Yeast, Gluten and Carbon Dioxide

You make sourdough bread from the same basic ingredients that you use for any other bread. The two most important ingredients are flour and yeast.

• Yeast is a single-cell fungus that breaks down the starches in wheat flour, forming sugar. This is fermentation. When the yeast works on the starch and sugar molecules, it gives off carbon dioxide gas and alcohol. Yeast is a leavening agent for bread. It is what makes the bread rise.

• Flour comes from any kind of ground grain, but most bread contains wheat flour. Two proteins found in wheat flour, gliadin and glutenin, form a stretchy substance called gluten. When you knead dough, you help gluten form long, threadlike chains. These gluten chains help hold the carbon dioxide gas in, creating those tiny holes that create the airy texture of bread.

The big difference between sourdough bread and the "normal" bread you buy or bake today is the source of the yeast. Most bakers today use cultivated yeast that comes in a package. The package contains live yeast fungi in suspended animation! The yeast has been dried, preserved and formed into a powder. You add flour, water, sugar and salt to the yeast to make a loaf of bread. The water re-activates the yeast fungi, which feeds on the sugar and starch to make the bread rise.

Sourdough bread deals with yeast in a completely different way. Sourdough yeast fungi are actually kept alive constantly in a liquid medium called a starter. The baker either captures wild yeast that floats in the air to create starter from scratch or gets a cup of active starter from a friend and expands it.

It turns out that the starter is what gives sourdough bread its distinctive taste...