Yeast Basics

Leavening.  There are three ways baked goods are leavened; i.e., caused to expand or rise:  (1) chemicals, (2) living organisms - organic method, (3) air and steam - physical method.  Prior to baking, pizza dough uses living organisms, specifically yeast, to rise.  During baking, it uses air and steam.  

Yeast.  Yeast is a single-celled microscopic plant which consumes carbohydrates (sugar) in the dough and changes them into (a) carbon dioxide gas and (b) alcohol.  This is called "fermentation."  The carbon dioxide, when trapped by a flour's gluten, causes the dough to rise.  

Since it is alive, yeast is temperature sensitive.

   45º             Yeast is inactive; this is a storage temperature for yeast.
   60º to 70º    Yeast acts slowly.
   70º to 90º    Yeast grows best; good temperature for bread dough to rise (proof).
   Above 100º  Yeast acts slowly.
   140º            Yeast is killed.

Gluten.  Gluten consists of long elastic strands of protein.  Gluten forms when two proteins in flour - glutenin and gliadin - absorb water, and then are mixed or kneaded.  The higher the percentage of protein in a flour, the more gluten that will form.  All-purpose flour has between 11% and 12% protein; bread flour between 12% and 13% protein.  Gluten traps gas in tiny pockets, much like bubble gum holds air when you blow in it; this causes dough to rise.  When baked, gluten solidifies giving support to the shape of baked goods.

Fat.  Fats surround gluten strands and keep them from sticking together.  In other words, they shorten gluten strands.  That is why they are called "shortening."  They decrease the effect of the gluten, making baked goods more tender.  French bread and pizza doughs have no or little fat; shortening is not desirable.

Ovenspring.  When pizza is baked, the air, carbon dioxide, alcohol, and moisture in the dough cause the dough to rise.  The air and carbon dioxide enlarge; the alcohol and water turn into steam and expand tremendously.  When water turns into steam, its volume increases 1,600 times!  That is why a good dough has adequate water and is somewhat sticky.  The rising of the dough during baking is called "oven spring."