Understanding Baking: How Yeast Works

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? 

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.
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 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 the 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.  

http://www.preparedpantry.com/howyeastworks.aspx


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