All About Yeast From King Aurthur Flour

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.

You can increase the size of most bread recipes by simply doubling, tripling, etc. all of the ingredients, including the yeast. Depending on the recipe and rising time, you may use as little as 1 teaspoon, or up to 2 1/4 teaspoons (sometimes more) of instant yeast per pound (about 4 cups) of flour.

That being said, many home recipes, particularly older ones, use more yeast than this; so when you double or triple the yeast, you may find that your dough is rising too fast—faster than you can comfortably deal with it. In addition, if you've increased your recipe by 5 times or more, and also increased the yeast by 5 times, keep in mind the time it will take you to shape the dough. You may find the rising dough outpaces your ability to get it shaped and baked. If that's the case, make a note to reduce the amount of yeast next time.

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 1/4 cup per 3 cups of flour—slows yeast down.

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.

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.

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.

In days gone by there was a significant difference between active dry yeast and instant yeast. Today, the difference is minimal, and the two can be used interchangeably—with slightly different results. Let's look at ADY first.

Active dry yeast: The classic ADY manufacturing process dried live yeast cells quickly, at a high temperature. The result? Only about 30% of the cells survived. Dead cells “cocooned” around the live ones, making it necessary to “proof” the yeast—dissolve it in warm water—before using.

These days, ADY is manufactured using a much gentler process, resulting in many more live cells. Thus, it’s no longer necessary to dissolve ADY in warm water before using—feel free to mix it with the dry ingredients, just as you do instant yeast.

However, if you want to make sure that your ADY is alive and ready to work in your bread dough, proof it first, as follows:

Place ½ cup of 110°F water (slightly warmer than lukewarm) in a 1-cup liquid (glass or plastic) measure. Add the yeast called for in the recipe, plus 1/2 teaspoon of sugar, stirring to dissolve. Wait 10 minutes; the yeast is active and healthy if the foam has risen to the 1-cup mark. If you don't see any activity, buy a fresh supply of yeast. Once you’re sure the yeast is active, continue with your recipe.

ADY, compared to instant yeast, is considered more “moderate.” It gets going more slowly, but eventually catches up to instant—think of the tortoise and the hare. Many bread-bakers appreciate the longer rise times ADY encourages; it’s during fermentation of its dough that bread develops flavor.

Fleischmann’s and Red Star are the two brands of active dry yeasts you’re most likely to see in your supermarket.

Instant yeast is manufactured to a smaller granule size than ADY. Thus, with more surface area exposed to the liquid in a recipe, it dissolves more quickly, and gets going faster than ADY. While you can proof it if you like, it’s not necessary; like ADY, simply mixing it into your bread dough along with the rest of the dry ingredients works just fine.

One caveat: in dough that’s high in sugar (generally, more than ¼ cup sugar per 3 cups of flour), the sugar evens things out, and instant yeast and ADY will perform the same.

SAF instant yeast, produced by France’s LeSaffre company, leads the way among instant yeast brands, with Red Star also commanding a good percentage of the market. LeSaffre is the largest yeast producer in the world, responsible for fully one-third of the total amount of yeast manufactured yearly.

SAF Red is an all-purpose instant yeast widely used by professionals everywhere—including the bakers in the King Arthur Bakery and test kitchens.

SAF Gold, another SAF variety, is an "osmotolerant" yeast, perfect for sweet breads, and any dough with a high amount of sugar. SAF Gold works best when the amount of sugar is between 10% and 30% of the amount of the flour, by weight (this is called a "baker's percentage"). So, for a 3-cup-flour loaf (12 3/4 ounces flour), you'd choose SAF Gold if the sugar is greater than 3 tablespoons, or up to about a heaping 1/2 cup. Understand that the greater the amount of sugar, the more slowly your dough will rise.

How does SAF Gold work? Sugar likes to absorb water; and when sugar's in bread dough, it pulls water away from yeast, leaving the yeast thirsty. The yeast cells in SAF Gold are bred to require less liquid to function; so they're better able to withstand sugar's greedy ways with water.

SAF Gold is best used in sweet breads; it won't do well in "lean" doughs (low in sugar and fat).

Bread machine yeast and instant yeast are the same yeast. RapidRise, Flesichmann’s branded instant yeast, is also an instant yeast, but a different strain than SAF or Red Star.

Personally, we find RapidRise is faster out of the gate than SAF or Red Star, but gives out sooner. And since we like to give our loaves leisurely rises (a long rise brings out bread's flavor), we like SAF/Red Star.