Did you ever wonder why flour tastes like sawdust but a French or Italian bread made with that same flour and little else has a pleasant, sweet taste? The yeast in the bread works to break down the starches, expanding the bread with gases and alcohol that give our breads that sweet, yeasty flavor we love.

Yeast: A Living Organism

Bread wouldn’t be bread without yeast and yeast can’t work without sugars.  Yeast is alive—living organisms—and living organisms need food for fuel, in this case, simple sugars.  But flour is mostly starch and table sugar (sucrose) is too complex for the yeast before fermentation. 

Amylase and invertase, enzymes present in the flour or created by the yeast, break down the starch molecules into sugars.  Some of these simple sugar molecules become food for the yeast; others create the sweet flavor we find in a fine bread—even a French bread where there is no sugar added.

As the yeast feeds on the sugar, it creates two digestive byproducts—alcohol (ethanol) and carbon dioxide.  The carbon dioxide is what leavens the bread—carbon dioxide gases filter through the dough creating loft.  The alcohol is evaporated in baking.

Yeast and Fermentation

The biological and chemical actions taking place as the bread ages and rises are called fermentation.  Generally, a long, slow fermentation makes for better flavor, texture, and moisture retention.  Many fine breads call for “retarding” or slowing down the growth of the yeast with refrigeration. 

If the dough is refrigerated, the yeast grows more slowly.  Fermentation still takes place as the amylase enzymes work within the dough and sugar is released albeit at a slower rate.  When the dough is warmed and the growth of the yeast takes off, there is plenty of sugar present for the yeast and an excess of sugar to sweeten the bread.

When yeast grows more slowly, we find the richer, fuller flavor of breads made with retarded dough.  In a previous article, we discussed a focaccia that uses refrigeration to slow down the growth of the yeast and create the desired crumb and flavor.  Is it a good bread without retarding?  Yes, but retarding does give it desirable flavor overtones and a more open crumb.

Four Key Facts You Need to Know About Yeast

1. Hot water or water with too much chlorine or salt will kill the yeast. You might try bottled water from the store. Make sure your water is under 120 degrees.
2. What looks like yeast is not really yeast. They are little maltodextrin or other balls encapsulating yeast spores. The yeast spores are microscopic. Those little balls dissolve in the water to release the spores.
3. The yeast that you put in your dough, that’s just seed yeast to get the process started. Those little critters are growing and multiplying–at the right temperature doubling every ten minutes.
4. Let the dough rise–a lot. In most applications, let it rise until it has doubled in size and then form the loaves. Forming the loaves forces much of the gas out and redistributes the yeast. Don’t handle the bread too much. You don’t need to force all the gas out.

(Updated from May 24, 2014)