Know Your Yeast

WHAT IS YEAST?
Yeast is a single-celled microorganism related to mushrooms. About 1,500 species exist but, in the kitchen, we use mostly just one - Saccharomyces cerevisiae (which means “sugar-eating fungi”). Used to make bread rise, it’s available in various forms which differ mostly by moisture content.

Cake yeast (aka fresh yeast or compressed yeast) is made from a slurry of yeast and water with enough of its moisture removed so that the yeast can be compressed into blocks. Experienced bakers swear by its superior leavening and the nuanced slightly sweet flavor it gives baked goods. Cake yeast is highly perishable and lasts only about two weeks in the refrigerator.

Active dry yeast was developed by the Fleischmann’s company during World War II so that the U.S. Army could make bread without keeping yeast refrigerated. Partially dehydrated and formed into granules, it contains dormant yeast cells that keep at room temperature for several months. To use active dry yeast, rehydrate it first in warm water (about 105°F) along with a pinch of sugar to feed the yeast. The resulting foam is the ingredient that helps to strengthen the gluten or soften the crumb.

Yeast should be checked before every recipe as a habit. Better to spend a few minutes to check the yeast before beginning than make bread that doesn't rise. If the yeast is active, it will dissolve completely into the water and the liquid will start bubbling.


HOW DOES YEAST MAKE BREAD RISE?
As bread dough is mixed and kneaded millions of air bubbles are trapped and dispersed throughout the dough. Meanwhile, the yeast in the dough metabolizes the starches and sugars in the flour, turning them into alcohol and carbon dioxide gas. This gas inflates the network of air bubbles causing the bread to rise. During rising, the yeast divides and multiplies producing more carbon dioxide. As long as there is ample air and food (carbohydrates) in the dough, the yeast will multiply until its activity is stopped by the oven’s heat.