°F is WAY hotter than is comfortable. If you stepped into a bathtub of 139°F water, you’d leap out fast. So long as the water you combine with your yeast feels comfortable to you, it’ll be comfortable for the yeast, too.

When doubling a recipe, should I double the yeast too?

You can increase the size of most bread recipes simply by doubling, tripling, etc. all of the ingredients—except yeast.

If you increase the amount of yeast at the same rate you increase everything else, you may find yourself with a lot of dough on your hands and not enough time to deal with it. For example, by the time you’ve shaped the eighth loaf, the first may be well on its way to doubled in size, which makes the whole process harder to time and handle.

Most home bread bakers prefer to stick with 2 to 3 teaspoons yeast for up to 8 loaves (24 cups of flour—about 6 pounds), simply giving the bread a longer, slower rise. Not only does this improve the flavor, it slows down the rising dough so that you can work with it more easily.

What factors affect how well yeast works?

If you’ve ever baked bread, you’ll have noticed that sometimes yeast seems to work more quickly than other times. Yeast, like any living organism, is happiest when it’s in a comfortable environment. For yeast, this means plenty of food and moisture; the right pH (acid balance); and the right amount of warmth. Yeast prefers temperatures between 70°F and 100°F; for convenience’s sake, and to produce the most flavorful loaf, it’s best to keep rising conditions cooler, rather than warmer.

Salt and sugar can both slow down yeast activity. Each of them are osmotic, meaning they can pull moisture out of yeast cells, thus adversely affecting how the yeast functions. We add salt to yeast dough both for flavor, and to moderate yeast’s work; we don’t want our loaves rising TOO fast. Sugar is optional; a little bit makes yeast happy, but too much—generally, more than ¼ cup per 3 cups of flour—slows yeast down.

King Arthur Flour Company

King Arthur Flour is America’s oldest flour company, founded in Boston in 1790 to provide pure, high-quality flour for residents of the newly formed United States. More than 220 years later, we’re the nation’s premier baking resource, offering everything from top-quality baking products to inspiring educational programs—all backed by the passion and commitment of our dedicated employee-owners.