Yeast has been the driving force behind leavened bread and fermented beverages for thousands of years, but this tiny organism can only survive and grow under certain conditions. In baking, both sugar and salt affect how yeast works, and too much of either can be damaging. With an understanding of the factors that influence yeast activity in dough, you can control how quickly the dough rises and avoid situations that prevent the dough from rising at all.
How Yeast Works
Yeast is a one-celled fungus that metabolizes simple sugar into carbon dioxide and alcohol. In dough, the gases that the yeast expels get trapped in air bubbles, causing the dough to inflate. Traditionally, people harnessed wild yeasts already present on the surface of the grains, but modern producers grow yeast in tanks to sell specifically for baking. Yeast comes in the form of active dry yeast, instant dry yeast and fresh cake yeast.
As the sole food source for yeast, sugars are essential to its activity. Starch in the flour provides enough sugar for the yeast to feed on, but adding a small amount of sugar makes the yeast more active. Too much sugar in the dough has the opposite effect, because it dehydrates the yeast cells. To compensate, sweet recipes with more than 10 percent sugar -- by weight -- often call for greater quantities of yeast.
Slow With Salt
Salt plays a vital role in the flavor, structure and color of breads, but it retards yeast activity. Like excess sugar, salt draws moisture out of yeast cells, which limits the amount of moisture available to help the yeast absorb nutrients. Slowing down yeast activity isn’t necessarily a bad thing. Long, slow fermentation allows the dough to develop a more complex flavor. For most recipes, the amount of salt should equal about 2 percent of the flour’s weight.
Don't Forget Temperature
Temperature is another important factor in yeast activity. Most yeast is most active at about 95 degrees Fahrenheit, but it thrives at any temperature between 50 and 115 F. Some bakers opt for long, slow rising times by refrigerating the dough. However, freezing the dough or exposing it to temperatures above 140 F can damage or kill the yeast. To keep the yeast active, ensure the other ingredients are not too hot or cold before mixing them into the dough.
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